The Local Wind Forecast and Its Uncertainty

(Completing the Wind Forecast)

Florida Governor Hurricane ConferenceMay 11-16

2014

Pablo Santos

Meteorologist In Charge

NWS Miami/South Florida Forecast Office

2014 – FLGHC

TS 23

Motivation

• What is there in it for me in this presentation?

– A better appreciation of the approach used by local offices to downscale NHC’s forecast to provide you more detailed locally driven information.

– A better understanding of the limitations behind deterministic only based information and basing decisions on those alone.

– A better understanding of the wind speed probabilities available to you nationally and at the local level and:

• What they mean

• What questions you might have that THEY CAN answer

• How NWS can help you make better decisions using this data

• And potential advanced applications.

NHC Wind Forecasts

Full Radii Only Out to 36 Hrs

50/34 kts Radii Only Out to 72 Hrs

No Radii Days 96/120 Hrs

NHC Official Forecast

WFO Wind Grid Forecasts

WFO Wind Grid Forecasts

NHC/WFO Wind Forecasts

• So when making forecast of Wind locally using NHC guidance, NWS offices then have to refine that forecast to create a wind grid based on:

– Conceptual models– Climatology-based data – Local knowledge– Understanding of smaller scale processes– Diagnostic data for present conditions – Other tools such as high resolution models, when available

• Intensity changes are depicted as a steady trend in space and time.

• Wind Speeds can be artificially reduced prior to landfall as they are interpolated within 12-hour forecast points. They are worse in days 4 and 5.

• We are required to make wind grids forecasts at the hourly scale out to 48 hours, 3 hourly out to 72 hours, and 6 hourly out to 120 hours with this guidance on a 2.5 km grid.

• The limitations (which translate into errors) of the assumptions listed so far are magnified the higher the resolution of the wind grids.

• This is how the local offices create a local wind forecast from the NHC forecasts. ASSUMPTIONS AND LIMITATIONS RESULT IN GREATLY MAGNIFIED ERRORS when looking at deterministic wind forecasts alone. HURRIVAC has the same limitations.

NHC/WFO Wind Forecasts Limitations

Local Wind ForecastWhat is the Point?

• Approach currently used is science-based, but still limited with uncertainty and many sources of errors. All of these limitations also apply to Hurrevac.

• We are delivering information with greater precision, BUT IT DOES NOT IMPLY ACCURACY. Science is not where we would like it to be; service is outpacing the state of the science.

• This is in part why NHC delivers their advisory forecasts the way they do, but the need to provide more detailed local level info is pushing the envelope.

• Message From a Decision Making Perspective:

– AVOID OVER-RELYING ON DETERMINISTIC ONE SCENARIO/BEST GUEST ALONE!

• It is bad enough to rely in one scenario alone only without considering the issues raised so far.

– Be well informed about these issues and embrace a combined official forecast with probabilistic-based information.

D-Day Give me the Probabilities

1111

Wind Speed Probability Text Product(abbreviated)In addition to:

- Onset Probabilities: Text

- Cumulative Wind Speed Probability: Text/Graphic

11

Onset Period ProbabilitiesCumulative Probabilities

What Questions do They Answer?

• Cumulative (Available in NHC Graphic and PWSAT#; coarse time resolution) – What arethe chances that tropical storm or hurricane conditions will occur between hour 00 andXX out to 120 hours/5days with this event at my location?

• Onset (Available in PWSAT#; coarse time resolution) – What are the chances thattropical storm or hurricane conditions will begin during a particular time period at mylocation? And given that, what is the most likely period of onset of these conditions?

• Incremental (Not available from NHC; but from NWS offices) – What are the chancesthat tropical storm or hurricane conditions will be experienced during a particularperiod at my location? How likely is the event to happen during that period? How likelyis it to last? At what values is the event becoming more plausible (likely) than justpossible?

