review and approval statement - the national .... irwin climo.pdf2004/03/21 · robert j. falvey, lt...

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REVIEW AND APPROVAL STATEMENT

AFCCC/CCD-04/006, National Training Center (NTC), Fort Irwin, California–A Full-YearClimatology,has been reviewed and is approved for public release. There is no objection tounlimited distribution of this document to the public at large, or by the Defense Technical InformationCenter (DTIC) to the National Technical Information Service (NTIS).

ROBERT J. FALVEY, Lt Col, USAFDirector of Operations

JOHN D. GRAYScientific and Technical InformationProgram Manager

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2. Report Date: 21 March 2004

3. Report Type: Country Climatological Digest

4. Title: National Training Center (NTC), Fort Irwin, California–A Full-Year Climatology

6. Authors: Mrs. Melody Higdon

7. Performing Organization Names and Address: Air Force CombatClimatology Center (AFCCC), Asheville NC 28801-5002

8. Performing Organization Report Number: AFCCC/CCD–04/006

12. Distribution/Availability Statement: Approved for public release;distribution is unlimited.

13. Abstract: This country climatology digest is a climatological study ofNational Training Center (NTC), Fort Irwin, California. After describing thegeography and major meteorological features of the entire region, the studydiscusses in detail the climatic controls of National Training Center’s’weather. Each “season” is defined and discussed in considerable detailwith emphasis on general weather, hazards, clouds, visibility, winds,precipitation, and temperature.

14. Subject Terms: CLIMATOLOGY, METEOROLOGY, WEATHER,GEOGRAPHY, TOPOGRAPHY, CLOUDS, SKY COVER,TEMPERATURE, DEW POINT, RELATIVE HUMIDITY, ATMOSPHERICPRESSURE, PRECIPITATION, VISIBILITY, WINDS, FORT IRWIN,CALIFORNIA

15: Number of Pages: 32

17. Security Classification of Report: UNCLASSIFIED

18. Security Classification of this Page: UNCLASSIFIED

19. Security Classification of Abstract: UNCLASSIFIED

20. Limitation of Abstract: UL

REPORT DOCUMENTATION PAGE

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TABLE OF CONTENTS

CONVENTIONS ------------------------------------------------------------------------------------- 7

TERRAIN ---------------------------------------------------------------------------------------------- 7

ALL SEASONS -------------------------------------------------------------------------------------- 8Air Pollution ----------------------------------------------------------------------------------- 8

WINTER (NOVEMBER - FEBRUARY) -------------------------------------------------------- 9

General Weather ---------------------------------------------------------------------------- 9Sky Cover ------------------------------------------------------------------------------------- 10Visibility --------------------------------------------------------------------------------------- 10Winds ------------------------------------------------------------------------------------------ 10Precipitation ---------------------------------------------------------------------------------- 12Temperature ---------------------------------------------------------------------------------- 13

SPRING (MARCH - MAY) ------------------------------------------------------------------------- 16


SUMMER MONSOON (JUNE-SEPTEMBER) ---------------------------------------------- 24


FALL TRANSITION (OCTOBER) --------------------------------------------------------------- 32


BIBLIOGRAPHY ------------------------------------------------------------------------------------ 39

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LIST OF FIGURES

Figure 1. Terrain Map of the Area. ------------------------------------------------------------- 8

Figure 2. January Wind Rose for Ft Irwin/Bicycle Lake. ---------------------------------- 11

Figure 3a. January Mean Precipitation. ------------------------------------------------------- 12

Figure 3b. January Mean Precipitation Days. ------------------------------------------------ 13

Figure 4a. January Mean High Temperature. ------------------------------------------------- 14

Figure 4b. January Extreme High Temperature. --------------------------------------------- 14

Figure 4c. January Mean Low Temperature. ------------------------------------------------- 15

Figure 4d. January Extreme Low Temperature. ---------------------------------------------- 15

Figure 4e. January Days with Temperature Less Than or Equal to 32F.---------------- 16

Figure 5. April Wind Rose for Ft Irwin/Bicycle Lake. -------------------------------------- 18

Figure 6a. April Extreme High Temperature. ------------------------------------------------- 19

Figure 6b. April Days with Temperatures Equal to or Greater Than 90F. --------------- 20

Figure 6c. April Mean Low Temperature. ----------------------------------------------------- 20

Figure 6d. April Extreme Low Temperature. -------------------------------------------------- 21

Figure 7. Typical Monsoon Set Up for the Region. ---------------------------------------- 22

Figure 8. August Wind Rose for Ft Irwin/Bicycle Lake. ----------------------------------- 24

Figure 9a. August Mean High Temperature. -------------------------------------------------- 25

Figure 9b. August Days with Temperatures Equal to or Greater Than 90F. ------------ 26

Figure 9c. August Mean Low Temperature. -------------------------------------------------- 26

Figure 9d. August Extreme Low Temperature. ----------------------------------------------- 27

