Prognostic Meteorological Discussion
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FXUS21 KWNC 211846
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 21 2018

SYNOPSIS: AN ACTIVE PATTERN IS EXPECTED TO BRING WET WEATHER TO MUCH OF THE
EASTERN U.S. DURING THE FORECAST PERIOD AND THE HIGHEST RAINFALL IS EXPECTED TO
BE IN THE SOUTHEAST. A LOW PRESSURE SYSTEM WITH TROPICAL ORIGINS IS FORECAST TO
BRING SIGNIFICANT RAINFALL TO THE SOUTHEAST THIS WEEKEND AND RESIDUAL RAINFALL
IS LIKELY TO CONTINUE THROUGH THE BEGINNING OF NEXT WEEK. ADDITIONAL LOW
PRESSURE MAY BRING EVEN MORE RAINFALL TO THE SOUTHEAST NEXT WEEKEND, WHICH
COULD LEAD TO LOCALIZED FLOODING. A SERIES OF UPPER-LEVEL LOW PRESSURE SYSTEMS
WILL MOVE THROUGH THE GULF OF ALASKA THIS WEEK AND COULD BRING SIGNIFICANT
RAINFALL TO THE ALASKAN PANHANDLE ON SATURDAY.

HAZARDS

HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST AND THE LOWER MISSISSIPPI VALLEY,
THU-FRI, MAY 24-MAY 25.

HEAVY RAIN ACROSS PORTIONS OF THE MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY,
THE CENTRAL APPALACHIANS, THE TENNESSEE VALLEY, THE NORTHEAST, THE SOUTHERN
APPALACHIANS, THE SOUTHEAST, THE GREAT LAKES, AND THE OHIO VALLEY, SAT-MON, MAY
26-MAY 28.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE PACIFIC NORTHWEST, THE
NORTHERN ROCKIES, AND THE NORTHERN GREAT BASIN.

FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE NORTHERN PLAINS AND THE
NORTHERN ROCKIES.

FLOODING LIKELY ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE NORTHERN GREAT
BASIN.

FLOODING LIKELY ACROSS PORTIONS OF THE NORTHERN ROCKIES.

HEAVY RAIN ACROSS PORTIONS OF THE ALASKA PANHANDLE, SAT, MAY 26.

SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL
PLAINS, THE CENTRAL ROCKIES, THE LOWER MISSISSIPPI VALLEY, THE SOUTHERN
ROCKIES, THE SOUTHERN PLAINS, AND THE SOUTHWEST, TUE-MON, MAY 29-JUN 4.

SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE LOWER MISSISSIPPI
VALLEY, THE TENNESSEE VALLEY, THE MID-ATLANTIC, THE SOUTHERN APPALACHIANS, THE
SOUTHEAST, AND THE SOUTHERN PLAINS, TUE-THU, MAY 29-MAY 31.

SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHEAST, THE
MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY, THE SOUTHERN APPALACHIANS, AND THE
TENNESSEE VALLEY, SAT-MON, JUN 2-JUN 4.

SEVERE DROUGHT ACROSS THE CENTRAL PLAINS, THE CENTRAL ROCKIES, THE CENTRAL
GREAT BASIN, THE SOUTHERN ROCKIES, CALIFORNIA, THE SOUTHEAST, THE SOUTHERN
PLAINS, AND THE SOUTHWEST.

DETAILED SUMMARY

FOR THURSDAY MAY 24 - MONDAY MAY 28: A SERIES OF UPPER-LEVEL TROUGHS IS
FORECAST TO AFFECT ALASKA AND THE WEST COAST OF THE CONUS THROUGHOUT THE
FORECAST PERIOD. A SURFACE CYCLONE ASSOCIATED WITH AN UPPER-LEVEL TROUGH WILL
BEGIN THE PERIOD OVER THE ALEUTIANS AND MOVE INTO THE GULF OF ALASKA BY THE END
OF THE WEEKEND. THIS WILL BRING HEAVY RAINFALL TO THE ALASKAN SOUTH COAST AND
PANHANDLE. ALTHOUGH MOST OF THE SOUTH COAST RAINFALL SHOULD STAY BELOW OUR
HAZARDOUS CRITERIA, MODELS INDICATE THAT ISOLATED AREAS ALONG THE PANHANDLE
COULD SEE MORE THAN 3 INCHES OF RAIN ON SATURDAY, MAY 26.



