665
FXUS02 KWBC 290659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT THU AUG 29 2024
VALID 12Z SUN SEP 01 2024 - 12Z THU SEP 05 2024
..OVERVIEW
AN UPPER TROUGH CROSSING THE GREAT LAKES/NORTHEAST AROUND THE START
OF THE WEEK WILL GIVE WAY TO RISING HEIGHTS THEREAFTER AS A PORTION
OF INITIAL ROCKIES RIDGING DRIFTS INTO THE PLAINS/MIDWEST. THE
REMAINDER OF THE RIDGE SHOULD SETTLE INTO THE SOUTHWEST AS AN
EASTERN PACIFIC UPPER LOW TRACKS THROUGH THE NORTHWESTERN STATES.
THERE HAS BEEN A LOT OF SPREAD FOR DETAILS OF INCOMING PACIFIC FLOW
AND ITS POSSIBLE INTERACTION WITH EJECTING UPPER LOW ENERGY AFTER
TUESDAY, BUT THE MOST COMMON THEME IS FOR SOME DEGREE OF RIDGING TO
REBUILD OVER THE NORTHWEST U.S. INTO WESTERN CANADA. ACROSS THE
SOUTHERN TIER OF THE LOWER 48 FROM THE HIGH PLAINS EASTWARD,
GUIDANCE STILL DIFFERS IN RESOLVING LOWER-PREDICTABILITY SHORTWAVE
FEATURES THAT WILL INFLUENCE RAINFALL COVERAGE COVERAGE AND
AMOUNTS, WITH NORTHERN STREAM FLOW UNCERTAINTIES POSSIBLY PLAYING A
ROLE AS WELL. AT THE VERY LEAST, A COUPLE FRONTS SETTLING INTO THE
SOUTH AND A PERSISTENT SURFACE TROUGH/EMBEDDED WAVES NEAR THE
WESTERN GULF COAST SHOULD PROVIDE A FOCUS FOR SOME OF THE ACTIVITY.
AN AREA OF WELL ABOVE NORMAL TEMPERATURES SHOULD PROGRESS FROM THE
NORTHWEST INTO NORTHERN HIGH PLAINS SUNDAY- TUESDAY FOLLOWED BY A
REBOUND IN TEMPERATURES OVER THE WEST COAST STATES BY WEDNESDAY-
THURSDAY. MODERATELY BELOW NORMAL DAYTIME HIGHS WILL EXPAND FROM
THE PLAINS TOWARD THE EAST COAST EARLY IN THE WEEK, FOLLOWED BY
MODERATION OVER MOST AREAS EXCEPT FOR THE SOUTHEAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
FORECAST ISSUES CAN BE MOST BROADLY BROKEN DOWN INTO TWO ASPECTS,
ONE BEING UNCERTAIN SPECIFICS OF INCOMING PACIFIC FLOW BY
MIDWEEK/POTENTIAL INTERACTION WITH EJECTING PACIFIC-NORTHWEST U.S.
UPPER LOW ENERGY, AND POSSIBLE EVOLUTION/PROGRESSION OF SOUTHERN
PLAINS ENERGY AND RELATIVE DOMINANCE OF NORTHERN STREAM FLOW (WITH
SOME DEPENDENCE ON DEVELOPING SPREAD TO THE NORTH).
REGARDING THE PACIFIC/NORTHERN TIER PART OF THE FORECAST, DYNAMICAL
AND MACHINE LEARNING (ML) MODELS CONTINUE TO SHOW SIGNIFICANT
DISAGREEMENTS AMONG EACH OTHER AND IN CONSECUTIVE RUNS. ENSEMBLE
SPAGHETTI PLOTS DEVELOP A LOT OF SPREAD AS WELL. STRAY RUNS LIKE
THE 18Z GFS AND OLD 00Z/28 ECMWF WERE ON THE EXTREME SIDE OF THE
SPREAD FOR HOW MUCH INCOMING FLOW AMPLIFIES INTO THE NORTHERN TIER
BEHIND THE EJECTING NORTHWEST UPPER LOW. AT LEAST FOR THE MOMENT
THERE IS RELATIVELY GREATER CLUSTERING TOWARD THE IDEA OF LESS
NORTHERN STREAM INFLUENCE ON THE NORTHWEST UPPER LOW, ULTIMATELY
CORRESPONDING TO THE FEATURE REACHING THE NORTHERN PLAINS/NORTH-
CENTRAL ROCKIES BY AROUND NEXT THURSDAY. THE 12Z CMC WAS A SLOW
EXTREME IN LEAVING THE ENERGY STUCK OVER THE NORTHWEST. IN
ADDITION, A DEVELOPING FORECAST PROBLEM UPSTREAM BY THURSDAY
INVOLVES WHETHER PACIFIC TROUGH ENERGY SPLITS OR REMAINS PHASED.
