Prognostic Meteorological Discussion
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936
FXUS21 KWNC 181909
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 18 2017

SYNOPSIS: A COLD FRONT IS FORECAST TO DEVELOP ACROSS THE NORTHERN PLAINS AND
UPPER MISSISSIPPI VALLEY, PUSHING SOUTHEASTWARD ACROSS THE COUNTRY THROUGHOUT
WEEK-1. SURFACE HIGH PRESSURE IS EXPECTED TO BUILD ACROSS MUCH OF THE EASTERN
HALF OF THE LOWER 48 STATES IN ITS WAKE. 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 U.S. IS FAVORED,
WITH UPPER-LEVEL LOW PRESSURE ACROSS THE EASTERN HALF OF THE LOWER 48 AND SOUTH
OF ALASKA.

HAZARDS

HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL PLAINS, THE MIDDLE AND UPPER
MISSISSIPPI VALLEY, AND THE GREAT LAKES, MON-TUE, AUG 21-AUG 22.

SEVERE WEATHER ACROSS PORTIONS OF THE GREAT LAKES AND THE OHIO VALLEY, TUE, AUG
22.

HEAVY RAIN ACROSS PORTIONS OF THE EASTERN CAROLINAS, WED-THU, AUG 23-AUG 24.

HEAVY RAIN ACROSS PORTIONS OF FLORIDA, TUE-FRI, AUG 22-AUG 25.

FLOODING POSSIBLE ACROSS PORTIONS OF THE CENTRAL PLAINS AND THE SOUTHERN PLAINS.

SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT BASIN,
THE NORTHERN PLAINS, THE NORTHERN ROCKIES, CALIFORNIA, THE PACIFIC NORTHWEST,
AND THE SOUTHWEST, SUN-FRI, AUG 27-SEP 1.

MODERATE RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN
ROCKIES AND THE NORTHERN GREAT BASIN, SUN-TUE, AUG 27-AUG 29.

SEVERE DROUGHT ACROSS THE CENTRAL PLAINS, THE NORTHERN PLAINS, HAWAII, THE
NORTHERN ROCKIES, THE MIDDLE MISSISSIPPI VALLEY, CALIFORNIA, AND THE UPPER
MISSISSIPPI VALLEY.

DETAILED SUMMARY

FOR MONDAY AUGUST 21 - FRIDAY AUGUST 25: A SURFACE LOW IS ANTICIPATED TO FORM
OVER THE NORTHERN PLAINS LATE THIS WEEKEND. AS THIS SURFACE LOW AND ITS
ASSOCIATED COLD FRONT (STRETCHING FROM THE NORTHERN PLAINS TO THE UPPER MIDDLE
MISSISSIPPI VALLEY) MOVE EASTWARD, ASSOCIATED HAZARDS ARE ANTICIPATED. HEAVY
RAIN (>1 INCH IN 24 HOURS) IS LIKELY FOR PARTS OF THE CENTRAL PLAINS
NORTHEASTWARD THROUGH THE GREAT LAKES ON MONDAY AND TUESDAY, WHILE SPC DEPICTS
A SEVERE WEATHER THREAT FOR PARTS OF THE OHIO VALLEY AND GREAT LAKES ON
TUESDAY, AUGUST 22. AS THIS COLD FRONT APPROACHES THE CAROLINAS AND BEGINS TO
STALL, PERIODS OF HEAVY RAIN ARE LIKELY FOR COASTAL SECTIONS OF THE CAROLINAS
ON WEDNESDAY AND THURSDAY. IN THE WAKE OF THIS COLD FRONT, COOLER AND DRIER
CONDITIONS ARE EXPECTED FOR THE END OF THE WEEK OVER MUCH OF THE CENTRAL AND
EASTERN CONUS.



THE NATIONAL HURRICANE CENTER (NHC) IS CURRENTLY MONITORING TROPICAL STORM
HARVEY AS WELL AS TWO OTHER DISTURBANCES IN THE ATLANTIC. ONE DISTURBANCE IS
FORECAST TO APPROACH FLORIDA DURING THE PERIOD, POSSIBLY AS A TROPICAL
DEPRESSION. THIS, ALONG WITH AN APPROACHING COLD FRONT, LEADS TO INCREASED
CHANCES FOR HEAVY RAIN FOR SOUTHERN PARTS OF THE FLORIDA PENINSULA FOR AUG
22-25.



