946
FXUS02 KWBC 281914
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
314 PM 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. BUT THERE IS NOTABLE 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 A
SOUTHERN HIGH PLAINS UPPER WEAKNESS THAT EVENTUALLY DRIFTS
EASTWARD, WHILE GFS RUNS REMAIN SUPPRESSED/FARTHER SOUTH THAN OTHER
GUIDANCE, BUT THEY SEEM TO BE THE OUTLIER, AS NON-NCEP GUIDANCE
INCLUDING THE EC-BASED AI/ML MODELS SHOW NOTABLE CONVECTION INTO
MUCH OF TEXAS. 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. EVENTUALLY THE
SOUTHERN STREAM DIFFERENCES LEAD TO SOUTHEAST QPF DIFFERENCES, AND
ALSO COMPLICATING MATTERS IS THE POTENTIAL FOR ENERGIES ALONG THE
WEST-CENTRAL GULF COAST.
RECENT MODEL GUIDANCE HAS SHOWN REASONABLE AGREEMENT WITH
NORTHEAST TROUGHING. BUT IN THE EASTERN PACIFIC TO NORTHWEST,
GUIDANCE SPREAD INCREASES BY MONDAY-WEDNESDAY, AT FIRST TO THE
NORTH OF AND THEN INTERACTING WITH THE UPPER LOW FEATURE MOVING
INTO NORTHERN CALIFORNIA. THE NEWER 12Z MODELS HAVE SHOWN A MARKED
SHIFT TO SHOWING LESS NORTHERN TROUGHING THAN THE PREVIOUS CYCLES.
THIS KEEPS THE UPPER LOW FEATURE MOVING NORTHEAST SEPARATE FROM THE
NORTHERN FLOW THAT HAS FLIPPED TO RIDGING FOR A NOTABLE PATTERN
CHANGE FOR THE NORTHWESTERN U.S./SOUTHWESTERN CANADA. THIS WILL
HAVE TO BE MONITORED TO DETERMINE IF THIS TREND REMAINS IN FUTURE
CYCLES.
THE WPC FORECAST USED A MULTI-MODEL DETERMINISTIC BLEND FOR THE
EARLY TO MID-PERIOD, WITH THE BLENDING PROCESS TAKING CARE OF
SMALLER SCALE DIFFERENCES. AS SPREAD INCREASED LATER IN THE PERIOD,
THE PROPORTION OF ENSEMBLE MEANS REACHED HALF BY DAY 7.
..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. A TREND TOWARD SLIGHTLY SLOWER FRONTAL TIMING LED
TO ADJUSTING THE ERO TO COVER NORTHERN NEW ENGLAND AS WELL. BY DAY
5/SUNDAY, A MARGINAL REMAINS IN PLACE 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. THE FRONT GRADUALLY TRACKING SOUTH INTO THE WORKWEEK
SHOULD LEAD TO STORMS ACROSS THE SOUTHEAST.
THE 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. AS
MENTIONED ABOVE, FAVORED THE MORE AGREEABLE NON-NCEP GUIDANCE
INSTEAD OF THE SOUTHERN/SUPPRESSED GFS. THUS HAVE MARGINAL RISKS
OUTLOOKED FOR DAYS 4 AND 5 FOR WESTERN PARTS OF TEXAS INTO
SOUTHEAST NEW MEXICO, WHERE ABOVE NORMAL MOISTURE IS IN PLACE AND
THERE MAY BE SOME ENERGY ALOFT. FOR DAY 4/SATURDAY, WESTERN PARTS
OF THE PREVIOUSLY ISSUED MARGINAL WERE TRIMMED OUT FROM THE
SOUTHWEST-- THOUGH CONVECTION IS LIKELY TO EXIST ACROSS THE
MOGOLLON RIM IN PARTICULAR, THERE IS LIKELY LESS THAN A 5 PERCENT
CHANCE (THE MARGINAL THRESHOLD) OF FLASH FLOODING.
MEANWHILE, GUIDANCE STILL SHOWS THE POTENTIAL FOR CONVECTION ALONG
PARTS OF THE LOUISIANA/TEXAS COAST. THE DAY 4 ERO HAS MAINTAINED A
MARGINAL RISK AREA BASED ON THIS PERSISTENCE AND EXPECTED RAINFALL
FROM PRIOR DAYS, AND A MARGINAL IS INTRODUCED FOR DAY 5/SUNDAY AS
WELL. CONFIDENCE IN SPECIFICS LIKE HOW MUCH RAIN WILL FALL
OFFSHORE/ONSHORE IS STILL NOT VERY HIGH.
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 NORTHERN 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.
TATE/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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