Figure 10. October Wind Rose for Ft Irwin/Bicycle Lake. ---------------------------------- 29

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Figure 11a. October Mean High Temperature. ------------------------------------------------ 30

Figure 12b. October Days with Temperatures Equal to or Greater Than 90F. ---------- 31

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CONVENTIONS: The spelling of place namesand geographical features are those used bythe National Geospatial-Intelligence Agency(NGA). All distances are in nautical miles (NM)and kilometers (km), except for visibility, whichis in statute miles and meters. Elevations arein feet above mean sea level (MSL), with ametric conversion following. Temperatures arein degrees Fahrenheit (F) and Celsius (C).Wind speeds are in knots. Cloud bases areabove ground level (AGL) unless otherwisestated; tops, when provided, are above meansea level (MSL). Precipitation amounts are ininches, with a millimeter (mm) or centimeter(cm) conversion following. Precipitation valuesgiven are liquid equivalent unless statedotherwise. Any graphics provided tosupplement the text will not include conversions.Standard pressure levels are expressed inmillibars (mb). Time is reported either inCoordinated Universal Time (UTC) (also knownas Zulu or Z), or Local (L).

TERRAIN. The National Training Center is inthe heart of the Mojave Desert, 30 NM (56 km)northeast of Barstow, Calif. Bicycle Lake AAFis 2.6 NM (5 km) northeast of the main post.The field elevation is 2,350 feet (716 meters).The entire reservation encompasses more than642,000 acres of training area with the northernboundary less than 1.7 NM (3 km) from DeathValley National Monument. The San Bernadinoand San Gabriel Mountains are oriented eastto west 73 NM (135 km) southwest of BicycleLake. The Sierra Nevada Mountains, orientednorth to south, are to the west. Elevations in

excess of 10,000 feet (3,050 meters) arecommon in these ranges.

Broad valleys surrounded by rugged mountainpeaks dominate the NTC. Dry lakebeds arefound in nearly every internal basin. Elevationsrange from more than 6,000 feet (1,830 meters)in the northeast corner (the Avawatz Mountains)to 1,300 feet (400 meters) in the southeastcorner. This type of terrain is typical of most ofthe Mojave Desert.

Bicycle Lake is in a basin at the intersection ofseveral mountain ranges. The basin is 16 NM(30 km) long and extends northeast tosouthwest. It is 3.5 NM (6 km) wide at BicycleLake. It slopes gradually 9 NM (17 km)southwest to Paradise Range, which is at 3,430feet (1,046 meters). The Granite Mountains start11 NM (20 km) north. Elevations are 3,700-4,000 feet (1,130-1,220 meters). Just 0.9 NM(2 km) east, the Tiefort Mountains rise abruptlyto 5,000 feet (1,520 meters) and 4 NM (7 km)west, the foothills of Goldstone Range rise to3,500 feet (1,070 meters). There is a narrowpass between two hills 4.3 NM (8 km) west-northwest.

The lake is dry most of the time. Almost allprecipitation that falls on the mountains runs offinto the basins. Bicycle Lake acts as a drainagepoint for the surrounding high ground. Evenwhen it does contain water, it is very shallow.Water may remain in the lake for up to 2 weeksafter the surrounding mountains receive heavyrain. Figure 1 is a terrain map of the area.

National Training Center (NTC), Fort Irwin, CaliforniaA Full-Year Climatology

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Figure 1. Terrain Map of the Area. The box includes the working area around the NTC.

ALL SEASONS

Air Pollution. Smog from the Los Angeles areaoccasionally spills over the San GabrielMountains into the Mojave Desert. This is mostcommon in September through March. Visibilitygenerally remains at or above 5 miles (8,000

meters). Increasing industrial activity in thedesert to the south and west of the training centeradds its share of pollution. The haze and smoggenerally stay in southern areas of the desertand usually have no impact on conditions at thetraining center.

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WINTER (NOVEMBER-FEBRUARY)

General Weather. During winter more low-pressure systems (storms) move through thearea than at any other time of year; however,rainfall is still minimal in this desert region. TheNorth Pacific and Bermuda highs both retreatout of the area, which sets up the cycle of shortwave systems that move through the long wavetrough over the western US. Despite the numberof storm systems that can reach the area, fewproduce more than brief periods of cloudinessand even fewer produce any rainfall in thissheltered desert area. Storms that develop inthe Aleutian low track down the western side ofthe long wave trough to hit the US western coastand then continue eastward. These Gulf ofAlaska lows generally stay north of the area untillate December, but their associated cold frontscan reach here by late November. During ElNiño years, rainfall and cloud cover are bothgreater in the region than in other years, butconditions still remain largely dry because ofthe sheltering mountains to the west.

Wave trains (series of storms) that move throughthe zonal (straight-line) large-scale flowoccasionally set up and a cycle of cloudy andclear skies that continues until the zonal flowdisappears, usually in 3-5 weeks. Themountains to the west protect this area frommost of the effects of these storms, but someprecipitation still occurs. Cold fronts associatedwith lows that sweep past to the north producelimited rainshowers or rain, but storms thatmove up from the southwest can bring heavierrain to the area.

