017
FXUS02 KWBC 241918
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
317 PM EDT WED MAY 24 2023
VALID 12Z SAT MAY 27 2023 - 12Z WED MAY 31 2023
...COASTAL LOW OFFERS 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
MODEL GUIDANCE HAS CONVERGED THE PAST COUPLE OF DAYS ON SHOWING A
REX BLOCK TYPE PATTERN IN THE EAST, AS AN UPPER LOW DEVELOPS ATOP
THE SOUTHEAST OVER THE WEEKEND WHILE UPPER RIDGING POTENTIALLY
CUTS OFF AN UPPER HIGH FOR A TIME TO ITS NORTH WITHIN A BROADER
SOUTHWEST TO NORTHEAST ORIENTED RIDGE. THIS PATTERN WILL HELP
DIRECT A SURFACE LOW IN THE WESTERN ATLANTIC WESTWARD INTO THE
CAROLINAS EARLY NEXT WEEK, ALONG WITH THE POTENTIAL FOR HEAVY
RAIN. LOW PRESSURE MAY LINGER INTO THE WESTERN ATLANTIC IN SOME
FORM THROUGH THE FIRST HALF OF NEXT WEEK. THE SURFACE LOW(S) STILL
SEEM MOST LIKELY TO STAY FRONTAL/EXTRATROPICAL, BUT WITH PERHAPS A
NONZERO CHANCE FOR SOMETHING SUBTROPICAL TO COME OUT OF ESPECIALLY
THE FIRST LOW FOR A TIME, THE POTENTIAL FOR WHICH THE NATIONAL
HURRICANE CENTER IS TRACKING. THERE REMAIN SOME MODEL DIFFERENCES
WITH THE TRACK AND TIMING OF THE SURFACE LOW, THOUGH RECENT RUNS
ARE AT LEAST ARE MORE AGREEABLE FOR THE FIRST LOW TO MOVE INLAND.
THE GFS WAS REALLY THE FIRST MODEL TO TAKE THIS TYPE OF TRACK WITH
OTHER MODELS CONVERGING TOWARD IT, BUT NOW IS AMONG THE
FASTER/FARTHER NORTH SOLUTIONS WITH THE LOW TRACK, WITH
ECMWF/UKMET/CMC A LITTLE MORE AGREEABLE. THESE DIFFERENCES ARE
GENERALLY WITHIN REASON FOR THE FORECAST LEAD TIME BUT AFFECT THE
LOCATION AND DISTRIBUTION OF HEAVIEST QPF.
MEANWHILE IN THE WEST, PERIODS OF TROUGHING ARE LIKELY. MORE
SPECIFICALLY, A TROUGH AXIS ATOP THE GREAT BASIN OVER THE WEEKEND
LOOKS TO REFORM A BIT WEST OVER THE WEST COAST STATES INTO NEXT
WEEK AS UPSTREAM ENERGY MAY DIG TROUGHING THERE AS THE INITIAL
ENERGY LIFTS INTO THE NORTH-CENTRAL U.S./SOUTH-CENTRAL CANADA.
MODELS ARE SHOWING BETTER CLUSTERING WITH THIS EVOLUTION OVERALL,
ALBEIT WITH DIFFERENCES WITH POTENTIAL FOR A CLOSED UPPER LOW
WITHIN THE TROUGH EARLY NEXT WEEK. UKMET RUNS ARE PERHAPS A MINOR
OUTLIER WITH WEAKER WEST COAST TROUGHING AT THAT POINT. THIS
REDEVELOPED TROUGH MAY SHIFT SLOWLY EAST, WHILE SOME SHORTWAVE
IMPULSES COULD BE PRESENT EMBEDDED WITHIN CENTRAL U.S. TO
MIDWEST/GREAT LAKES RIDGING WITH LOW PREDICTABILITY.
WITH MORE AGREEABLE OPERATIONAL GUIDANCE FOR THIS FORECAST THAN IN
PREVIOUS DAYS ON THE LARGE SCALE AT LEAST, THE WPC FORECAST WAS
ABLE TO USE A BLEND OF DETERMINISTIC 00/06Z GUIDANCE FOR MUCH OF
THE FORECAST PERIOD, WITH THE BLENDING PROCESS SERVING TO REDUCE
INDIVIDUAL VARIATIONS BETWEEN MODELS. THE LATTER DAYS HAD GRADUAL
INCLUSION OF THE GEFS AND ECMWF ENSEMBLE MEANS TO FURTHER TEMPER
MODEL DIFFERENCES. THIS LED TO GENERALLY GOOD CONTINUITY FROM THE
OVERNIGHT FORECAST AS WELL.