TROPICAL STORM HARVEY IS FORECAST TO MAKE LANDFALL OVER NEAR BELIZE DURING THE
PERIOD, BUT COULD REEMERGE OVER THE SOUTHERN GULF OF MEXICO BY THE END OF THE
WEEK. IF SO, THERE IS SOME CHANCE THAT IT COULD IMPACT PARTS OF SOUTH TEXAS AT
THE VERY END OF WEEK-1 OR EARLY IN WEEK-2. INTERESTED PARTIES SHOULD MONITOR
THE LATEST NHC FORECAST ADVISORIES.



LOCALLY HEAVY RAIN IS POSSIBLE OVER PARTS OF SOUTHEASTERN ALASKA AT THE VERY
BEGINNING OF THE PERIOD DUE TO A ROBUST UPPER-LEVEL TROUGH FORECAST TO MOVE
THROUGH THE GULF OF ALASKA. THE LATEST GUIDANCE, HOWEVER, SUGGESTS THAT HEAVY
PRECIPITATION IS NOT LIKELY TO REACH HAZARDOUS CRITERIA OVER ANY WIDESPREAD
REGION.

FOR SATURDAY AUGUST 26 - FRIDAY SEPTEMBER 01: THE COLD FRONT ACROSS THE
SOUTHEAST FROM THE END OF THE 3 TO 7 PERIOD IS LIKELY TO PROGRESS FURTHER
SOUTHWARD. THIS COLD FRONT IN ADDITION TO ENHANCED MOISTURE FLOW ASSOCIATED
WITH POSSIBLE TROPICAL WAVES IN THE ATLANTIC MAY SUPPORT HEAVY RAINFALL ACROSS
THE GULF COAST, FLORIDA AND COASTAL SOUTHEAST. THE LATEST ENSEMBLE GUIDANCE,
HOWEVER, DOES NOT SUPPORT THE DEPICTION OF A DETERMINISTIC HAZARD, WITH FAIRLY
LARGE DISCREPANCIES BETWEEN THE GEFS AND ECMWF SYSTEMS.



THE LATEST ENSEMBLE GUIDANCE ALSO SUGGESTS A WEAKER TROUGH MOVING THROUGH THE
WESTERN CONUS EARLY IN WEEK-2. LARGE DISCREPANCIES LIKEWISE EXISTS OVER THE
CENTRAL CONUS BETWEEN THE ECMWF AND GEFS, WITH THE ECMWF AND THE LATEST 12Z
GEFS BEING LESS WET OVER THE PLAINS. THE HAZARD ISSUED YESTERDAY FOR THIS
REGION IS THEREFORE REMOVED IN TODAY`S FORECAST, THOUGH ODDS OF ABOVE-NORMAL
PRECIPITATION REMAIN ELEVATED.



IN GENERAL, BROAD RIDGING IS EXPECTED ACROSS THE WESTERN CONUS THROUGHOUT
WEEK-2. THIS PATTERN FAVORS A SLIGHT CHANCE OF MUCH ABOVE NORMAL TEMPERATURES
ACROSS MUCH OF THE WESTERN THIRD OF THE CONUS AUG 27 TO SEP 1. THE GEFS
REFORECAST TOOL SHOWS A 20 PERCENT CHANCE OR GREATER OF THE HIGHLIGHTED AREA
REACHING THE 85TH PERCENTILE OR GREATER OF DAILY MAXIMUM TEMPERATURES, AND
LOCALIZED TEMPERATURES REACHING OR EXCEEDING 95 DEG F, WITH AREAS ACROSS
CALIFORNIA AND THE SOUTHWEST REACHING 100 DEG F OR GREATER. THE BEST ODDS FOR
EXCEEDING THE 85TH PERCENTILE OR GREATER OF DAILY MAXIMUM TEMPERATURES ARE OVER
THE INTERIOR NORTHWEST, WHERE A MODERATE RISK IS DENOTED FOR DAYS 9-11.



THE U.S. DROUGHT MONITOR VALID ON AUG 15 INDICATES THAT SEVERE TO EXCEPTIONAL
(D2-D4) DROUGHT COVERAGE DECREASED SLIGHTLY FROM 5.38 PERCENT TO 5.33 PERCENT
ACROSS THE CONTINENTAL U.S.

FORECASTER: STEPHEN BAXTER

$$




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