Cut-off lows sometimes form off the southernCalifornia coast (San Diego vicinity). The longerthese storms remain in place, the more rainfallthey can bring to the area. They are almostexclusively a winter feature and bring steady lightrain from the west or southwest. Cut-off lowsthat form between the Rockies and the easternPacific Ocean and those that occur far enough

south can bring rainfall directly to the area frommid-level clouds that stream over the mountainsor indirectly in wrap-around moisture that isadvected in behind the passing low once itmoves eastward.

Cold fronts that approach from the northeast arecalled “Back-door” cold fronts. They developon the southern rim of a shallow cold high overthe Salt Lake area and can bring strong, gustywinds to the area. The area is most vulnerableto these fronts in January and February.Mountain passes that are oriented northeast tosouthwest can enhance these winds becauseof funneling. Very high pressure over the GreatBasin area is a good indicator of anapproaching back door front.

The Pineapple Express is the name for a longband of moisture that stretches across theeastern Pacific from south of Hawaii to the westcoast. Although this normally brings heavyrainfall to the northwestern states, the band ofmoisture occasionally shifts southward to causeheavy rain, unusually warm temperatures for theseason, and potentially, flooding. Themountains shelter the training center from theworst effects but do not totally protect the area.Snowmelt in the mountains can also causeflooding in the lowlands.

October-March is the main period for theoccurrence of Santa Ana (also seen asSantana) winds. Santa Anas are hot, extremelydry foehn-like desert winds that blow from thenorth or northeast across the desert. They entersouthern California through mountain passes,where they are further enhanced as mountaingap winds. Santa Anas develop under strongpressure gradients between a high centeredover the Great Basin (Nevada) and a low offthe southern California coast. They seldompersist more than 36 hours at a time, but thereare have been events that persisted for days.The winds blow, sometimes with great force(gale force), from the deserts, and they may

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carry a tremendous dust load. The combinationof heat, aridity and strong winds increase thepotential for serious fires.

Gale force winds can also occur when highpressure is off the coast and low pressure isover the Great Basin. The tight pressuregradient between the two systems causesstrong winds to blow from the north or northwestover the Mojave Desert. Winds have beenreported as high as 85 mph in some places.Sand an dust storms are more likely to occur inthe NTC area with these winds than with SantaAnas. Visibility in the areas most intenselyimpacted drops to just a few feet andsandblasting can seriously damage engines,rotors and other equipment. These wind eventsare most likely to occur in late winter and spring,but they can occur any time the samecircumstances (high off the coast and low overthe Great Basin) develop. Sandblasting is mostsevere within 6 feet of the ground. There areseveral documented instances in this area inwhich wind-driven sand wore away telephone/power poles until they snapped.

Sky Cover. Most cloud cover in this area is inmiddle and high clouds. The mean cloud coveris scattered all season, but there is more inJanuary and February than in November andDecember. Low clouds, mostly instratocumulus and stratus, rarely produceceilings except for brief periods underrainshowers. Ceilings below 25,000 feet occur15-20 percent of the time all day in November.In December through February, they occur 25-35 percent of the time, with the peak rate at 09-17L. Ceilings below 10,000 feet occur 4-6percent of the time all day in November and 15-20 percent of the time all day in December-February. Ceilings below 3,000 feet do notoccur in November and occur 2-3 percent ofthe time all day in December-February. Ceilingsbelow 1,000 feet are very rare all day in Januaryand at 06-14L in February and are not reported

the rest of the season. Ceilings below 200 feetare not reported at any time of the year.

Visibility. Visibility is generally excellent. Whilefog is more likely in winter than in any otherseason, it is still not at all common. If it occurs,it is most likely to happen under a dome of highpressure the morning after rain has occurred.In areas downwind of dust sources, dust liftedby strong winds is more likely to causerestrictions than fog. Fog restricts visibilitybelow 7 miles (11,000 meters) on an averageof 1-2 days per month all season. January hasthe most days. For the area, wind blown dustrestricts visibility below 11,000 meters on anaverage of 1 day or less per month, but man-made dust occurs whenever the range is in use.Because there is little vegetation, vehicle trafficcauses dust restrictions in the area of activity;the heavier the vehicles and traffic, the worsevisibility becomes. Once the surface hardpanis broken, the soil is quickly pulverized into dust.Under light winds, visibility in the area of activitycan remain poor for some time as dust lingersin the air.

At the observation site, visibility below 11,000meters occurs 1-3 percent of the time all winterexcept at 00-08L in January, when it occurs 4-5 percent of the time. Visibility below 3 miles(4,800 meters) is very rare at 00-05L in Januaryand is not reported the rest of the season.Visibility below 1 mile (1,600 meters) is notreported all season.

