822
FXUS02 KWBC 271913
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
313 PM EDT TUE AUG 27 2024
VALID 12Z FRI AUG 30 2024 - 12Z TUE SEP 03 2024
..OVERVIEW
GUIDANCE EXPECTS A SLOW EASTWARD DRIFT OF THE MEAN PATTERN OVER THE
COURSE OF THE PERIOD, WITH A RIDGE CROSSING THE WEST AND SETTLING
OVER THE ROCKIES/HIGH PLAINS BY EARLY NEXT WEEK WHILE THE
DOWNSTREAM CANADA/NORTHERN TIER U.S. TROUGH PUSHES INTO EASTERN
NORTH AMERICA. MEANWHILE AN EASTERN PACIFIC UPPER LOW MAY EJECT
INTO THE NORTHWEST BY EARLY NEXT WEEK. THE FORECAST PATTERN
EVOLUTION WILL SERVE TO MODERATE THE HAZARDOUS HEAT INITIALLY OVER
PARTS OF THE EAST, WITH INCREASING COVERAGE OF NEAR TO BELOW NORMAL
HIGHS EAST OF THE ROCKIES BY NEXT WEEK. A LEADING FRONT CROSSING
THE EAST WILL PRODUCE SHOWERS AND THUNDERSTORMS LATE WEEK INTO THE
WEEKEND FOLLOWED BY A DRYING TREND, WHILE MORE PERSISTENT CHANCES
FOR RAIN OF VARYING INTENSITY WILL EXIST ACROSS PORTIONS OF THE
SOUTHERN TIER--ESPECIALLY FROM THE SOUTHERN ROCKIES TO WEST-CENTRAL
GULF COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST DYNAMICAL AND MACHINE LEARNING (ML) MODELS ARE FAIRLY
AGREEABLE WITH IMPORTANT FEATURES FOR ABOUT THE FIRST HALF OF THE
PERIOD. FROM ABOUT SUNDAY ONWARD, GUIDANCE BEGINS TO DIFFER WITH
SPECIFICS OF LOWER-PREDICTABILITY ENERGY THAT MAY REACH/EVOLVE OVER
THE SOUTHERN ROCKIES AND VICINITY. OVER THE PAST COUPLE DAYS THE
GFS HAS BEEN THE MOST AGGRESSIVE WITH THE DEPTH OF THIS ENERGY, AND
WHILE RECENT TRENDS OF ML MODELS HAVE RAISED THE POTENTIAL FOR THE
EXISTENCE OF THIS ENERGY ALOFT, THEY SUGGEST THAT IT SHOULD BE
WEAKER AND MORE PROGRESSIVE THAN DEPICTED BY THE GFS. THE MAIN
OUTLIER FROM THE 00/06Z CYCLE WAS THE CMC, WHICH SHOWED MORE
EXTREME SOUTHWESTWARD ELONGATION OF EASTERN TROUGH ENERGY, LEADING
TO A DIFFERENT SOUTHERN PLAINS TO EAST PATTERN THAN OTHER GUIDANCE.
THE NEWER 12Z CMC SEEMS IN BETTER ALIGNMENT. THEN, MODEL SPREAD
BEGINS AS PRETTY REASONABLE FOR THE EASTERN PACIFIC UPPER LOW,
PERHAPS HIGHER CONFIDENCE THAN NORMAL FOR THAT REGION, UNTIL
MONDAY-TUESDAY WHEN SPREAD INCREASES AT LEAST PARTLY DUE TO
ADDITIONAL TROUGHING UP NEAR ALASKA. PREFERRED THE ENSEMBLE MEANS
BY THAT POINT AS AN INTERMEDIATE SOLUTION.
BASED ON THE ARRAY OF GUIDANCE, THE UPDATED FORECAST STARTED WITH
THE STANDARD 00Z/06Z OPERATIONAL MODEL COMPOSITE FOR THE FIRST HALF
OF THE PERIOD AND THEN TRANSITIONED TO A MODEL/ENSEMBLE MEAN MIX
(GEFS/ECENS) FOR THE LATTER PERIOD AS SPREAD INCREASED.
..WEATHER/HAZARDS HIGHLIGHTS
AS A FIRST FRONT TRACKS ACROSS THE EASTERN TO SOUTH-CENTRAL PARTS
OF THE COUNTRY, AMPLE MOISTURE AND INSTABILITY POOLING ALONG AND
AHEAD OF IT COULD LEAD TO LOCALLY HEAVY RAIN CAUSING A FLASH
FLOODING THREAT. THE BIGGEST CHANGE TO THE DAY 4/FRIDAY ERO WAS TO
COMBINE THE MARGINAL RISKS FROM THE GREAT LAKES REGION
SOUTHWESTWARD INTO THE OHIO VALLEY AND MID-MISSISSIPPI VALLEY
THROUGH THE SOUTHERN PLAINS TO JOIN WITH THE NEW MEXICO ONE. MODELS
SHOW SOME VARIABILITY IN THE PLACEMENT/FOCUS, BUT FOR THE CENTRAL
PART OF THE COUNTRY, THE FRONT STARTING TO LAY OUT MORE WEST TO
EAST COULD LEAD TO TRAINING CONVECTION WITH HEAVY RAIN RATES.
