511
FXUS02 KWBC 280658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 AM EDT MON APR 28 2025
VALID 12Z THU MAY 01 2025 - 12Z MON MAY 05 2025
..OVERVIEW
BEHIND A DEVELOPING STORM SYSTEM TRACKING FROM THE MIDWEST THROUGH
THE GREAT LAKES AND SOUTHEASTERN CANADA LATE THIS WEEK (WITH
ASSOCIATED FRONTS AND RAINFALL OF VARYING INTENSITY), GUIDANCE
CONTINUES TO SHOW A LOWER 48 PATTERN ALOFT THAT BECOMES MORE
BLOCKY HEADING INTO THE WEEKEND AND EARLY NEXT WEEK. AN UPPER
TROUGH REACHING THE WEST COAST BY EARLY SATURDAY WILL DISPLACE A
LEADING WEAK UPPER LOW CROSSING THE SOUTHWEST AND SHOULD ULTIMATELY
CLOSE OFF A FAIRLY DEEP LOW BY SUNDAY-MONDAY, WHILE A RIDGE LIKELY
SETS UP OVER THE PLAINS AND THEN MISSISSIPPI VALLEY. THIS PATTERN
WILL FAVOR A BROAD AREA OF RAIN AND HIGHER ELEVATION SNOW MOVING
ACROSS THE WEST WHILE PARTS OF THE SOUTHERN HALF OF THE PLAINS
SHOULD SEE MULTIPLE DAYS WITH EPISODES OF RAIN/THUNDERSTORMS. IN
CONTRAST TO THE RELATIVE CONSENSUS OVER THE WESTERN-CENTRAL U.S.
LATE IN THE PERIOD, MODELS AND ENSEMBLES ARE ALL OVER THE PLACE
REGARDING HOW MUCH TROUGHING (AND PERHAPS EVEN AN EMBEDDED UPPER
LOW) MAY LINGER NEAR THE EAST COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THERE IS STILL SOME MODERATE SPREAD FOR DETAILS OF THE MIDWEST
THROUGH SOUTHERN CANADA SYSTEM, WITH DEPENDENCE ON SPECIFICS OF THE
SHORTWAVE INITIALLY EJECTING THROUGH THE PLAINS/MISSISSIPPI VALLEY
AND INFLUENCE FROM A TRAILING NORTHERN STREAM TROUGH. CMC RUNS HAVE
BEEN A BIT ON THE SLOW SIDE RELATIVE TO OTHER DYNAMICAL MODELS AS
WELL AS MACHINE LEARNING (ML) MODELS. MEANWHILE, BY EARLY FRIDAY
THE ML MODELS LEAN WEAKER THAN MOST DYNAMICAL GUIDANCE AND THEIR
AVERAGE DEPTH IS EVEN WEAKER THAN AN AVERAGE OF THE VARIOUS 12Z/18Z
ENSEMBLE MEANS. AN AVERAGE OF THE DYNAMICAL MODELS CONTINUES TO
LOOK BEST AT THIS TIME.
THE SUPPORTING UPPER TROUGH THAT REACHES INTO THE EAST SHOWS DECENT
CLUSTERING INTO EARLY SATURDAY BUT THEN SOLUTIONS RAPIDLY DIVERGE
WITH DIFFERENCES IN THE CHARACTER OF UPSTREAM ENERGY STREAMING INTO
THE MEAN TROUGH. LATEST ECMWF RUNS AND THE 00Z GFS/ICON SHOW RAPID
PROGRESSION OF THE SATURDAY TROUGH IN FAVOR OF MUCH MORE SHALLOW
CYCLONIC FLOW OVER NEW ENGLAND THEREAFTER. ON THE OTHER EXTREME,
CMC RUNS AND THE 00Z UKMET DROP AN UPPER LOW INTO THE
NORTHEAST/MID-ATLANTIC. THE FULL ENSEMBLE SPREAD IS QUITE VARIED AS
WELL, BUT WITH THE RESULTING MEANS HOLDING ONTO SOME EAST COAST
TROUGHING INTO MONDAY. RECENT ML MODELS DIFFER AS WELL BUT WITH A
RELATIVE MAJORITY SUPPORTING A TROUGH WITH POSSIBLE UPPER LOW OVER
THE NORTHEAST. THE NEW 00Z AIFS HAS COME IN WITH AN UPPER LOW JUST
OFF THE NEW JERSEY COAST FOR EARLY MONDAY. PUTTING TOGETHER THE
GENERAL THEME OF ML GUIDANCE, PLUS TELECONNECTION SUPPORT FOR SOME
DEGREE OF TROUGHING ALIGNED NEAR THE EAST COAST BASED ON A POSITIVE
HEIGHT ANOMALY CENTER LIKELY TO BE JUST WEST OF LAKE SUPERIOR BY
THE END OF THE PERIOD, PREFERENCE LEANS TO THE EAST COAST TROUGH OF
THE ENSEMBLE MEANS WITH MODEST INPUT OF THE 18Z/12Z GFS THAT ALSO
HAD TROUGHING. WITHIN THE TROUGH CLUSTER, THIS IS SOMEWHAT
CONSERVATIVE IN THAT IT KEEPS ANY POTENTIAL UPPER LOW NORTH OF
MAINE FOR NOW.