Winds. The complex terrain at the NTC causesfunneling of the winds that can increaseprevailing speeds by 10-15 knots. Thestrongest winds occur at the ends of valleys andpasses. Wind direction is steered by terrainand may be very different from place to placethroughout the training area. Calm conditionsoccur 25 percent of the time all season butoccurrence rates will vary widely with location.Sheltered sites have the highest rates of calmconditions.

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Figure 2. January Wind Rose for Ft Irwin/Bicycle Lake.

Funneled winds through valleys can cause windgusts up to 35 knots. The occasional frontalsystem that moves through the area canproduce wind gusts of 25-35 knots. Winds thatblow across ridges and peaks will cause light-to-moderate turbulence over the ridges and upto 50 NM (93 km) downwind. The air is very dry

so rotor clouds will not usually form to give visualclues to this hazard. The training center isvulnerable to strong gradient winds such asSanta Anas and Great Basin winds. Figure 2is the January wind rose for Ft Irwin/BicycleLake, which is representative for the season inthis area.

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Figure 3a. January Mean Precipitation.

Precipitation. Although rainfall occurs in winter,it is typically light and with little accumulation.Light drizzle or rain that persists for a few hoursis most common with lows from the southwest(such as cut-off lows), but rainshowers occur withcold fronts. Thunderstorms are uncommon butcan occur with fast-moving cold fronts. Coldfronts and their parent storm systems moveprogressively farther south as the seasonadvances and typically do not bring much rainto the area until late in December. During ElNiño years, however, they bring in rain as earlyas mid to late October.

Snow is uncommon here. The local mountainpeaks are more likely to get snow than thelowlands, but it does linger for more than a fewhours below 2,500 feet (760 meters) elevation.There are exceptions. The winter of 1948-1949brought a drastic departure from the usualweather. The entire western United Statesexperienced very cold temperatures andunusual snowfall. George AFB at Victorville, tothe southwest, recorded 17 inches (43 cm) ofsnow in January while other nearby locationsreported up to 14 inches (36 cm) of snow. Thiswas a very unusual event, but these conditionsare always possible, if not probable. Figures3a-b depict January precipitation statistics forthe area.

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Figure 3b. January Mean Precipitation Days.

Temperatures. Winter is generally mild tocool. Cold air outbreaks behind back door coldfronts produce the lowest temperatures andsouthwesterly winds and clear skies producethe warmest days. Record highs reached 86F(30C) in November and February and 74F

(23C) in December and January. Record lowsreached 11 to 18F (-12 to –8C) all season, withthe coldest records in December and January.Figures 4a-e depict January temperaturestatistics, which are representative for winter inthis area.

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Figure 4a. January Mean High Temperature.

Figure 4b. January Extreme High Temperature.

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Figure 4c. January Mean Low Temperature.

Figure 4d. January Extreme Low Temperature.

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Figure 4e. January Days with Temperature Less Than or Equalto 32F.

SPRING (MARCH-MAY)

General Weather. Spring weather is a true mixof winter and summer systems. As the NorthPacific and Bermuda high build back into theregion, the cycle of storm systems that movedown from the Gulf of Alaska slowly shiftsnorthward. The North Pacific high ischaracterized by strong subsidence on theeastern side. This causes the air to becomevery hot and dry and contributes to the desertcharacteristics of the region. All the stormsystems that occur in winter still occur in spring,but summer systems begin to take over by mid-April. Fronts occasionally move through butusually produce little weather. Winds increase12 hours before the frontal passage to gust upto 25-35 knots, generally from the west. Skieswill be overcast with ceilings of 4,000-5,000 feetwith frontal passage, but these conditionsusually last only 2-6 hours. March weather ismuch like that of winter and May weather is likethat of summer. Rainfall remains limited in

March and decreases in April and May towardthe annual minimum in June. Temperatureswarm quickly and diurnal temperature spreadsincrease as humidity decreases.

Santa Ana winds, while more prevalent InOctober-March have occurred all through springin some years. These strong, hot winds areextremely dry and can carry a lot of dust intothe area. Slant range visibility aloft is often moreseverely restricted, but ground visibility can besharply reduced as well. Santa Anas increasethe potential for serious fire hazards, can causeproblems with equipment due to dust, and aredangerous to ground troops because of heatand dehydration issues.

Great Basin gale force winds blow from thenorth or northwest over the Mojave Desert.Winds have been reported as high as 85 mphin some places. Sand and dust storms aremore likely to occur in the NTC area with thesewinds than with Santa Anas. Visibility in the

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areas most intensely impacted drops to just afew feet and sandblasting can seriouslydamage engines, rotors and other equipment.These wind events are most likely to occur inlate winter and spring, but they can occur anytime the same circumstances (high off the coastand low over the Great Basin) develop. Asevere sand and dust storm event occurred in30 April 1954.

Sky Cover. Mean cloud cover is scattered allseason but there is less in May than in April orMarch. The sky is clear more than 50 percentof the time. The cloud types change from mostlystratiform types to mostly cumuliform clouds andhigh, thin cirrus. Rainshower and thunderstormcloud bases, already high, rise as thetemperatures warm and aridity increases.When thunderstorms develop over themountains, the cirrus canopy, known as theanvil, spreads downwind of the storm.

