722
FXUS02 KWBC 290658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 AM EDT TUE APR 29 2025
VALID 12Z FRI MAY 02 2025 - 12Z TUE MAY 06 2025
..OVERVIEW
BEHIND A SYSTEM TRACKING ACROSS SOUTHEASTERN CANADA ON FRIDAY WITH
SOME RAIN AND THUNDERSTORMS ACROSS THE EAST/SOUTH, GUIDANCE
CONTINUES TO SHOW A MORE BLOCKY PATTERN SETTLING OVER THE LOWER 48.
WITH ONLY MODERATE DETAIL DIFFERENCES/ADJUSTMENTS, MOST SOLUTIONS
HAVE BEEN FAIRLY WELL BEHAVED AS AN UPPER TROUGH APPROACHES THE
WEST COAST BY SATURDAY AND CLOSES OFF AN UPPER LOW THAT SHOULD
DRIFT INTO THE SOUTHWESTERN U.S. EARLY NEXT WEEK, SPREADING RAIN
AND HIGH ELEVATION SNOW ACROSS THE WEST. THIS SYSTEM MAY START TO
GENERATE HEAVIER SOUTHERN HIGH PLAINS RAINFALL TOWARD THE END OF
THE PERIOD. A LEADING RIDGE SHOULD SETTLE OVER THE PLAINS INTO THE
MISSISSIPPI VALLEY. IN CONTRAST TO THE WESTERN CONSENSUS,
INDIVIDUAL MODELS AND ENSEMBLES CONTINUE TO SHOW REMARKABLY
DIFFERENT IDEAS FOR THE ULTIMATE EVOLUTION OF ENERGY WITHIN AN
UPPER TROUGH REACHING THE EAST DURING THE WEEKEND INTO EARLY NEXT
WEEK. THIS SPREAD KEEPS CONFIDENCE MUCH LOWER THAN AVERAGE FOR
DETAILS OF EASTERN U.S. SENSIBLE WEATHER.
..GUIDANCE/PREDICTABILITY ASSESSMENT
OVER THE EAST, WHILE INDIVIDUAL ENSEMBLE MEMBERS DIVERGE
CONSIDERABLY AFTER SATURDAY, THE RESULTING MEANS STILL PROVIDE THE
MOST STABLE SOLUTION WITH AN UPPER TROUGH AXIS LINGERING NEAR THE
EAST COAST. THIS ALIGNMENT ALSO REFLECTS TELECONNECTION TENDENCIES
RELATIVE TO THE CORE OF POSITIVE HEIGHT ANOMALIES EXPECTED TO BE
WEST/NORTHWEST OF LAKE SUPERIOR LATE IN THE PERIOD. GUIDANCE SPREAD
AS OF THE 12Z/18Z RUNS FAVORED HOLDING ONTO A SOLUTION CLOSEST TO
THE ENSEMBLE MEANS WHILE AWAITING A BETTER OPPORTUNITY FOR MORE
CONFIDENT ADJUSTMENTS. AS FOR OTHER GUIDANCE, CMC/UKMET RUNS AND
THE 12Z ECMWF/ICON CLOSED OFF AN UPPER LOW WITHIN THE EXPECTED MEAN
TROUGH NEAR THE EAST COAST BUT AT VERY DIFFERENT LATITUDES. MOST
12Z/18Z MACHINE LEARNING (ML) MODELS ADDED EVEN MORE INTRIGUE,
CLOSING OFF INITIAL UPPER MIDWEST/CENTRAL PLAINS SHORTWAVE ENERGY
TO THE WEST OF THE APPALACHIANS (ANYWHERE BETWEEN THE GREAT LAKES
AND TENNESSEE VALLEY). THE NEW 00Z ICON HAS SHIFTED TO THE ML MODEL
CLUSTER WHILE THE 00Z ECMWF HAS SHIFTED SOMEWHAT IN THAT DIRECTION
AS WELL. LATEST GUIDANCE LEAVES THE 18Z AND 00Z GFS RUNS, WHICH
COMPLETELY CLEAR THE TROUGH FROM THE EAST COAST, AS VERY PRONOUNCED
EXTREMES. AT LEAST THE 00Z GEFS MEAN STILL HOLDS ONTO SOME EAST
COAST TROUGHING.
