645
FXUS02 KWBC 170703
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 AM EDT TUE SEP 17 2024
VALID 12Z FRI SEP 20 2024 - 12Z TUE SEP 24 2024
...HEAVY RAIN WITH POSSIBLE FLOODING IS FORECAST FOR THE NORTH-
CENTRAL PLAINS TO UPPER MIDWEST THIS WEEKEND...
..OVERVIEW
UPPER TROUGHING OVER THE WEST LATE WEEK WILL PIVOT EASTWARD AND
PROVIDE SUPPORT FOR LIKELY HEAVY RAIN ACROSS PORTIONS OF THE
CENTRAL U.S. THAT LOOKS TO PEAK OVER THE WEEKEND. THE TROUGH
PUSHING EAST ALONG WITH FRONTAL BOUNDARIES SHOULD GRADUALLY COOL
THE CENTRAL U.S. CLOSER TO NORMAL AFTER A WARM PERIOD THROUGH LATE
WEEK. MEANWHILE IN THE EAST, UPPER TROUGHING WILL LINGER FOR LATE
WEEK INTO THE WEEKEND, WHICH ALONG WITH WEAK SURFACE FRONTAL
SYSTEMS OVER THE WESTERN ATLANTIC MAY LEAD TO SHOWERS ACROSS PARTS
OF THE EASTERN SEABOARD. A STALLING FRONT MAY FOCUS RAIN OVER SOUTH
FLORIDA AT TIMES FOR PERHAPS NONZERO FLOODING CONCERNS OVER THE
URBAN CORRIDOR.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE BEGINS THE PERIOD FRIDAY IN GOOD AGREEMENT WITH THE
OVERALL PATTERN, INDICATING UPPER TROUGHING OVER THE WEST WITH A
POSSIBLE EMBEDDED UPPER LOW OVER CALIFORNIA/NEVADA, RIDGING OVER
MUCH OF THE CENTRAL U.S. THOUGH LIMITED IN ITS NORTHWARD EXTENT DUE
TO NORTHERN STREAM ENERGY, AND EAST COAST TROUGHING. GENERALLY
THIS PATTERN WILL SHIFT EASTWARD OVER THE WEEKEND, AND AN AVERAGE
OF THE 18Z GFS AND 00Z CMC/ECMWF WORKED THROUGH THE WEEKEND WHILE
THE 12Z UKMET SEEMED TOO SLOW IN THE WEST. MODEL SPREAD INCREASES
CONSIDERABLY REGARDING THE EVOLUTION OF THE WESTERN TROUGH/LOW AND
HOW IT INTERACTS WITH NORTHERN STREAM SHORTWAVES, IN THE 12/18Z
MODEL CYCLE AND THE NEWER 00Z MODEL CYCLE AS WELL AS THE AI MODELS.
THE 00Z ECMWF IS GENERALLY STRONGER WITH A SHORTWAVE DEVELOPING A
COMPACT CLOSED LOW MOVING THROUGH SOUTH-CENTRAL CANADA SUNDAY
COMPARED TO ITS PREVIOUS RUN, WHILE GFS RUNS HAVE HAD THIS FEATURE
BUT WITH TIMING DIFFERENCES. MEANWHILE THE NEW 00Z CMC DEVIATED
CONSIDERABLY FROM ITS PREVIOUS RUN AND FROM OTHER GUIDANCE IN
PHASING THE NORTHERN AND SOUTHERN STREAM TROUGHING ENERGIES OVER
THE WEEKEND TO CREATE ONE UPPER LOW IN THE NORTH-CENTRAL U.S.
SUNDAY, WHICH SEEMS LIKE A LOW PROBABILITY SOLUTION SINCE OTHER
GUIDANCE DOES ANY PHASING OF THE STREAMS LATER. THE UPSTREAM
PATTERN IS ALSO RATHER UNCERTAIN INTO EARLY NEXT WEEK AS GUIDANCE
VARIES ON RIDGING OR TROUGHING COMING INTO THE NORTHWEST. THE 18Z
GFS SEEMED LIKE AN OUTLIER AT FIRST BLUSH COMPARED TO OTHER
DETERMINISTIC MODELS AS IT DUG ENERGY MORE DEEPLY INTO THE NORTHERN
HIGH PLAINS BY TUESDAY, BUT IT HAD SUPPORT FROM THE ENSEMBLES AND
THE NEWER 00Z MODELS DID TREND THIS WAY SOMEWHAT.
SOME GUIDANCE, ESPECIALLY RECENT CANADIAN AND GFS MODEL RUNS,
OFFER INCREASED TROPICAL POTENTIAL UP FROM THE CARIBBEAN INTO EARLY
NEXT WEEK. UNCERTAIN POSITION AND DIGGING OF UPPER TROUGHING BY
THEN TO THE SOUTHEAST WOULD AFFECT ANY LOWER LATITUDE INTERACTIONS,
SO WPC AND NHC WILL CONTINUE TO MONITOR.
