198
FXUS02 KWBC 271848
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
248 PM EDT FRI SEP 27 2024
VALID 12Z MON SEP 30 2024 - 12Z FRI OCT 04 2024
..OVERVIEW
A BROAD UPPER TROUGH SWINGING ACROSS THE PLAINS EARLY NEXT WEEK
WILL FINALLY DISLODGE THE PERSISTENT EASTERN U.S. UPPER TROUGH/LOW
RESPONSIBLE FOR MULTIPLE DAYS OF RAINFALL ENHANCED BY HELENE'S
MOISTURE. THE NEW PATTERN WILL FAVOR PROGRESSIVE MEAN FLOW ACROSS
THE NORTHERN TIER, PUSHING ALONG A COUPLE FRONTAL SYSTEMS, AND
SOUTHERN TIER RIDGING (THOUGH WITH POSSIBLE BREAKS IN PLACES). THE
MID-ATLANTIC/NORTHEAST WILL GRADUALLY TREND DRIER IN THE LATER HALF
OF NEXT WEEK WHILE PRECIPITATION CHANCES TREND UP IN FLORIDA AND
ALONG THE GULF COAST. FRONTS ASSOCIATED WITH NORTHEASTERN PACIFIC
SYSTEMS COULD BRING LIGHT RAIN TO THE NORTHERN PACIFIC NORTHWEST.
EXPECT THE MOST NOTABLE TEMPERATURE ANOMALIES NEXT WEEK TO BE ON
THE WARM SIDE OF NORMAL OVER THE SOUTHERN HALF OF THE WEST ON A
CONSISTENT BASIS AND MORE BRIEFLY OVER PARTS OF THE CENTRAL U.S.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE IS IN GOOD AGREEMENT MONDAY THROUGH WEDNESDAY, WITH
AN UPPER TROUGH SWINGING ACROSS THE PLAINS. THIS PATTERN WILL PUSH
A COLD FRONT ACROSS THE CENTRAL U.S. AND INTO THE EAST WHILE A
WAVE EVOLVES AHEAD OF IT ALONG A FRONT LINGERING NEAR THE EAST
COAST. A STORM TRACKING TOWARD THE ALASKA PANHANDLE SHOULD BRING
ANOTHER FRONT INTO THE NORTHWEST LATE TUESDAY OR EARLY WEDNESDAY.
MODEL SOLUTIONS GRADUALLY DIVERGE NEXT THURSDAY AND FRIDAY AS THE
MODELS STRUGGLE WITH THE EVOLUTION OF THE UPPER TROUGH AS IT MOVES
OVER THE EAST AND WITH AN UPPER LOW/TROUGH MOVING FROM THE GULF OF
ALASKA DOWN THE CANADIAN WEST COAST. THE CMC SEEMED TO FALL THE
FURTHEST FROM THE GENERAL MODEL CONSENSUS/MEANS, AND IS THE ONLY
MODEL TAKING THE WESTERN LOW/TROUGH SOUTHWEST INTO THE PACIFIC
INSTEAD OF DOWN THE CANADIAN COAST, WHICH IMPACTS THE DOWNSTREAM
EVOLUTION OF THE UPPER LEVEL PATTERN. THE GFS AND THE ECMWF
DYNAMICAL AND AI MODELS SEEM TO FALL MORE IN LINE WITH THE
CONSENSUS WITH THE LOW/TROUGH ENTERING THE NORTHWEST ON THURSDAY,
UPPER RIDGING OVER THE INTERMOUNTAIN WEST, AND WEAK TROUGHING OVER
THE CENTRAL/EASTERN U.S.
LATE IN THE PERIOD, GUIDANCE ALSO DIFFERS REGARDING SIGNALS FOR
TROPICAL DEVELOPMENT IN THE NORTHWESTERN CARIBBEAN/GULF OF MEXICO,
WITH GFS/CMC RUNS SHOWING GREATER POTENTIAL THAN OTHER GUIDANCE.
REMAINING SOLUTIONS THAT HAVE SOME INDICATION OF LOW PRESSURE OVER
THE REGION SUGGEST IT MAY BE WEAKER AND OVER THE SOUTHWESTERN GULF
BY NEXT THURSDAY/FRIDAY.
