Synoptic-Scale Weather Systems of the

Intermountain West

Dr. David Schultz

NOAA/National Severe Storms LaboratoryNorman, Oklahoma

schultz@nssl.noaa.govhttp://www.nssl.noaa.gov/~schultz

What Might Be the Forecasting Challenges in February 2002?• 1–2 February 1989: SLC Airport greatest 24-h Feb

snowfall of 11.9”, associated with strong cold front. 40–60-mph south winds ahead of front.

• 10 February 1975: 65-mph winds derailed chair lift at Park City Ski Resort. Gondolas shut down.

• 12–13 February 1986: 3 feet of snow in the Wasatch Mts. Avalanche in the Sundance area.

• 13–19 February 1985: Dense fog caused multiple vehicle accidents (30 car and semi pileup on 13th) and 3 deaths.

What are the Winter-Weather Problems in Northern Utah?

• low pressure systems and frontal passages

• snowstorms (mountain and valley)

• winds (synoptic-scale and canyon winds)

• fog due to synoptic-scale ridging

• bitter cold from arctic outbreak

What are the Winter-Weather Problems in Northern Utah?

• low pressure systems and frontal passages

• snowstorms (mountain and valley)

• winds (synoptic-scale and canyon winds)

• fog due to synoptic-scale ridging

• bitter cold from arctic outbreak

TO BE DISCUSSED IN THIS TALK

Planetary-Scale ClimatologyIn the wintertime, a planetary-scale ridge and subtropical high are usually positioned over the western US, implying that the passage of cyclonic storms usually is inhibited.

Leads to high incidence ofanticyclones, cyclolysis, frontolysis

In this regime (positive PNA pattern), forecast models are usually more predictable (Palmer 1988),probably due to persistence.

(Lackmann et al. 1996)

Zishka and Smith: anticyclones

Zishka and Smith (1980)

Zishka and Smith: cyclones

Zishka and Smith (1980)

Nevada Lee Cyclogenesis

Favored Feb.–May, with a secondary maximum in Nov.

Lee (1995) identified two kinds of Nevada lee cyclones: SW (75%) and NW (25%)

SW cases begin with lee troughing, then when forcing aloft overspreads trough, cyclogenesis occurs and cyclone becomes mobile away from lee of Sierra Nevada.

Tiros Lee: Nevada lee cyclogenesisL

L

LL

SW Nevada Lee Cyclogenesis

Composite 500-mb height, QGPV and sfc highs/lows

(Lee 1995)

–24 h

+24 h +48 h

0 h

Tiros Lee: Nevada lee cyclogenesis

NW Nevada Lee Cyclogenesis

Composite 500-mb height, QGPV and sfc highs/lows

(Lee 1995)

H

LL

L

H

H

–24 h

+24 h +48 h

0 h

Atallah and Bosart (1996)

Frequency of 700-mb vorticity maxima passages through western U.S.

Atallah and Bosart (1996)

Frequency of 700-mb vorticity maxima passages through western U.S.

British Columbia track

Columbia River Valley track

Arizona and CaliforniaCentral Valley tracks

Tracking Cyclones and Upper-Level Forcing

• Lows typically don’t move through the West continuously.• Schultz and Doswell (2000) suggested that tracking the

occurrence of a mobile pressure minimum (a signal of the upper-level forcing) may assist in analysis.

L1

L3L2

primary low

Fraser Rivertrough

lee low

Tracking Cyclones and Upper-Level Forcing

• Look for pressure-check signatures in time series of SLP or altimeter setting, or the location of the zero isallobar

Frontal Passages in the West-I• Upstream topography tears fronts apart: Steenburgh and Mass (1996)

• Fronts passing through the west can be poorly defined at the surface for many reasons.

TEMPERATURE: - trapped cold air in valleys masks frontal movement aloft - diurnal heating/cooling effects - different elevations of stations (use potential temperature) - frontal retardation/acceleration by topography - precipitation (diabatic) effects - upslope/downslope adiabatic effects (e.g., Chinooks)

PRESSURE: - diurnal pressure variations - sea level pressure reduction problems WINDS: - diurnal mountain/valley circulations

- topography channels the wind down the pressure gradient, therefore the wind is not nearly geostrophic

Modification of Geostrophic Balance by Topography

Rossby radius of deformation (lR) is a measure of the horizontal extent to which modification of the force balances takes place.

lR=Nh/f

lR is about 100–200 km for the Wasatch.

