357
FXUS02 KWBC 240657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 AM EDT SAT AUG 24 2024
VALID 12Z TUE AUG 27 2024 - 12Z SAT AUG 31 2024
..OVERVIEW
EXPECT THE STRONG UPPER RIDGE OVER THE MIDDLE MISSISSIPPI VALLEY
AS OF EARLY TUESDAY TO DRIFT SOUTHEAST WITH TIME, GRADUALLY
WEAKENING AS IT DRIFTS OFF THE SOUTHEAST COAST TOWARD THE END OF
THE WEEK. THIS SHOULD PROMOTE A SIMILAR DRIFT AND EVENTUAL
MODERATION OF TUESDAY'S HOT TEMPERATURES OVER THE EAST-CENTRAL U.S.
MEANWHILE A NORTHERN PLAINS SHORTWAVE SHOULD ROUND THE TOP OF THE
RIDGE BUT STILL WITH A FAIR DEGREE OF UNCERTAINTY FOR SPECIFICS AS
IT BRUSHES THE GREAT LAKES/NORTHEAST. MOST GUIDANCE STILL HAS A
STRONG UPPER TROUGH COMING INTO THE NORTHWEST U.S. AND WESTERN
CANADA BY TUESDAY AND CONTINUING ONWARD THEREAFTER, BRINGING SOME
COOL DAYTIME HIGHS TO AREAS FROM THE NORTHWEST INTO NORTHERN HIGH
PLAINS TUESDAY- THURSDAY. UPSTREAM ENERGY MAY LEAD TO SOME
BROADENING OF THE OVERALL MEAN TROUGH BY NEXT SATURDAY. AN UPPER
RIDGE WILL LIKELY BUILD OVER MORE OF THE WEST BY LATE WEEK AFTER
THE INITIAL TROUGH'S DEPARTURE. THE LEADING NORTHERN TIER
SHORTWAVE/SURFACE REFLECTION AND THEN THE COLD FRONT ASSOCIATED
WITH THE ADVANCING WESTERN NORTH AMERICA TROUGH WILL PRODUCE AREAS
OF RAINFALL OVER THE EASTERN HALF OF THE COUNTRY. DIURNAL
CONVECTION SHOULD CONTINUE OVER THE SOUTHERN ROCKIES AND ALONG THE
GULF COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
BASED ON 12Z/18Z GUIDANCE, THE UPDATED FORECAST MAINTAINED A
PREFERENCE FOR RECENT ECMWF/UKMET/CMC RUNS AND THE ECENS/CMCENS
MEANS. GEFS MEANS OFFER DECENT COMPARISONS FOR THE OVERALL PATTERN
TOWARD THE END OF THE PERIOD THOUGH. GFS RUNS HAVE CONTINUED TO
HAVE DIFFICULTY IN RESOLVING THE LEADING SHORTWAVE TRACKING OUT OF
THE NORTHERN PLAINS AND EVENTUAL EVOLUTION OF THE VIGOROUS TROUGH
TRACKING EASTWARD FROM WESTERN NORTH AMERICA.
THE 18Z GFS HAD A FAIRLY NOVEL SOLUTION OF COMBINING ITS
EXCESSIVELY PROGRESSIVE NORTHERN TIER SHORTWAVE ENERGY WITH
AMPLIFYING EASTERN CANADA DYNAMICS TO PRODUCE A DEEP AND SLOW
MOVING UPPER LOW NEAR NEW ENGLAND AND NOVA SCOTIA LATE IN THE WEEK.
OTHER DYNAMICAL AND MACHINE LEARNING (ML) MODELS OFFERED MINIMAL
SUPPORT FOR THAT SCENARIO. THE NEW 00Z GFS HAS FINALLY ADJUSTED A
LOT CLOSER TO OTHER GUIDANCE IN PRINCIPLE. STILL, THERE IS QUITE A
BIT OF SPREAD FOR SHORTWAVE SPECIFICS AS IT CONTINUES
EAST/SOUTHEAST--RANGING BETWEEN SHEARING OUT TO STAYING FAIRLY WELL
DEFINED AS IT CROSSES NEW ENGLAND.
