329
FXUS02 KWBC 281851
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 PM EDT SAT SEP 28 2024
VALID 12Z TUE OCT 01 2024 - 12Z SAT OCT 05 2024
..OVERVIEW
MOST GUIDANCE SHOWS PROGRESSIVE MEAN FLOW ACROSS THE NORTHERN
LOWER 48 DURING THE PERIOD, PUSHING ALONG A SERIES OF FRONTAL
SYSTEMS. ONE SHOULD PUSH THROUGH THE EAST AROUND MIDWEEK WHILE
ANOTHER MOVES INTO THE NORTHWEST, WITH THE LATTER FRONT REACHING
THE CENTRAL AND EVENTUALLY EASTERN U.S. LATE IN THE WEEK. ANOTHER
FRONT MAY REACH THE NORTHWEST TOWARD THE END OF THE WEEK. THESE
FRONTS MAY PRODUCE MOSTLY LIGHT TO MODERATE RAINFALL OVER SOME
AREAS. SEPARATE UPPER RIDGES WILL PREVAIL OVER THE SOUTHERN HALF OF
THE WEST AND OVER/SURROUNDING FLORIDA, WITH THE FORMER IN
PARTICULAR PRODUCING WELL ABOVE NORMAL TEMPERATURES. THERE IS
SIGNIFICANT UNCERTAINTY OVER HOW A POTENTIAL WEAKNESS BETWEEN THE
TWO RIDGES MAY EVENTUALLY DRAW INCREASING MOISTURE NORTHWARD INTO
THE GULF COAST REGION, WITH SOME SIGNAL FOR EMBEDDED TROPICAL
DEVELOPMENT AS WELL.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE PERIOD BEGINS AS THE UPPER LOW THAT HAS BEEN LINGERING OVER
THE OHIO/TENNESSEE VALLEY THIS WEEKEND AND WAS ASSOCIATED WITH THE
REMNANTS OF HELENE SHEARS OUT AND MOVES EASTWARD AS AN UPPER WAVE
JUST OFFSHORE THE EAST COAST MID-NEXT WEEK, WITH A LINGERING
WEAKNESS TRAILING SOUTHWEST TOWARDS THE GULF COAST. THIS CONTINUES
SOUTH OF AMPLIFYING UPPER RIDGING OVER NEW ENGLAND AS A NORTHERN
STREAM UPPER TROUGH BEGINS TO DROP SOUTH OVER THE UPPER
MIDWEST/GREAT LAKES REGION. A COASTAL LOW THAT DEVELOPS WITH THE
UPPER WAVE MOVING OFF THE COAST HAS BEEN TRENDING MORE OFFSHORE IN
THE LATEST RUNS OF THE GUIDANCE, ESPECIALLY IN THE LAST FEW RUNS OF
THE GFS. THE GFS IS ALSO A BIT MORE AGGRESSIVE WITH THE STRENGTH
OF THE LOW, INCLUDING THE 12Z RUN, AND TAKES A MORE NORTHERLY PATH
COMPARED TO THE 00Z ECMWF. ENSEMBLES INDICATE A GENERAL CLUSTERING
JUST OFFSHORE AS WELL. THE MORE EASTERN SOLUTIONS OFFSHORE LEAD TO
LESS ONSHORE FLOW AHEAD OF THE UPPER-TROUGH AND EXPECTED
APPROACHING FRONTAL SYSTEM TO THE WEST AND RESULTS IN A SIGNIFICANT
DECREASE OF QPF OVER THE NORTHEASTERN U.S., WHICH IS SHARED BY THE
NBM. DIFFERENCES IN THE LOW POSITION/STRENGTH ALSO LEAD TO SPATIAL
DISPARITY IN QPF TOTALS, WITH HIGHER AMOUNTS INTO NEW ENGLAND IN
THE GFS AND THE MID-ATLANTIC IN THE ECMWF. OPTED TO KEEP A BIT OF
CONTINUITY IN THE FORECAST AS WELL AS BLEND IN SOME ECENS MEAN
GIVEN QPF FOOTPRINT DIFFERENCES TO AVOID A MORE DRASTIC CHANGE FROM
THE PRIOR FORECAST, THOUGH STILL INDICATE A DOWNWARD TREND.
