836
FXUS02 KWBC 180701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT MON SEP 18 2023
VALID 12Z THU SEP 21 2023 - 12Z MON SEP 25 2023
..OVERVIEW
AN UPPER LEVEL CLOSED LOW DROPPING THROUGH THE WEST THURSDAY AND
FRIDAY SHOULD EVENTUALLY SHIFT EAST AS ADDITIONAL TROUGHING MOVES
INTO THE PACIFIC NORTHWEST NEXT WEEKEND. GUIDANCE SUGGESTS THE
UPPER LOW WILL LINGER ACROSS THE PLAINS FOR AT LEAST A FEW DAYS AS
UPPER RIDGING BUILDS INTO THE WESTERN ATLANTIC CREATING A BLOCKY
PATTERN ACROSS THE CONUS THE SECOND HALF OF THE PERIOD. ELSEWHERE,
A SURFACE LOW FORMING OFF THE FLORIDA COAST LATE THIS WEEK BRINGS
POTENTIAL FOR HEAVY COASTAL RAINS TO PARTS OF THE SOUTHEAST AS
MID-ATLANTIC AS IT MOVES SLOWLY UP THE COAST.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
OVERALL, THERE'S GOOD AGREEMENT ON THE UPPER PATTERN ACROSS THE
CONUS, BUT A LOT OF SPREAD IN THE DETAILS, EVEN EARLY IN THE
PERIOD WHICH HAVE BIG IMPLICATIONS FOR RAINFALL AMOUNTS/QPF. WITH
THE UPPER LOW INTIALLY OVER THE WEST, A GENERAL MODEL BLEND
SUFFICED FOR THURSDAY AND FRIDAY, BUT THERE IS DIVERGENCE IN THE
GUIDANCE AS THE LOW SHIFTS INTO THE PLAINS NEXT WEEKEND REGARDING
PLACEMENT AND TIMING. THE 12Z/SEP 17 ECMWF SEEMS MOST DISPLACED
WITH THE LOW AS IT BRINGS IT WELL INTO THE LOWER MISSISSIPPI
VALLEY, WHILE THE GFS AND CMC (AND ENSEMBLE MEANS) SUGGEST
SOMETHING DEEPER AND FARTHER NORTH ACROSS THE CENTRAL PLAINS. THE
CMC IS ALSO MUCH FASTER LIFTING THE SURFACE LOW UP THE SOUTHEAST
COAST AND THE ENSEMBLE CONSENSUS WOULD SUGGEST SOMETHING SLOWER
AND CLOSER TO THE GFS/ECMWF. THE UKMET IS ALSO AN OUTLIER EARLY IN
THE PERIOD AS IT TRIED TO INTIALLY FORM THE SURFACE LOW WELL WEST
INTO THE EASTERN GULF OF MEXICO. THE NHC IS MONITORING THIS SYSTEM
FOR POTENTIAL SUB-TROPICAL DEVELOPMENT, BUT REGARDLESS OF
CHARACTERISTICS, HAS THE POTENTIAL TO BRING SIGNIFICANT RAINFALL
TO PARTS OF THE SOUTHEAST AND MID-ATLANTIC COAST NEXT WEEKEND.
..WEATHER/HAZARDS HIGHLIGHTS
ANAMOLOUS MOISTURE ASSOCIATE WITH THE UPPER LOW IN THE WEST LATE
THIS WEEK WILL ALLOW FOR POSSIBLY HEAVY RAINFALL FROM THE NORTHERN
ROCKIES INTO THE PLAINS. THERE REMAINS A LOT OF SPREAD IN THE
DETAILS AND PLACEMENT THOUGH, SO ONLY A BROAD MARGINAL RISK
REMAINS FOR NOW ON THE DAY 4/THURSDAY EXCESSIVE RAINFALL OUTLOOK.
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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