385
FXUS02 KWBC 120712
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
312 AM EDT SUN MAY 12 2024
VALID 12Z WED MAY 15 2024 - 12Z SUN MAY 19 2024
..ANOTHER SOUTHERN TIER HEAVY RAINFALL THREAT MID-LATE WEEK
..OVERVIEW
A SYSTEM REACHING THE MID-ATLANTIC BY EARLY WEDNESDAY AND A COLD
FRONT JUST TO ITS NORTH/WEST WILL SUPPORT A WET MIDWEEK OVER
PORTIONS OF THE EAST BEFORE THE SURFACE LOW CONTINUES EASTWARD INTO
THE ATLANTIC. FARTHER WEST, SHORTWAVE ENERGY EMERGING FROM THE
ROCKIES SHOULD SUPPORT A WAVY FRONTAL SYSTEM PUSHING EASTWARD FROM
THE PLAINS, SPREADING ANOTHER BROAD AREA OF RAINFALL OVER THE
CENTRAL/EASTERN U.S. WITH HIGHEST TOTALS STILL EXPECTED TO BE
ACROSS THE SOUTHERN TIER. GUIDANCE IS STILL HAVING DIFFICULTY IN
RESOLVING SOME IMPORTANT DETAILS THOUGH. POTENTIAL STALLING OF THE
SYSTEM'S TRAILING FRONT NEAR THE GULF COAST COULD SUPPORT AT LEAST
A LOCALIZED HEAVY RAINFALL THREAT INTO THE WEEKEND.
MODELS/ENSEMBLES STILL DIFFER FOR SPECIFICS OF FLOW ROUNDING
EASTERN PACIFIC RIDGING FROM WEDNESDAY ONWARD WITH EFFECTS
TRANSLATING EASTWARD WITH TIME, BUT AT LEAST THE PRIOR EXTREMELY
BROAD SOLUTION ENVELOPE HAS NARROWED SOMEWHAT.
..GUIDANCE/PREDICTABILITY ASSESSMENT
COMPARISONS OF 12Z/18Z DYNAMICAL GUIDANCE AND ECMWF-INITIALIZED
MACHINE LEARNING MODELS, WITH DIFFERENT SOLUTIONS CLUSTERING BETTER
FOR DIFFERENT FEATURES, ULTIMATELY LED TO AN EVEN WEIGHT OF THE
GFS/ECMWF/UKMET/CMC FOR ABOUT THE FIRST HALF OF THE PERIOD AND THEN
A TRANSITION TO 30-40 PERCENT TOTAL 18Z GEFS/12Z ECENS WEIGHT
WHILE MAINTAINING INPUT FROM THE GFS/ECMWF/CMC.
FOR THE SYSTEM TRACKING OFF THE MID-ATLANTIC, GFS RUNS IN
PARTICULAR HAVE HAD MORE STRAY SOLUTIONS RECENTLY COMPARED TO OTHER
GUIDANCE AND THE ENSEMBLE MEANS (WITH CONSENSUS SAYING THE UPPER
DYNAMICS SHOULD STAY MORE SEPARATED FROM FLOW TO THE NORTH).
12Z/18Z RUNS WERE SOMEWHAT NORTH AND FAST BUT THE NEW 00Z RUN FITS
WITH THE MEANS BETTER, WHILE NOW THE 00Z UKMET STRAYS TO THE NORTH
SIDE OF THE SPREAD.
REMAINING DETAILS OF THE FORECAST REMAIN AMBIGUOUS, WITH A
COMBINATION OF DIFFERENCES FOR HOW SHORTWAVE ENERGY EMERGING FROM
THE PLAINS/ROCKIES EVOLVES, DETAILS OF A SEPARATE SOUTHERN STREAM
SHORTWAVE TRACKING OUT OF THE SOUTHWEST (PLUS POTENTIAL FOR
ADDITIONAL LINGERING ENERGY THAT COULD EJECT LATER), AND FLOW
COMING INTO NORTH AMERICA AROUND THE TOP OF EASTERN PACIFIC
RIDGING. AMIDST ALL THESE DETAIL ISSUES THERE IS AT LEAST A GENERAL
THEME OF THE FLOW EVOLVING TOWARD A MORE WARM-SEASON PATTERN WITH
THE WESTERLIES BECOMING BETTER DEFINED ALONG THE NORTHERN TIER U.S.