Blazek thesis

Steenburgh and Blazek (2001)

Frontal Passages in the West-II• Warm-frontal passages are often not well defined at the surface, although regions of warm advection are likely to be occurring aloft. (Williams 1972)

• “The strength of the potential temperature gradient associated with the front is strongly modulated by differential sensible heating across the front. An estimate of the contribution to frontogenesis from differential diabatic heating . . . shows that it is several times greater than the contribution from the surface winds alone.” (Hoffman 1995)

• Advection of postfrontal air through the complex topography is difficult to accomplish. Therefore you may not see classic frontal passages at the surface, but the baroclinic zone may be advancing aloft. The temperature decrease (if any) behind the cold front may be a result of downward mixing of the colder air. Isallobars may be useful to follow these elevated frontal passages through the west.

• Larry Dunn has described some frontal passages in the West as split fronts. This concept may be useful and is in qualitative agreement with the results described above. In these cases, the precipitation may be out ahead of the surface position of the front.

Failure of the Norwegian Cyclone Model

• lack of warm fronts

• occluded fronts sometimes act as cold fronts

• deformation of fronts by topography

• precipitation is often unrelated to surface features

• disconnect between upper-level systems and low-level systems (e.g., IPEX IOP 3)

IPEX IOP 3: 1800 UTC 12 Feb 2000 Cross Section

NSSL 5 sounding

P-3 flight track

NSSL 4 sounding

Mt. Ogden

Great Salt Lake

(Courtesy of Justin Cox)

W E

trough at 700 mb

Forecasting Snowstorms in Utah• Favorable track: Nevada cyclogenesis with track of surface low through SLC or just north of SLC

• Track of surface low south of SLC favors downslope flow along Wasatch, holding snowfall down

• Well-defined shortwave trough aloft

• Difference between 5–10-inch and >10-inch snowstorms: DURATION, either by a slow-moving trough or multiple shortwaves in a long-wave trough

• Be aware of warm-advection snowstorms from southwest, with stationary/cold front draped across state.

• Synoptic-scale banding (“warm seclusion”–Dunn): snow in valleys>=snow in mts. (e.g., IPEX IOP 5)

IPEX IOP 5: 17 February 2000• Surface cyclone south of SLC

• Weak flow field at all levels

• Snowband northwest of cyclone

• 4–12 in. snow in Tooele Valley

500 hPa

SURFACE

6-h median 6-h median reflectivity from reflectivity from KMTXKMTX

yellow maxima yellow maxima are 20-25 dBare 20-25 dBZZ

(Horel)

700-hPa FRONTOGENESIS

500-hPa omega500-hPa omega

700-hPa theta700-hPa theta

shadingshading700-hPa frontogenesis700-hPa frontogenesis

700-hPa winds700-hPa winds

RUC-2: 1500 UTCRUC-2: 1500 UTC

LL

Using MesoWest to Aid Synoptic Analysis

• Use of multiple stations to confirm frontal passage

• Use of multiple elevations for interpreting vertical structure of weather systems (e.g., Promontory Point is 2700 feet above SLC)

• Generating time series of a particular station (e.g., looking for pressure minima, frontal passages)

Even if you were able to predict the liquid equivalent perfectly

• . . . you’d still have to know the snow density.• Usually this is assumed to be 10 inches of

snow to 1 inch of liquid water (snow ratio), higher for “the greatest snow on earth”

• The following graph is snow ratios from 2273 snowfall events greater than 2 mm liquid from 1980–1989 for 29 U.S. stations.

ratio of snow to liquid equivalent

per

cen

t

10 to 1 ratio

(Roebber, Bruening, Schultz and Cortinas)

ratio of snow to liquid equivalent

Nu

mb

er o

f ev

ents

119 eventsSLC 1980-89

ratios of 5–15 account for

57% of events

(Roebber, Bruening, Schultz and Cortinas)

REFERENCESHill, C. D., 1993: Forecast problems in the Western Region of the United States. Wea. Forecasting, 8, 158–165.

Schultz, D. M., and C. A. Doswell III, 2000: Analyzing and forecasting Rocky Mountain lee cyclogenesis often associated with strong winds. Wea. Forecasting, 15, 152-173.

Steenburgh, W. J., and T. R. Blazek, 2001: Topographic distortion of a cold front over the Snake River Plain and central Idaho mountains. Wea. Forecasting, 16, 301-314.

Williams, P., Jr., 1972: Western region synoptic analysis--Problems and methods. NOAA NWS Western Region Tech. Memo. NWSTM WR-71, 71 pp. [Available from NOAA NWS Western RegionHeadquarters, 125 S. State Street, Rm. 1311, Salt Lake City, UT 84138-1102.]

http://www.wrh.noaa.gov/Saltlake/projects/indexWorkArea.html

http://www.nssl.noaa.gov/~schultz/ipex/refs.html

http://www.nssl.noaa.gov/~schultz/wwt/