424
FXUS02 KWBC 070700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT SUN APR 7 2024
VALID 12Z WED APR 10 2024 - 12Z SUN APR 14 2024
...HEAVY RAIN AND FLASH FLOODING THREAT ACROSS THE SOUTHERN U.S.
NEXT WEEK WITH LESS EXTREME BUT POTENTIALLY SIGNIFICANT RAIN OVER
OTHER PARTS OF THE EAST...
..OVERVIEW
GUIDANCE CONTINUES TO SHOW VIGOROUS LOW PRESSURE DEVELOPMENT FROM
THE LOWER MISSISSIPPI VALLEY THROUGH THE GREAT LAKES INTO CANADA
WEDNESDAY ONWARD AS AN UPPER LOW INITIALLY TRACKING THROUGH THE
SOUTHERN PLAINS PHASES WITH AMPLIFYING NORTHERN STREAM FLOW TO
YIELD A LARGE SCALE UPPER TROUGH OVER THE EAST BY FRIDAY-SATURDAY
AND FINALLY STARTING TO DEPART BY NEXT SUNDAY. GULF INFLOW WILL
INTERACT WITH THE UPPER DYNAMICS/SURFACE FRONTS TO PRODUCE
HEAVY/EXCESSIVE RAINFALL ACROSS THE SOUTH WITH A BROAD AREA OF LESS
EXTREME BUT STILL POTENTIALLY SIGNIFICANT RAIN OVER SOME AREAS
FARTHER NORTH AS WELL AS ALONG MUCH OF THE EAST COAST. A TRAILING
UPPER RIDGE CROSSING THE LOWER 48 WILL BRING A PRONOUNCED WARMING
TREND TO THE WEST AND THEN THE PLAINS/MISSISSIPPI VALLEY. MEANWHILE
GUIDANCE HAS BEEN RAPIDLY CHANGING FOR HOW MUCH UPPER TROUGHING
MAY MOVE INTO THE WEST TOWARD THE END OF THE WEEK OR NEXT WEEKEND,
WITH TRENDS TOWARD A DEEPER TROUGH LEADING TO EXPANDING
PRECIPITATION COVERAGE AND HIGHER TOTALS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE UPDATED FORECAST BASED ON 12Z/18Z GUIDANCE STARTED WITH AN
EMPHASIS ON THE 12Z ECMWF WITH SMALLER INPUT FROM OTHER GUIDANCE
(AND LESS THAN USUAL GFS WEIGHT) EARLY IN THE PERIOD, FOLLOWED BY A
TREND TOWARD MORE BALANCED GFS/ECMWF INPUT ALONG WITH SOME
INCORPORATION OF THE 18Z GEFS/12Z ECENS MEANS.
FOR THE SYSTEM AFFECTING THE EASTERN HALF OF THE COUNTRY, THE MAIN
CONSIDERATION IS THAT AN OVERWHELMING MAJORITY OF GUIDANCE,
INCLUDING DYNAMICAL MODELS/MEANS AND ECMWF-INITIALIZED MACHINE
LEARNING MODELS (MLS), SUGGESTS THAT THE GFS CONTINUES TO RUN ABOUT
6-12 HOURS FAST WITH THE DEVELOPING SURFACE LOW DURING WEDNESDAY-
THURSDAY. DECELERATION OF THE GFS AFTER THURSDAY ALLOWS OTHER
GUIDANCE TO CATCH UP BY FRIDAY, WITH BETTER CLUSTERING OBSERVED
FROM THAT TIME ONWARD.