FOR THE WPC AFTERNOON FORECAST, A BLEND OF THE AVAILABLE
DETERMINISTIC MODELS WAS USED FOR MONDAY/TUESDAY, WITH MORE WEIGHT
PLACED ON THE GFS AND ECMWF THAN THE CMC AND UKMET. ENSEMBLE MEANS
FROM THE GEFS AND ECENS WERE ADDED IN INCREASING AMOUNTS FOR
WEDNESDAY THROUGH FRIDAY WHILE THE CMC AND UKMET WERE GRADUALLY
PHASED OUT OF THE BLEND.
..WEATHER/HAZARDS HIGHLIGHTS
THE LINGERING UPPER LOW OVER THE EAST AND ACCOMPANYING MOISTURE
FROM HELENE WILL CONTINUE TO PRODUCE SOME SHOWERS AND THUNDERSTORMS
OVER PORTIONS OF THE CENTRAL APPALACHIANS AND MID-ATLANTIC INTO
MONDAY, THOUGH MUCH LIGHTER IN INTENSITY COMPARED TO THE
ONGOING/SHORT-TERM RAINFALL. THERE IS A MARGINAL RISK AREA DEPICTED
ON THE DAY 4/MONDAY EXCESSIVE RAINFALL OUTLOOK (ERO) THAT INCLUDES
PORTIONS OF PENNSYLVANIA, MARYLAND, WEST VIRGINIA, VIRGINIA, AND
DC. HEAVY RAINFALL DURING THIS PERIOD SHOULD NOT BE AS WIDESPREAD
BUT WET CONDITIONS FROM PRIOR DAYS OF RAINFALL MAY BE AN
AGGRAVATING FACTOR IN TERMS OF THE FLOODING POTENTIAL.
BY DAY 5/TUESDAY, THE GUIDANCE DEVELOPS A MORE SPREAD FOR FINER
DETAILS AT THE SURFACE AND ALOFT, LEADING TO A MORE DIFFUSE SIGNAL
FOR WHERE ANY POCKETS OF ENHANCED RAINFALL MAY EXIST. THEREFORE,
THE DAY 5 ERO DEPICTS NO RISK AREAS AT THIS TIME. BEYOND TUESDAY,
SOME RAIN MAY CONTINUE NORTHEASTWARD FROM THE APPALACHIANS/MID-
ATLANTIC BUT WITH LOW CONFIDENCE IN THE COVERAGE AND AMOUNTS.
ELSEWHERE, FLORIDA AND EVENTUALLY PARTS OF THE GULF COAST FARTHER
TO THE WEST MAY SEE SOME RAINFALL AT TIMES. NORTHERN PARTS OF THE
PACIFIC NORTHWEST MAY SEE A COUPLE EPISODES OF LIGHT RAIN, ONE WITH
A FRONT REACHING THE REGION ON TUESDAY AND ANOTHER AROUND FRIDAY.
TEMPERATURES WILL REMAIN QUITE WARM TO HOT ACROSS THE SOUTHERN HALF
OF THE WEST NEXT WEEK, ESPECIALLY MONDAY THROUGH WEDNESDAY WHEN
HIGH TEMPERATURES ARE FORECAST TO BE 10-15 DEGREES ABOVE AVERAGE,
WITH NUMEROUS DAILY RECORDS POSSIBLE. MORE TRANSIENT ANOMALOUS
WARMTH WILL BE LIKELY OVER PARTS OF THE CENTRAL U.S. AHEAD OF COLD
FRONTS, INCLUDING AREAS FROM THE UPPER MIDWEST SOUTHWESTWARD ON
MONDAY AND IN PLAINS ON WEDNESDAY (AND LINGERING TO SOME DEGREE
INTO THURSDAY OVER SOUTHERN AREAS). THE NORTHWEST AND NORTHERN HIGH
PLAINS WILL TEND TO BE NEAR TO BELOW NORMAL. THE EAST WILL TEND TO
SEE HIGHS VARY SOMEWHAT BUT WILL GENERALLY BE NEAR NORMAL.
RAUSCH/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 (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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