UNCERTAINTY INCREASES BY LATE NEXT WEEK WITH THE TIMING OF
ADDITIONAL NORTHERN STREAM SHORTWAVES IN PROGRESSIVE FLOW ALONG THE
U.S./CANADIAN BORDER, AND WITH THE HANDLING AN UPPER-TROUGH OVER
THE NORTHEASTERN PACIFIC DROPPING INTO WESTERN CANADA AND THE
NORTHWESTERN U.S. THE GFS HAS TRENDED MORE AMPLIFIED, DIGGING
DEEPER INTO THE WESTERN U.S. AND SLOWER IN TIMING THROUGH THE 00Z
RUN, BEFORE SHIFTING FASTER AND BACK A BIT EASTWARD IN THE 06Z AND
12Z RUNS. TIMING AND DEPICTION OF A LOWER AMPLITUDE WAVE REMAINING
MORE IN LINE WITH THE U.S./CANADIAN BORDER IS MORE CONSISTENT IN
THE LAST FEW RUNS OF THE ECMWF, BUT STILL SHOWS VARYING DIFFERENCES
IN THE SOLUTIONS. THE 00Z ECENS AND GEFS MEANS OFFER A COMPROMISE
SOLUTION INDICATING A LESS-AMPLIFIED UPPER-TROUGH OVER THE
NORTHWESTERN U.S. WITH THE ECMWF THE MOST SIMILAR. THE MORE
AMPLIFIED GFS SOLUTION WOULD NOT SURPRISINGLY LEAD TO MORE
EXPANSIVE PRECIPITATION CHANCES INTO THE NORTHWESTERN U.S. BOTH THE
00Z CMC AS WELL AS THE CMC ENSEMBLE MEAN WERE OUT OF PHASE WITH
THE OTHER GUIDANCE DURING THIS TIMEFRAME.
THE POTENTIAL FOR TROPICAL DEVELOPMENT ALSO LEADS TO ANOTHER
REGION OF UNCERTAINTY IN QPF ALONG THE GULF COAST LATE NEXT WEEK
AND INTO NEXT WEEKEND. THE LATEST RUNS OF THE GUIDANCE SHOW
SIGNIFICANT DIFFERENCES IN THE TIMING AND PATH OF A POTENTIAL
TROPICAL SYSTEM. THE CMC AND GFS ARE MORE AGGRESSIVE IN THE
DEVELOPMENT OF A SYSTEM IN THE NORTHWEST CARIBBEAN THAT EVENTUALLY
MOVES INTO THE GULF OF MEXICO LATE NEXT WEEK/EARLY NEXT WEEKEND,
WITH THE CMC FASTER IN BRINGING THE SYSTEM UP TOWARDS THE GULF
COAST. THE NATIONAL HURRICANE CENTER NOW ANTICIPATES AN AREA OF LOW
PRESSURE LIFTING NORTH OUT OF THE NORTHWESTERN CARIBBEAN AND INTO
THE CENTRAL GULF BY NEXT SATURDAY. REGARDLESS OF THE EXACT NATURE
OF ANY DEVELOPMENT, INCREASING MOISTURE SHOULD LEAD TO RAINFALL
ACROSS THE REGION, BUT GUIDANCE SHOWS SIGNIFICANT DIFFERENCES, WITH
THE GFS MUCH HEAVIER COMPARED TO THE ECMWF. OPTED FOR A BLEND OF
THE NBM AND ENSEMBLE MEANS WHICH KEEPS AMOUNTS ON THE LESS
EXTENSIVE SIDE FOR NOW GIVEN THE DEGREE OF UNCERTAINTY AND HOW
HIGHER QPF AMOUNTS WOULD LIKELY BE TIED TO TROPICAL DEVELOPMENT,
BUT THIS WILL NEED TO BE CLOSELY MONITORED FOR BOTH HEAVY RAINFALL
POTENTIAL AND ANY OTHER ADDITIONAL IMPACTS.