REGARDING FLOW AROUND THE PACIFIC RIDGE, GUIDANCE HAS BEEN
TRENDING TOWARD AN INTERMEDIATE SOLUTION OVER THE NORTHWEST MID-
LATE WEEK, WITH UPPER RIDGING HOLDING ON A LITTLE LONGER THAN THE
AVERAGE OF PRIOR SPREAD (BUT NOT AS STRONGLY AS SOME GFS/CMC RUNS)
AND THEN STAYING NEAR THE MIDDLE OF THE ENVELOPE LATER IN THE
PERIOD. SOME ECMWF-INITIALIZED MACHINE LEARNING MODELS SUGGEST THE
SYSTEM TRACKING ALONG THE CANADIAN BORDER LATE WEEK INTO THE
WEEKEND COULD BE A LITTLE STRONGER THAN THE DYNAMICAL MODEL
AVERAGE. FARTHER EAST, SURFACE DETAILS WILL DEPEND ON LOWER-
PREDICTABILITY DETAILS OF SHORTWAVE ENERGY. LATEST GUIDANCE HAS
SHIFTED NORTHWARD WITH THE MAIN AREA OF LOW PRESSURE AFTER REACHING
THE MISSISSIPPI VALLEY BY EARLY FRIDAY BUT THEN CONSENSUS SUGGESTS
THE TRIPLE POINT WAVE MAY BECOME THE PRIMARY LOW UPON REACHING THE
WESTERN ATLANTIC.
..WEATHER/HAZARDS HIGHLIGHTS
THE PRIMARY FOCUS FOR HEAVY RAINFALL POTENTIAL STILL LOOKS TO BE IN
THE SOUTHERN HALF OF THE PLAINS BY THE WEDNESDAY-WEDNESDAY NIGHT
TIME FRAME COVERED BY THE DAY 4 EXCESSIVE RAINFALL OUTLOOK.
REFLECTING SOME DEVELOPING SPREAD FOR APPROACHING DYNAMICS ALOFT
(FROM THE ROCKIES, AS WELL AS A SEPARATE SOUTHWEST SHORTWAVE THAT
MAY HAVE SOME INTERACTION WITH NORTHERN STREAM FLOW),
MODEL/ENSEMBLE GUIDANCE CONTINUES TO DIFFER FOR CONVECTIVE
SPECIFICS OVER THIS REGION. THE GFS/ECMWF AND THEIR ENSEMBLE MEANS
MAINTAIN A THEME OF INSTABILITY AND INCREASING MOISTURE OVER THIS
REGION, ALONG WITH MULTIPLE SURFACE FEATURES THAT MAY HELP TO FOCUS
CONVECTION. THERE HAS BEEN SOME LINGERING SUPPORT FOR ONE AREA OF
EMPHASIS OVER TEXAS WHERE GROUND CONDITIONS ARE ON THE WETTER SIDE
SO FOR NOW THE ERO MAINTAINS THE SLIGHT RISK AREA CLOSE TO
CONTINUITY. HOWEVER THERE IS A DEVELOPING SIGNAL FROM SOME
DYNAMICAL AND MACHINE LEARNING OUTPUT TOWARD BETTER CONVECTIVE
EMPHASIS A LITTLE FARTHER NORTH--OVER AN AREA WHERE SHORT TERM
FORECAST RAINFALL COULD LEAD TO WETTER SOILS HEADING INTO DAY 4
THAN CURRENTLY OBSERVED. PREFER TO WAIT ANOTHER RUN TO GAIN BETTER
CONFIDENCE IN ANY POTENTIAL ADJUSTMENTS. MEANWHILE CONTINUITY LOOKS
GOOD FOR THE LINGERING MARGINAL RISK OVER NORTHERN FLORIDA AS THE
FRONT REACHING THE AREA STALLS, ACCOMPANIED BY INSTABILITY AND
ABOVE NORMAL MOISTURE.
