640
FXUS02 KWBC 171921
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
321 PM EDT SUN SEP 17 2023
VALID 12Z WED SEP 20 2023 - 12Z SUN SEP 24 2023
..OVERVIEW
THE RIDGE OVER THE CENTRAL U.S. WILL CONTINUE TO AMPLIFY AS THE
UPPER TROUGH LIFTS OUT OF THE NORTHEAST ON WEDNESDAY. ACROSS THE
NORTHWEST A DIGGING SHORTWAVE TROUGH IS
EXPECTED TO PHASE WITH A LOW LOCATED OFFSHORE THE CALIFORNIA COAST
TO BECOME A DEEP LOW OVER THE GREAT BASIN BY THURSDAY/FRIDAY. THIS
FEATURE WILL EXIT INTO THE PLAINS BY THE WEEKEND, KEEPING COOL AND
UNSETTLED CONDITIONS ACROSS THE WEST FOR A MAJORITY OF THE
EXTENDED 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
THE LATEST RUNS OF THE GUIDANCE AND ENSEMBLE MEANS CONTINUED TO
SHOW FAIRLY GOOD AGREEMENT ON THE OVERALL AMPLIFICATION OF THE
LARGE SCALE PATTERN AND ITS EVOLUTION THROUGH THE EXTENDED PERIOD.
LIKE THEIR PREVIOUS CYCLES, THE UKMET REMAINS DISPLACED A BIT TO
THE SOUTH AND EAST WITH THE UPPER LOW, AND BY NEXT WEEKEND THE 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 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. THE
PREFERRED MODEL CONSENSUS UTILIZED A HEAVIER WEIGHTING OF THE
ECWMF BUT ALSO INCLUDED THE GFS, CMC AND UKMET. THE INCLUSION OF
BOTH THE UKMET AND CMC WERE REDUCED THEN OMITTED BEYOND THE MIDDLE
AND LATER PERIODS AND WERE REPLACED BY THE GEFS MEAN AND EC
ENSEMBLE MEANS. THIS BLEND PROVIDED A SENSE OF CONTINUITY FROM THE
PREVIOUS FORECAST.
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.
..WEATHER/HAZARDS HIGHLIGHTS
RETURN FLOW FROM THE GULF OF MEXICO WILL TRANSPORT PW VALUES OF
1.25 INCHES NORTHWARD INTO THE PLAINS AHEAD OF THE AMPLIFYING
TROUGH OVER THE WEST. THIS STEADY STREAM OF MOISTURE 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
INHERITED DAY 4 EXCESSIVE RAINFALL OUTLOOK FEATURED 2 AREAS
HIGHLIGHTED BY MARGINAL RISKS. THE SOUTHERN ONE SPANS FROM
NORTHEAST TEXAS TO NORTHERN MISSOURI WHERE MOISTURE ANOMALIES ARE
GREATEST ALONG A SLOW MOVING/STATIONARY BOUNDARY. WITH THE LATEST
TREND IN THE GUIDANCE MINOR EXPANSIONS WERE MADE WESTWARD ALONG
THE TEXAS AND OKLAHOMA BORDER AND EASTWARD ALONG THE MISSOURI AND
ARKANSAS BORDER. THE NORTHERN RISK AREA SPANS FROM THE EASTERN
DAKOTAS TO THE MINNESOTA/WISCONSIN BORDER. NO CHANGES WERE MADE TO
THIS AREA AT THIS TIME. THE AMPLIFYING UPPER LOW OVER THE WEST BY
MIDWEEK SHOULD LEAD TO PROLONGED PRECIPITATION ACROSS THE
NORTHERN/CENTRAL INTERMOUNTAIN WEST AND NORTHERN ROCKIES. AS THE
LOW DEEPENS THE RAINFALL IS EXPECTED TO INCREASE KEEPING THE
THREAT FOR EXCESSIVE RAINFALL AND LOCAL FLASH FLOODING ELEVATED.
THE DAY 5 ERO HAS A LARGE MARGINAL RISK SPANNING FROM EASTERN
NEVADA NORTHWARD TO MONTANA AND EAST TO NORTHERN MINNESOTA. 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. INCREASING POTENTIAL FOR MORE WIDESPREAD
HEAVIER RAIN DEVELOPS NEXT WEEKEND ACROSS THE NORTHERN/CENTRAL
PLAINS INTO PARTS OF THE MISSISSIPPI VALLEY AS THE WEST UPPER LOW
BEGINS TO PUSH INTO THE REGION. MINOR ADJUSTMENTS WERE MADE TO THE
WESTERN BOUNDS OVER CENTRAL IDAHO AND CENTRAL NEVADA AND BROUGHT
FURTHER NORTHEAST INTO NORTHERN MINNESOTA.
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.
CAMPBELL/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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