073
FXUS02 KWBC 170620
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
219 AM EDT SUN SEP 17 2023
VALID 12Z WED SEP 20 2023 - 12Z SUN SEP 24 2023
..OVERVIEW
AN UPPER TROUGH LIFTS OUT OF THE NORTHEAST WEDNESDAY AS A RIDGE
AMPLIFIES OVER THE CENTRAL PART OF THE COUNTRY AND A SHORTWAVE
DIGGING INTO THE PACIFIC NORTHWEST WEDNESDAY. THIS SHORTWAVE
SHOULD PHASE WITH A LOW CURRENTLY OFF CALIFORNIA AND CLOSE INTO A
DEEP LOW, SETTLING OVER NORTHERN NEVADA THURSDAY-FRIDAY BEFORE
GETTING KICKED EASTWARD INTO THE NORTHERN PLAINS NEXT WEEKEND.
THIS KEEPS WEATHER RATHER UNSETTLED AND COOL ACROSS THE WEST MUCH
OF THE PERIOD. AN UPPER LOW SETTLING OVER THE SOUTHEAST/FLORIDA
LATER THIS WEEK BRINGS POTENTIAL FOR HEAVY COASTAL RAINS NEXT
WEEKEND ALONG THE SOUTHEAST AND MID-ATLANTIC COASTS.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THERE WAS GOOD AGREEMENT WEDNESDAY-THURSDAY ON THE AMPLIFICATION
OF THE NORTHWEST TROUGH AND PHASING WITH THE CA LOW INTO A
SPRAWLING LOW TO USE A BLEND FAVORING THE DETERMINISTIC MODELS.
AFTER THIS, THE UKMET IS DISPLACED A BIT TO THE SOUTH AND EAST
WITH THE UPPER LOW, AND BY NEXT WEEKEND THE 12Z/SEP 16 CMC IS MUCH
SLOWER TO MOVE THE LOW EAST INTO THE NORTHERN PLAINS DUE TO A MORE
BLOCKY PATTERN UPSTREAM ACROSS THE EAST PACIFIC. THE CMC WAS NOT
USED IN THE LATE PERIOD BLEND. HOWEVER, THE NEW 00Z CMC (AVAILABLE
AFTER FORECAST GENERATION TIME) DID COME IN MUCH MORE IN LINE WITH
THE GFS AND ECMWF BOTH OF WHICH ARE FASTER TO KICK THE SYSTEM INTO
THE NORTHERN PLAINS AND ARE MORE CONSISTENT WITH THE ENSEMBLE
MEANS. BY LATE WEEK, THERE IS DECENT AGREEMENT THAT AN AREA OF LOW
PRESSURE MAY FORM OFF THE EASTERN FLORIDA COAST, WITH MUCH MORE
UNCERTAINTY ON WHAT, IF ANY, TROPICAL CHARACTERISTICS THIS MAY
EXHIBIT AS IT LIFTS NORTH ALONG THE SOUTHEAST COAST NEXT WEEK. THE
WPC FORECAST CONTINUES TO FAVOR A WEAKER FRONTAL WAVE, WHICH
REGARDLESS OF CHARACTERIZATION, HAS POTENTIAL TO SPREAD HEAVY
RAINFALL UP THE SOUTHEAST AND MID-ATLANTIC COASTS NEXT WEEK. GIVEN
THE INCREASING UNCERTAINTY IN THE PATTERN NEXT WEEKEND BOTH IN THE
WEST AND THE SOUTHEAST, THE WPC BLEND FAVORED A BLEND CLOSEST TO
THE ENSEMBLE MEANS, PLUS THE DETERMINISTIC ECMWF.
..WEATHER/HAZARDS HIGHLIGHTS
A FETCH OF MOISTURE FROM THE WESTERN GULF AHEAD OF AN AMPLIFYING
WESTERN TROUGH SHOULD ALLOW AREAS OF HEAVIER AND POSSIBLY
ORGANIZED CONVECTIVE THUNDERSTORMS TO DEVELOP ACROSS THE PLAINS,
BUT THERE REMAINS A LOT OF UNCERTAINTY AND SPREAD IN THE LOCATION
OF HEAVIEST AMOUNTS. THE BROAD MARGINAL RISK INHERITED ON THE DAY
4/WEDNESDAY EXCESSIVE RAINFALL OUTLOOK WAS BROKEN UP INTO TWO
AREAS -- FROM NORTHEAST TEXAS INTO MISSOURI WHERE INSTABILITY
SHOULD BE STRONGEST, AND ANOTHER AREA ACROSS PARTS OF THE UPPER
MIDWEST, WHERE MOISTURE ANOMALIES ARE GREATEST ALONG A SLOW
MOVING/STATIONARY BOUNDARY. THE AMPLIFYING UPPER LOW OVER THE WEST
BY MIDWEEK SHOULD BRING PROLONGED PRECIPITATION TO THE
NORTHERN/CENTRAL INTERMOUNTAIN WEST AND NORTHERN ROCKIES. RAINFALL
SHOULD GET HEAVIER WITH TIME AS THE LOW DEEPENS AND THERE IS A
GENERAL FLOODING RISK FOR THE TERRAIN INTO THE NORTHERN PLAINS. A
LARGE MARGINAL RISK WAS INTRODUCED ON THE DAY 5/THURSDAY ERO
TONIGHT ACROSS THIS REGION. HIGHER ELEVATIONS IN THE NORTHERN
ROCKIES ARE LIKELY TO SEE SNOW THAT THE EXTENT TO WHICH DEPENDS ON
THE POSITION AND DEPTH OF THE UPPER TROUGH/DEVELOPING LOW.
MEANWHILE, A STALLED FRONTAL BOUNDARY WILL MAINTAIN A FAIRLY WET
PATTERN OVER THE FLORIDA PENINSULA THROUGH MUCH OF THE WEEK,
THOUGH CONFIDENCE IN WHERE THAT WOULD FOCUS IS LOW ENOUGH TO NOT
RAISE AN EXCESSIVE RAIN OUTLOOK AREA IN DAYS 4 OR 5 AT THIS TIME.
THE POSSIBLE SURFACE LOW OFF THE COAST BY FRIDAY COULD TAP
TROPICAL MOISTURE, INCREASING THE POTENTIAL FOR HEAVIER RAINS
ALONG THE ATLANTIC COAST FROM FLORIDA TO THE MID-ATLANTIC. AMOUNTS
AND EXTENT INLAND OF THE HEAVIER RAINS REMAINS EXTREMELY UNCERTAIN
AT THIS POINT.
AN AREA OF MAX TEMPERATURES GENERALLY 10-15F ABOVE NORMAL WILL
SHIFT EAST WITH THE BROAD RIDGE AXIS FROM THE UPPER MIDWEST ON
WEDNESDAY THROUGH THE GREAT LAKES THURSDAY-SATURDAY MODERATING
SOME WITH TIME. THE UPPER LOW OVER THE WEST LOOKS TO BRING A
COOLING TREND ACROSS MUCH OF THE WEST, WITH HIGH TEMPS AROUND 15F
BELOW NORMAL FORECAST FROM MONTANA THROUGH CALIFORNIA THROUGH AT
LEAST SATURDAY. MODEST ABOVE NORMAL TEMPERATURES MAY RETURN TO
PARTS OF TEXAS THIS WEEK, WHILE MUCH OF THE EASTERN U.S. SHOULD BE
FAIRLY NEAR NORMAL.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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