320
FXUS02 KWBC 240711
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
310 AM EDT WED MAY 24 2023
VALID 12Z SAT MAY 27 2023 - 12Z WED MAY 31 2023
...COASTAL LOW OFFERS LINGERING HEAVY RAIN AND COASTAL/MARITIME
THREATS FOR THE SOUTHEAST/MID-ATLANTIC...
...BROAD MULTI-DAY THUNDERSTORM AND LOCAL HEAVY RAIN THREAT FROM
THE ROCKIES TO PLAINS...
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
AN WARMING UPPER RIDGE/HIGH WILL SPREAD FROM THE CENTRAL U.S. TO
ATOP THE MIDWEST AND NORTHERN MID-ATLANTIC OVER FOR THE MEMORIAL
HOLIDAY WEEKEND AS PART OF A REX BLOCK, ADDITIONALLY REBUILDING
INTO NEXT WEEK WITH MEAN TROUGHING ANCHORING OVER THE WEST COAST
AND SOUTHEAST, RESULTING IN LITTLE OVERALL MOTION TO THE ENTIRE
SYNOPTIC PATTERN ACROSS THE U.S. THROUGH NEXT MIDWEEK. WITHIN THIS
BLOCKY PATTERN, THERE HAVE BEEN CONSIDERABLE DIFFERENCES FROM
MODEL TO MODEL AND RUN TO RUN EARLIER THIS WEEK, BUT RECENT MODEL
GUIDANCE SEEMS TO BE STABILIZING. CONFIDENCE HAS INCREASED FOR A
CLOSED/CUTOFF SOUTHEAST UPPER LOW TO DEVELOP AND LINGER AS AN
UPPER HIGH CUTS OFF FOR A TIME NORTH OF IT WITHIN A BROADER
SOUTHWEST TO NORTHEAST ORIENTED RIDGE. A SURFACE LOW LOOKS TO
DEVELOP OFFSHORE OF THE SOUTHEAST COAST. THE SURFACE EVOLUTION
STILL OFFERS DIFFERENCES WITH POSITION AND ULTIMATE EJECTION.
DURATION, PROXIMITY AND ANY REFORMATIONS OFFER COASTAL/MARITIME
THREATS AND SUGGEST HEAVIEST RAINS WILL SPREAD INLAND INTO THE
CAROLINAS/MID-ATLANTIC. THE SURFACE LOW(S) STILL SEEMS MOST LIKELY
TO STAY FRONTAL/EXTRATROPICAL, BUT WITH PERHAPS A NONZERO CHANCE
FOR SOMETHING SUBTROPICAL TO COME OUT OF IT FOR A TIME.
OUT WEST, UPPER TROUGHING REMAINS LIKELY TO REMAIN A FOCUS THROUGH
THIS FORECAST PERIOD, BUT THEN VARIOUS UPSTREAM ENERGIES FROM THE
PACIFIC AND WESTERN CANADA STILL SHOW CONSIDERABLE VARIABILITY IN
THEIR PHASING/INTERACTION POTENTIAL. HOWEVER, RECENT GUIDANCE HAS
BECOME BETTER CLUSTERED WITH FLOW EVOLUTION. REGARDLESS OF THE
DETAILS, BROAD AREAS FROM THE INTERIOR WEST/ROCKIES TO THE PLAINS
ARE LIKELY TO SEE CONTINUED UNSETTLED CONDITIONS WITH BROAD
CONVECTIVE DEVELOPMENTS/LOCAL DOWNPOURS AS ANOMALOUS MOISTURE AND
IMPULSES CHANNELS BETWEEN THE UPPER TROUGH POSITION AND THE
DOWNSTREAM UPPER RIDGE.
OVERALL, THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED
FOR ALL AREAS FROM BEST CLUSTERED GUIDANCE FROM THE 18 UTC
GFS/GEFS MEAN AND THE 12 UTC ECMWF/ECMWF ENSEMBLE MEAN ALONG WITH
THE 01 UTC NATIONAL BLEND OF MODELS. THIS MAINTAINS MAX WPC
PRODUCT CONTINUITY AND THE LATEST 00 UTC GUIDANCE VERSIONS OVERALL
REMAIN IN LINE FOR THE MOST PART WITH THIS FORECAST PLAN, AT LEAST
AS PREDICTABILITY INCREASES TO MORE AVERAGE LEVELS THROUGH THE
UPCOMING HOLIDAY WEEKEND.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND STORMS ARE LIKELY OVER SOUTHERN FLORIDA INTO THE
HOLIDAY WEEKEND WITH THE POTENTIAL FOR A DEVELOPING SURFACE LOW
SYSTEM, AND RAIN IS LIKELY TO SPREAD FARTHER NORTH ACROSS PARTS OF
THE EASTERN SEABOARD DEPENDENT ON THE LOW TRACK. THIS FORECAST
UPDATE CONTINUES TO SHOW THAT THE HEAVIEST RAIN IS LIKELY ACROSS
THE COASTAL CAROLINAS HEADING INTO THE WEEKEND, THEN LINGERING.
