993
FXUS02 KWBC 151840
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 PM EDT FRI SEP 15 2023
VALID 12Z MON SEP 18 2023 - 12Z FRI SEP 22 2023
..OVERVIEW
AN UPPER TROUGH AND SURFACE COLD FRONT WILL MOVE OFF THE EAST
COAST EARLY NEXT WEEK WHILE THE TAIL END OF THE FRONT LINGERS OVER
THE FLORIDA PENINSULA AND NORTHERN GULF OF MEXICO. THIS SYSTEM
WILL PRODUCE PRECIPITATION CHANCES IN NEW ENGLAND ON MONDAY AND
TUESDAY AND PROVIDE FOCUS FOR DAILY SHOWERS AND THUNDERSTORMS IN
FLORIDA THROUGH THE WEEK. IN THE WEST, UNSETTLED WEATHER IS
FORECAST TO DEVELOP BY MID-WEEK AS UPPER LEVEL TROUGHING AMPLIFIES
OVER THE REGION. A COUPLE OF FRONTAL SYSTEMS WILL MOVE ACROSS THE
NORTHWEST AND EMERGE IN THE CENTRAL U.S. DURING THE SECOND HALF OF
THE WEEK, WHICH WILL CREATE PRECIPITATION CHANCES ACROSS THE
NORTHWEST, ROCKIES, AND PLAINS. BELOW AVERAGE TEMPERATURES WILL
FILTER INTO THE WEST IN THE WAKE OF THE FRONTAL SYSTEMS.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GUIDANCE SHOWED GOOD AGREEMENT ON THE UPPER LEVEL PATTERN THROUGH
THE MEDIUM RANGE PERIOD. UPPER LEVEL TROUGHING WILL EXIT THE EAST
COAST EARLY IN THE WEEK, FOLLOWED BY A RIDGE MOVING OVER THE
EASTERN U.S. MID-WEEK, AND TROUGHING WILL AMPLIFY OVER THE WEST
COAST DURING THE SECOND HALF OF NEXT WEEK. A DETERMINISTIC MODEL
BLEND ON THE GFS, ECMWF, CMC, AND UKMET WERE USED FOR DAYS 3-5,
BUT MODEL DIFFERENCES INCREASE DURING THE LATER HALF OF THE
PERIOD. THE GFS ADVANCES THE WESTERN TROUGH INLAND FASTER THAN THE
ECMWF AND CMC, BUT THE ENSEMBLE MEANS SEEM TO AGREE ON A SLOWER
SOLUTION. TO SMOOTH OUT DIFFERENCES IN THE TIMING AND POSITION OF
THE WESTERN TROUGH, THE GEFS AND ECENS MEANS WERE ADDED TO THE
MODEL BLEND FOR DAYS 6 AND 7.
..WEATHER/HAZARDS HIGHLIGHTS
THE RISK FOR HEAVY RAINFALL LEADING TO FLASH FLOODING IS FORECAST
TO BE LOW EARLY NEXT WEEK. A FRONTAL BOUNDARY STALLING OVER THE
FLORIDA PENINSULA WILL MAINTAIN A FAIRLY WET PATTERN. HOWEVER,
MOISTURE ANOMALIES ARE ONLY MODERATELY ABOVE NORMAL, FLASH FLOOD
GUIDANCE VALUES ARE HIGH, AND MODELS DO NOT SHOW MUCH CLUSTERING
FOR ANY RELATIVELY HEAVIER RAIN AMOUNTS THAT MAY FALL. SHORTER
TERM GUIDANCE MAY ULTIMATELY HELP TO REFINE ANY AREAS THAT COULD
AT LEAST MERIT A MARGINAL RISK. WRAP AROUND PRECIPITATION ON THE
BACKSIDE OF A SURFACE LOW EXITING MAINE MAY PRODUCE SOME MODERATE
RAINFALL ON MONDAY IN NEW ENGLAND, AND GIVEN ALREADY WET SOILS IN
THAT REGION, IT WOULDN'T TAKE MUCH TO CREATE AT LEAST LOCALIZED
FLOODING CONCERNS. STILL, THERE'S TOO MUCH UNCERTAINTY IN THE
LOCATION OF HEAVIER RAINFALL AMOUNTS TO WARRANT ADDING A MARGINAL
RISK FOR THE DAY 4/MONDAY EXCESSIVE RAINFALL OUTLOOK.
THE AMPLIFYING TROUGH OVER WEST NEXT WEEK SHOULD BRING SOME
PRECIPITATION TO THE PACIFIC NORTHWEST/NORTHERN ROCKIES AND
EVENTUALLY THE PLAINS. HIGH ELEVATIONS IN THE NORTHERN ROCKIES
COULD SEE SOME SNOW IF THE UPPER TROUGH AND POSSIBLE EMBEDDED LOW
ARE DEEP ENOUGH. AHEAD OF THE WESTERN TROUGH/LOW, AREAS OF RAIN
WILL LIKELY DEVELOP ACROSS THE SOUTH-CENTRAL PLAINS
TUESDAY-THURSDAY. WITH A PROLONGED FETCH FROM THE WESTERN GULF
DURING THIS TIME, HEAVY RAINFALL MAY BECOME ORGANIZED,
PARTICULARLY AROUND MID-WEEK. A MARGINAL RISK AREA HAS BEEN ADDED
FOR PARTS OF KANSAS AND OKLAHOMA FOR THE DAY 5/TUESDAY EXCESSIVE
RAINFALL OUTLOOK.
AN AREA OF HIGH TEMPERATURES UP TO 15F ABOVE NORMAL WILL SHIFT
FROM THE NORTHERN PLAINS ON MONDAY TO THE GREAT LAKES BY THURSDAY.
THE UPPER TROUGH FORECAST TO AMPLIFY OVER THE WEST NEXT WEEK
SHOULD PROMOTE A COOLING TREND OVER THE REGION STARTING TUESDAY,
WITH HIGH TEMPS 5-15F BELOW NORMAL STRETCHING FROM MONTANA TO
SOUTHERN CALIFORNIA NEXT WEDNESDAY TO FRIDAY. OTHERWISE, MOST
TEMPERATURES FROM THE SOUTH TO THE EAST WILL BE NEAR SEASONABLE
VALUES THROUGH MUCH OF THE WEEK.
SANTORELLI/DOLAN
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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