FOR THE DAY 5 ERO (THURSDAY-THURSDAY NIGHT), AFOREMENTIONED SPREAD
FOR DETAILS ALOFT CONTINUE TO INFLUENCE CONVECTIVE DETAILS FROM
THE PLAINS INTO THE GULF COAST STATES. THE OUTLOOK STARTS WITH A
SLIGHT RISK AREA EXTENDING FROM SOUTHEASTERN TEXAS INTO SOUTHWEST
MISSISSIPPI, WHICH REPRESENTS THE AVERAGE OF ENSEMBLE PROBABILITIES
FOR GREATEST RAINFALL (OR SPECIFICALLY THE 12Z ECENS, WITH THE
CMCENS A LITTLE SOUTHEAST AND THE GEFS A LITTLE NORTH) ALONG WITH
LATEST ECMWF/CMC RUNS PLUS 12Z MACHINE LEARNING MODELS. EVEN THOUGH
GFS DETAILS ALOFT LEAD TO MUCH LESS QPF, IT IS MORE SIMILAR TO
OTHER MODELS/MEANS IN DEPICTING A PRONOUNCED SURGE OF MOISTURE INTO
THIS REGION WITH ADEQUATE INSTABILITY AS WELL. A FRONT FARTHER
NORTH MAY HELP TO FOCUS LESS EXTREME BUT STILL LOCALLY HEAVY
RAINFALL, WHICH IS ACCOUNTED FOR BY A MARGINAL RISK AREA THAT
EXTENDS INTO PARTS OF THE SOUTH- CENTRAL PLAINS AND MIDWEST.
EXPECT THE MOISTURE SHIELD TO CONTINUE ACROSS THE EASTERN U.S. LATE
IN THE WEEK, WITH HEAVIEST RAINFALL MOST LIKELY EXTENDING ACROSS
THE SOUTHEAST. DEPENDING ON SPECIFICS OF UPPER FLOW, THE FRONT
TRAILING FROM THE SURFACE SYSTEM ASSOCIATED WITH THIS EPISODE COULD
STALL NEAR THE GULF COAST AND PROVIDE A FOCUS FOR ADDITIONAL
RAINFALL DURING THE WEEKEND.
AN EASTERN PACIFIC UPPER RIDGE EXTENDING INTO THE WEST SHOULD
SUPPORT ABOVE NORMAL TEMPERATURES OVER MOST OF THE WEST COAST
STATES AND PARTS OF THE GREAT BASIN DURING WEDNESDAY-THURSDAY, WITH
SOME AREAS SEEING PLUS 10-15F ANOMALIES. THEREAFTER, EXPECT THE
WARM READINGS TO PERSIST OVER CALIFORNIA/NEVADA WITH EVENTUAL
EXPANSION INTO THE SOUTHERN ROCKIES WHILE THE NORTHWEST TRENDS
CLOSER TO NORMAL (POSSIBLY BELOW NORMAL OVER MONTANA). MOST OF
FLORIDA MAY SEE SEVERAL DAYS WITH HIGHS UP TO 5-10F ABOVE NORMAL,
IN A WARM PATTERN TO THE SOUTH OF A COUPLE FRONTS EXPECTED TO STALL
UPON REACHING THE FAR NORTHERN PART OF THE STATE. COMBINED WITH
HIGH DEW POINTS, THESE TEMPERATURES MAY BRING HEAT INDICES AS HIGH
AS 105-110 ACROSS SOUTHEAST/SOUTHERN FLORIDA, SIGNALING THE FIRST
HAZARDOUS HEAT DAYS OF THE SUMMER. OTHERWISE, AREAS TO THE EAST OF
THE ROCKIES WILL TEND TO BE WITHIN A FEW DEGREES OF NORMAL FOR
HIGHS, ASIDE FROM BELOW NORMAL OVER THE SOUTHERN PLAINS ON
THURSDAY AND POSSIBLY AN AXIS OF MODESTLY ABOVE NORMAL READINGS
DROPPING SOUTHEAST FROM THE NORTHERN PLAINS FRIDAY ONWARD. MOST OF
THE EASTERN U.S. SHOULD SEE SOMEWHAT ABOVE NORMAL LOWS THROUGH THE
PERIOD.
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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