MARGINAL RISKS EXCESSIVE RAINFALL OUTLOOK (ERO)RISK AREAS ARE IN
PLACE FOR THE DAYS 4-5 (SATURDAY-SUNDAY) GIVEN THE POTENTIAL FOR
HEAVY RAIN. ONE OF THE MORE UNCERTAIN PARTS OF THE FORECAST IS
THAT GUIDANCE VARIES ON HOW FAR INLAND TO SPREAD THE
PRECIPITATION. IF RECENT TRENDS CONTINUE, HEAVIER RAIN MAY BE SEEN
INTO INTERIOR PORTIONS OF THE CAROLINAS/MID-ATLANTIC AS WELL.
AMPLE MOISTURE MAY BE FURTHER DIRECTED INTO THE MID-ATLANTIC THIS
WEEKEND ON THE EAST/NORTH SIDES OF THE SOUTHEAST UPPER LOW
FORMING. AMOUNTS AND PLACEMENT REMAIN UNCERTAIN AT THIS POINT
THOUGH. BEACH HAZARDS FOR THE UPCOMING MEMORIAL DAY WEEKEND MAY
NOT BE RULED OUT EITHER CONSIDERING THIS PATTERN, WITH POTENTIAL
FOR GUSTY WINDS AND UNSETTLED MARITIME CONDITIONS. SHOWERS AND
STORMS MAY FORM FARTHER INLAND ACROSS THE TENNESSEE VALLEY AND
VICINITY TOO. WITH THE UPPER TROUGH/LOW AND THE POTENTIAL
CLOUDINESS AND RAIN CHANCES, TEMPERATURES ARE LIKELY TO BE COOLER
THAN NORMAL ON AVERAGE THROUGH THE WEEKEND ACROSS MUCH OF THE
EAST.
OTHERWISE, THE AFOREMENTIONED MAIN UPPER RIDGE POSITION SLOW
TRANSLATION SHOULD SUPPORT SUMMERTIME TEMPERATURES WITH VALUES
UPWARDS TO 10-15 DEGREES ABOVE NORMAL INTO LATE WEEK THAT SHOULD
PROVE SLOW TO SPREAD SLOWLY EAST WITH TIME ACROSS THE
NORTH-CENTRAL U.S. TO THE MIDWEST. THE HEAT MAY BE TEMPERED
LOCALLY BY SCATTERED AND POTENTIALLY STRONG THUNDERSTORM ACTIVITY
BOTH WITH EJECTION OF IMPULSES FROM THE WEST AND WITH INTERACTION
WITH NORTHERN STREAM SYSTEM/FRONTAL APPROACHES FROM SOUTHERN
CANADA. LOCALLY HEAVY THUNDERSTORM ACTIVITY MAY ALSO FOCUS HEAVY
LOCAL DOWNPOURS DOWN OVER THE ROCKIES AND HIGH PLAINS AS WELL WITH
MOISTURE/INSTABILITY POOLING AHEAD OF A GENERAL DRYLINE PATTERN
WITH SOUTHERN STREAM IMPULSES UNDER FAVORABLE UPPER DIFLUENCE AND
INSTABILITY. ABOVE TO MUCH ABOVE NORMAL OVERNIGHT TEMPERATURES AND
A PATTERN FAVORING SCATTERED TO WIDESPREAD SHOWERS AND
THUNDERSTORMS ARE ALSO EXPECTED TO LINGER ALL WEEK OVER MUCH OF
INTERIOR WEST THROUGH THE ROCKIES IN MOISTENED FLOW CHANNELED
BETWEEN WEST COAST UPPER TROUGHING AND THE DOWNSTREAM CENTRAL U.S.
UPPER RIDGE. SINCE MOISTURE LEVELS WILL BE ABOVE AVERAGE AND
MOISTURE SHOULD BE POOLING AHEAD OF SLOW-MOVING FRONTS/BOUNDARIES,
PLAN TO HAVE BROAD ERO MARGINAL RISKS FOR THE WEEKEND EROS ACROSS
MUCH OF THE HIGH PLAINS AND BACK TOWARD THE NORTHERN ROCKIES FOR
SOME POTENTIAL FOR FLASH FLOODING WITH SOME DOWNPOURS OF HEAVY
RAIN THAT COULD BE SLOW-MOVING. THE DETAILS AND LOCAL FOCUS WILL
BE DEPENDENT ON THE ULTIMATE SHAPE OF THE STILL QUITE UNCERTAIN
LATER WEEK UPPER TROUGH POSITIONS AND FLOW INTERACTIONS, AS WELL
AS SMALLER-SCALE BOUNDARIES THAT HAVE LITTLE PREDICTABILITY AT
THIS FORECAST LEAD TIME.
SCHICHTEL
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES ARE
AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
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