Prognostic Meteorological Discussion
Issued by NWS

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FXUS21 KWNC 231850
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MAY 23 2017

SYNOPSIS: A SURFACE LOW PRESSURE SYSTEM IS EXPECTED TO EXIT NEW ENGLAND AT THE
BEGINNING OF THE PERIOD, WHILE A TRAILING COLD FRONT DISSIPATES OVER THE
FLORIDA STRAITS. A SURFACE LOW AND ITS ASSOCIATED COLD FRONT ARE FORECAST TO
CROSS THE CENTRAL U.S. DURING THE MEMORIAL DAY WEEKEND. A SURFACE LOW PRESSURE
SYSTEM IS FORECAST TO ADVANCE EAST ACROSS THE ALEUTIANS AND WEAKEN ONCE IT
REACHES THE GULF OF ALASKA. DURING WEEK-2, AN AREA OF UPPER-LEVEL HIGH (LOW)
PRESSURE IS EXPECTED TO PERSIST ACROSS THE PACIFIC NORTHWEST (NORTHEASTERN
U.S.).

HAZARDS

HEAVY RAIN FOR PARTS SOUTHEAST OKLAHOMA, NORTHEAST TEXAS, THE LOWER TO MIDDLE
MISSISSIPPI VALLEY, AND OHIO AND TENNESSEE VALLEYS, SAT-SUN, MAY 27-28.

HEAVY RAIN FOR PARTS OF TEXAS AND THE LOWER MISSISSIPPI VALLEY, MON-TUE, MAY
29-30.

MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF WESTERN OREGON AND NORTHERN
CALIFORNIA, SUN-TUE, MAY 28-30.

MUCH BELOW-NORMAL TEMPERATURES FOR PARTS OF SOUTH-CENTRAL AND WESTERN MAINLAND
ALASKA, FRI-SAT, MAY 26-27.

A SLIGHT RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF THE PACIFIC
NORTHWEST AND NORTHERN CALIFORNIA, WED-FRI, MAY 31-JUN 2.

A MODERATE RISK OF MUCH ABOVE-NORMAL TEMPERATURES FOR PARTS OF EASTERN
WASHINGTON, WED-THU, MAY 31-JUN 1.

FLOODING OCCURRING, IMMINENT, OR POSSIBLE FOR PARTS OF ILLINOIS AND THE LOWER
MISSISSIPPI VALLEY.

FLOODING OCCURRING, IMMINENT, LIKELY, OR POSSIBLE FOR PARTS OF IDAHO, UTAH, AND
WASHINGTON.

FLOODING POSSIBLE ALONG THE WALKER RIVER IN WESTERN NEVADA.

SEVERE DROUGHT ACROSS PARTS OF THE SOUTHEAST, TEXAS, AND SOUTHERN CALIFORNIA.

DETAILED SUMMARY

FOR FRIDAY MAY 26 - TUESDAY MAY 30: AN UPPER-LEVEL TROUGH IS FORECAST TO EJECT
FROM THE WESTERN U.S. LATER THIS WEEK AND LEAD TO THE INITIATION OF CONVECTION
ALONG A DRYLINE LOCATED ACROSS THE CENTRAL AND SOUTHERN HIGH PLAINS. HOWEVER, A
STRONG CAPPING INVERSION MAY LIMIT THE NUMBER OF SEVERE THUNDERSTORMS AND A
SEVERE WEATHER HAZARD IS NOT DEPICTED ON THE MAP AT THIS TIME.



SURFACE LOW DEVELOPMENT IS EXPECTED ACROSS THE CENTRAL OR SOUTHERN GREAT PLAINS
BY MAY 26 DUE TO THE EJECTING UPPER-LEVEL TROUGH. THIS SURFACE LOW ALONG WITH
AN EVOLVING COLD FRONT INCREASE THE RISK OF HEAVY RAINFALL ACROSS THE
SOUTH-CENTRAL U.S., BEGINNING THIS WEEKEND. THE 6Z DETERMINISTIC GFS MODEL
INDICATES THAT PRECIPITABLE WATER VALUES INCREASE TO MORE THAN 1.75 INCHES FROM
TEXAS NORTHEAST TO THE LOWER OHIO VALLEY WHERE HEAVY RAIN (48-HOUR AMOUNTS OF 1
TO 3 INCHES) IS EXPECTED ON MAY 27 AND 28. A COLD FRONT, PROVIDING A FOCUS FOR
CONVECTION AND HEAVY RAINFALL (48-HOUR AMOUNTS OF 1 TO 3 INCHES), IS FORECAST
TO SLOWLY PUSH SOUTH ACROSS TEXAS AND THE LOWER MISSISSIPPI VALLEY ON MAY 29
AND 30. BOTH OF THESE HEAVY RAIN AREAS WILL HAVE TO BE MONITORED FOR THE
POTENTIAL OF SEVERE THUNDERSTORMS.