Ceilings below 25,000 feet occur 20-30 percentof the time in March, with the highest rates at09-17L. In April, they occur 15 percent of thetime or less most of the day and 26 percent ofthe time at 12-20L. In May, they occur 10-15percent of the time all day with the highest ratesat 12-20L. Ceilings below 10,000 feet occurless than 10 percent of the time most of thedays in March and a maximum of 15 percent ofthe time at 09-17L. In April and May, they occur5 percent of the time or less all day, with little to

no diurnal variation. Ceilings below 3,000 feetare very rare at all hours in March and April andare not reported at all in May. Ceilings below1,000 feet are not reported in this season.

Visibility. Visibility is typically excellent allseason. Although a light fog at sunrise is stillpossible along rivers in March and early April, itis not at all common and it quickly dissipatesonce the sun is well up. Dust is much more likelyto restrict visibility, especially where humanactivity is greatest. Wheeled and trackedvehicles that move off road can cause dustrestrictions of 3 miles (4,800 meters) or lessthat persist for hours in the immediate area ofthe activity. On windy days, they can kick up adust haze that spreads over a larger areadownwind and cause slight restrictions for aslong as they continue to move. In periods ofintense vehicle activity, visibility can bedegraded to below 1 mile (1,600 meters) in theimmediate area.

Fog that restricts visibility below 7 miles (11,000meters) occurs on an average of less than 1day per month in March and April and not at allin May. Blowing dust restricts visibility below11,000 meters on an average of 1 day permonth. At the observation site, visibility below11,000 meters occurs 1-2 percent of the timeat all hours in March and April and not at all inMay. Visibility below 4,800 meters is notreported all season.

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Figure 5. April Wind Rose for Ft Irwin/Bicycle Lake.

Winds. The complex terrain at the NTC causesfunneled winds that can increase prevailingspeeds by 10-15 knots. Calm conditions occur10-15 percent of the time all season with themost occurrences at sunrise. Santa Ana andGreat Basin winds are generally the strongestpersistent winds but thunderstorm down rushgusts cause winds up to 55 knots. The strongest

winds occur at the ends of valleys and passeswhere funneling occurs. Wind direction islargely governed by terrain and may be verydifferent from place to place throughout thetraining area. Figure 5 is the April wind rose forBicycle Lake, which is representative of theseasonal winds for this area.

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Precipitation. Rainfall decreases quickly inMarch as the winter storm track shifts northwardout of the region. The summer monsoon is notyet in effect and there is only limited convection(rainshowers and thunderstorms) with passinglate season cold fronts. Mean precipitationranges from 0 to 0.5 inches (0 to 13 mm).

Temperatures. The days heat up quickly inthis season and the average high is above 90F(32C) before mid April. The relative humiditydecreases as temperatures rise, which

increases the diurnal spread between the dailyhigh and low temperatures. The progressivelyhotter and drier conditions begin to makeground troops vulnerable to disablingdehydration and heat-related illness. Mean hightemperatures range from 70 to 80F (21 to 27C).Record highs reached 88F (31C) in March, 96F(36C) in April and 107F (42C) in May. Recordlows reached 30F (-1C) in March, 34F (1C) inApril and 38F (3C) in May. Figures 6a-d depicttemperature statistics for April, a representativespring month.

Figure 6a. April Extreme High Temperatures.

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Figure 6c. April Mean Low Temperatures.

Figure 6b. April Days with Temperatures Equal to or GreaterThan 90F.

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Figure 6d. April Extreme Low Temperatures.

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Figure 7. Typical Monsoon Set Up for the Region.

SUMMER MONSOON (JUNE-SEPTEMBER)

General Weather. The North Pacific andBermuda highs expand over the region andbring frequent clear skies and hightemperatures. Summer is hot and dry in theDesert Southwest even though the summermonsoon brings rainfall to the region in the formof rainshowers and thunderstorms. Themountains in the area get the most rain, as thisis where convective activity typically developsand expends most of its energy. Thunderstormsare high-based and often produce little morethan virga (rain that evaporates before it hitsthe ground) over the lowlands because the airis so dry. Spectacular lightning displaysfrequently light the night horizon in late Junethrough early September. The lightning fromthese dry thunderstorms can spark fires.

