421
FXUS02 KWBC 040620
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
220 AM EDT WED SEP 4 2024
VALID 12Z SAT SEP 07 2024 - 12Z WED SEP 11 2024
...HEAVY RAINFALL THREATS TO IMPACT NORTHERN FLORIDA AND THE
NORTHEAST ON SATURDAY...
..OVERVIEW
AN AMPLIFIED UPPER-LEVEL PATTERN WILL BE IN PLACE BY THE WEEKEND
WITH AN UPPER RIDGE OVER THE WEST THAT SHIFTS EAST WITH TIME, AND
AN UPPER LOW SLOWLY MOVING THROUGH THE NORTHEAST. THIS WILL SUPPORT
A PERIOD OF ENHANCED RAINFALL OVER PORTIONS OF NEW ENGLAND THIS
WEEKEND, WITH A FALL- LIKE DRY AND COOL AIR MASS FILTERING IN
BEHIND IT ACROSS MUCH OF THE EASTERN HALF OF THE COUNTRY. A
LINGERING FRONT WILL KEEP RAIN CHANCES FROM THE CENTRAL GULF COAST
EASTWARD TO FLORIDA, WITH THE BEST CHANCE FOR HEAVY RAIN ON
SATURDAY ACROSS NORTHERN FLORIDA. TROPICAL MOISTURE MOVING INTO THE
SOUTHWESTERN GULF COULD BRING HEAVY RAIN INTO SOUTHERN TEXAS BY
EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MAIN AREA OF UNCERTAINTY DURING THE PERIOD CONTINUES TO
SURROUND THE DEEP UPPER LOW/TROUGH AS IT SHIFTS OUT OF THE GREAT
LAKES. THE ECMWF CONTINUES TO BE THE SOUTHERN OUTLIER PULLING THE
UPPER LOW THROUGH THE NORTHEAST, WITH THE GFS AND CMC HAVING A MORE
NORTHWARD TRACK WITH THE SYSTEM THROUGH FAR EASTERN CANADA. THE
ENSEMBLE MEANS AND RECENT AI MODELS ALSO SUPPORTED THIS FARTHER
NORTH TRACK, WHICH PULLS THE RAINFALL OUT OF THE NORTHEAST QUICKER.
THERE ARE ALSO TIMING INCONSISTENCIES WITH THE NEXT AMPLIFIED
TROUGH IN THE EASTERN PACIFIC NEXT WEEK, WHICH GUIDANCE SHOWS
POTENTIAL FOR A CUTOFF LOW TO FORM OFF THE WEST COAST NEXT WEEK.
THE TIMING OF THIS, WHICH IS VERY UNCERTAIN AT THESE LATER RANGES,
WILL AFFECT THE OVERALL TIMING OF THE ARRIVAL OF THE LARGER SCALE
TROUGH INTO THE WEST.
THE FORECAST FOR TONIGHT USED A BLEND OF THE GFS, CMC, AND ECMWF
FOR DAYS 3 AND 4, BEGINNING TO INCORPORATE MINOR AMOUNTS OF THE
ENSEMBLE MEANS INTO THE BLEND BY DAY 5 AS UNCERTAINTY WITH THE
NORTHEAST LOW INCREASED. BY DAY 7, THE BLEND CONSISTED OF 70
PERCENT OF THE ENSEMBLE MEANS WHICH SHOWED GOOD CONSISTENCY WITH
EACH OTHER IN AN ATTEMPT TO MITIGATE DIFFERENCES AMONGST THE
DETERMINISTIC SOLUTIONS.
..WEATHER/HAZARDS HIGHLIGHTS
THE MAIN HEAVY RAINFALL FOCUS DURING THE PERIOD WILL BE ACROSS THE
NORTHEAST ON SATURDAY AS THE UPPER LOW/ASSOCIATED TROUGH PIVOTS
THROUGH THE NORTHEAST, AIDED BY WRAP BACK MOISTURE FROM A SURFACE
LOW ALONG THE COAST. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK FOR NOW
CONTINUES TO SHOW JUST A MARGINAL, BUT THE POTENTIAL EXISTS FOR AN
EVENTUAL UPGRADE TO A SLIGHT RISK IF MODELS COME INTO BETTER
AGREEMENT ON EXACTLY WHERE POCKETS OF HEAVIER RAIN WILL SET UP.
FARTHER SOUTH, A LINGERING FRONT ACROSS NORTHERN FLORIDA AND ALONG
THE GULF COAST WILL CONTINUE TO FOCUS SHOWERS AND STORMS, LOCALLY
HEAVY, THROUGH THE WEEKEND. THE BETTER CHANCE FOR HEAVIER QPF IS ON
SATURDAY, COVERED BY A MARGINAL RISK ON THE DAY 4 ERO. DEPENDING
ON WHAT FALLS WHERE DURING THE SHORT RANGE PERIOD, A MARGINAL RISK
MAY BE NEEDED ON SUNDAYS ERO AS WELL, BUT FOR NOW THE QPF DOES NOT
SUPPORT ANY RISK. FROM SUNDAY INTO EARLY NEXT WEEK, MUCH OF THE
COUNTRY SHOULD DRY OUT, THOUGH SHORTWAVE ENERGY WITHIN AND AROUND
WESTERN U.S. UPPER RIDGING MAY PRODUCE ISOLATED TO SCATTERED
RAINFALL WITH MOSTLY LIGHT AMOUNTS. MEANWHILE, TROPICAL MOISTURE
MOVING INTO THE SOUTHWESTERN GULF COULD BRING AN INCREASING THREAT
OF HEAVY RAIN INTO SOUTHERN TEXAS BY EARLY NEXT WEEK, AND RAINFALL
MAY MOVE BACK INTO THE NORTHWEST EARLY TO MID NEXT WEEK AHEAD OF
THE UPPER TROUGH.
HAZARDOUS HEAT ACROSS THE SOUTHWEST AND INTO CALIFORNIA DURING THE
SHORT RANGE PERIOD SHOULD BE WANING BY THIS WEEKEND AS THE UPPER
RIDGE BEGINS TO SHIFT OUT OF THE WEST. ABOVE NORMAL TEMPERATURES
WILL ALSO SLIDE EAST WITH TIME FROM THE WEST INTO THE CENTRAL U.S.,
HOWEVER IT DOES NOT LOOK TO BE PARTICULARLY HAZARDOUS AT THIS
TIME. IN CONTRAST, A MUCH COOLER AIRMASS WILL FILTER EASTWARD FROM
THE NORTHERN PLAINS AND GREAT LAKES INTO THE EAST THIS WEEKEND AND
EARLY NEXT WEEK, WITH DAYTIME HIGHS 10-20 DEGREES BELOW 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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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