THE WPC FORECAST WAS BASED ON A BLEND OF THE 18Z GFS AND 12Z
ECMWF/CMC EARLY, TRANSITIONING TO FAVOR MORE ENSEMBLE MEANS BY THE
LATER PERIOD GIVEN INCREASING SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
LOW PRESSURE/FRONTAL SYSTEMS OVER THE CENTRAL U.S. WILL HELP
PRODUCE SCATTERED RAIN AND THUNDERSTORMS THERE ON FRIDAY. THERE IS
LOW CONFIDENCE IN ANY AREA OF FOCUS FOR HEAVY RAIN, SO CONTINUED TO
LEAVE THE DAY 4/FRIDAY ERO BLANK PENDING BETTER MODEL AGREEMENT.
HOWEVER, BY SATURDAY, DYNAMICAL SUPPORT FOR THUNDERSTORMS WITH
HEAVY RAIN WILL INCREASE AS THE UPPER LOW WITHIN THE TROUGH
APPROACHES. MODELS THEREFORE SHOW A CONSIDERABLE UPTICK IN RAINFALL
AMOUNTS ON THE CUSP OF THE INSTABILITY GRADIENT, WITH 3-5 INCHES
LOCALLY. FOR THE DAY 5/SATURDAY ERO, HAVE OUTLINED A MARGINAL RISK
CENTERED OVER NEBRASKA/KANSAS AND VICINITY. THERE MAY BE A NEED
FOR HIGHER RISK LEVELS WITHIN THE MARGINAL IN THE FUTURE IF MODELS
ALIGN MORE AGREEABLY ON AN HEAVY RAIN AXIS (AND THAT AXIS IS NOT
OVER THE NEBRASKA SANDHILLS THAT ARE NOT PRONE TO FLASH FLOODING).
MODERATE TO HEAVY RAIN IS EXPECTED GRADUALLY SHIFT INTO THE MIDWEST
SUNDAY-MONDAY.
UPSTREAM, THE QUESTIONS WITH THE PATTERN ALOFT CREATE UNCERTAINTY
WITH THE QPF IN THE NORTHWEST INTO THE WEEKEND AND EARLY NEXT WEEK,
BUT THERE IS A GROWING SIGNAL TO FOCUS A DEEPER INFLUX OF MOISTURE
AND SYSTEM ENERGY INTO THE REGION MONITOR.
IN THE EAST, TROUGHING REMAINING OVERHEAD AND WEAK ATLANTIC
FRONTS/SURFACE LOWS COULD PROMOTE SHOWERS OVER PARTS OF THE EASTERN
SEABOARD LATE THIS WEEK INTO EARLY NEXT WEEK. THESE SHOWERS LOOK
TO REMAIN LIGHT. THE FLORIDA PENINSULA COULD ALSO SEE SOME FOCUSED
CONVECTION DUE TO A FRONTAL BOUNDARY IN A MOIST ENVIRONMENT. THIS
FRONT STALLING INTO THE WEEKEND AND BEYOND COULD INCREASE RAIN
TOTALS, WHICH MAY CREATE FLOODING PROBLEMS ESPECIALLY FOR THE
TYPICALLY SENSITIVE URBAN CORRIDOR OF SOUTHEAST FLORIDA.
INITIAL TROUGHING ALOFT IN THE WEST WILL LEAD TO CONTINUED BELOW
NORMAL TEMPERATURES THERE THROUGH LATE WEEK, THOUGH MODERATING
COMPARED TO THE SHORT RANGE TO ONLY 5-10 BELOW NORMAL. THE WEST
COAST, ESPECIALLY CALIFORNIA, LOOKS TO WARM TO A BIT ABOVE NORMAL
BY EARLY NEXT WEEK PENDING THE UPPER LEVEL PATTERN UNCERTAINTIES.
COOLER THAN AVERAGE TEMPERATURES WILL MIGRATE INTO THE NORTH-
CENTRAL PLAINS UNDER THE TROUGH EARLY NEXT WEEK. WARMER THAN NORMAL
TEMPERATURES, INCLUDING 90S TO NEAR 100F IN THE SOUTHERN PLAINS,
WILL LAST INTO LATE WEEK AND THEN GRADUALLY MODERATE. THE SCOPE OF
ABOVE NORMAL TEMPERATURES WILL DECREASE INTO THE WEEKEND AND MAINLY
FOCUS ACROSS THE UPPER GREAT LAKES TO THE OHIO VALLEY. THE EASTERN
SEABOARD LOOKS TO BE AROUND NORMAL ON AVERAGE THROUGH THE PERIOD,
WITH PARTS OF THE MID-ATLANTIC COOLER THAN USUAL FOR HIGHS.
TATE
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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