THE UPDATED WPC FORECAST CONSISTS OF A COMPOSITE BLEND EARLY IN
THE PERIOD GIVEN GOOD CLUSTERING OF THE DETERMINISTIC GUIDANCE. AN
INCREASING CONTRIBUTION FROM THE ECENS AND GEFS MEANS IS INCLUDED
FOR THE LATTER HALF OF THE PERIOD GIVEN THE GREATER DIFFERENCES
WITH THE PROGRESSIVE FLOW IN THE NORTHERN STREAM AND THE EVENTUAL
UPPER-TROUGH APPROACHING SOUTHWEST CANADA AND THE NORTHWESTERN U.S.
THE DETERMINISTIC 00Z ECMWF IS EMPHASIZED DURING THIS PERIOD GIVEN
HOW IT BEST FALLS WITHIN THE ENVELOPE OF THE TWO MEAN SOLUTIONS.
THE GFS CONTRIBUTION VARIES IN THE MID- TO LATE PERIOD, WITH A
PREFERENCE FOR THE 00Z RUN INITIALLY GIVEN SIMILAR UPPER WAVE
TIMING ALONG THE U.S./CANADIAN BORDER, AND THEN A PREFERENCE FOR
THE 06Z RUN AT THE END OF THE PERIOD DUE TO A CLOSER SOLUTION TO
THE ECMWF/ENSEMBLE MEANS WITH RESPECT TO THE NORTHEASTERN PACIFIC
UPPER-TROUGH COMING INTO THE NORTHWESTERN U.S.
..WEATHER/HAZARDS HIGHLIGHTS
FORECASTS HAVE BEEN SHOWING RAINFALL PROGRESSING FROM THE GREAT
LAKES/APPALACHIANS/MID-ATLANTIC INTO NEW ENGLAND FROM TUESDAY
THROUGH MIDWEEK, WITH VARIOUS IDEAS FOR COVERAGE AND AMOUNTS
DEPENDING ON THE TRACK OF WESTERN ATLANTIC LOW PRESSURE AND HOW
MUCH INTERACTION OCCURS WITH AN APPROACHING FRONT. 00Z MODEL
GUIDANCE HAS TRENDED TOWARD MORE SEPARATION OF THESE TWO FEATURES,
LEADING TO LOWER RAINFALL TOTALS. THUS THE EXCESSIVE RAINFALL
OUTLOOKS FOR DAYS 4-5/TUESDAY-WEDNESDAY DEPICT NO RISK AREAS.
EXPECT RAINFALL TO INCREASE ACROSS THE GULF COAST REGION DURING THE
LATTER HALF OF NEXT WEEK, BUT WITH LOW CONFIDENCE REGARDING THE
TIMING, COVERAGE, AND AMOUNTS DUE TO ITS CONNECTION TO POSSIBLE
TROPICAL DEVELOPMENT OVER THE GULF OF MEXICO. MONITOR FORECASTS
OVER THE COMING DAYS FOR BETTER RESOLUTION OF THE RAINFALL/TROPICAL
FORECAST ACROSS THIS REGION. A FRONTAL SYSTEM MAY BRING LIGHT TO
LOCALLY MODERATE RAINFALL TO THE PACIFIC NORTHWEST BY FRIDAY AS
WELL, WITH SOME LOW PRECIPITATION CHANCES SPREADING INTO THE
NORTHERN ROCKIES BY SATURDAY.
TEMPERATURES WILL REMAIN QUITE WARM TO HOT ACROSS THE SOUTHERN
HALF OF THE WEST NEXT WEEK WITH FAIRLY BROAD COVERAGE OF HIGHS
10-15F ABOVE AVERAGE. RECORD HIGHS SHOULD BE MOST NUMEROUS THROUGH
WEDNESDAY BUT THERE MAY STILL BE A FEW LINGERING INTO LATE WEEK.
PORTIONS OF THE PLAINS WILL ALSO BE ABOVE NORMAL ONE OR MORE DAYS
FROM WEDNESDAY ONWARD IN THE WARM SECTOR AHEAD OF A FRONT. THE
NORTHWEST WILL TEND TO BE CLOSER TO NORMAL. TEMPERATURES OVER THE
EAST WILL VARY WITH FRONTAL PROGRESSION, WITH HIGHS AVERAGING OUT
TO NEAR NORMAL AND MORNING LOWS AVERAGING ABOVE NORMAL.
PUTNAM/RAUSCH
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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