506
FXUS02 KWBC 250701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT THU MAY 25 2023
VALID 12Z SUN MAY 28 2023 - 12Z THU JUN 01 2023
...COASTAL LOW OFFERS HEAVY RAIN AND COASTAL/MARITIME THREATS FOR
THE SOUTHEAST/MID-ATLANTIC...
...BROAD MULTI-DAY THUNDERSTORM AND LOCAL HEAVY RAIN THREAT
CONTINUES FOR THE PLAINS...
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE FOR THE MEDIUM-RANGE FORECAST PERIOD (12 Z SUNDAY -
12 Z THURSDAY) BEGINS WITH A REX BLOCK PATTERN OVER THE EASTERN
CONUS AS A CLOSED RIDGE SETTLES IN OVER THE GREAT LAKES WHILE A
CUTOFF LOW LINGERS OVER THE SOUTHEAST. AN ASSOCIATED LOW PRESSURE
SYSTEM AT THE SURFACE OFFSHORE OF THE SOUTHEASTERN U.S. LOOKS TO
TRACK NORTHWESTWARD INTO THE CAROLINAS. THE NATIONAL HURRICANE
CENTER HAS BEEN MONITORING THE FORECAST FOR THIS SYSTEM BUT KEEPS
THE CHANCES OF ANY TROPICAL DEVELOPMENT VERY LOW. MEAN TROUGHING
WILL BE IN PLACE OVER PORTIONS OF THE WEST, CENTERED ON THE WEST
COAST, THROUGH AT LEAST THE MID-PERIOD, WITH WEAK FLOW ALOFT OVER
THE CENTER OF THE COUNTRY WITH THE NORTHERN STREAM SHUNTED
NORTHWARD INTO CANADA. SUBTLE SHORTWAVES TRAVERSE THROUGH A WEAK
SOUTHERN STREAM RIDGE OVER THE SOUTHERN PLAINS, WHICH WILL LIKELY
HELP PROMOTE SCATTERED PRECIPITATION CHANCES ACROSS THE AREA.
LATER IN THE PERIOD, THE CUTOFF LOW OVER THE SOUTHEAST SLOWLY
TRANSLATES NORTHEASTWARD UP THE EAST COAST. THE MEAN TROUGHING
OVER THE WEST COAST SHIFTS A BIT WESTWARD AS THE NORTHERN STREAM
AMPLIFIES NORTHWESTWARD WITH A RIDGE BUILDING OVER THE NORTHERN
ROCKIES.
CLUSTERING SOLUTIONS AND RUN-TO-RUN CONSISTENCY SUGGEST MOST OF
THE DETERMINISTIC (GFS/ECMWF/CMC) AND ENSEMBLE MEAN (GEFS/ECENS)
GUIDANCE HAS A GOOD HANDLE ON THE PATTERN EVOLUTION THROUGHOUT THE
PERIOD IN A BROAD SENSE, WITH MOST DIFFERENCES RELATED TO
SMALLER-SCALE SHORTWAVES AND OTHER SUCH FEATURES THAT HAVE
EXPECTEDLY LOWER PREDICTABILITY. HOWEVER, IT IS WORTH NOTING THAT
THE RAINFALL FORECAST FOR THE LOW PRESSURE SYSTEM OVER THE
CAROLINAS REMAINS A BIT MORE UNCERTAIN. MOST OF THE GUIDANCE, WITH
THE EXCEPTION OF THE 18Z GFS, AGREES UPON THE LOCATION, BUT THE
AMOUNTS DO VARY A BIT. ALSO, SUBTLE DIFFERENCES IN HEIGHT RISES
OVER THE CENTRAL/EASTERN CONUS LOOK TO HAVE AN IMPACT ON FORECAST
HIGH TEMPERATURES NEXT WEEK. THE 12Z ECMWF IS NOTICEABLY WARMER
WITH TEMPERATURES THAN THE 18Z GFS. OTHERWISE, MORE BROADLY, ONE
NOTABLE DIFFERENCE EARLY IN THE FORECAST PERIOD IS WEAKER MEAN
TROUGHING OVER THE WEST IN THE 12Z UKMET. THIS LENDS WELL TO
FOLLOWING A GENERAL MODEL BLEND OF THE 06Z GFS AND 12Z CMC/EMCWF,
SIMILAR TO THE PRIOR WPC FORECAST, BUT WITH THE UKMET NOT INCLUDED
GIVEN THE NOTED LARGE-SCALE DIFFERENCES. A CONTRIBUTION FROM THE
18Z GEFS AND 12Z ECENS MEANS IS ADDED DURING THE LATE PERIOD AS
THE DETERMINISTIC MODEL SOLUTIONS BEGIN TO DIVERGE MORE,
SPECIFICALLY WITH RESPECT TO THE DEGREE OF THE NOTED HEIGHT RISES
OVER THE CENTRAL/EASTERN CONUS AS WELL AS WITH RESPECT TO THE
RIDGING OVER THE NORTHERN ROCKIES. THE 18Z GFS IS MORE AGGRESSIVE
COMPARED TO THE 12Z ECMWF/CMC AND THE ENSEMBLE MEANS WITH THIS
PROGRESSION. THE RUN-TO-RUN CONSISTENCY IN THE GUIDANCE AND
SIMILAR GUIDANCE BLEND TO THE PRIOR WPC FORECAST KEEPS THIS
FORECAST UPDATE VERY SIMILAR.