The monsoon is a period of thunderstorm andrainshower activity that resultsfrom the prevalent southeasterlyflow in the region in July throughearly September. The flow comesfrom the Bermuda high, whichreaches its maximum extent andintensity in this period. It carriestremendous moisture westwardfrom the Gulf of Mexico at 8,000-12,000 feet. Combined with thevalley breezes that flow upmountains and hills, and risingconvective currents in blisteringdaytime heating, this moist flowfuels high-based convection thattypically starts on mountain tops.It rapidly expands as outflowboundaries from these first

storms converge and fire off more. The thermaltrough that is over the Mexican and Americandeserts and separates the Bermuda and NorthPacific highs contributes to the instability of theregime. Early markers for the onset of themonsoon are rising humidity and afternoon“popcorn” cumulus that begins to develop onthe mountains. As the season gets closer, thecumulus gets larger and larger and eventuallydevelops into thunderstorms. Another indicatoris the steady westward spread of thunderstormactivity across Texas and then New Mexico,Arizona and, finally, California. Despite this,rainfall is still only comparatively heavy, relativeto normally bone-dry conditions, in the valleysand basins. The moisture seldom reaches thedesert floor, but helps produce thunderstormsover the San Gabriel Mountains to the south.Rain from these storms may cause flash floodsin the training area. Figure 7 depicts thegeneralized large-scale flow during themonsoon season.

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Thunderstorms can slide down the mountainsinto the valleys below after sunset but just asoften, remain in the mountains where lightningfills the night sky for hours. The thunderstormspresent a potential ground hazard apart fromlightning, strong down rush gusts andoccasional hail. Even if they never move downfrom the mountains, they still fill gullies, river andstreams that are normally dry. This can happenvery quickly and flash floods can occur in a valleyor basin because of storms too far away to seeor hear. It is not safe to camp or travel in dryriverbeds at any time of year, but it is particularlydangerous in the monsoon season. This iswhere the term “gullywasher,” a term for flashflood-causing heavy rains, was born. Monsoonthunderstorms have dropped as much as 3-4inches of rain within 24 hours, most typicallywithin 6 hours, in this region.

On rare occasions, easterly waves that travelwestward along the base of the Bermuda highhave tracked into the area and produced heavy,even severe convective activity during themonsoon season. These rare storms tend tobe the ones that produce the heaviest rainfallevents. They are generally seen as lines ofconvection that are oriented north to south andtravel rapidly westward.

Tropical cyclones that track northward from thesouth or southwest bring widespreadprecipitation and thunderstorm activity to thearea. The storm tracks are those that movestorms directly up the Gulf of California or upthe Pacific coast toward southern California.Those storms that track directly up the Gulf ofCalifornia produce the most rainfall and floodingin the desert southwest. These storm tracksare not the usual ones, however, there is a clearlink between these tracks and strong El Niñoevents. During strong El Niños, the coldupwelling current is displaced northward andallows the storms that track up the coast toremain over warm water longer and, thus, bemore intense and survive intact longer. Oncethere, they can be swept eastward by a passing

long wave trough, which advects the stormmoisture into the Southwest. Once again, themountains absorb most of the moisture fromthese storms on their windward slopes butsome rain does make it into the area.

Santa Anas or Great Basin winds, while nottypical of summer, can occur in this seasonunder the right conditions (see winter generalweather description). These winds can rapidlyspread fires started by dry thunderstorms.Winds can be very strong and funneling throughnorthwest or north facing mountain passescontributes to the problem because it canincrease winds by 15 knots or more. Sand anddust storms created by these winds can reducevisibility to near zero in the most intenselyimpacted areas, and cause serious mechanicalproblems with equipment.

Sky Cover. Mean sky cover is clear toscattered, with most cloud cover inthunderheads over the mountains or high cirrus.Low clouds are unknown outside of theoccasional monsoon rainshowers orthunderstorms that reach peak activity in Julyand August. Middle clouds tend to occur onlywith rainshowers and thunderstorms. Ceilingsbelow 25,000 feet occur 5-15 percent of the timeall season. The highest rates occur at 15-20Land the lowest occur at 21-05L. Ceilings below10,000 feet occur 5 percent of the time or lessall season with lower rates overnight than duringthe day. Ceilings below 3,000 feet are notreported all season.

Visibility. Visibility is generally good toexcellent. Although dust is the main cause ofrestrictions to visibility, it does not naturally limitit much below 5 miles (8,000 meters) on windydays or at all on calm or light wind days. Dust ismuch more likely to restrict visibility wherehuman activity is greatest. Wheeled andtracked vehicles that move off road over thedesert can cause dust restrictions of 3 miles(4,800 meters) or less that persist for hours inthe immediate area of the activity. On windy

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Figure 8. August Wind Rose for Ft Irwin/Bicycle Lake.

days, they can kick up a dust haze that spreadsover a larger area downwind and cause slightrestrictions for as long as they continue to move.In periods of intense vehicle activity, visibility canbe degraded to below 1 mile (1,600 meters) inthe immediate area. At the observation site,visibility below 11,000 meters is very rare or isnot reported all season. Visibility below 3 miles(4,800 meters) is not reported all season.

Winds. The prevailing winds in summer comefrom the south or south-southeast at 7-8 knotsall season. Winds from these directions occur30-40 percent of the time and reflect the summermonsoon winds that flow around the base of theBermuda high. Calm conditions occur 10-15percent of the time in June-August and 20percent of the time in September. Calms aremost likely at sunrise. Peak gusts reached 55-60 knots. The strongest winds of summertypically occur in down rush gusts underconvection (rainshowers and thunderstorms).

