Threats Assessment Discussion

475 
FXUS21 KWNC 161748
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 16 2017
SYNOPSIS: A COLD FRONT STRETCHING FROM THE TENNESSEE VALLEY TO THE NORTHEAST IS 
FORECAST TO SINK SOUTHWARD TO THE SOUTHEAST THROUGHOUT WEEK-1. ELSEWHERE, 
SURFACE HIGH PRESSURE IS FORECAST NEAR THE FOUR CORNERS AND OVER THE GULF OF 
MEXICO IN WEEK-1. PERIODIC DISTURBANCES ARE FORECAST IN THE GULF OF ALASKA AND 
CHUKCHI SEA THROUGHOUT WEEK-1. DURING WEEK-2, A TENDENCY TOWARDS MID-LEVEL HIGH 
PRESSURE ACROSS MUCH OF THE WESTERN TWO THIRDS OF THE CONTIGUOUS 48 STATES IS 
FAVORED.
HAZARDS
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HEAVY RAIN ACROSS PORTIONS OF THE UPPER AND MIDDLE MISSISSIPPI VALLEY AND GREAT 
LAKES, MON, AUG 21.
HIGH WINDS FOR COASTAL PORTIONS OF NORTHERN CALIFORNIA, SAT-SUN, AUG 19-AUG 20.
HEAVY RAIN ACROSS PORTIONS OF THE SOUTH COAST OF ALASKA, SUN, AUG 20.
SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN 
PLAINS, THU-SUN, AUG 24-AUG 27.
SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF THE SOUTHERN 
PLAINS, THU-SUN, AUG 24-AUG 27.
SEVERE DROUGHT ACROSS PARTS OF THE GREAT PLAINS, WESTERN CORN BELT, NORTHERN 
ROCKIES, ARIZONA, CALIFORNIA, AND HAWAII.
DETAILED SUMMARY
FOR SATURDAY AUGUST 19 - WEDNESDAY AUGUST 23: A SURFACE LOW IS ANTICIPATED TO 
FORM OVER THE NORTHERN PLAINS BY AUG 20. SOME MODELS INDICATE THE POSSIBILITY 
FOR THIS LOW TO BRING HEAVY RAIN (ONE INCH OR GREATER OF RAINFALL IN A 24-HOUR 
PERIOD) TO PARTS OF THE MIDDLE MISSISSIPPI VALLEY ON AUG 20. DUE TO SIGNIFICANT 
MODEL DIFFERENCES AND THE EXPECTATION THAT HEAVY RAINFALL WOULD BE VERY 
LOCALIZED, A RELATED HEAVY RAIN AREA IS NOT DESIGNATED AT THIS TIME.
EARLY IN WEEK-1, A COLD FRONT IS PREDICTED TO STRETCH ACROSS THE NORTHERN 
PLAINS SOUTHWESTWARD TO THE NORTHERN ROCKIES BY AUG 20. AS THIS COLD FRONT 
MOVES EASTWARD, PARTS OF THE UPPER AND MIDDLE MISSISSIPPI VALLEY AND GREAT 
LAKES MAY RECEIVE HEAVY RAINFALL AUG 21. LOCALIZED AREAS MAY RECEIVE UP TO 2 
INCHES OF RAINFALL OR GREATER IN A 24-HOUR PERIOD. THERE IS A POTENTIAL FOR 
LOCALIZED REGIONS ACROSS THE OHIO VALLEY AND GREAT LAKES TO ALSO RECEIVE HEAVY 
RAINFALL AUG 22. HOWEVER, DUE TO SIGNIFICANT MODEL UNCERTAINTY REGARDING 
RAINFALL AMOUNTS ON AUG 22 AND MODELS SHOWING EXTREMELY LOCALIZED AREAS OF 
HEAVY RAINFALL, THE HEAVY RAIN HAZARD IS NOT EXTENDED TO AUG 22 AT THIS TIME.
A SURFACE TROUGH IS PREDICTED TO DEVELOP OVER COASTAL NORTHERN CALIFORNIA 
THROUGHOUT THE 3 TO 7 DAY PERIOD. THIS PATTERN MAY SUPPORT HIGH WINDS (40 KNOTS 
OR GREATER) ACROSS COASTAL PORTIONS OF NORTHERN CALIFORNIA AUG 20.
SOME MODELS SHOW THE POSSIBILITY OF A SURFACE LOW PRESSURE SYSTEM APPROACHING 
THE SOUTH COAST OF ALASKA BY AUG 21. THIS STORM MAY BRING HEAVY RAIN (2 INCHES 
OF RAINFALL OR GREATER) TO PARTS OF THE SOUTH COAST OF ALASKA ON AUG 20. IT 
SHOULD BE NOTED THAT THE 6Z DETERMINISTIC GFS INDICATES A MUCH STRONGER STORM 
THAN THE 0Z DETERMINISTIC ECMWF AND CONSEQUENTLY SIGNIFICANTLY GREATER RAINFALL 
AMOUNTS. 
CURRENT HURRICANE GERT IS FORECAST TO BE LOCATED IN THE NORTHERN ATLANTIC FAR 
FROM THE EASTERN SEABOARD BY WEEK-1. THERE ARE CURRENTLY THREE OTHER 
DISTURBANCES IN THE ATLANTIC BEING MONITORED BY NHC. NO IMPACTS TO HAZARDS ARE 
EXPECTED AT THIS TIME ASSOCIATED WITH THESE FEATURES. A TROPICAL DISTURBANCE IN 
THE EAST PACIFIC IS BEING MONITORED FOR TROPICAL CYCLONE DEVELOPMENT OVER THE 
COMING DAYS, BUT EACH AREA IS ANTICIPATED TO TRACK WEST-NORTHWESTWARD AND AWAY 
FROM BOTH THE CONUS AND HAWAII.
FOR THURSDAY AUGUST 24 - WEDNESDAY AUGUST 30: IN GENERAL, BROAD RIDGING IS 
EXPECTED ACROSS THE WESTERN TWO-THIRDS OF THE CONUS THROUGHOUT WEEK-2. THIS 
PATTERN FAVORS A SLIGHT CHANCE OF MUCH ABOVE NORMAL TEMPERATURES ACROSS 
PORTIONS OF THE SOUTHERN PLAINS AUG 24 TO 27. THESE AREAS MAY REACH 
TEMPERATURES OF 95 DEGREES F OR GREATER, WITH CENTRAL PARTS OF TEXAS REACHING 
GREATER THAN 100 DEG F. THIS RIDGE MAY ALSO SUPPORT A SLIGHT CHANCE FOR MUCH 
ABOVE NORMAL TEMPERATURES ACROSS PARTS OF THE NORTHERN PLAINS AUG 24 TO 27. THE 
GEFS REFORECAST TOOL SHOWS A 20 PERCENT CHANCE OR GREATER OF THE HIGHLIGHTED 
NORTHERN PLAINS REGION REACHING THE 85TH PERCENTILE OR GREATER OF DAILY MAXIMUM 
TEMPERATURES, AND TEMPERATURES REACHING OR EXCEEDING 95 DEG F.
THE U.S. DROUGHT MONITOR VALID ON AUG 8 INDICATES THAT SEVERE TO EXCEPTIONAL 
(D2-D4) DROUGHT COVERAGE INCREASED SLIGHTLY FROM 5.23 PERCENT TO 5.38 PERCENT 
ACROSS THE CONTINENTAL U.S. THIS INCREASE IS RELATED TO AN EXPANSION OF SEVERE 
DROUGHT ACROSS PARTS OF THE WESTERN CORN BELT AND MONTANA.
FORECASTER: MELISSA OU 
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