THE GFS HAD BEEN STARTING TO GET CLOSER TO OTHER GUIDANCE FOR THE
ULTIMATE EVOLUTION OF THE SOUTHERN CANADA/NORTHERN TIER U.S. UPPER
TROUGH AND EMBEDDED LOW BY THE LATTER HALF OF THE WEEK, BUT THE NEW
00Z GFS HAS RETURNED TO SPLITTING ENERGY IN A FASHION NOT
SUGGESTED BY OTHER DYNAMICAL AND ML MODELS OR THE ENSEMBLE MEANS
(INCLUDING THE GEFS). THIS ISSUE ALSO AFFECTS SOME FINER DETAILS OF
THE PATTERN OVER THE ROCKIES BY THURSDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE STRONG UPPER RIDGE INITIALLY OVER THE MISSISSIPPI VALLEY WILL
SUPPORT A BROAD AREA OF ANOMALOUS HEAT ON TUESDAY, FOLLOWED BY A
VERY GRADUAL SOUTHEASTWARD SUPPRESSION CORRESPONDING TO THE
RIDGE'S MOVEMENT AND EVENTUAL WEAKENING. SOME AREAS MAY SEE ONE OR
MORE DAYS WITH HIGHS UP TO 10-15 ABOVE NORMAL (AND A FEW DAILY
RECORDS) TUESDAY INTO THURSDAY OR FRIDAY, WITH GREATEST ANOMALIES
FINALLY DECLINING TO PLUS 5-10F RANGE BY NEXT SATURDAY.
EXPERIMENTAL HEATRISK CONTINUES TO SHOW A FAIRLY BROAD AREA OF
MODERATE TO MAJOR LEVEL IMPACTS WITH SMALL EMBEDDED POCKETS OF
EXTREME. IN CONTRAST, THE STRONG UPPER TROUGH PROGRESSING EASTWARD
FROM THE NORTHWEST PRODUCE BELOW NORMAL HIGHS FROM THE NORTHWEST
INTO THE NORTHERN HIGH PLAINS TUESDAY-THURSDAY WITH SOME MINUS
10-15F ANOMALIES. EXPECT TEMPERATURES TO REBOUND TO ABOVE NORMAL
LEVELS OVER THE NORTHWEST BY LATE WEEK AS UPPER RIDGING BUILDS INTO
THE REGION.
DURING THE 12Z TUESDAY-12Z WEDNESDAY PERIOD COVERED BY THE DAY 4
EXCESSIVE RAINFALL OUTLOOK, A MARGINAL RISK AREA REMAINS IN PLACE
OVER PORTIONS OF THE SOUTHERN ROCKIES GIVEN ADEQUATE
MOISTURE/INSTABILITY, GFS/ECMWF QPF SIGNALS, AND LIKELY EXISTENCE
OF SHORTWAVE IMPULSES ALOFT. BY DAY 5 (WEDNESDAY-WEDNESDAY NIGHT)
MOST GUIDANCE SUGGESTS THAT CONVECTION OVER THE REGION SHOULD TREND
SOMEWHAT LIGHTER. THE EXCEPTION COMES FROM THE 00Z GFS WHOSE MORE
FAVORABLE UPPER PATTERN HAS LITTLE SUPPORT FROM CONSENSUS. THUS NO
RISK AREA IS DEPICTED FOR NOW IN THE DAY 5 ERO.
FARTHER NORTH, A LEADING NORTHERN TIER SHORTWAVE MAY STILL PRODUCE
SOME CONVECTION NEAR THE UPPER GREAT LAKES EARLY TUESDAY BUT
WITHOUT A SUFFICIENTLY STRONG SIGNAL FOR A RISK AREA IN THE DAY 4
ERO. SOME CONVECTION WILL LIKELY EXTEND EASTWARD/SOUTHEASTWARD
INTO PARTS OF THE NORTHEAST/MID-ATLANTIC BY WEDNESDAY, BUT AT THIS
TIME GUIDANCE SIGNALS FOR LOCATION AND MAGNITUDE OF RAINFALL ARE
TOO INCOHERENT TO MERIT A RISK AREA IN THE DAY 5 ERO. ON THE OTHER
HAND, THE DAY 5 ERO HAS INTRODUCED A MARGINAL RISK AREA COVERING
MOST OF MINNESOTA/EASTERN NORTH DAKOTA AND FAR NORTHEASTERN SOUTH
DAKOTA AS THE STRONG WESTERN NORTH AMERICA UPPER TROUGH APPROACHES,
WITH THE ASSOCIATED COLD FRONT AS WELL AS A LEADING WARM FRONT
HELPING TO FOCUS CONVECTION.
CONTINUED EASTWARD PROGRESSION OF THE PLAINS COLD FRONT AFTER EARLY
THURSDAY SHOULD BRING AREAS OF RAIN AND THUNDERSTORMS INTO THE
EASTERN PART OF THE COUNTRY BY LATE WEEK INTO THE WEEKEND. SOME
CONVECTION MAY PERSIST OVER AND NEAR THE SOUTHERN ROCKIES LATE IN
THE WEEK. EXPECT EPISODES OF SHOWERS AND THUNDERSTORMS ALONG THE
GULF COAST THROUGH THE PERIOD, WHILE SOUTHERN FLORIDA COULD SEE
SOME INCREASE OF MOISTURE TOWARD THE END OF THE WEEK.
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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