High-based or distant thunderstorms are stillcapable of producing surprisingly strong surfacewinds in the region.

The complex terrain at the NTC causesfunneling of the winds that can increaseprevailing speeds by 10-15 knots. Thestrongest winds occur at the ends of valleys andpasses. Wind direction is largely governed byterrain and may be very different from place toplace throughout the training area. Strongsurface heating and funneling of winds throughvalleys can cause wind gusts up to 35 knots.Although they are uncommon in this season,Santa Anas or Great Basin winds cause strongwinds that can persist for days at a time. Theyweaken somewhat at night and reach peakstrength in the afternoons. Figure 8 is the Augustwind rose for Ft Irwin/Bicycle Lake. It isrepresentative of the winds for the summerseason.

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Figure 9a. August Mean High Temperature.

Precipitation. The area is typically pretty dry.Nearly all the rainfall occurs in rainshowers andthunderstorms over the mountains. Should athunderstorm or heavy rainshower occuroverhead, it is possible to get as much rain inan hour as is normal for the whole year. Theground is hard and resists absorbing water, solarge puddles can develop quickly and onlygradually disappear by evaporation under thehot sun. Distant storms or storms over themountains may not produce rainfall on aparticular site but can still produce flooding,particularly flash flooding there. Riverbeds,gullies and arroyos that are normally dry can fillto raging torrents with little to no warning andmust be regarded as hazardous at all time.Mean precipitation ranges from 0 to 0.5 inches(0 to 13 mm) during August, which is arepresentative month for the summer.

Temperatures. Conditions are hot and dry allseason and the occasional rainshowers and

thunderstorms do little to nothing to break theheat. The higher relative humidity the rainfallbrings makes it more uncomfortable but is nota long-term problem in this arid area. Thediurnal temperature spread is typically 30Fahrenheit (11 Celsius) degrees or morebetween the daily high and low temperatures.Since daytime temperatures are so high andthe air is so dry, heat-related illnesses anddehydration are serious threats to groundtroops. During the day, it is important toremember that the soil surface may be as muchas 30 Fahrenheit (11 Celsius) degrees hotterthan the air just above it. Area temperaturesare above 100F (38C) every day. Record highsreached 107 to 113F (42 to 45C) all season.Record lows reached 45 to 46F (7 to 8C) inJune and September and 56F (13C) in July andAugust. Figures 9a-d depict Augusttemperature statistics, which represent summerconditions.

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Figure 9b. August Days with Temperature Greater Than or Equalto 90F.

Figure 9c. August Mean Low Temperature.

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Figure 9d. August Extreme Low Temperature.

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FALL TRANSITION (OCTOBER)

General Weather. Summer heat lasts nearlythrough this whole month yet the first signs ofthe coming winter season begin to be seen.Thunderstorm activity decreases to practicallynothing as the monsoon flow disappears byearly in the month. The retreat of the moisteasterly flow from around the base of theBermuda high can be followed with by theeasterly retreat of the thunderstorm activity. TheNorth Pacific and Bermuda highs weaken andretreat away from the area. This opens the doorfor deep troughs to begin to push farther andfarther south as the Aleutian low deepens andexpands. The first of the winter cold fronts sweepthrough north of Ft Irwin but get progressivelycloser as the month goes on. Cut-off lows andwave trains start to develop rainfall on the westcoast and gradually shift farther and farther southin October.

The first of the cold season Santa Anas occurin October and these hot, dry winds can be verystrong even now. They develop when a highover the Great Basin and a low off the southernCalifornia coast occur at the same time. Thepressure gradient between the two systemssends powerful north or northeast winds acrossthe deserts and through mountain passes intosouthern California. The passes augment thewinds because of funneling and gale-forcewinds can persist for days or weeks at a time.The winds weaken at night and reach peakspeed in the afternoons. Santa Anassometimes carry a lot of dust and can restrictvisibility both on the ground and in the air.

Great Basin winds can cause strong winds toblow from the north or northwest over the MojaveDesert. Sand/dust storms are more likely tooccur in the NTC area with these winds thanwith Santa Anas. Visibility in the areas mostintensely impacted drops to just a few feet andsandblasting by sand can seriously damageengines, rotors and other equipment. Althoughthis is more likely to occur in winter than in other

seasons, they can occur any time the samecircumstances (high off the coast and low overthe Great Basin) develop.

Sky Cover. The mean cloud cover is scatteredthrough October, but there is a slight increasein high and middle cloud by the end of the month.The cloud types change from cumuliform tomostly stratiform types by the end of the monthas well. Ceilings below 25,000 feet occur lessthan 10 percent of the time at 18-05L and 15percent of the time or less the rest of the day.Ceilings below 10,000 feet occur 5 percent ofthe time all day. Ceilings below 3,000 feet arenot reported in this month.

