064
FXUS02 KWBC 180719
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
319 AM EDT MON SEP 18 2023
VALID 12Z THU SEP 21 2023 - 12Z MON SEP 25 2023
...HEAVY RAINFALL POTENTIAL ACROSS PARTS OF THE PLAINS AS WELL AS
THE SOUTHEAST COAST LATE WEEK AND NEXT WEEKEND...
..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 INITIALLY 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 INITIALLY 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
ANOMALOUS MOISTURE ASSOCIATE WITH THE UPPER LOW IN THE WEST LATE
THIS WEEK WILL ALLOW FOR POSSIBLY HEAVY RAINFALL FROM THE NORTHERN
ROCKIES INTO THE HIGH PLAINS AND DEPENDING ON DEPTH OF THE UPPER
LOW, THERE COULD BE SOME SNOW IN THE HIGHEST ELEVATIONS TOO. AS
THE LOW MOVES EAST INTO THE PLAINS, RAINFALL SHOULD BEGIN TO
EXPAND IN COVERAGE ACROSS THE CENTRAL AND NORTHERN PLAINS THROUGH
SATURDAY. THE SET UP IS FAVORABLE FOR HEAVY TO EXCESSIVE RAINFALL,
GIVEN AMPLE MOISTURE AND INSTABILITY, BUT THERE REMAINS TOO MUCH
UNCERTAINTY IN THE GUIDANCE FOR ANYTHING HIGHER THAN A BROAD
MARGINAL RISK RIGHT NOW ON THE DAYS 4 AND 5 EXCESSIVE RAINFALL
OUTLOOKS.
A STALLED FRONTAL BOUNDARY WILL CONTINUE A FAIRLY WET PATTERN OVER
THE FLORIDA PENINSULA INTO LATE WEEK, THOUGH STILL LOW CONFIDENCE
IN PLACEMENT AND AMOUNTS OF RAINFALL TO INCLUDE EVEN A MARGINAL
RISK ON THE EROS. THE SURFACE LOW OFF THE COAST BY FRIDAY COULD
TAP TROPICAL MOISTURE, INCREASING THE POTENTIAL FOR HEAVY RAINFALL
ALONG THE ATLANTIC COAST FROM FLORIDA TO THE MID-ATLANTIC. AMOUNTS
AND EXTENT INLAND OF THE HEAVIER RAINS REMAINS EXTREMELY UNCERTAIN
AT THIS POINT, BUT DID INTRODUCE A MARGINAL RISK IN THE ERO ALONG
THE SOUTHEAST COAST FOR FRIDAY.
TEMPERATURES AS MUCH AS 10-20 DEGREES BELOW NORMAL ASSOCIATED WITH
THE DEEP WESTERN UPPER LOW FROM CALIFORNIA TO MONTANA SHOULD
MODERATE BY SUNDAY AS THE LOW SHIFTS EAST. ELSEWHERE, TEMPERATURES
COULD BE MODERATELY ABOVE NORMAL ACROSS THE UPPER MIDWEST/GREAT
LAKES AND PARTS OF TEXAS THURSDAY AND FRIDAY. BY MONDAY, MUCH OF
THE COUNTRY SHOULD BE NEAR NORMAL OR WITHIN A FEW DEGREES OF
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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