THIS WEEKEND`S FORECAST OVER THE EASTERN CONUS IS TRICKY BECAUSE THE ECMWF AND
GFS DETERMINISTIC MODELS DON`T AGREE. THE DETERMINISTIC ECMWF AND THE MAJORITY
OF BOTH THE GFS AND ECMWF ENSEMBLE MEMBERS SUGGEST THAT A TROPICAL LOW WILL
FORM IN THE GULF OF MEXICO AND MOVE INTO LOUISIANA AT THE END OF THE WEEK. THIS
WILL LEAD TO HEAVY RAINFALL OVER THE GULF COAST AND INTERIOR SOUTHEAST CONUS
THROUGH THE WEEKEND. THERE ARE A COUPLE OF ENSEMBLE MEMBERS, INCLUDING THE
DETERMINISTIC GFS, THAT FORECAST THE LOW TO FORM EAST OF FLORIDA AND MOVE INTO
SOUTH CAROLINA OVER THE WEEKEND. ASSUMING THAT EACH ENSEMBLE MEMBER IS EQUALLY
LIKELY TO OCCUR, THE ODDS ARE HEAVILY TILTED TOWARDS THE GULF OF MEXICO
SOLUTION AND WE`VE CHOSEN THAT AS THE BASIS FOR THIS FORECAST.



HEAVY RAINFALL IS LIKELY ALONG THE INTERIOR GULF COAST DURING MAY 24-25 IN
ASSOCIATION WITH THE AFOREMENTIONED LOW. SOME AREAS WITHIN THIS REGION COULD
SEE MORE THAN 2 INCHES OF RAINFALL IN THIS TIME FRAME. AS THE LOW PRESSURE
SYSTEM INTENSIFIES AND MOVES INLAND OVER THE WEEKEND THE AREA OF HEAVY RAINFALL
WILL EXPAND INWARD OVER MOST OF THE SOUTHEAST. LOUISIANA, FLORIDA, AND THE
SOUTHERN PARTS OF MISSISSIPPI, ALABAMA, AND GEORGIA COULD RECEIVE MORE THAN 2
INCHES OF RAIN EACH DAY OVER THE WEEKEND. AREAS FURTHER NORTH WITHIN THE HAZARD
AREA COULD RECEIVE MORE THAN 1 INCH OF RAIN EACH DAY OVER THE WEEKEND. WE ALSO
EXPECT HEAVY RAINFALL IN EASTERN NEW YORK AND WESTERN NEW ENGLAND DURING MAY
26-28.



MODELS ARE IN GOOD AGREEMENT THAT A SOUTHERLY FLOW THIS WEEKEND WILL LEAD TO
WARM TEMPERATURES OVER MUCH OF TEXAS AND OKLAHOMA. THE FORECAST TEMPERATURES
ARE SLIGHTLY LOWER THAN OUR HAZARDOUS THRESHOLD AND AREN`T ON THE FORECAST MAP,
BUT MANY REGIONS COULD REACH TEMPERATURES GREATER THAN 100 DEGREES F.

FOR TUESDAY MAY 29 - MONDAY JUNE 04: THE ECMWF SUGGESTS THAT THE TROPICAL LOW
WILL STALL OVER THE SOUTHEAST FOR A FEW DAYS AND RAINFALL ASSOCIATED WITH IT IN
THE SOUTHEAST CONUS AT THE END OF THE 3-7 DAY PERIOD WILL CONTINUE THROUGH THE
END OF WEEK-2 OVER MOST OF LOUISIANA, MISSISSIPPI, ALABAMA, GEORGIA, FLORIDA,
AND THE CAROLINAS.



MODEL GUIDANCE ALSO SUGGESTS THAT A BROAD TROUGH WILL MOVE THROUGH THE
SOUTHEAST DURING JUNE 2-4. OUR PRECIPITATION TOOLS BASED ON THE GFS AND ECMWF
ENSEMBLES SUGGEST A SLIGHT RISK OF SIGNIFICANT PRECIPITATION OVER MUCH OF THE
AREA. THE ADDITIONAL RAINFALL COULD BE ESPECIALLY PROBLEMATIC TO THIS REGION IF
THE FORECAST FOR EXCESSIVE RAINFALL EARLIER IN THE PERIOD IS CORRECT.



THE GFS ENSEMBLE LEANS TOWARD PERSISTENT RIDGING DURING WEEK-2 OVER MOST OF THE
CONUS, WHICH IS LIKELY TO LEAD TO AREAS OF MUCH ABOVE NORMAL TEMPERATURES.
TEMPERATURES ARE FORECAST TO BE GREATER THAN THE CLIMATOLOGICAL 85TH PERCENTILE
FROM MAY 29 THROUGH JUNE 4 OVER MUCH THE SOUTHERN PLAINS. THE WARMEST
TEMPERATURES WILL LIKELY BE OVER SOUTHERN TEXAS.



THE US DROUGHT MONITOR, VALID ON MAY 15, INDICATES THAT THE COVERAGE OF SEVERE
TO EXCEPTIONAL DROUGHT INCREASED SLIGHTLY FROM 15.92 PERCENT LAST WEEK TO 16.71
PERCENT THIS WEEK.

FORECASTER: KYLE MACRITCHIE

$$



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