Visibility. Visibility is generally good toexcellent all season. The main cause of visibilityrestrictions continues to be dust. Wind blowndust is far less of a problem than dust raised bytracked and wheeled vehicles over the aridterrain. Depending on the amount of vehicletraffic and the strength of the wind, visibility canbe reduced to a mile (1,600 meters) or less inthe immediate area of activity and to less than3 miles (4,800 meters) within 1-3 miles (1/2-5km) of the area. Dust lifted by wind alone rarelydecreases visibility below 5 miles (8,000meters). Although morning fog is possible, it isquite uncommon even around the seasonalrivers. It is light, and dissipates quickly soonafter sunrise. Fog restricts visibility below 7miles (11,000 meters) on an average of lessthan 1 day in October. Blowing dust restrictsvisibility below 11,000 meters) on an averageof less than 1 day all month as well. At theobservation site, visibility below 11,000 metersis rare all day and visibility below 3 miles (4,800meters) is not reported.

Winds. The end of the summer monsoon ismarked by the change of southerly winds tonortherly. This occurs in late September or earlyOctober. The prevailing winds in October arelight and calms occur often, roughly 25-30percent of the time. This rate depends onlocation. The more sheltered a site is, the higher

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Figure 10. October Wind Rose for Ft Irwin/Bicycle Lake.

the rate of calm conditions. The peak gustsreached 42 knots. The strongest wind gustsstill occur in down rush flow under high-basethunderstorms, but winds behind cold frontsbegin to increase by late in the month. SantaAnas or Great Basin winds have the highestpersistent winds. Funneled winds can exceedprevailing speeds by 10-15 knots. Thestrongest winds occur at the ends of valleys and

passes. Wind direction is largely governed byterrain and may be very different from place toplace throughout the training area. Varyingwinds can be a hazard to low-level airoperations. Funneling of winds through valleyscan cause wind gusts up to 35 knots. Figure10 is the October wind rose for Ft Irwin/BicycleLake, which is representative of winds in thearea.

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Figure 11a. October High Temperatures.

Precipitation. Summer monsoonthunderstorms disappear in October assoutherly winds retreat eastward. It is stillpossible for a thunderstorm or rainshower toproduce a brief period of heavy rainfall over asmall area, which would cause localizedflooding. The retreat of the monsoon rains canbe tracked eastward across Arizona into NewMexico and then Texas. By the middle of themonth, the monsoon convection is gone and thefirst of the early winter storm systems begin tosweep cold fronts north of the area. These rarelymake it far enough south to reach this area withrainfall, but precipitation has occurred onoccasion. El Niño years are when cold frontsare most likely to reach this far south in October.Mean precipitation ranges from 0 to 0.5 inches(0 to 13 mm).

Temperatures. It is still hot in October, but theworst is generally over by mid month. As themonsoon southerly winds disappear, the relativehumidity drops and the comfort level rises a bit.The diurnal temperature spread is still quitelarge, an average of 30 Fahrenheit (11 Celsius)degrees between the daily highs and lows.Although temperatures are beginning to cooldown, the threat of dehydration and heat-relatedillness still exists for ground troops. The recordhigh at Ft Irwin reached 100F (38C) and therecord low reached 31F (-1C). Extreme highsin the area range from 107 to 115F (42 to 46C)at most places. Mean low temperatures rangefrom 40 to 50F (4 to 10C). Figures 11a-b depictOctober temperature statistics.

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Figure 11b. October Days with Temperatures Greater Than or Equal to 90F.

32

BIBLIOGRAPHY

Bryson R.A., et al, World Survey of Climatology Vol. 9, Climates of North America, ElsevierPublishing Company, Amsterdam, the Netherlands, 1974

Det 13,25 WS, 5WW, Terminal Forecast Reference Notebook, Davis-Monthan AFB, 1943 to1979

Higdon, M., Narrative: National Training Center, Ft Irwin, California, A Full Year ClimatologyOverview, Air Force Combat Climatology Center, June 1997

National Training Center (Bicycle Army Airfield), Terminal Forecast Reference Notebook, FtIrwin AAF website

Ramage, Colin S., Forecasters Guide to Tropical Meteorology, Air Weather Service TechnicalReport 240, Air Weather Service, Scott Air Force Base, Ill, 1995

Satellite Monitoring of the Climate, Climatic-Monitoring Atlas: 1986-1994, METEO FRANCE,Paris, 1994

Schmidli, R.J., et al, NOAA Technical Memorandum NWS WR-177, Climate of Phoenix,Arizona, NOAA, Phoenix, Arizona, July 1996

Smith, Walter, The Effects of Eastern North Pacific Tropical Cyclones on the SouthwesternUnited States, NOAA, Salt Lake City, Utah, August 1986

US Marine Corps, OH 0-57, A Study of Windborne Sand and Dust in Desert Areas, QuanticoVirginia, August 1990

review and approval statement - the national .... irwin climo.pdf2004/03/21 · robert j. falvey, lt...

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