OVER RECENT DAYS THE TROUGH NEARING THE WEST COAST BY SATURDAY HAS
BEEN EXHIBITING A SLIGHTLY SLOWER TREND. AS THE UPPER LOW CLOSES
OFF OVER/NEAR CALIFORNIA, THE NEW 00Z GFS FAVORABLY ADJUSTED
NORTHEASTWARD FROM THE 12Z/18Z RUNS TO THE CONSENSUS TRACK. THEN BY
MONDAY-TUESDAY THE 00Z GFS STILL LOOKS MORE FAVORABLE IN CONTRAST
TO FARTHER NORTH EARLIER RUNS. THE 12Z/18Z ML MODELS SHOWED TYPICAL
SPREAD BUT GENERALLY LEANED TO THE SOUTHERN HALF OF THE DYNAMICAL
MODEL ENVELOPE.
GUIDANCE CONSIDERATIONS LED TO STARTING THE UPDATED FORECAST WITH
A 12Z/18Z OPERATIONAL MODEL COMPOSITE FROM FRIDAY INTO EARLY
SUNDAY, FOLLOWED BY A RAPID SHIFT TOWARD 60-70 PERCENT ENSEMBLE
INPUT (18Z GEFS AND 12Z ECENS/CMCENS) FOR MONDAY-TUESDAY.
..WEATHER/HAZARDS HIGHLIGHTS
EXPECT SOME SHOWERS AND THUNDERSTORMS AHEAD OF A COLD FRONT
PUSHING THROUGH THE EAST ON FRIDAY. LATEST GUIDANCE SIGNALS HAVE
BECOME A LITTLE MORE DIFFUSE REGARDING HOW MUCH FLASH FLOOD
POTENTIAL THERE MAY BE OVER THE CENTRAL AND NORTHERN APPALACHIANS
WHERE GROUND CONDITIONS ARE ON THE WETTER SIDE, BUT ENOUGH
INSTABILITY MAY EXIST TO PROMOTE RAIN RATES THAT COULD CAUSE
LOCALIZED RUNOFF ISSUES. THUS FOR NOW MAINTAINED THE EXISTING
MARGINAL RISK AREA IN THE DAY 4 EXCESSIVE RAINFALL OUTLOOK. MODELS
ARE STARTING TO DEVELOP SOME CLUSTERING FOR THE IDEA OF FRONTAL
WAVINESS PRODUCING A BAND OF ENHANCED RAINFALL OVER PARTS OF THE
NORTHEAST ON SATURDAY. THIS IS AN EMERGING TREND AND THERE IS A
QUESTION OF HOW MUCH INSTABILITY MAY EXIST IN THIS RAIN BAND, SO
INITIALLY OPTED FOR NO RISK AREA IN THE DAY 5 ERO FOR THIS
ACTIVITY. WILL CONTINUE TO MONITOR THIS ASPECT OF THE FORECAST IN
FUTURE RUNS. PRECIPITATION COVERAGE AND AMOUNTS OVER THE EAST
BECOME INCREASINGLY UNCERTAIN AFTER SATURDAY, DEPENDING ON
EVOLUTION OF UPPER TROUGHING AND POSSIBLE CLOSED LOW.
