848
FXUS02 KWBC 071855
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
255 PM EDT SUN APR 7 2024
VALID 12Z WED APR 10 2024 - 12Z SUN APR 14 2024
***HEAVY RAIN AND STRONG STORMS FOR THE GULF COAST AND SOUTHEAST
STATES ON WEDNESDAY, THEN THE MID-ATLANTIC AND NORTHEAST ON
THURSDAY***
19Z UPDATE: MODEL GUIDANCE HAS IMPROVED ON THE MORE AMPLIFIED
SOLUTION ACROSS THE WESTERN U.S. FOR LATER IN THE FORECAST PERIOD,
WITH A WELL DEFINED UPPER LOW BECOMING MORE LIKELY. THE CMC IS
SLIGHTLY SLOWER WITH THE PASSAGE OF THAT TROUGH, WHEREAS THE ECMWF
IS MORE PROGRESSIVE AND A LITTLE WEAKER, WITH THE GFS A MIDDLE-
GROUND SOLUTION. FOR THE STORM SYSTEM ACROSS THE EASTERN U.S.
WEDNESDAY THROUGH FRIDAY, THERE IS DECENT OVERALL MODEL AGREEMENT,
EVEN THOUGH THE GFS/GEFS MEAN ARE FASTER BRINGING THE LOW TOWARDS
THE GREAT LAKES. THE UKMET DIFFERED WITH THE SURFACE PRESSURE
PATTERN ACROSS THE PLAINS BY BRINGING THE SURFACE HIGH FARTHER
NORTH INTO THE MIDWEST BY FRIDAY COMPARED TO THE MODEL CONSENSUS,
WHICH INDICATES THIS REGION WILL STILL BE MORE UNDER THE INFLUENCE
OF THE DEPARTING SURFACE LOW. TAKING THESE FACTORS INTO ACCOUNT,
THE UPDATED FORECAST WAS DERIVED MAINLY FROM A CMC/ECMWF/GFS BLEND
THROUGH FRIDAY, ALONG WITH SOME UKMET ON WEDNESDAY. SINCE MODEL
AGREEMENT HAS IMPROVED OVERALL THROUGH NEXT WEEKEND, THE ENSEMBLE
MEANS INCREASED TO JUST 30 PERCENT OF THE FORECAST ALONG WITH SOME
PREVIOUS WPC CONTINUITY. THE PREVIOUS DISCUSSION, ALONG WITH THE
UPDATED ERO DISCUSSION, ARE APPENDED BELOW. /HAMRICK
----------------
..GENERAL 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 THE 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
HEAVY RAINFALL WILL LIKELY BE ONGOING ACROSS THE CENTRAL GULF COAST
AND DEEP SOUTH ON WEDNESDAY, WITH DEVELOPING LOW PRESSURE
BEGINNING ADVECTING GULF MOISTURE NORTHWARD WHILE DYNAMICS ALOFT
BEGIN TO PHASE INTO A LARGER SCALE UPPER TROUGH. FOR THE WEDNESDAY-
WEDNESDAY NIGHT TIME PERIOD FOR THE DAY 4 EXCESSIVE RAINFALL
OUTLOOK, THE BROAD SLIGHT RISK AREA REMAINS VALID WITH A SLIGHT
EXPANSION TO THE NORTH TO INCLUDE SOUTHERN TENNESSEE AND FARTHER
NORTHWEST IN MISSISSIPPI. THE MODELS ARE SHOWING THE POTENTIAL FOR
TWO MAIN AXES OF ENHANCED QPF, WITH ONE ACROSS NORTHERN MISSISSIPPI
AND ALABAMA, AND A SECOND AND POTENTIALLY HEAVIER BAND CLOSER TO
THE FLORIDA PANHANDLE. THERE IS STILL ENOUGH MODEL VARIANCE IN THE
12Z GUIDANCE WITH THE LOCATION OF MAXIMUM QPF TO PRECLUDE
INTRODUCTION OF AN EMBEDDED MODERATE RISK FOR NOW, BUT THE
LOCALIZED MAXIMA DO SUGGEST SUCH POTENTIAL ONCE BETTER CLUSTERING
DEVELOPS IN FUTURE MODEL RUNS. THERE IS ALSO A SEPARATE ENHANCED
QPF AXIS NORTH OF THE LOW TRACK FROM THE ARKLATEX NORTHEASTWARD
THROUGH THE OHIO VALLEY, BUT STILL WITH LIMITED AGREEMENT FOR
SPECIFICS. THE SURROUNDING MARGINAL RISK AREA ACCOMMODATES THIS
AREA OF POTENTIALLY ENHANCED RAINFALL.
WITH THE STORM SYSTEM LIFTING TO THE NORTHEAST BY THURSDAY, THE
DAY 5 ERO MAINTAINS THE SLIGHT RISK AREA, BUT EXPANDED TO INCLUDE
NORTHWESTERN NORTH CAROLINA AND ALSO SOUTHERN PORTIONS OF VERMONT
AND NEW HAMPSHIRE. THIS REGION SHOWS THE BEST GUIDANCE OVERLAP AND
CONTINUITY FOR RELATIVELY HIGHER RAINFALL TOTALS AS WELL AS HIGHER
ANTECEDENT SOIL MOISTURE AND CURRENT 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 THE 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 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/HAMRICK
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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