AS FOR THE SOUTHERN TIER, THERE IS A CONTINUED SIGNAL FOR SOME
AMOUNT OF ENERGY ALOFT TO EVOLVE OVER OR NEAR THE SOUTHERN PLAINS
BUT IN A VARIETY OF WAYS AND DIFFERING DEGREES OF PROGRESSION. AN
ADDITIONAL COMPLICATION IS THAT DIFFERENT DYNAMICAL AND ML MODELS
VARY IN HOW MUCH NORTHERN STREAM INFLUENCE THERE MAY BE. BY LATE
IN THE PERIOD THIS LEADS TO A RANGE OF SOLUTIONS FROM A PROGRESSIVE
SOUTHERN TIER SHORTWAVE THAT BRINGS A LOT OF MOISTURE THROUGH THE
SOUTHEASTERN QUADRANT OF THE LOWER 48 AND PERHAPS EXTENDING EVEN A
LITTLE NORTHWARD (00Z CMC/ICON, 18Z GFS) TO GREATER NORTHERN
STREAM DOMINANCE OVER THE EAST AND A MUCH MORE SUPPRESSED RAINFALL
PATTERN (LATEST ECMWF RUNS AND 00Z GFS). ML MODELS DO NOT FAVOR THE
NORTHWARD EXTREME OF RAINFALL COVERAGE BUT OTHERWISE DO NOT FALL
INTO A PARTICULAR SIDE OF THE ENVELOPE. ENSEMBLE MEANS/BLENDED
SOLUTIONS WITH MAX AMOUNTS TEMPERED IN THE NORTHERN PART OF THE
RAINFALL SHIELD SEEM TO PROVIDE A REASONABLE ACCOUNT FOR THE HIGH
UNCERTAINTY AT THIS TIME.
THE LATEST FORECAST STARTED WITH A 12Z/18Z OPERATIONAL MODEL
COMPOSITE EARLY IN THE PERIOD. WITH THE 18Z GFS BECOMING MORE
UNFAVORABLE MID-LATE PERIOD, THE GFS COMPONENT SWITCHED TO THE 12Z
RUN. ALSO 18Z GEFS/12Z ECENS INPUT WAS INTRODUCED FOR THE LATTER
HALF OF THE PERIOD, REACHING HALF WEIGHT BY NEXT THURSDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS COVERING THE SUNDAY
THROUGH MONDAY NIGHT PERIOD FOCUS ON THREE PRIMARY REGIONS OF
INTEREST, ALL WITH MARGINAL RISK AREAS BOTH DAYS. ONE IS OVER THE
SOUTHERN MID-ATLANTIC WEST INTO THE APPALACHIANS WHERE A COUPLE OF
FRONTS WILL BE SAGGING SOUTHWARD OVER THE COURSE OF THE PERIOD,
WITH ASSOCIATED INSTABILITY AND MODERATELY ANOMALOUS MOISTURE.
MEANWHILE RAINFALL SHOULD CONTINUE ALONG PARTS OF THE
LOUISIANA/TEXAS COAST, WITH PRIOR DAYS OF RAINFALL PROVIDING WET
GROUND CONDITIONS HEADING INTO THE DAYS 4-5 PERIOD. THERE IS STILL
UNCERTAINTY IN THE COVERAGE OF SHOWERS AND THUNDERSTORMS OVER THE
SOUTHERN HIGH PLAINS AND VICINITY BUT STILL ENOUGH OF A SIGNAL TO
FAVOR MAINTENANCE OF THE MARGINAL RISK. AT THE MOMENT THERE SEEMS
TO BE A LITTLE BETTER AGREEMENT ON DAY 5 RELATIVE TO DAY 4, WITH
BETTER CONSENSUS WITH RESPECT TO ANOMALOUS MOISTURE.
PRECIPITATION COVERAGE AND TOTALS ACROSS THE SOUTHERN TIER AFTER
EARLY TUESDAY REMAIN VERY UNCERTAIN DUE TO SIGNIFICANT DIFFERENCES
IN UPPER FLOW DETAILS. SOME AREAS OF LOCALLY HEAVY RAINFALL APPEAR
LIKELY GIVEN THE FRONTS SETTLING OVER THE SOUTH HELPING TO SERVE AS
A FOCUS, BUT WITH VERY LOW CONFIDENCE IN RESOLVING THE DETAILS OF
COVERAGE AND TOTALS. EXPECT THE NORTHWEST TO SEE SCATTERED
RAINFALL EARLY NEXT WEEK AS THE EASTERN PACIFIC UPPER LOW EJECTS
INLAND. SOME SCATTERED RAINFALL COULD REACH THE NORTHERN PLAINS BY
MIDWEEK OR SO.
UPPER RIDGING WILL SUPPORT AN AREA OF WELL ABOVE NORMAL
TEMPERATURES DRIFTING FROM THE NORTHWEST INTO THE NORTHERN HIGH
PLAINS DURING SUNDAY-TUESDAY, WITH SOME LOCATIONS SEEING HIGHS
10-15F OR SO ABOVE NORMAL. THIS WARM AIR SHOULD QUICKLY MODERATE
FARTHER EASTWARD. A RETURN OF UPPER RIDGING OVER THE WEST COAST
STATES BY WEDNESDAY-THURSDAY SHOULD PROMOTE ABOVE NORMAL
TEMPERATURES WITH ANOMALIES FOR HIGHS AGAIN REACHING PLUS 10-15F AT
SOME LOCATIONS. BELOW NORMAL HIGHS OVER THE CENTRAL U.S. ON SUNDAY
WILL EXPAND TOWARD THE EAST COAST EARLY NEXT WEEK, SUPPRESSING WHAT
HEAT LINGERS OVER THE SOUTHEAST TO START THE WEEK. MOST OF THE
CENTRAL/EASTERN U.S. SHOULD TREND CLOSER TO NORMAL BY NEXT THURSDAY
BUT THE SOUTHERN MID-ATLANTIC/SOUTHEAST MAY REMAIN BELOW NORMAL FOR
HIGHS.
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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