971
FXUS02 KWBC 161840
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 PM EDT SAT SEP 16 2023
VALID 12Z TUE SEP 19 2023 - 12Z SAT SEP 23 2023
..OVERVIEW
AN UPPER TROUGH LIFTS OUT OF THE NORTHEAST TUESDAY AS A RIDGE
AMPLIFIES OVER THE CENTRAL PARTS OF THE COUNTRY AND A TROUGH FROM
THE GULF OF ALASKA DIGS INTO THE PACIFIC NORTHWEST TUESDAY NIGHT
THAT LIKELY PHASES WITH A LOW CURRENTLY OFF CALIFORNIA AND CLOSES
INTO A DEEP LOW AND SETTLES OVER NORTHERN NEVADA THURSDAY. THE
SURFACE COLD FRONT FROM THE EASTERN TROUGH STALLS OVER THE FLORIDA
PENINSULA AND NORTHERN GULF OF MEXICO AND PROVIDES FOCUS FOR DAILY
THUNDERSTORMS IN FLORIDA THROUGH THE WEEK. MOISTURE FLOWING UP THE
PLAINS BRINGS ORGANIZED THUNDERSTORM RISKS TUESDAY AND WEDNESDAY.
IN THE WEST, UNSETTLED WEATHER IS FORECAST TO DEVELOP AHEAD OF
THIS DEVELOPING TROUGH/LOW WITH PROLONGED PRECIPITATION CHANCES
ACROSS THE NORTHERN INTERMOUNTAIN WEST, NORTHERN ROCKIES, AND
EVENTUALLY THE NORTHERN PLAINS. BELOW AVERAGE TEMPERATURES WILL
SPREAD THROUGH THE WEST WITH THIS DEVELOPING SYSTEM.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THERE WAS GOOD AGREEMENT THROUGH THURSDAY AMONG THE 00Z
ECMWF/CMC/UKMET ON THE AMPLIFICATION OF THE NORTHWEST TROUGH AND
PHASING WITH THE CA LOW INTO A SPRAWLING LOW TO USE A BLEND
FAVORING THESE DETERMINISTIC MODELS. THE 06Z GFS WAS MUCH QUICKER
WITH THE SOUTHWARD PROGRESSION OF THE TROUGH AND IN EJECTING THE
CA LOW TO BE CONSIDERED AN OUTLIER. THE 12Z VERSIONS OF THESE
MODELS HAVE COME CLOSER WITH A BIT MORE PROGRESSIVE TROUGH/LOW
(WITH THE GFS A BIT SLOWER), BUT THE 12Z GFS IS STILL THE FAST
OUTLIER. STILL, THERE IS GOOD DETERMINISTIC AGREEMENT ON THE
DEVELOPING LOW TO STALL OVER THE INTERIOR NORTHWEST OR NORTHERN
GREAT BASIN THROUGH FRIDAY BEFORE EJECTING EAST (AND THE ENSEMBLE
MEANS ARE GENERALLY QUICKER TO EJECT EAST) THAN THE FORECAST BLEND
STILL LEANS HEAVILY ON DETERMINISTICS THROUGH DAY 6 BEFORE
FEATURING MORE ENSEMBLE MEANS FROM THE 00Z ECENS/06Z GEFS FOR DAY
7. FOR QPF, THE 13Z NBM PROVIDED A GOOD STARTING POINT WITH
ENHANCEMENTS MADE WITH THE 00Z ECMWF/06Z GFS FOR DAYS 4/5
(FOCUSING ON SOUTH-CENTRAL PLAINS RAIN) BEFORE LEANING HEAVILY ON
THE NBM FOR DAYS 6/7.
..WEATHER/HAZARDS HIGHLIGHTS
A PROLONGED 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
SOUTH-CENTRAL PLAINS TUESDAY. THE DAY 4 MARGINAL RISK AREA IS
MAINTAINED FOR MUCH OF OKLAHOMA AND EASTERN KANSAS WITH A BIT OF A
NORTHEASTWARD EXPANSION INTO MISSOURI PER THE 12Z GUIDANCE
CONSENSUS TODAY. RAINFALL SHOULD EXPAND BOTH NORTH AND SOUTH
THROUGH THE PLAINS WEDNESDAY ALONG A LINGERING BOUNDARY AS WELL AS
THE ENHANCED MOISTURE TRANSPORT EAST OF THE AMPLIFYING TROUGH/LOW.
THE ECMWF HAS FOCUSED QPF FARTHER SOUTH OVER TEXAS WHERE
INSTABILITY AND MOISTURE ARE GREATER WITH A SEPARATE AREA OF HEAVY
QPF OVER THE UPPER MIDWEST IN ASSOCIATION WITH A COLD FRONT
CROSSING THE NORTHERN PLAINS, WHILE THE GFS HAS FAVORED HEAVY RAIN
REPEATING OVER OKLAHOMA THROUGH IOWA ALONG WITH A SIMILAR SEPARATE
AREA OVER THE UPPER MIDWEST. FOR NOW, THE DAY 5 MARGINAL RISK
INCORPORATES BOTH APPROACHES WITH EXPANDED COVERAGE FROM NORTHEAST
TEXAS THROUGH EASTERN IOWA AND THEN BACK AROUND THE NORTHEASTERN
GREAT PLAINS THROUGH NORTHERN MINNESOTA. A STRONG POLEWARD SURGE
OF MOISTURE UP THE PLAINS IS ANTICIPATED, BUT THE HEAVY RAIN AREAS
WILL LIKELY BECOME MORE FOCUSED THROUGH THE COMING DAYS.
A STALLED FRONTAL BOUNDARY WILL MAINTAIN A FAIRLY WET PATTERN OVER
THE FLORIDA PENINSULA THROUGH NEXT WEEK, INCLUDING THE RISK OF LOW
PRESSURE DEVELOPMENT OVER/EAST OF THE PENINSULA. HOWEVER,
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 AMPLIFYING TROUGH INTO A LOW OVER THE WEST BY MIDWEEK SHOULD
BRING SOME PROLONGED PRECIPITATION TO THE NORTHERN INTERMOUNTAIN
WEST AND NORTHERN ROCKIES STARTING BY WEDNESDAY NIGHT. THERE IS A
GENERAL FLOODING RISK FOR THE TERRAIN ONTO THE HIGH PLAINS WITH
HIGHER ELEVATIONS IN THE NORTHERN ROCKIES LIKELY TO SEE SNOW THAT
THE EXTENT TO WHICH DEPENDS ON THE POSITION AND DEPTH OF THE UPPER
TROUGH/DEVELOPING LOW.
AN AREA OF MAX TEMPERATURES GENERALLY 10-15F ABOVE NORMAL WILL
SHIFT EAST WITH THE BROAD RIDGE AXIS FROM THE NORTHERN PLAINS ON
TUESDAY THROUGH THE GREAT LAKES BY THURSDAY AND EASTERN CANADA
INTO NEXT WEEKEND. THE UPPER TROUGH FORECAST TO AMPLIFY INTO A LOW
OVER THE WEST THROUGH THE MIDDLE OF NEXT WEEK LOOKS TO BRING A
COOLING TREND ACROSS MUCH OF THE WEST STARTING TUESDAY, WITH HIGH
TEMPS AROUND 15F BELOW NORMAL FORECAST FROM MONTANA THROUGH
CALIFORNIA FOR THURSDAY AND FRIDAY. OTHERWISE, MOST TEMPERATURES
FROM THE SOUTH TO THE EAST WILL BE NEAR SEASONABLE VALUES THROUGH
MUCH OF THE WEEK.
JACKSON
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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