MULTIPLE DAYS OF LOW LEVEL FLOW COMING IN FROM THE GULF, ALONG
WITH SOME WEAK SHORTWAVE ENERGY APPROACHING FROM THE SOUTHWEST AND
EVENTUALLY HEIGHT FALLS AHEAD OF THE DEEPER WESTERN UPPER LOW, WILL
SUPPORT MULTIPLE EPISODES OF RAINFALL OVER PORTIONS OF THE
SOUTHERN AND EVENTUALLY ALSO CENTRAL PLAINS. WITH A FAIR AMOUNT OF
VARIANCE IN THE DETAILS, GUIDANCE CONTINUES TO MAINTAIN SUPPORT IN
PRINCIPLE FOR THE EXISTING MARGINAL RISK AREA FROM SOUTHEASTERN NEW
MEXICO INTO TEXAS ON DAY 4/FRIDAY. LATEST SOLUTIONS ARE INCREASING
POTENTIAL FOR IMPACTFUL RAIN TO EXTEND FARTHER EAST ALONG A
SURFACE FRONT, NOW FAVORING EXTENSION OF THE MARGINAL RISK AREA
INTO PARTS OF LOUISIANA. MOISTURE/INSTABILITY AND PATTERN
PERSISTENCE SHOULD LEAD TO ADDITIONAL POCKETS OF ENHANCED RAINFALL
OVER THE SOUTHWESTERN HALF OF TEXAS INTO SATURDAY, MERITING A
MARGINAL RISK OVER THAT REGION ON DAY 5. THE SOUTHWEST UPPER LOW
STILL LOOKS TO DRIFT FAR ENOUGH EASTWARD TO START GENERATING
HEAVIER RAINFALL OVER THE SOUTHERN HALF OF THE HIGH PLAINS DURING
THE FIRST HALF OF NEXT WEEK.
THE PACIFIC SYSTEM MOVING INTO THE WEST WILL BRING RAIN AND HIGH
ELEVATION SNOW INTO THE NORTHWEST AROUND FRIDAY, WITH PRECIPITATION
THEN CONTINUING ACROSS THE SIERRA NEVADA/GREAT BASIN/NORTHERN
PARTS OF THE SOUTHWEST AND INTO THE NORTHERN-CENTRAL ROCKIES.
HIGHEST TOTALS WILL BE DEPENDENT ON THE EXACT TRACT OF THE FORECAST
EVOLVING UPPER LOW, AND ARE CURRENTLY MOST LIKELY TO BE OVER THE
GREAT BASIN AND NORTH-CENTRAL ROCKIES. LATEST GUIDANCE IS KEYING ON
POTENTIAL FOR A BAND OF LOCALLY HEAVY RAINFALL OVER NEVADA ON DAY
5/SATURDAY AS THE UPPER LOW CLOSES OFF OVER CALIFORNIA, WITH
ANOMALOUS MOISTURE AND SOME INSTABILITY. THE DAY 5 ERO DEPICTS A
MARGINAL RISK AREA TO REFLECT THIS POSSIBLE THREAT. LEADING WEAK
SHORTWAVE/UPPER LOW ENERGY MAY PRODUCE LIGHTER PRECIPITATION FROM
THE SIERRA NEVADA EAST INTO THE ROCKIES ON FRIDAY.
WELL ABOVE NORMAL TEMPERATURES SHOULD MIGRATE FROM THE NORTHERN
HALF OR SO OF THE WEST ON FRIDAY INTO THE NORTHERN PLAINS BY THE
WEEKEND AND EARLY NEXT WEEK. PLUS 15-25F ANOMALIES SHOULD BE MOST
COMMON OVER FAR NORTHERN AREAS DURING FRIDAY-SUNDAY. IN CONTRAST,
THE UPPER LOW TRACKING INTO THE SOUTHWEST WILL BRING MUCH COOLER
TEMPERATURES TO THE REGION. HIGHS SHOULD BE 10-20F BELOW NORMAL ON
SUNDAY-MONDAY, WITH SOME RECORD COLD HIGHS POSSIBLE NEAR THE UPPER
LOW'S PATH. PERSISTENT CLOUDS AND PRECIPITATION SHOULD KEEP THE
SOUTHERN HIGH PLAINS HIGHS BELOW NORMAL AS WELL. LOCATIONS NEAR THE
EAST COAST WILL SEE A WARM DAY ON FRIDAY AHEAD OF A COLD FRONT,
FOLLOWED BY A TREND TO NEAR OR SLIGHTLY BELOW NORMAL READINGS.
HOWEVER CONFIDENCE IN SPECIFICS FROM SUNDAY ONWARD IS LOWER THAN
AVERAGE DUE TO VERY UNCERTAIN UPPER LEVEL FLOW DETAILS.
RAUSCH
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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