We consider trend from advisory to advisory

Cumulative

Incremental – 12HR

Onset

Onset

Tropical

Storm

Conditions

Onset

Hurricane

Conditions

Threshold:

45%

Threshold:

35%

Threshold:

30%

Threshold:

25%

Threshold:

22.5%

Threshold:

20%

Threshold:

17.5%Incremental

Tropical

Storm

Threshold:

25%

Threshold:

20%

Threshold:

15%

Threshold:

12.5%

Threshold:

10%

Threshold:

8%

Threshold:

7%Incremental

Hurricane

Local ScaleWind Speed Probabilities

Consider Trend From Advisory to

Advisory

Applications

• Communicate Risk.

• Provide objective measure of uncertainty that can be used to create new targeted products.

– Threat/Potential Impact Graphics• Briefings

• Can be used as decision making aid by planners (example follows).

• Communicate to the public graphically degree of preparation needed.

• Can be used by EM and Media along with NWS to help communicate the bottom line message to the public.

– Expressions of Uncertainty• Used to enhance forecast information during tropical cyclones (Expressions of Uncertainty).

• Can be used as decision making aid by planners.

• For trend analysis from advisory to advisory for proper risk assessment.

Cumulative: Note that

chances of hurricane-

force winds at Tampa

Bay and Port Charlotte

are both around 30%!

Tampa

Port Charlotte

IncrementalNotice: highest right

up Port Charlotte

Communicate RISK

Example: Forecast Applications: Expressions of UncertaintyAdvisory Time: 20040902_1500 (25-36 Hours – Third Period)

FRANCES (ZFP) FRANCES (Click Point)

Official in 2014

Example: A major hurricane approximately 36-hours from

projected landfall (Hurricane Warning issued). Deterministic

NHC track/intensity forecast used

by WFO’s to create local wind

forecasts (location/timing specific)

NHC

WFO’s

NHC 34 kt probability

NHC 64 kt probability

Probabalistic

NHC wind probabilities

also incorporated to

produce Impact Graphics

Threat/Potential Impact Graphics

Official Forecast Actual Threat/Potential Impacts

Example: Approaching major hurricane at the onset of the warning

period (~ 36 hours). Remember to consider the trend with these from

advisory to advisory also. Notice implicit use of probabilistic information.

QUESTION: When advocating the measure of protective

actions according to wind impacts, which is better?2014 - FLGHC

Threat/Potential Impact Graphics

Storm Intensity

Cat 3(96-113 Kt)

Threat?

OR

FORECAST VALID XX/XX00Z 31.4N 77.5W

MAX WIND XXX KT...GUSTS XXX KT.

Neal Batista

Decision Making Guide

Clearance Times

Forecast impact ≤33hrs?

FORECAST VALID XX/XX00Z 31.4N 77.5W

MAX WIND XXX KT...GUSTS XXX KT.

Available 2012 season

Neal Batista

Wind Probabilities

64KtIncremental wind

prob ≥25%?

Neal Batista

More on Trend –Wilma Example

Impact Graphics use

implicitly probabilistic

guidance. What about

their trend from

advisory to advisory

also?

2014

Tropical

Workshop

• Advisory 30• Advisory 30

• Advisory 31

• Advisory 32

• Advisory 35

Take Away

• Be aware of the data upon which you base your decisions (whether national or local):

– Your NWS office can help you sort through it and target your needs with their proper application.

• Be aware of the limitations of deterministic only based information.

– And making decisions solely based on deterministic or even “alternate scenarios” only.

• Be aware of all probabilistic data available to you including

– Cumulative, Onset, Incremental

– Questions they answer

– Their temporal resolution

– Be knowledgeable of their significance and trend behavior.

– Again your local NWS office can help you sort through this

• Be aware of the potential applications of using combined deterministic/probability data and their utility in communicating the message and making decisions.

– Again, your local NWS office can help you with this

Questions…Pablo Santos

Meteorologist In ChargeNWS Miami/South Florida Forecast Office

305-220-4500pablo.santos@noaa.gov