486
FXUS02 KWBC 211924
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
324 PM EDT MON APR 21 2025
VALID 12Z THU APR 24 2025 - 12Z MON APR 28 2025
..MULTI-DAY HEAVY RAIN THREAT FOR THE SOUTH-CENTRAL CONUS
..OVERVIEW
THE MEDIUM RANGE PERIOD BEGINS THURSDAY WITH SHORTWAVES MOVING
THROUGH THE CENTRAL U.S. LEADING TO ROUNDS OF THUNDERSTORMS THAT
COULD CAUSE HEAVY RAIN. RAIN IS LIKELY TO SPREAD INTO THE EAST-
CENTRAL AND EASTERN U.S. WITH A LOW PRESSURE/FRONTAL SYSTEM LATE
WEEK. MEANWHILE UPPER TROUGHING WILL MOVE FROM THE EASTERN PACIFIC
INTO THE WESTERN U.S. AND PROMOTE MODEST PRECIPITATION IN THE
NORTHWEST, AND MOIST INFLOW AHEAD OF THE TROUGH WILL INCREASE RAIN
AND THUNDERSTORM CHANCES IN THE CENTRAL U.S. ONCE AGAIN OVER THE
WEEKEND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS IN OKAY AGREEMENT ON THE LARGE SCALE, BUT
WITH DIFFERENCES ON THE SMALLER SCALE THAT COULD LEAD TO SENSIBLE
WEATHER DIFFERENCES EVEN EARLY IN THE PERIOD. SHORTWAVES IN THE
CENTRAL TO EASTERN U.S. LATE WEEK LEAD TO CONSIDERABLE QPF
DISCREPANCIES. THE 00Z CMC APPEARED TO BE AN OUTLIER AS ITS
SHORTWAVE AND ITS ASSOCIATED SURFACE LOW FRIDAY-SATURDAY WERE
FARTHER WEST COMPARED TO THE MODEL CONSENSUS. MEANWHILE THE 06Z GFS
TRACKED EASTWARD RATHER THAN NORTHEAST WITH THE LOW. THE 00Z ECMWF
WAS ALIGNED WELL WITH THE AI/ML MODELS AND THIS SEEMED MOST
FAVORABLE. HOWEVER, THE 12Z CMC REMAINED SLOW/WEST WHILE THE 12Z
ECMWF JUMPED TO BE SLOWER AS WELL. THUS STILL PLENTY OF UNCERTAINTY
THERE.
AN UPPER TROUGH MOVING THROUGH THE EASTERN PACIFIC TOWARD THE WEST
SHOWS SOME SPREAD EVEN AS THE PERIOD BEGINS REGARDING HOW MUCH
ENERGY SPLITS SOUTH VERSUS STAYS NORTH IN THE NORTHEAST PACIFIC.
THESE YIELD TIMING AND DEPTH DIFFERENCES AS THE TROUGH MOVES EAST.
FORTUNATELY INCOMING 12Z MODELS LOOK TO HAVE SOME CONVERGENCE ON
THE TIMING AND TRACK OF THIS TROUGH.
THE WPC FORECAST USED A MODEL (FAVORING THE ECMWF) AND ENSEMBLE
MEAN BLEND EARLY IN THE FORECAST PERIOD. BUT GIVEN THE MODEL
DISCREPANCIES, INCREASED THE PROPORTION OF ENSEMBLE MEANS TO OVER
HALF BY DAY 5 AND ALMOST THE WHOLE BLEND BY DAY 7, TO SMOOTH OUT
THE INDIVIDUAL MODEL DIFFERENCES.
..WEATHER/HAZARDS HIGHLIGHTS
INTO LATER WEEK, AN ILL-DEFINED SERIES OF GENERALLY PROGRESSIVE
UPPER FEATURES AND SURFACE BASED SYSTEMS WILL CONTINUE TO WORK OVER
THE LOWER 48 THIS WEEK TO FOCUS LOCAL WEATHER/CONVECTION. LIKELY
MULTIPLE ROUNDS OF THUNDERSTORMS ARE FORECAST TO FOCUS IN THE
SOUTH-CENTRAL U.S. ON THURSDAY AS THE PATTERN SUPPORTS A MOIST AND
UNSTABLE FLOW FOR A FAVORABLE, AT LEAST LOCAL, FLASH FLOOD SETUP.
RECENT AND ONGOING HEAVY RAINFALL OVER PORTIONS OF THESE REGIONS
WILL MAKE SOME AREAS PARTICULARLY SUSCEPTIBLE TO ADDITIONAL RAINS.
IN COORDINATION WITH THE AFFECTED LOCAL FORECAST OFFICES, WILL SHOW
A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE DAY 4 (12Z THURSDAY TO
12Z FRIDAY) ERO FROM KANSAS AND NORTHERN OKLAHOMA INTO FAR WESTERN
MISSOURI. THEN ON FRIDAY, A LOW PRESSURE SYSTEM AND ITS TRAILING
FRONT IS LIKELY TO MOVE ACROSS THE MISSISSIPPI VALLEY TOWARD THE
GREAT LAKES, SPREADING THUNDERSTORM CHANCES INTO THOSE AREAS.
CONTINUE TO INDICATE A MARGINAL RISK FROM ARKANSAS THROUGH PORTIONS
OF THE TENNESSEE AND OHIO VALLEYS FOR LOCALIZED FLOODING WITH
HEAVY RAIN, WITH MINOR ADJUSTMENTS FROM CONTINUITY. SOME GUIDANCE
SUGGESTS HEAVY RAINFALL FARTHER NORTHEAST INTO THE UPPER OHIO
VALLEY INTO THE LOWER GREAT LAKES REGION, SO WILL CONTINUE TO
MONITOR.
MEANWHILE, DESPITE SOME MODEL GUIDANCE UNCERTAINTIES IN THE
DETAILS, UPPER TROUGH AMPLIFICATION AND DIGGING IN THE EASTERN
PACIFIC LATE THIS WEEK MOVING INTO THE WEST THIS WEEKEND, ALONG
WITH SURFACE SYSTEM GENESIS AND SLOW BUT STEADY SYSTEM TRANSLATION
STILL OFFERS INCREASED LIGHT TO MODERATE WIDESPREAD RAIN CHANCES
AND MOUNTAIN SNOWS. RENEWED MOISTURE STREAMING IN EAST OF THE
TROUGH PLUS LINGERING BOUNDARIES IN THE AREA WILL ALLOW FOR
ADDITIONAL ROUNDS OF POSSIBLY HEAVY RAIN AND THUNDERSTORMS IN THE
PLAINS AND MISSISSIPPI VALLEY BY THE WEEKEND.
TEMPERATURES ARE LIKELY TO VARY SOMEWHAT FROM DAY TO DAY IN THE
CENTRAL AND EASTERN U.S. AS FRONTS PROGRESS THROUGH, BUT MOSTLY BE
CLOSE TO OR ABOVE AVERAGE. A HANDFUL OF RECORD OR NEAR RECORD HIGHS
ARE POSSIBLE IN THE SOUTHEAST AND PARTICULARLY OVER THE WEST COAST
OF FLORIDA. COOLER THAN AVERAGE TEMPERATURES, PARTICULARLY FOR
HIGHS, ARE LIKELY FOR THE WEST COAST STARTING LATE WEEK AND MOVING
INTO THE INTERMOUNTAIN WEST BY EARLY NEXT WEEK UNDERNEATH UPPER
TROUGHING.
TATE/SCHICHTEL
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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