THE EASTERN PACIFIC/WESTERN U.S. FORECAST HAS BEEN CHANGING VERY
RAPIDLY IN RECENT GUIDANCE RUNS. 24-36 HOURS AGO THE MLS WERE
SUGGESTING THAT THE INCOMING UPPER TROUGH COULD BE DEEPER/SHARPER
THAN MOST DYNAMICAL GUIDANCE, AND THIS TREND HAS CONTINUED INTO
TODAY. IN FACT IN 18Z/00Z RUNS THERE IS AN INCREASING SIGNAL FOR
THIS TROUGH TO CONTAIN AN EMBEDDED UPPER LOW. THE ML FORECASTS AT
LEAST OFFERED MORE SUPPORT FOR INCLUDING THE 18Z GFS/12Z UKMET IN
THE FORECAST BLEND THAN MIGHT HAVE BEEN THE CASE OTHERWISE. THE NEW
00Z ECMWF HAS SHIFTED TO AN EMBEDDED UPPER LOW AS WELL, SO IT IS
LIKELY THE MANUAL DETERMINISTIC FORECAST TO DEPICT ADDITIONAL
CHANGES.
..WEATHER/HAZARDS HIGHLIGHTS
EXPECT HEAVY RAINFALL TO BE IN PROGRESS ACROSS THE SOUTH AT THE
START OF THE FORECAST PERIOD ON WEDNESDAY, WITH DEVELOPING LOW
PRESSURE BEGINNING TO TRACK NORTHEASTWARD THROUGH THE LOWER
MISSISSIPPI VALLEY ON WEDNESDAY-WEDNESDAY NIGHT HELPING TO PULL
GULF MOISTURE NORTHWARD WHILE DYNAMICS ALOFT BEGIN TO PHASE INTO A
LARGER SCALE UPPER TROUGH. BY THE WEDNESDAY-WEDNESDAY NIGHT VALID
PERIOD FOR THE DAY 4 EXCESSIVE RAINFALL OUTLOOK, THE HEAVIEST
CONVECTION SHOULD BEGIN MOVING EASTWARD INTO THE LOWER MISSISSIPPI
VALLEY AND SOUTHEAST WHERE A SLIGHT RISK AREA CONTINUES TO BE
DEPICTED. THE MAIN ADJUSTMENT FROM THE PRIOR DAY 5 OUTLOOK WAS TO
EXPAND THE SLIGHT RISK AREA A LITTLE EASTWARD OVER GEORGIA TO
ACCOUNT FOR A COUPLE OPERATIONAL MODELS THAT TRENDED FASTER, AND
THE OCCASIONAL BIAS OF GUIDANCE TO BE TOO SLOW WITH CONVECTIVE
PROGRESSION. THERE IS STILL ENOUGH SCATTER WITH THE LOCATION OF
MAXIMUM QPF TO PRECLUDE INTRODUCTION OF AN EMBEDDED MODERATE RISK
BUT THE LOCALIZED MAXIMA DO SUGGEST SUCH POTENTIAL ONCE BETTER
CLUSTERING DEVELOPS. ALSO OF NOTE, GUIDANCE HAS BEEN CONSISTENT IN
DEPICTING A SEPARATE ENHANCED QPF AXIS TO THE NORTH OF THE LOW
TRACK FROM THE ARKLATEX NORTHEASTWARD THROUGH THE OHIO VALLEY, BUT
STILL WITH POOR AGREEMENT FOR SPECIFICS. THE SURROUNDING MARGINAL
RISK AREA ACCOMODATES THIS AREA OF POTENTIALLY ENHANCED RAINFALL.
