359
FXUS02 KWNH 070645
PREEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
243 AM EDT WED MAY 7 2025
VALID 12Z SAT MAY 10 2025 - 12Z WED MAY 14 2025
..MULTI-DAY SOUTHEAST U.S. HEAVY RAINFALL/FLOODING THREAT
..OVERVIEW
CLOSED LOW ENERGY WILL MEANDER OVER THE SOUTHEAST, FINALLY LIFTING
OUT OF THE REGION AROUND NEXT TUESDAY-WEDNESDAY. THIS WILL SUPPORT
DAILY HEAVY RAINFALL THREATS THROUGH THE WEEKEND AND EARLY NEXT
WEEK OVER THE SOUTHEAST AND NORTHERN FLORIDA. TO THE NORTH, A
NORTHERN STREAM SHORTWAVE WILL CLOSE OFF A NEW COMPACT UPPER LOW
OVER THE NORTHEAST EARLY IN THE PERIOD. ELONGATED UPPER TROUGHING
OFF THE WEST COAST, STEMMING FROM AN UPPER LOW MOVING OUT OF THE
GULF OF ALASKA, WILL SEND AN INITIAL SHORTWAVE INTO THE
WEST/NORTHWEST SUNDAY. UPSTREAM CLOSED LOW DEVELOPMENT OFF THE
NORTHWEST COAST WILL LEAD INTO AN AMPLIFIED UPPER TROUGH SLOW PUSH
INLAND OVER THE WEST COAST BY NEXT TUESDAY.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE OVERALL
PATTERN, BUT PLENTY OF UNCERTAINTY IN THE DETAILS. THE ONE FEATURE
THAT HAS THE BIGGEST IMPACT ON SENSIBLE WEATHER IS THE STALLING
UPPER LOW OVER THE SOUTHEAST FROM SATURDAY TO TUESDAY BEFORE IT
FINALLY LIFTS NORTH OUT OF THE REGION. DIFFERENCES IN THE EXACT
PLACEMENT OF THE LOW AND ENERGY VORTS ROTATING AROUND IT HAVE HUGE
IMPACTS ON EXACTLY WHERE HEAVIEST RAINFALL WILL BE LOCATED. THE
BIGGEST OUTLIER WITH THIS WAS THE UKMET WHICH WAS DISPLACED NOTABLY
FURTHER SOUTH AND WEST THAN CONSENSUS.
ELSEWHERE, THERE ARE SOME LINGERING TIMING DIFFERENCES WITH A
COMPACT CLOSED LOW LIFTING OUT OF THE NORTHEAST NEXT WEEK, BUT A
GENERAL MODEL COMPROMISE SEEMED TO WORK BEST AS A STARTING POINT.
OUT WEST, GUIDANCE SHOWS SOME DIFFERENCES AS THE MAIN TROUGH
CROSSES THE WEST COAST NEXT MONDAY, AND THEN HOW QUICKLY IT OPENS
UP OVER THE WEST.
THE WPC FORECAST FOR TONIGHT UTILIZED A GENERAL MODEL BLEND FOR
THE FIRST HALF OF THE PERIOD, GRADUALLY INCREASING WEIGHTING OF THE
ENSEMBLE MEANS FROM DAYS 5-7, WITH SOME CONTRIBUTIONS FROM
DETERMINISTIC GUIDANCE FOR ADDED SYSTEM DEFINITION.
..WEATHER/HAZARDS HIGHLIGHTS
A WAVY FRONTAL SYSTEM AND A CLOSED UPPER LOW STALLING OVER THE
SOUTHEAST WILL CONTINUE TO SUPPORT A MULTI-DAY HEAVY
RAINFALL/RUNOFF THREAT AS FUELED BY POOLING ANOMALOUS MOISTURE.
RELATIVELY DRY ANTECEDENT CONDITIONS AND HIGHER FLASH FLOOD
GUIDANCE MAY LIMIT THE OVERALL FLASH FLOODING POTENTIAL AT LEAST
INITIALLY. FOR NOW, OPTED TO MAINTAIN WPC EXCESSIVE RAINFALL
OUTLOOK MARGINAL RISKS FOR DAYS 4/5 (SATURDAY AND SUNDAY)
ESPECIALLY CONSIDERING LINGERING UNCERTAINTY IN THE DETAILS.
HOWEVER, REPEAT CELLS/TRAINING ALONG WITH SUBSEQUENT CUMULATIVE
EFFECTS FROM PRIOR DAYS MAY LEAD TO THREAT LEVEL UPGRADES AS THE
SUPPORTING GUIDANCE SIGNAL CONTINUES TO INCREASE. RAINFALL MAY
GRADUALLY LIFT NORTHWARD WITH TIME INTO THE MID-ATLANTIC BY EARLY
NEXT WEEK.
ELSEWHERE, RAINFALL WILL BE EXITING THE NORTHEAST AFTER SATURDAY
WITH THE UPPER LOW. OUT WEST, RAIN AND HIGHER ELEVATIONS SNOWS WILL
BECOME MORE WIDESPREAD THROUGH THE WEEKEND AND INTO NEXT WEEK WITH
AMPLIFIED UPPER TROUGHING AND SURFACE SYSTEM APPROACH.
GENERAL TROUGHING ACROSS THE SOUTH WILL KEEP TEMPERATURES NEAR OR
BELOW NORMAL AT LEAST THROUGH THE WEEKEND AND LONGER FOR THE
SOUTHEAST. UPPER RIDGING OUT WEST WILL SUPPORT MUCH ABOVE NORMAL
TEMPERATURES WITH DAYTIME HIGHS 20-25+ DEGREES ABOVE NORMAL IN THE
NORTHERN PLAINS. ABOVE NORMAL TEMPERATURES MAY ALSO EXTEND ACROSS
THE UPPER MIDWEST/GREAT LAKES/NORTHEAST BY NEXT WEEK. PARTS OF THE
SOUTHWEST SHOULD APPROACH 100 DEGREES WITH AT LEAST SOME LOCALIZED
HEAT THREAT GIVEN EARLIER IN THE SEASON TIMING.
SANTORELLI
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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