213
FXUS02 KWBC 021901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
301 PM EDT FRI MAY 2 2025
VALID 12Z MON MAY 05 2025 - 12Z FRI MAY 09 2025
...BLOCKY PATTERN WITH CLOSED LOWS OVER THE WEST AND THE EAST WITH
AREAS OF HEAVY RAINFALL POTENTIAL EARLY TO MID NEXT WEEK...
..OVERVIEW
A BLOCKY PATTERN OVER THE LOWER 48 WILL BE IN PLACE BY THE
BEGINNING OF THE MEDIUM RANGE PERIOD ON MONDAY. THE DOMINANT
FEATURES WILL BE TWO CLOSED UPPER LOWS, ONE OVER THE WEST/SOUTHWEST
AND ONE OVER THE EAST, THAT WILL BOTH SLOWLY DRIFT AROUND
WEAKENING THROUGH WEDNESDAY/THURSDAY. THESE SYSTEMS ARE EXPECTED TO
PRODUCE AREAS OF SIGNIFICANT RAINFALL OVER PORTIONS OF THE CENTRAL
AND SOUTHERN PLAINS, NORTHEAST, AND MID-ATLANTIC, AS WELL AS HIGH
ELEVATION SNOWS IN THE WEST. EVENTUALLY, THE EASTERN UPPER LOW
SHOULD INTERACT WITH A NORTHERN STREAM TROUGH THAT WILL SWING
ACROSS SOUTHERN CANADA, RESULTING IN THE LOW BEING SWEPT UP TOWARDS
THE CANADIAN MARITIMES LATER NEXT WEEK. THE WESTERN LOW WILL
LIKELY REMAIN ISOLATED AS IT DRIFTS EAST OVER THE PLAINS AND
WEAKENS. THE REMNANTS OF THE WESTERN LOW MAY EVENTUALLY COMBINE
WITH ANOTHER TROUGH DEVELOPING OVER THE EAST COAST LATE THURSDAY
INTO FRIDAY. BY WEDNESDAY, ANOTHER TROUGH AND FRONTAL SYSTEM WILL
APPROACH THE WEST COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
TWO DISTINCT CUT-OFF LOWS WILL DOMINATE THE SYNOPTIC PATTERN ACROSS
THE CONTINENTAL U.S. THROUGH MID TO LATE WEEK, AND THE LATEST
DETERMINISTIC AND ENSEMBLE GUIDANCE ARE IN GOOD OVERALL AGREEMENT
WITH THE DEPICTION OF THEIR EVOLUTIONS. WITH THAT BEING SAID, THE
EVOLUTION OF SMALLER SCALE DETAILS, PARTICULARLY WITH REGARDS TO
EMBEDDED VORT MAXES AND SHORTER WAVELENGTH TROUGHS, BECOMES
INCREASINGLY UNCERTAIN BEYOND DAY 5, WHICH WILL LIKELY HAVE AN
IMPACT ON THE SPATIAL AND TEMPORAL DISTRIBUTION OF QPF AND THE
PLACEMENT OF FRONTAL SYSTEMS AND FEATURES. GIVEN THE ABOVE DECIDED
TO ROLL WITH A GENERAL MODEL BLEND FEATURING THE 00Z ECMWF, 00Z
CMC, 00Z ECENS, 00Z GEFS, AND THE 06Z GFS. UNLIKE THE OVERNIGHT
BLEND, THE 00Z UKMET WAS THE OUTLIER WITH THIS CYCLE, MAINLY WITH
HOW IT HANDLED THE EVOLUTION OF A NORTHERN STREAM SHORTWAVE
DIGGING ACROSS THE NORTHERN PLAINS IN THE BEGINNING OF THE PERIOD.
ALSO UNLIKE THE PREVIOUS BLEND, THE 00Z CMC WAS MUCH MORE USABLE
THIS GO AROUND. IN FACT, THE 00Z CMC WAS VERY CLOSE TO THE 00Z EC
AIFS AND THE 06Z GFS, WHICH ADDED CONFIDENCE FOR ITS INCLUSION. BY
THE END OF THE PERIOD, THE EC AND GEFS ENSEMBLE MEANS OFFERED
STABILITY AND FAVORABLY MATCHED CONTINUITY DESPITE SMALLER SCALE
DIFFERENCES FROM DETERMINISTIC GUIDANCE.
