Prognostic Meteorological Discussion
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FXUS21 KWNC 231921
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 23 2017

SYNOPSIS: TROPICAL DEPRESSION HARVEY IS FORECAST TO INTENSIFY AND THEN MOVE
INLAND INTO SOUTHEAST TEXAS ON AUGUST 26. ITS REMNANT LOW IS EXPECTED TO REMAIN
ACROSS THE WESTERN GULF COAST REGION THROUGH EARLY NEXT WEEK. A TROPICAL
CYCLONE MAY DEVELOP NEAR THE SOUTHEAST COASTLINE EARLY NEXT WEEK WITH A
SUBSEQUENT TRACK TO THE NORTHEAST. UPPER-LEVEL HIGH PRESSURE IS FORECAST TO
BUILD ACROSS THE WESTERN U.S. DURING THE FINAL WEEK OF AUGUST. UPPER-LEVEL LOW
PRESSURE IS LIKELY TO REMAIN CENTERED OVER WESTERN ALASKA AND THE ALEUTIANS
THROUGH WEEK-2.

HAZARDS

HEAVY RAIN AND FLOODING FOR THE TEXAS GULF COAST, INLAND AREAS OF SOUTHEAST
TEXAS, AND THE LOWER MISSISSIPPI VALLEY, SAT-WED, AUG 26-30.

SIGNIFICANT WAVES AND STORM SURGE ALONG THE TEXAS GULF COAST, SAT, AUG 26.

HIGH WINDS FOR THE TEXAS GULF COAST, SAT, AUG 26.

HEAVY RAIN FOR PARTS OF CENTRAL AND SOUTHERN FLORIDA, SAT, AUG 26.

HIGH WINDS FOR THE OUTER BANKS OF NORTH CAROLINA, MON-TUE, AUG 28-29.

MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF THE PACIFIC NORTHWEST, NORTHERN
GREAT BASIN, AND NORTHERN ROCKIES, SUN-WED, AUG 27-30.

HEAVY RAIN FOR THE ALASKA PENINSULA, KENAI PENINSULA, AND PRINCE WILLIAM SOUND,
MON-WED, AUG 28-30.

MODERATE RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF IDAHO, NEVADA,
OREGON, AND WASHINGTON, THU, AUG 31.

SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR MUCH OF THE WESTERN U.S.,
THU-WED, AUG 31-SEP 6.

FLASH FLOODING POSSIBLE FOR THE DESERT SOUTHWEST, THU-WED, AUG 31-SEP 6.

FLOODING LIKELY FOR PARTS OF NORTHWEST NEW MEXICO.

FLOODING OCCURRING, IMMINENT, LIKELY, OR POSSIBLE FOR PARTS OF KANSAS AND
MISSOURI.

SEVERE DROUGHT ACROSS PARTS OF THE GREAT PLAINS, WESTERN CORN BELT, NORTHERN
ROCKIES, AND HAWAII.

DETAILED SUMMARY

FOR SATURDAY AUGUST 26 - WEDNESDAY AUGUST 30: AS OF 2PM EDT ON AUG 23, TROPICAL
DEPRESSION HARVEY IS LOCATED OVER THE BAY OF CAMPECHE. HARVEY IS LIKELY TO
INTENSIFY PRIOR TO THIS PERIOD AND COULD BECOME A HURRICANE WHEN IT MAKES
LANDFALL ALONG THE TEXAS GULF COAST LATER THIS WEEK. BASED ON THE LATEST MODEL
SOLUTIONS, HIGH WINDS (POTENTIALLY HURRICANE FORCE) AND STORM SURGE ARE
FORECAST ALONG THE TEXAS GULF COAST THROUGH AUG 26. AFTER MAKING LANDFALL, THE
REMNANT LOW IS EXPECTED TO MOVE ERRATICALLY ACROSS THE WESTERN GULF REGION. THE
6Z GFS AND 0Z ECMWF ENSEMBLE MEANS ARE IN BETTER AGREEMENT TODAY, MAINTAINING A
REMNANT LOW NEAR THE WESTERN GULF COAST THROUGH EARLY NEXT WEEK. THEREFORE, A
PROLONGED PERIOD OF HEAVY RAINFALL AND FLOODING IS LIKELY FOR THE TEXAS GULF
COAST AND LOWER MISSISSIPPI VALLEY THROUGH AUGUST 30. THE DETERMINISTIC 0Z
ECMWF MODEL INDICATES 7 TO 10 INCHES, LOCALLY MORE, OF RAINFALL FROM AUG 26 TO
30 ACROSS THE OUTLINED HEAVY RAIN AREA. MAXIMUM RAINFALL AMOUNTS MAY EXCEED 15
INCHES ACROSS SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA WITH AN INCREASING RISK
OF MAJOR FLOODING. PLEASE REFER TO THE NATIONAL HURRICANE CENTER AND LOCAL NWS
OFFICES FOR THE LATEST UPDATES AND FORECASTS.



