294
FXUS02 KWBC 160653
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
253 AM EDT SAT SEP 16 2023
VALID 12Z TUE SEP 19 2023 - 12Z SAT SEP 23 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 MAY
CONTINUE TO PRODUCE PRECIPITATION CHANCES IN NORTHERN NEW ENGLAND
ON 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 A LIKELY CLOSED UPPER LOW DROPS
THROUGH 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 THIS SYSTEM.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THERE WAS GOOD ENOUGH AGREEMENT THE FIRST HALF OF THE PERIOD AS
THE TROUGH EXITS THE NORTHEAST AND TROUGHING BEGINS TO AMPLIFY
OVER THE WEST. A GENERAL DETERMINISTIC MODEL BLEND WAS USED. BY
THURSDAY AND BEYOND THOUGH, WHILE THERE IS GENERAL AGREEMENT IN
THE PRESENCE OF WHAT COULD BE A DEEP UPPER LOW, THERE ARE A LOT OF
DIFFERENCES IN TIMING/STRENGTH/PLACEMENT OF THIS FEATURE OVER THE
WEST. MODELS EXHIBIT A LOT OF RUN TO RUN VARIABILITY OF HOW FAR
SOUTH THIS LOW GETS, AMONG OTHER DETAILS, MOSTLY RELATED TO
ANOTHER AMPLIFIED SHORTWAVE UPSTREAM DROPPING THROUGH THE EAST
PACIFIC WHICH MAY KICK THE LOW EAST INTO THE PLAINS NEXT WEEKEND.
18Z/SEP 15 GFS NOTICEABLY MUCH QUICKER TO DO THIS AND WAS NOT
PREFERRED LATE PERIOD. ENSEMBLE MEANS ARE EXTREMELY WASHED OUT
WITH THIS FEATURE, AS THERE IS A LOT OF SPREAD IN THE INDIVIDUAL
ENSEMBLE MEMBERS. THIS PROVIDED A CHALLENGE FOR THE LATE PERIOD
WPC FORECAST BLEND. RELIED HEAVILY ON THE ENSEMBLE MEANS FOR THE
DAY 6 AND 7 FORECAST, GIVEN UNCERTAINTY, BUT DID INCLUDE MINOR
AMOUNTS OF THE ECMWF AND CMC (WHICH WERE CLOSEST IN PLACEMENT) FOR
SOME ADDED DEFINITION. THIS ALSO STICKS CLOSE TO PREVIOUS WPC
CONTINUITY, WHICH IS A SAFE PLACE TO BE AT THIS POINT.
..WEATHER/HAZARDS HIGHLIGHTS
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. THE AMPLIFYING TROUGH OVER THE WEST BY MID WEEK
SHOULD BRING SOME PRECIPITATION TO THE PACIFIC NORTHWEST/NORTHERN
ROCKIES. 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 OF MOISTURE FROM THE WESTERN GULF DURING THIS
TIME, HEAVY RAINFALL MAY BECOME ORGANIZED, PARTICULARLY AROUND
MID-WEEK. A MARGINAL RISK AREA MAINTAINED FOR PARTS OF KANSAS AND
OKLAHOMA FOR THE DAY 4/TUESDAY EXCESSIVE RAINFALL OUTLOOK.
RAINFALL SHOULD SPREAD NORTHWARD AS THE ASSOCIATED FRONT MOVES
TOWARDS THE REGION. MOISTURE AND INSTABILITY ARE FAVORABLE AHEAD
OF THIS, BUT STILL PLENTY OF UNCERTAINTY ON HOW ORGANIZED RAINFALL
MAY BE AND PLACEMENT OF HIGHEST AMOUNTS. ADDED A MARGINAL RISK
FROM EASTERN OK/KS NORTHWARD INTO THE UPPER MISSISSIPPI VALLEY FOR
THE DAY 5/WEDNESDAY ERO.
AN AREA OF HIGH TEMPERATURES GENERALLY 10-15F ABOVE NORMAL WILL
SHIFT FROM THE NORTHERN PLAINS ON TUESDAY 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
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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