..WEATHER/HAZARDS HIGHLIGHTS
A LOW PRESSURE SYSTEM OFF THE SOUTHEAST COAST IS FORECAST TO TRACK
INLAND OVER THE CAROLINAS THIS WEEKEND, BRINGING HEAVY RAIN ALONG
WITH GUSTY WINDS AND BEACH RELATED HAZARDS TO THE COAST INTO
MEMORIAL DAY. WHILE MORE SIGNIFICANT IMPACTS LOOK TO OCCUR ON
SATURDAY, JUST PRIOR TO THE CURRENT FORECAST PERIOD, DAY 4/5
MARGINAL RISKS OF EXCESSIVE RAINFALL ARE IN PLACE OVER THE REGION
AS WIDESPREAD, LOCALLY HEAVY RAINFALL SHOULD CONTINUE TO LEAD TO
AT LEAST ISOLATED INSTANCES OF FLASH FLOODING. THIS IS ESPECIALLY
TRUE FOR LOCATIONS THAT SEE RAIN OVER MULTIPLE DAYS CONTRIBUTING
TO WETTER SOILS. THERE IS A DEGREE OF UNCERTAINTY WITH BOTH THE
EXACT TRACK OF THE LOW, AND THUS FOOTPRINT OF HEAVY RAINFALL, AS
WELL AS WITH THE EXPECTED AMOUNTS, AND IT IS POSSIBLE THAT A
GREATER THREAT OF FLASH FLOODING MAY CONTINUE FROM SATURDAY INTO
SUNDAY. TO THE NORTH, UPPER-LEVEL RIDGING OVER THE GREAT LAKES
WILL PUSH HIGH TEMPERATURES WELL ABOVE AVERAGE BY AS MUCH AS 15-20
DEGREES FROM THE NORTHERN PLAINS THROUGH THE GREAT LAKES AND INTO
THE INTERIOR NORTHEAST NEXT WEEK, EVENTUALLY SPREADING FURTHER
EASTWARD INTO THE NORTHEAST AND MID-ATLANTIC. FRONTAL ACTIVITY
ALONG THE NORTHERN STREAM MAY BRING THE CHANCE FOR SOME ISOLATED
TO SCATTERED STORMS, PARTICULARLY FOR THE NORTHERN PLAINS/UPPER
MIDWEST, BUT CONDITIONS SHOULD OTHERWISE REMAIN DRY. DEPENDING ON
THE EVOLUTION OF BROADER HEIGHT RISES OVER THE CENTRAL/EASTERN
CONUS, TEMPERATURES COULD TREND EVEN WARMER MORE BROADLY OVER THE
EASTERN HALF OF THE COUNTRY.
MOIST, SOUTHEASTERLY UPSLOPE FLOW WILL REMAIN IN PLACE OVER THE
WESTERN PLAINS/HIGH PLAINS, LEADING TO DAILY ROUNDS OF
THUNDERSTORM DEVELOPMENT AND SUBSEQUENT HEAVY RAINFALL CHANCES
ACROSS THE REGION. A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN
PLACE SUNDAY AS MOISTURE REMAINS ANOMALOUSLY HIGH OVER THE REGION
AND WEAK FLOW ALOFT LEADS TO SLOW STORM MOTIONS. THE SUBTLE HEIGHT
RISES MORE BROADLY OVER THE CENTRAL/EASTERN CONUS AND THE
TEMPERING OF LOW-LEVEL JET ACTIVITY OVERNIGHT SHOULD DECREASE
MOISTURE TRANSPORT AND STORM COVERAGE/INTENSITY, BRINGING DOWN THE
THREAT OF LOCALLY HEAVY RAINFALL NEXT WEEK. MEAN TROUGHING WILL
REMAIN OVER THE WEST AT THE START OF THE PERIOD, WITH THE
SUBSEQUENT STORMY PATTERN OVER THE NORTHERN ROCKIES/GREAT BASIN
THIS WEEK CONTINUING INTO THE WEEKEND AND NEXT WEEK. STORM
COVERAGE AND RAINFALL AMOUNTS SHOULD TREND DOWNWARD AS TROUGHING
ALOFT BEGINS TO SHIFT WEST AND RIDGING BUILDS NORTHWESTWARD OVER
THE NORTHERN ROCKIES, BUT AN ISOLATED INSTANCE OR TWO OF FLASH
FLOODING IN TERRAIN SENSITIVE AREAS OR BURN SCARS WILL REMAIN
POSSIBLE. HIGH TEMPERATURES ACROSS MOST OF THE WEST WILL REMAIN
AROUND AVERAGE BEFORE A WARMING TREND OVER THE NORTHWEST AS THE
RIDGING BUILDS IN OVER THE REGION. COASTAL CALIFORNIA WILL BE
COOLER AS A CUTOFF LOW SITS OVER THE COAST, WITH ONSHORE FLOW IN
PLACE.
PUTNAM
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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