OVERALL MOST OF THE GUIDANCE IS MUCH BETTER BEHAVED FOR THE UPPER
TROUGH REACHING THE WEST COAST BY AROUND EARLY SATURDAY AND THEN
CLOSING OFF AN UPPER LOW THAT DRIFTS OVER THE SOUTHERN/CENTRAL
PARTS OF THE WEST BY SUNDAY-MONDAY. BY MONDAY THE 12Z ECMWF STRAYED
A TAD FASTER THAN MOST OTHER DYNAMICAL/ML MODEL SOLUTIONS, WHILE
SOME ML MODELS SUGGEST THAT THE UPPER LOW COULD TAKE A SLIGHTLY
FARTHER SOUTH TRACK THAN THE AVERAGE OF DYNAMICAL MODELS/MEANS.
BASED ON 18Z/12Z GUIDANCE COMPARISONS, THE THURSDAY-FRIDAY
FORECAST EMPHASIZED AN AVERAGE OF THE 18Z GFS/12Z ECMWF WITH MODEST
INPUT FROM THE 12Z UKMET. THE UKMET COMPARED LESS FAVORABLY TO
CONSENSUS BY SATURDAY SO THE 18Z GEFS/12Z ECENS MEANS REPLACED IT
AT THAT TIME. THEN THE BLEND RAPIDLY SHIFTED TOWARD 60 PERCENT
WEIGHT OF THE GEFS/ECENS/CMCENS MEANS WITH THE REST CONSISTING OF
THE 12Z/18Z GFS.
..WEATHER/HAZARDS HIGHLIGHTS
SURFACE LOW/FRONTAL PROGRESSION ACROSS THE EASTERN HALF OF THE
COUNTRY THURSDAY-FRIDAY WILL BRING A BROAD AREA OF RAINFALL WITH
VARYING INTENSITY, BUT GENERALLY WITH LIGHTER TOTALS THAN EXPECTED
ON PRIOR DAYS OVER THE PLAINS. ADEQUATE INSTABILITY AND MOISTURE
MAY STILL PRODUCE SOME POCKETS OF RAIN RATES GREAT ENOUGH TO CAUSE
RUNOFF ISSUES, ESPECIALLY WHERE THERE ARE WETTER GROUND CONDITIONS
LEADING INTO THE EVENT. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK
MAINTAINS A BROAD MARGINAL RISK AREA FROM PARTS OF THE LOWER
MISSISSIPPI VALLEY AND TENNESSEE VALLEY INTO THE GREAT LAKES. WHAT
THREAT OF EXCESSIVE RAINFALL EXISTS FARTHER EAST BY DAY 5 WILL BE
MORE NARROW, WITH BEST RELATIVE POTENTIAL OVER THE CENTRAL AND
NORTHERN APPALACHIANS WHERE GROUND CONDITIONS ARE ON THE WETTER
SIDE AND A SUBSET OF GUIDANCE SHOWS LOCALLY ENHANCED RAINFALL. A
MARGINAL RISK COVERS THIS AREA OF INTEREST. SOME CONVECTION MAY
ALSO EXTEND NEAR THE GULF COAST BUT WITH POOR AGREEMENT FOR
COVERAGE AND AMOUNTS. MOST OF THE EAST SHOULD TREND DRIER BY THE
WEEKEND BUT AN UNCERTAIN LINGERING UPPER TROUGH COULD PRODUCE SOME
CLOUDS/SHOWERS IF IT IS SUFFICIENTLY DEEP.
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. GUIDANCE SIGNALS ARE
SUFFICIENTLY GOOD TO SUPPORT A MARGINAL RISK AREA OVER MUCH OF THE
SOUTHWESTERN TWO-THIRDS OF TEXAS INTO FAR SOUTHEAST NEW MEXICO ON
DAY 5/FRIDAY. SOME LOCALLY HEAVY TOTALS ARE POSSIBLE THEREAFTER,
WITH MORE ORGANIZED HEAVY RAINFALL BECOMING POSSIBLE EARLY NEXT
WEEK DEPENDING ON THE EASTWARD DRIFT OF THE WESTERN UPPER LOW.
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 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 NORTHERN ROCKIES.
LEADING WEAK SHORTWAVE/UPPER LOW ENERGY MAY PRODUCE LIGHTER
PRECIPITATION FROM THE SIERRA NEVADA EAST INTO THE ROCKIES
THURSDAY-FRIDAY.
MOST OF THE EAST EXCEPT FOR FAR NORTHERN AREAS SHOULD SEE ABOVE
NORMAL TEMPERATURES LATE THIS WEEK WITH SOME PLUS 10-15F
ANOMALIES. COLD FRONTAL PASSAGE AND ARRIVAL OF UPPER TROUGHING
SHOULD BRING NEAR TO BELOW NORMAL TEMPERATURES EASTWARD FROM THE
CENTRAL U.S. INTO THE EAST BY THE WEEKEND. MEANWHILE ABOVE NORMAL
TEMPERATURES WILL MIGRATE FROM THE NORTHWEST INTO THE
NORTHERN/CENTRAL PLAINS DURING THE PERIOD, CORRESPONDING TO
MOVEMENT OF AN UPPER RIDGE. PLUS 10-20F OR SO ANOMALIES WILL BE
POSSIBLE OVER NORTHERN LOCATIONS WITH WARMEST READINGS LIKELY TO BE
DURING FRIDAY-SUNDAY. THEN THE WEST WILL TREND SIGNIFICANTLY
COLDER NEXT WEEKEND INTO MONDAY WITH THE ARRIVAL OF THE DEEP UPPER
TROUGH/LOW, BRINGING HIGHS DOWN TO 5-20F BELOW NORMAL. THE MOST
EXTREME ANOMALIES SHOULD BE OVER THE SOUTHERN GREAT BASIN AND
SOUTHWEST.
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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