161
FXUS02 KWBC 280659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT WED AUG 28 2024
VALID 12Z SAT AUG 31 2024 - 12Z WED SEP 04 2024
..OVERVIEW
MODELS AND ENSEMBLES SHOW MODERATELY PROGRESSIVE NORTHERN STREAM
FLOW, WITH A PORTION OF AN INITIAL WESTERN U.S. RIDGE EVENTUALLY
REACHING THE NORTHERN PLAINS WHILE REINFORCING TROUGH ENERGY
DOWNSTREAM CROSSES THE GREAT LAKES/NORTHEAST, WITH ITS DEPARTURE
NEXT WEEK YIELDING WEAKER TROUGHING BY THE MIDDLE OF NEXT WEEK.
REMAINING WESTERN U.S. RIDGING WILL LIKELY SETTLE INTO THE
SOUTHERN HALF OF THE REGION BY THAT TIME. AN EASTERN PACIFIC UPPER
LOW SHOULD EJECT ACROSS THE NORTHWEST EARLY NEXT WEEK, BUT WITH
INCREASING UNCERTAINTY OVER DETAILS OF INCOMING PACIFIC FLOW THAT
COULD INTERACT WITH IT. MEANWHILE, GUIDANCE HAS BEEN SHOWING A LOT
OF DIFFICULTY IN RESOLVING LOWER-PREDICTABILITY SHORTWAVE FEATURES
FROM THE SOUTHERN ROCKIES EASTWARD, LEADING TO SIGNIFICANT
DIFFERENCES IN FORECAST RAINFALL ACROSS THE SOUTHERN HALF OF THE
COUNTRY. THE FORECAST PATTERN EVOLUTION WILL SUPPORT SOME WELL
ABOVE NORMAL TEMPERATURES CROSSING THE NORTHWEST WHILE A DECENT
PORTION OF THE CENTRAL/EASTERN U.S. WILL SEE MULTIPLE DAYS WITH
BELOW NORMAL HIGHS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
OVERALL THE DYNAMICAL AND MACHINE LEARNING (ML) GUIDANCE HAS
MAINTAINED GOOD AGREEMENT AND CONTINUITY FOR MOST ASPECTS OF THE
FORECAST INTO THE WEEKEND. THEN THERE IS INCREASING SPREAD AND
VARIABILITY FOR SOME ASPECTS OF THE FORECAST--LEADING TO
SIGNIFICANT DIFFERENCES IN RAINFALL COVERAGE/AMOUNTS ACROSS THE
SOUTHERN HALF OF THE LOWER 48.
IN PARTICULAR, THE DYNAMICAL AND ML MODELS DISAGREE AMONG EACH
OTHER FOR LOWER-PREDICTABILITY SHORTWAVE DETAILS BETWEEN THE
SOUTHERN ROCKIES AND EAST-CENTRAL U.S. LEADING TO SIGNIFICANT QPF
DIFFERENCES. FOR EXAMPLE RECENT ECMWF RUNS HAVE BEEN FAVORING
SOUTHERN HIGH PLAINS UPPER WEAKNESS THAT EVENTUALLY DRIFTS
EASTWARD, LEADING TO GREATER RAINFALL OVER TEXAS THAN A NUMBER OF
OTHER SOLUTIONS, WHILE THE 00Z GFS DROPS WEAK NORTHERN STREAM
ENERGY INTO THE SOUTHERN PLAINS TO SUPPRESS RAINFALL MORE THAN THE
MEAN OF GUIDANCE. ML MODELS DIFFER AS WELL BUT OVERALL SEEM TO SHOW
A GENERAL THEME OF AN UPPER WEAKNESS REACHING SOMEWHERE OVER THE
PLAINS/MISSISSIPPI VALLEY BY NEXT WEDNESDAY. IN CONTRAST, THE NEW
00Z ECMWF HAS TRENDED FASTER WITH ITS EVOLVING WEAK SHORTWAVE SO
THAT BY MIDWEEK ITS QPF OVER THE EASTERN HALF OF THE COUNTRY IS
MORE SUPPRESSED THAN PRIOR RUNS. THERE ARE ALSO MEANINGFUL
DIFFERENCES WITH A POTENTIAL UPPER FEATURE NEAR THE WEST-CENTRAL
GULF COAST, WITH NO COHERENT THEMES ASIDE FROM A SOMEWHAT BETTER
DEFINED SURFACE REFLECTION ALONG THE WESTERN GULF COAST.
ELSEWHERE, LATEST GFS RUNS STRAY A BIT TO THE FAST SIDE WITH THE
VIGOROUS SHORTWAVE CROSSING THE NORTHEAST WHILE GUIDANCE SPREAD
RAPIDLY INCREASES BY NEXT WEDNESDAY FOR WHAT NORTHERN STREAM FLOW
WILL LOOK LIKE IMMEDIATELY BEHIND THE PACIFIC UPPER LOW THAT EJECTS
INLAND. THE NEW 00Z GFS IS ON THE AMPLIFIED SIDE OF THE SPREAD WITH
ITS TROUGH REACHING INTO THE NORTHWEST BY MIDWEEK.