..WEATHER/HAZARDS HIGHLIGHTS
EARLY NEXT WEEK, TWO OCCLUDED FRONTAL SYSTEMS ARE EXPECTED TO
BRING MODERATE TO LOCALLY HEAVY RAINFALL TO PORTIONS OF THE WEST
AND EAST. IN THE WEST, ANOMALOUS MOISTURE ACCOMPANYING THE UPPER
LOW WILL SUPPORT HEIGHTENED PRECIPITATION FROM THE GREAT BASIN TO
THE CENTRAL AND SOUTHERN ROCKIES AND PLAINS. BY MONDAY, THE SYSTEM
WILL BECOME STATIONARY OVER THE PLAINS, ALLOWING FOR ANOMALOUS
MOISTURE TO CONTINUOUSLY POOL NEAR A BOUNDARY IN THE VICINITY.
SEVERAL ROUNDS OF MOISTURE-LADEN THUNDERSTORMS ARE LIKELY TO
RESULT IN A MULTI-DAY HEAVY RAINFALL EVENT ACROSS THE CENTRAL AND
SOUTHERN PLAINS AND INTO THE LOWER MISSISSIPPI VALLEY MONDAY
THROUGH WEDNESDAY. PORTIONS OF THE PLAINS STATES WILL BE ESPECIALLY
VULNERABLE TO HEAVY RAIN/FLOODING DUE TO RECENT HEAVY RAINS AND
SATURATED SOILS. THIS ACTIVITY IS HIGHLIGHTED IN THE DAY 4-5
EXCESSIVE RAINFALL OUTLOOKS (EROS) WITH A SLIGHT RISK AREA
HIGHLIGHTED ACROSS THE TEXAS AND OKLAHOMA PANHANDLES ON MONDAY. FOR
TUESDAY, A GROWING SIGNAL WITH HIGH CONFIDENCE OF WIDESPREAD 2-5"+
RAINFALL AMOUNTS ACROSS PORTIONS OF THE ARKLATEX AND LOWER
MISSISSIPPI VALLEY WARRANTED A BROADER EXPANSION OF THE SLIGHT RISK
AREA. HIGHER END PROBABILITIES OF EXCESSIVE RAINFALL ARE FOCUSED
ACROSS NORTHEAST TEXAS, EXTREME SOUTHEAST OKLAHOMA, SOUTHERN
ARKANSAS, WEST CENTRAL MISSISSIPPI AND NORTHERN LOUISIANA. GIVEN
RELATIVELY GOOD SPATIAL AND TEMPORAL MODEL AGREEMENT AND AMOUNTS
TRENDING HIGHER, WOULD NOT BE SURPRISED TO SEE AN EMBEDDED UPGRADE
TO A MODERATE RISK IN FUTURE FORECAST UPDATES.
UNDER THE EASTERN UPPER LOW, THE FRONTAL SYSTEM WILL VERY SLOWLY
PUSH EASTWARD EARLY NEXT WEEK, RESULTING IN A STREAM OF ONSHORE
MOISTURE OVER THE MID-ATLANTIC AND NORTHEAST AND PERSISTENT
PRECIPITATION ACROSS THESE REGIONS THROUGH TUESDAY. INITIALLY,
RAINFALL COULD BE BENEFICIAL AND BRING SOME RELIEF TO DROUGHT
CONDITIONS, BUT REPEATED ROUNDS OF RAINFALL COULD PRODUCE LOCALLY
HEAVY TOTALS AND MAY RESULT IN LOCALIZED FLOODING IN URBAN AND POOR
DRAINAGE AREAS. THE DAY 4-5 EROS SHOW A MARGINAL RISK AREA ACROSS
THESE REGIONS FOR MONDAY AND TUESDAY. BY WEDNESDAY, THE FRONTAL
SYSTEM SHOULD ACCELERATE TO THE EAST AS THE UPPER LOW INTERACTS
WITH THE NORTHERN STREAM, SHIFTING PRECIPITATION FARTHER INTO THE
NORTHEAST.
THE UPPER LOWS OVER THE EAST AND SOUTHWEST EARLY IN THE WEEK WILL
RESULT IN BELOW TO WELL BELOW NORMAL TEMPERATURES, ESPECIALLY THE
SOUTHWEST WHERE DAYTIME HIGHS COULD BE 15-20 DEGREES BELOW NORMAL.
THIS SHOULD MODERATE WITH TIME AS THE EAST RETURNS TO NEAR NORMAL
BY TUESDAY AND THE SOUTHWEST AREA SLOWLY SHIFTS EASTWARD. ABOVE
NORMAL TEMPERATURES TO BEGIN THE WEEK OVER THE NORTHERN PLAINS MAY
EXPAND INTO THE UPPER GREAT LAKES. UPPER RIDGING BUILDING OVER THE
WEST AS THE UPPER LOW SHIFTS EAST WILL ALLOW FOR EXPANDING COVERAGE
OF ABOVE NORMAL TEMPERATURES NEXT WEEK.
MILLER/SANTORELLI
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 FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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