541
FXUS02 KWBC 231900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT TUE MAY 23 2023
VALID 12Z FRI MAY 26 2023 - 12Z TUE MAY 30 2023
...WATCHING COASTAL FRONT/LOW FOR THE POSSIBILITY OF BRINGING
HEAVY RAIN TO THE EASTERN SEABOARD...
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
AN UPPER RIDGE/HIGH IS LIKELY TO BE PRESENT ATOP THE MIDWEST LATE
WEEK, REBUILDING INTO NEXT WEEK AS MEAN TROUGHING IS EXPECTED FOR
THE WEST AND EAST COASTS, RESULTING IN LITTLE OVERALL MOTION TO
THE ENTIRE SYNOPTIC PATTERN ACROSS THE U.S. THROUGH EARLY NEXT
WEEK. WITHIN THIS BLOCKY PATTERN, THERE HAVE BEEN CONSIDERABLE
DIFFERENCES FROM MODEL TO MODEL AND RUN TO RUN OVER THE PAST
COUPLE OF DAYS, BUT RECENT MODEL GUIDANCE SEEMS TO BE STABILIZING
FINALLY AT LEAST IN THE EAST. THE TREND FOR AN UPPER LOW TO CUT
OFF IN THE SOUTHEAST THIS WEEKEND IS BECOMING MORE STABLE. THE
00Z/06Z GFS AND 00Z UKMET WERE THE STRONGEST WITH SHOWING THIS,
WITH THE 00Z ECMWF LEANING THAT DIRECTION BUT EVOLVING IT A BIT
DIFFERENTLY. THE 00Z CMC WAS THE OUTLIER WITH MAINTAINING A
STRONGER UPPER LOW OVER THE NORTHEAST, LIKE OLDER GUIDANCE FROM A
DAY OR TWO AGO. BUT FINALLY THE 12Z CMC FLIPPED TO A SOLUTION MUCH
MORE LIKE THE CONSENSUS OF OTHER MODELS. THE 12Z ECMWF LOOKS IN
BETTER ALIGNMENT AS WELL WITH THE MORE PERSISTENT GFS/UKMET TYPE
RUNS. THUS CONFIDENCE IS INCREASING FOR THIS SOUTHEAST UPPER LOW
TO DEVELOP, AND AN UPPER HIGH MAY CUT 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 IN RESPONSE.
THE SURFACE EVOLUTION STILL SHOWS SOME CONFUSING DIFFERENCES,
HOWEVER. MOST GUIDANCE NOW (CONSIDERING THE 12Z RUNS) TAKES A
SURFACE LOW INLAND (WITH POTENTIALLY HEAVY RAIN SPREADING INLAND
TOO) INTO THE CAROLINAS BY EARLY NEXT WEEK, BUT SOME GUIDANCE LIKE
THE GFS RUNS SHOW ANOTHER SURFACE LOW THEN DEVELOPING OFFSHORE
AGAIN IN A SIMILAR PLACEMENT. 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.
FARTHER WEST, TROUGHING IS LIKELY ACROSS THE WEST LATE WEEK, BUT
THEN VARIOUS UPSTREAM ENERGIES OVER THE PACIFIC AND WESTERN CANADA
STILL SHOW CONSIDERABLE VARIABILITY IN THEIR PHASING/INTERACTION
POTENTIAL. THE INCOMING 12Z GUIDANCE LOOKS LIKE IT MAY BE A LITTLE
MORE AGREEABLE WITH THE PATTERN FORTUNATELY. REGARDLESS OF THE
DETAILS, THE WEST IS LIKELY TO SEE CONTINUED UNSETTLED CONDITIONS.
GIVEN THESE CONSIDERATIONS, THE WPC FORECAST USED A BLEND OF THE
DETERMINISTIC 06Z GFS/00Z ECMWF/00Z UKMET AND THE ENSEMBLE MEANS
FROM THE 06Z GEFS AND THE 00Z ECMWF THROUGH THE PERIOD, THOUGH
PHASING OUT THE UKMET AND USING MORE ENSEMBLE MEANS BY LATE IN THE
TIMEFRAME.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND STORMS ARE LIKELY OVER FLORIDA LATE THIS WEEK 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 BY
FRIDAY, THEN LINGERING. MARGINAL RISKS ARE IN PLACE FOR THE DAYS
4-5 (FRIDAY-SATURDAY) EROS GIVEN THE POTENTIAL FOR HEAVY RAIN,
WITH SOME MINOR SOUTHWARD ADJUSTMENTS TO THE PREVIOUS ISSUANCE
GIVEN A SLOWER TREND OF THE LOW. 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, AND THERE WAS SOME INCREASE IN THAT DIRECTION IN THIS
FORECAST. 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 DIFFLUENCE 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,
WILL MAINTAIN BROAD MARGINAL RISKS FOR THE LATE WEEK 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.
TATE/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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