BY THURSDAY-THURSDAY NIGHT, UPPER TROUGH PHASING AND CONTINUED
DEEPENING/NORTHWARD PROGRESSION OF LOW PRESSURE WILL PROMOTE A
BROAD SOUTHERLY FLOW OF MOISTURE ACROSS THE EAST. WITHIN THIS LARGE
AREA OF POTENTIALLY ENHANCED RAINFALL (BUT LIKELY WITH LESS EXTREME
TOTALS THAN FORECAST ACROSS THE SOUTH), THE DAY 5 ERO PROPOSES A
SLIGHT RISK AREA ENCOMPASSING PARTS OF THE MID- ATLANTIC AND
SOUTHERN NEW ENGLAND. THIS REGION SHOWS THE BEST GUIDANCE OVERLAP
AND CONTINUITY FOR RELATIVELY HIGHER RAINFALL TOTALS AS WELL AS
HIGHER ANTECEDENT SOIL MOISTURE/STREAM FLOWS. OTHER AREAS WITHIN
THE LARGE SURROUNDING MARGINAL RISK HAVE VARYING DEGREES OF
SENSITIVITY AND MIXED MESSAGES FROM THE GUIDANCE REGARDING
LOCATION/TIMING OF ENHANCED RAINFALL. RAINFALL MAY CONTINUE INTO
FRIDAY OVER THE NORTHEAST. THE DEEPENING STORM SYSTEM MAY ALSO
PRODUCE BRISK TO STRONG WINDS OVER SOME AREAS. THE STORM PREDICTION
CENTER IS MONITORING THE POTENTIAL FOR SEVERE WEATHER WITH THE
CONVECTION FORECAST TO TRACK ACROSS THE SOUTHERN TIER. CHECK THEIR
LATEST OUTLOOKS FOR MORE INFORMATION.
RECENT TRENDS FOR A STRONGER UPPER TROUGH TO MOVE INTO THE WEST
SUGGEST GREATER AREAL COVERAGE AND HIGHER TOTALS FOR PRECIPITATION
FROM THURSDAY INTO THE WEEKEND. THE BEST EMPHASIS FOR RAIN AND
HIGHER ELEVATION SNOW SHOULD BE FROM THE PACIFIC NORTHWEST INTO THE
NORTHERN-CENTRAL ROCKIES. THE MORE EXTREME SIDE OF THE ENVELOPE,
WHICH IS NOT OUT OF THE QUESTION GIVEN THE SIGNIFICANT MODEL
CHANGES IN THE PAST DAY OR SO, COULD EVEN BRING MEANINGFUL MOISTURE
INTO CALIFORNIA AND THE SOUTHWEST. THE UPPER TROUGH AND LEADING
SURFACE SYSTEM MAY REACH FAR ENOUGH EAST TO START DEVELOPING SOME
AREAS OF RAIN OVER THE CENTRAL U.S. BY NEXT SUNDAY.
THE UPPER LOW CROSSING THE SOUTHERN PLAINS ON WEDNESDAY WILL RESULT
IN SOME LINGERING MODERATELY BELOW NORMAL HIGHS BEFORE A MODERATING
TREND. TO THE EAST OF THE UPPER LOW AND PHASING NORTHERN STREAM
FLOW WITH ASSOCIATED SURFACE LOW AND COLD FRONT, A MAJORITY OF THE
EASTERN HALF OF THE COUNTRY WILL SEE NEAR TO ABOVE NORMAL
TEMPERATURES, ESPECIALLY FOR MORNING LOWS, DURING WEDNESDAY-
THURSDAY. COASTAL AREAS OVER THE NORTHEAST MAY BE EXCEPTION, WITH
COOLER HIGHS DUE TO ONSHORE FLOW NORTH OF A BACKDOOR FRONT
SETTLING INTO THE NORTHERN MID-ATLANTIC AROUND WEDNESDAY-THURSDAY.
PASSAGE OF A COUPLE FRONTS WILL BRING A COOLER TREND FROM WEST TO
EAST LATE IN THE WEEK. MEANWHILE AN UPPER RIDGE PROGRESSING FROM
THE WEST THROUGH THE PLAINS AND BEYOND WILL SUPPORT AN AREA OF WELL
ABOVE NORMAL TEMPERATURES (PLUS 10-20F ANOMALIES) MOVING FROM THE
WEST INTO THE PLAINS WEDNESDAY WEDNESDAY THROUGH SATURDAY, WITH
MORE MODERATE WARMTH EXTENDING INTO THE EAST BY NEXT SUNDAY. A
TRAILING UPPER TROUGH WITH LEADING COLD FRONT WILL THEN BRING A
COOLER TREND TO THE WEST AND THE NORTHERN HIGH PLAINS BY THE
WEEKEND.
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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