249
FXUS02 KWBC 261847
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
247 PM EDT THU SEP 26 2024
VALID 12Z SUN SEP 29 2024 - 12Z THU OCT 03 2024
19Z UPDATE: THE 12Z MODEL GUIDANCE SUITE MAINTAINS GOOD OVERALL
SYNOPTIC SCALE AGREEMENT ACROSS THE CONTINENTAL U.S. ON SUNDAY AND
INTO MONDAY. THE GUIDANCE STILL SHOWS THE UPPER TROUGH ARRIVING
OVER THE GREAT LAKES BY TUESDAY, WITH THE CMC DEVELOPING A STRONGER
TROUGH/CLOSED LOW OVER SOUTHERN ONTARIO BY WEDNESDAY COMPARED TO
THE OTHER GUIDANCE. THAT TRENDS ALSO CONTINUES GOING INTO THURSDAY
WITH THE CMC INDICATING A STRONGER LOW SOLUTION. TIMING AND
EVOLUTION BECOMES LESS CLEAR WITH THE NEXT TROUGH BEGINNING TO
DEVELOP OVER THE NORTHWESTERN U.S. LATE IN THE FORECAST PERIOD
MIDWEEK, SO THE ENSEMBLE MEANS WERE GRADUALLY INCREASED TO ABOUT
40% BY NEXT WEDNESDAY/THURSDAY, WHILE STILL MAINTAINING GOOD
OVERALL CONTINUITY FROM THE PREVIOUS FORECAST. THE PREVIOUS
FORECAST DISCUSSION IS APPENDED BELOW FOR REFERENCE. /HAMRICK
-------------------
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST GUIDANCE IS IN BROAD AGREEMENT THIS WEEKEND. A BLOCKY SPLIT
FLOW PATTERN OVER MUCH OF THE COUNTRY SHOULD BE HIGHLIGHTED ALOFT
BY A WARMING ROCKIES/SOUTHWEST RIDGE, A LOW/TROUGH OUT FROM THE
CANADIAN MARITIMES, AND A CUTOFF LOW MEANDERING OUT FROM THE MID-
SOUTH/OHIO VALLEY STATES. MOIST POST TROPICAL CYCLONE HELENE
WEAKENS INTO SUNDAY/MONDAY AS PER NHC NEAR TENNESSEE/KENTUCKY AS
ENTRAINED INTO THE CUTOFF LOW. SLOW FEATURE TRANSLATION SHOULD SUPPORT
A PESKY RAINFALL/RUNOFF THREAT IN PROXIMITY OF THE LOW AND
EASTWARD WITHIN A LINGERING MOISTURE FEED INTO SOIL DRENCHED
SOUTH-CENTRAL APPALACHIANS TERRAIN.
FORECAST SPREAD INCREASED SIGNIFICANTLY NEXT WEEK WITHIN A MORE
PROGRESSIVE AND ENERGETIC NORTHERN STREAM FLOW FROM THE NORTHEAST
PACIFIC THROUGH THE NORTHWEST U.S. AND DOWNSTREAM. GUIDANCE VARIES
WITH OVERALL TIMING AND IN PARTICULAR WITH THE EXTENT OF POTENTIAL
SOUTHEASTWARD DIGGING/AMPLITUDE OF UPPER TROUGH ENERGIES DOWNSTREAM
ACROSS THE CENTRAL AND EASTERN U.S.. THE PATTERN WAS INCREASINGLY
OUT OF SYNC AND UNCERTAIN NEXT WEEK, BUT NEWER 00 UTC MODEL GUIDANCE
HAS CONVERGED TO A DEGREE WITH CLOSER TO MIDDLE OF THE ROAD
AMPLITUDE IN LINE WITH AN ENSEMBLE COMPOSITE. GIVEN LINGERING
UNCERTAINTIES, PREFER AN ENSEMBLE MEANS AND NBM BASED FORECAST
APPROACH THROUGH MEDIUM RANGE TIME SCALES THAT MAINTAINS REASONABLY
GOOD WPC CONTINUITY AND FITS WELL WITH LATEST NHC HELENE GUIDANCE.
..WEATHER/HAZARDS HIGHLIGHTS
MOISTURE FROM WHAT WILL BE THE REMNANTS OF HELENE, IN COMBINATION
WITH A LINGERING UPPER LOW, WILL RESULT IN CONTINUED SHOWERS AND
SOME THUNDERSTORMS FROM CENTRAL KENTUCKY/TENNESSEE EASTWARD TO THE
CENTRAL APPALACHIANS AND MID-ATLANTIC REGION INTO MONDAY, ALBEIT
MUCH LIGHTER IN INTENSITY COMPARED TO ONGOING CURRENT RAINFALL.
THE MARGINAL RISK AREA FOR THE DAY 4 PERIOD WAS EXPANDED SOME TO
INCLUDE MORE OF VIRGINIA, MARYLAND, AND SOUTHERN PENNSYLVANIA GIVEN
A STRONGER MODEL QPF SIGNAL. THERE WAS ALSO AN EXTENSION NORTHWARD
INTO CENTRAL PA FOR DAY 5 WITH THE EXISTING MARGINAL RISK AREA,
WHILE MAINTAINING THE OUTLOOK AREA OVER THE CENTRAL APPALACHIANS AS
THE REMNANT LOW IS SLOW TO MOVE OUT OF THE REGION. EVEN THOUGH
HEAVY RAINFALL DURING THIS PERIOD SHOULD NOT BE AS WIDESPREAD, THE
EXTREMELY SATURATED SOILS WILL BE AN AGGRAVATING FACTOR IN TERMS OF
THE FLOODING POTENTIAL.
TEMPERATURES ARE EXPECTED TO REMAIN WARM ACROSS MUCH OF THE WESTERN
U.S. AND INTO THE NORTH-CENTRAL U.S. FOR THE UPCOMING WEEKEND, WITH
HIGHS RUNNING UP TO 15-25 DEGREES ABOVE AVERAGE FOR PORTIONS OF THE
DAKOTAS AND INTO EASTERN WYOMING. EXCESSIVE HEAT WILL ONCE AGAIN
MAKE WEATHER HEADLINES FOR THE LOWER ELEVATIONS OF THE DESERT
SOUTHWEST WITH HIGHS APPROACHING 110 DEGREES FOR THE NORMALLY
HOTTER LOCATIONS OF SOUTHEAST CALIFORNIA AND SOUTHERN ARIZONA. MOST
OF THE EASTERN U.S. SHOULD BE NEAR AVERAGE FOR HIGHS, AND ABOVE
AVERAGE FOR OVERNIGHT LOWS OWING GREATER CLOUD COVER AND HUMIDITY
THROUGH EARLY IN THE WEEK.
HAMRICK/SCHICHTEL
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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