AN UPPER-LEVEL RIDGE IS FORECAST TO STRENGTHEN (500-HPA HEIGHTS INCREASING TO
NEAR 582-DM) OVER THE PACIFIC NORTHWEST BY EARLY NEXT WEEK, LEADING TO A
WARMING TREND FOR THIS REGION. A MUCH ABOVE-NORMAL TEMPERATURE HAZARD IS POSTED
FOR PARTS OF WESTERN OREGON AND NORTHERN CALIFORNIA WHERE MAXIMUM TEMPERATURES
ARE FORECAST TO AVERAGE MORE THAN 12 DEGREES F ABOVE NORMAL AND WARM TO NEAR 90
DEGREES F OR ABOVE.



AN AMPLIFIED TROUGH IS EXPECTED TO RESULT IN MUCH BELOW-NORMAL TEMPERATURES
ACROSS PARTS OF SOUTH-CENTRAL AND WESTERN MAINLAND ALASKA ON MAY 26 AND 27.
MINIMUM TEMPERATURES COULD FALL INTO THE TEENS (DEGREES F) IN THE COLDEST
LOCATIONS OF THE OUTLINED HAZARDS AREA. EARLY NEXT WEEK, THE DETERMINISTIC
MODEL SOLUTIONS DIVERGE ACROSS ALASKA WITH THE 0Z ECMWF INDICATING A STRONG
SURFACE LOW AFFECTING THE ALEUTIANS AND THE 6Z GFS MODEL FEATURING SURFACE HIGH
PRESSURE ACROSS THIS REGION.

FOR WEDNESDAY MAY 31 - TUESDAY JUNE 06: THE 6Z GFS AND 0Z ECMWF ENSEMBLE MEANS
ARE IN REASONABLY GOOD AGREEMENT WITH AN UPPER-LEVEL RIDGE EXTENDING FROM THE
PACIFIC NORTHWEST NORTH TO EASTERN ALASKA AND AN UPPER-LEVEL TROUGH OVER THE
NORTHEAST DURING WEEK-2. THE UPPER-LEVEL RIDGE SUPPORTS A SLIGHT TO MODERATE
RISK OF MUCH ABOVE-NORMAL TEMPERATURES ACROSS PARTS OF THE PACIFIC NORTHWEST
FROM MAY 31 TO JUNE 2. THE GEFS REFORECAST TOOL INDICATES THAT MAXIMUM
TEMPERATURES HAVE A 40 PERCENT CHANCE, OR HIGHER, OF EXCEEDING THE 85TH
PERCENTILE AND 90 DEGREES F ACROSS EASTERN WASHINGTON ON MAY 31 AND JUNE 1.
ELSEWHERE ACROSS MOST OF THE CENTRAL AND EASTERN U.S., NEAR TO BELOW-NORMAL
TEMPERATURES ARE FAVORED.



ALTHOUGH THERE IS A SIGNAL FOR POTENTIALLY HEAVY RAINFALL ACROSS THE SOUTHERN
GREAT PLAINS AND LOWER MISSISSIPPI VALLEY EARLY IN WEEK-2, THE DETERMINISTIC
MODEL SOLUTIONS ARE DRIER FOR THESE AREAS. ALSO, THE 6Z GFS AND 0Z ECMWF
ENSEMBLE MEANS CONTINUE TO DIFFER ON THE MOST LIKELY AREA FOR HEAVY RAINFALL,
RANGING FROM KANSAS TO THE WESTERN GULF COAST. THEREFORE, A CATEGORICAL HEAVY
RAIN HAZARD IS NOT POSTED FOR THE SOUTH-CENTRAL U.S.



THE U.S. DROUGHT MONITOR RELEASED ON MAY 18 SHOWS D2-D4 DROUGHT COVERAGE OVER
THE CONUS WITH A SLIGHT INCREASE LAST WEEK TO 1.38%. THIS GROWTH IS DUE TO
DEGRADATION OF DROUGHT CONDITIONS THAT OCCURRED ACROSS THE SOUTHEAST, SOUTHERN
PLAINS, AND SOUTHWEST.

FORECASTER: BRAD PUGH

$$




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