THE UPDATED FORECAST STARTED WITH AN OPERATIONAL MODEL COMPOSITE
FOR THE WEEKEND AND THEN SPLIT THE 18Z GFS WITH THE GEFS MEAN TO
ACCOUNT FOR TROUGH PROGRESSION OVER THE NORTHEAST. BY LATE IN THE
PERIOD THE BLEND REACHED A HALF MODEL/HALF ENSEMBLE MEAN
(GEFS/ECENS) WEIGHT TO REFLECT THE MOST COMMON IDEAS OF THE LARGE
SCALE PATTERN.
..WEATHER/HAZARDS HIGHLIGHTS
ON DAY 4/SATURDAY, CONTINUE TO EXPECT LOCALLY HEAVY RAIN TO AFFECT
PARTS OF THE NORTHEAST INTO THE MID-ATLANTIC AND BACK INTO THE
APPALACHIANS PLUS PERHAPS THE TENNESSEE/OHIO VALLEYS AS A LEADING
FRONT PROGRESSES INTO THIS REGION, WITH MOISTURE AND INSTABILITY
SUPPORTING MAINTENANCE OF A MARGINAL RISK AREA IN THE EXCESSIVE
RAINFALL OUTLOOK. SOME GUIDANCE IS RAISING THE POTENTIAL FOR SOME
ENHANCED TOTALS OVER NORTHERN NEW ENGLAND BUT SO FAR WITH MINIMAL
INSTABILITY. THE SOUTHWEST/SOUTHERN ROCKIES/SOUTHERN HIGH PLAINS
SHOULD ALSO CONTINUE TO SEE THUNDERSTORMS INTO SATURDAY BUT WITH
DECLINING CONFIDENCE IN SPECIFICS DUE TO DIFFERENCES IN POTENTIAL
SHORTWAVE ENERGY. THE MARGINAL RISK AREA OVER THIS REGION IS
UNCHANGED BUT WITH POTENTIAL FOR FUTURE ADJUSTMENT DEPENDING ON
GUIDANCE TRENDS/CLUSTERING. MEANWHILE, GUIDANCE STILL SHOWS THE
POTENTIAL FOR CONVECTION ALONG PARTS OF THE LOUISIANA/TEXAS COAST.
THE DAY 4 ERO HAS INTRODUCED A MARGINAL RISK AREA BASED ON THIS
PERSISTENCE AND EXPECTED RAINFALL FROM PRIOR DAYS, THOUGH
CONFIDENCE IN SPECIFICS IS STILL NOT VERY HIGH. DURING DAY 5
SUNDAY, THE ONLY RISK AREA INITIALLY DEPICTED IS A MARGINAL OVER
PARTS OF THE SOUTHERN MID-ATLANTIC/APPALACHIANS AS
MOISTURE/INSTABILITY MAY BE SUFFICIENT TO PRODUCE LOCALLY HEAVY
RAINFALL ALONG THE WAVY FRONT REACHING THE AREA. THERE WILL BE
OTHER POSSIBILITIES FOR LOCALLY HEAVY RAINFALL ACROSS THE SOUTHERN
TIER BUT GUIDANCE SIGNALS APPEAR TOO DIFFUSE FOR DEPICTING ANY RISK
AREAS AT THIS TIME.
AFTER SUNDAY, UNCERTAIN DETAILS OF SOUTHERN TIER SHORTWAVE ENERGY
CONTINUE TO MAKE IT DIFFICULT TO RESOLVE SPECIFICS OF THE RAINFALL
FORECAST IN TERMS OF NORTHWARD EXTENT AND MAGNITUDE/LOCATION OF
HIGHEST TOTALS. THE FRONTS SETTLING INTO THE SOUTHERN TIER SHOULD
PROVIDE SOME FOCUS AT TIMES THOUGH, WHILE TRAILING HIGH PRESSURE
PROMOTES A DRYING TREND FARTHER NORTH. THE NORTHWEST MAY SEE
SCATTERED RAINFALL EARLY NEXT WEEK AS AN EASTERN PACIFIC UPPER LOW
EJECTS INLAND.
UPPER RIDGING TRACKING ACROSS THE NORTHWEST INTO THE HIGH PLAINS
WILL PRODUCE WELL ABOVE NORMAL TEMPERATURES OVER THAT PART OF THE
COUNTRY DURING THE PERIOD, WITH SOME LOCATIONS SEEING GREATER THAN
PLUS 10F ANOMALIES ON ONE OR MORE DAYS. MEANWHILE THERE WILL BE
SOME LINGERING HEAT OVER PARTS OF THE SOUTHERN HALF OR SO OF THE
EAST INTO THE WEEKEND BUT WITH HIGHS ONLY UP TO 5-10F ABOVE NORMAL.
OTHERWISE, A DECENT PORTION OF THE CENTRAL/EASTERN U.S. WILL TEND
TO SEE NEAR TO BELOW NORMAL HIGHS, WITH COOLEST ANOMALIES (SOME
POCKETS OF HIGHS AT LEAST 10F BELOW NORMAL) OVER THE PLAINS SUNDAY-
TUESDAY AND INTO THE EAST MONDAY-WEDNESDAY.
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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