..WEATHER/HAZARDS HIGHLIGHTS
THE LOW PRESSURE SYSTEM OFF THE SOUTHEAST COAST LATE WEEK IS
LIKELY TO MOVE INLAND EARLY NEXT WEEK, SPREADING WIDESPREAD RAIN
AND THUNDERSTORMS TO THE CAROLINAS AND VICINITY OVER THE MEMORIAL
DAY HOLIDAY WEEKEND. WHILE THE EXACT LOW TRACK AND TIMING AND THUS
THE PRECIPITATION FORECAST IS STILL SOMEWHAT UNCERTAIN, THE
POTENTIAL IS THERE FOR WIDESPREAD HEAVY RAIN CAUSING MULTIPLE
INSTANCES OF FLASH FLOODING. THUS THIS FORECAST CYCLE PLANS TO
INTRODUCE A SLIGHT RISK OF EXCESSIVE RAINFALL FOR DAY 4/SATURDAY
FROM THE COASTAL CAROLINAS INLAND TOWARD THE PIEDMONT TOWARD THE
SOUTHERN APPALACHIANS. THERE ARE SOME MODEL DIFFERENCES WITH THE
EVOLUTION BY SUNDAY AND FOR NOW JUST A MARGINAL RISK REMAINS
OUTLINED FOR DAY 5 IN NORTHERN PARTS OF THE SOUTHEAST/SOUTHERN
MID-ATLANTIC, BUT IF MODEL GUIDANCE STARTS TO ALIGN BETTER ON THE
PLACEMENT FOR THE HEAVIEST RAIN, AN EMBEDDED SLIGHT RISK COULD BE
NEEDED IN FUTURE FORECAST CYCLES. AREAS OF THE SOUTHEAST TO
MID-ATLANTIC COULD CONTINUE TO SEE RAIN INTO MEMORIAL DAY AND
TUESDAY BUT WITH PERHAPS LOWERING AMOUNTS. ADDITIONALLY, BEACH
HAZARDS WITH POTENTIAL FOR GUSTY WINDS AND UNSETTLED MARITIME
CONDITIONS INCLUDING RIP CURRENTS ARE LIKELY THIS MEMORIAL DAY
WEEKEND. WITH THE UPPER TROUGH/LOW AND THE CLOUDINESS AND RAIN
CHANCES, TEMPERATURES ARE LIKELY TO BE COOLER THAN NORMAL ON
AVERAGE THROUGH THE WEEKEND ACROSS THE SOUTHEAST, WITH HIGHS
POSSIBLY STAYING AS COOL AS THE 60S, WITH A GENERAL MODERATION
TOWARD NORMAL NEXT WEEK.
MEANWHILE FARTHER WEST, SCATTERED SHOWERS AND THUNDERSTORMS ARE
LIKELY ACROSS THE GREAT BASIN AND NORTHERN/CENTRAL ROCKIES AS
ENHANCED MOISTURE POOLS NEAR MULTIPLE ROUNDS OF FRONTAL SYSTEMS
THROUGH THE WEEKEND AND INTO NEXT WEEK. THE HIGH PLAINS CAN ALSO
EXPECT SHOWERS AND STORMS WITH MOISTURE/INSTABILITY POOLING AHEAD
OF A GENERAL DRYLINE PATTERN THAT WILL BE LONG-LASTING. PERIODIC
ENERGETIC IMPULSES ALOFT COULD HELP SUPPORT LIFT FOR THESE STORMS
IN MOIST ENVIRONMENTS WITH PRECIPITABLE WATER VALUES OVER THE
75TH, IF NOT 90TH, PERCENTILE. MARGINAL RISKS ARE IN PLACE FOR
THESE REGIONS IN THE WEEKEND EROS WITH THREATS FOR LOCALIZED HEAVY
DOWNPOURS THAT COULD BE SLOW-MOVING TO CAUSE FLOODING ISSUES. THE
EXACT DETAILS AND LOCAL FOCUS WILL BE DEPENDENT ON SHORTWAVE
IMPULSES AND SMALLER-SCALE BOUNDARIES THAT HAVE LITTLE
PREDICTABILITY AT THIS FORECAST LEAD TIME THOUGH. PRECIPITATION
MAY TEMPER HEAT LOCALLY, BUT OVERALL EXPECT GENERALLY WARMER THAN
AVERAGE TEMPERATURES FOR THE NORTHERN TIER OF THE COUNTRY BY
AROUND 10-15 DEGREES. THE MIDWEST/GREAT LAKES REGION COULD REACH
15-20F ABOVE AVERAGE BY THE MIDDLE OF NEXT WEEK WITH HIGHS IN THE
90S UNDER THE LINGERING UPPER HIGH/RIDGE.
TATE
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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