A TROUGH OF LOW PRESSURE AND ENHANCED MOISTURE ARE EXPECTED TO BRING HEAVY RAIN
(LOCALLY MORE THAN 2 INCHES) TO PARTS OF CENTRAL AND SOUTH FLORIDA ON AUG 26.
DETERMINISTIC MODEL SOLUTIONS CONTINUE TO INDICATE SLOW TROPICAL OR SUBTROPICAL
CYCLONE DEVELOPMENT OFFSHORE OF THE SOUTHEAST COASTLINE THIS WEEKEND. AN
INCREASING PRESSURE GRADIENT BETWEEN THIS AREA OF LOW PRESSURE AND A 1028-HPA
SURFACE HIGH OVER NEW ENGLAND IS EXPECTED TO RESULT IN HIGH WINDS (GUSTS ABOVE
35 KNOTS) ACROSS THE OUTER BANKS ON AUG 28 AND 29.



AN UPPER-LEVEL RIDGE IS FORECAST TO STRENGTHEN ACROSS THE WESTERN U.S. WITH
500-HPA HEIGHTS INCREASING TO MORE THAN 588DM EARLY IN THIS PERIOD. MUCH
ABOVE-NORMAL TEMPERATURES ARE POSTED FOR AREAS OF THE PACIFIC NORTHWEST,
NORTHERN GREAT BASIN, AND NORTHERN ROCKIES WHERE MAXIMUM TEMPERATURES ARE
FORECAST TO AVERAGE AROUND 12 DEGREES F ABOVE NORMAL AND EXCEED 90 DEGREES F.
ALSO, NUMEROUS WILDFIRES CONTINUE TO BURN IN THESE SAME AREAS WITH THE LARGEST
WILDFIRE (MORE THAN 68,000 ACRES BURNED) AT NENA SPRINGS IN NORTHERN OREGON.



ANOMALOUS SOUTHERLY FLOW WITH A TROPICAL CONNECTION IS EXPECTED TO AFFECT
SOUTHERN MAINLAND ALASKA NEXT WEEK AS A HIGHLY AMPLIFIED TROUGH (RIDGE)
DEVELOPS OVER THE ALEUTIANS (EASTERN ALASKA). THE ANOMALOUS SOUTHERLY, ONSHORE
FLOW IS LIKELY TO BRING HEAVY RAIN (MORE THAN 1.5 INCHES PER 24 HOURS) FROM THE
ALASKA PENINSULA EAST TO THE PRINCE WILLIAM SOUND. THE 0Z DETERMINISTIC ECMWF
MODEL INDICATES A 72-HOUR MAXIMUM OF MORE THAN 10 INCHES ALONG THE SOUTHERN
KENAI PENINSULA FROM AUG 28 TO 30.

FOR THURSDAY AUGUST 31 - WEDNESDAY SEPTEMBER 06: THE REMNANT LOW FROM TROPICAL
DEPRESSION HARVEY IS EXPECTED TO LIFT AWAY FROM THE GULF COAST AT THE BEGINNING
OF WEEK-2. ALTHOUGH THERE IS SUPPORT FOR AN AREA OF HEAVY RAIN SOMEWHERE ACROSS
THE EASTERN U.S. ON DAY 8, THE EXACT LOCATION VARIES LARGELY AMONG MODEL
GUIDANCE. THE 0Z ECMWF ENSEMBLE MEAN INDICATES GENERALLY LESS THAN 0.25 INCH
ACROSS MUCH OF THE EASTERN U.S. ON AUG 31, REPRESENTATIVE OF THE LARGE ENSEMBLE
SPREAD. ALSO, THE AREA FOR HEAVIEST RAIN VARIES AMONG RECENT GFS ENSEMBLE MEAN
SOLUTIONS. THEREFORE, A CATEGORICAL HEAVY RAIN HAZARD IS NOT POSTED BEYOND DAY
7.



THE GFS AND ECMWF ENSEMBLE MEANS MAINTAIN A STABLE LONGWAVE PATTERN THROUGH THE
BEGINNING OF SEPTEMBER WITH AN UPPER-LEVEL RIDGE (TROUGH) OVER WESTERN (EASTERN
NORTH AMERICA). THE MOST ANOMALOUS LONGWAVE TROUGH ACROSS THE NORTHERN
HEMISPHERE REMAINS THE UPPER-LEVEL TROUGH EXTENDING FROM THE BERING SEA TO
SOUTH OF THE ALEUTIANS. THE AMPLIFIED UPPER-LEVEL RIDGE SUPPORTS A SLIGHT (AUG
31-SEP 6) TO MODERATE (AUG 31) RISK OF MUCH ABOVE-NORMAL TEMPERATURES ACROSS
THE WESTERN U.S.



TROPICAL CYCLONE (TC) DEVELOPMENT IS FAVORED ACROSS THE EAST PACIFIC BY THE END
OF AUGUST. A NORTHWEST TRACK OF A TC CLOSE TO THE BAJA PENINSULA IS EXPECTED TO
BRING A SURGE OF MOISTURE NORTH FROM THE GULF OF CALIFORNIA INTO THE DESERT
SOUTHWEST DURING EARLY SEPTEMBER. THEREFORE, A FLASH FLOODING POSSIBLE HAZARD
IS POSTED FOR THE DESERT SOUTHWEST.



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: BRAD PUGH

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