ANTECEDENT CONDITIONS ARE NOT VERY WET THOUGH, WHICH COULD TEMPER
EFFECTS. THE NORTHERN PART OF THE RISK AREA COULD SEE QUICKER
MOVEMENT OF THE CONVECTION BUT TENDS TO BE MORE SENSITIVE TO RAIN.
BY DAY 5, LOCALLY HEAVY RAIN LOOKS TO AFFECT PARTS OF THE NORTHEAST
TO MID-ATLANTIC BACK INTO THE APPALACHIANS AND PERHAPS THE
TENNESSEE/OHIO VALLEYS AS THE FRONT PROGRESSES, SUPPORTING A
MARGINAL THERE. THE SOUTHWEST/SOUTHERN ROCKIES/SOUTHERN HIGH PLAINS
SHOULD ALSO CONTINUE TO SEE THUNDERSTORMS INTO SATURDAY AS
SHORTWAVE ENERGY APPROACHES FROM THE WEST. SOME EPISODES OF
CONVECTION THERE COULD OCCUR OVER SENSITIVE TERRAIN. MEANWHILE,
GUIDANCE IS PERSISTENT WITH THE POTENTIAL FOR CONVECTION ALONG
PARTS OF THE LOUISIANA/TEXAS COAST BUT WITH CONTINUED DIFFERENCES
FOR GREATEST AMOUNTS AND EXACT LOCATION, INCLUDING WHETHER INLAND
OR JUST OFFSHORE. SINCE NOTABLE RAIN MAY BE LIMITED TO THE
IMMEDIATE COAST OVER SANDY SOILS, AND CURRENTLY LOOKS TO STAY EAST
OF THE HOUSTON METRO AREA, CONTINUED TO HOLD OFF ON ANY EROS THERE.
ALSO, MOST BUT NOT ALL GUIDANCE SHOWS PRECIPITABLE WATER VALUES
GRADUALLY INCREASING OVER THE FLORIDA PENINSULA DURING FRIDAY-
SATURDAY WHICH COULD ENHANCE LOCALIZED AMOUNTS THERE, BUT AGAIN
WOULD LIKE TO SEE A MORE PRONOUNCED GUIDANCE SIGNAL BEFORE
INTRODUCING A RISK AREA AT THIS DISTANT TIME FRAME.
AFTER SATURDAY, UNCERTAIN DETAILS OF SHORTWAVE ENERGY THAT MAY
EVOLVE/PROGRESS OVER AND EAST OF THE SOUTHERN ROCKIES WILL PLAY A
ROLE IN CONVECTION OVER THE SOUTHERN ROCKIES/PLAINS, WITH SOME
LOCALLY HEAVY AMOUNTS POSSIBLE. RAINFALL MAY CONTINUE ALONG THE
WESTERN HALF OF THE GULF COAST. EASTERN U.S. RAIN WILL SETTLE INTO
THE SOUTHERN TIER ALONG A COUPLE COLD FRONTS AS 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.
HAZARDOUS HEAT WILL LINGER OVER PARTS OF THE EASTERN U.S. INTO
FRIDAY AHEAD OF A COLD FRONT ADVANCING FROM THE MIDWEST. BEST
POTENTIAL FOR HIGHS UP TO 10-15F ABOVE NORMAL WILL BE OVER AND JUST
SOUTH OF THE OHIO VALLEY, WITH A FEW LOCATIONS POSSIBLY
APPROACHING DAILY RECORDS. EXPERIMENTAL HEATRISK SHOWS MODERATE TO
MAJOR (AND ISOLATED EXTREME) LEVEL IMPACTS ON FRIDAY. CONTINUED
EROSION OF THE EASTERN UPPER RIDGE AND APPROACH/ARRIVAL OF THE COLD
FRONT SHOULD SUPPRESS THE HEAT MORE INTO THE SOUTH AND WITH ONLY
SINGLE- DIGIT ANOMALIES. THE CENTRAL U.S. WILL SEE NEAR TO
MODERATELY BELOW NORMAL HIGHS LATE THIS WEEK. A SECOND COLD FRONT
DROPPING THROUGH THE CENTRAL/EASTERN STATES SUNDAY-TUESDAY SHOULD
BRING MORE WIDESPREAD BELOW NORMAL HIGHS. MEANWHILE, EXPECT AN AREA
OF ABOVE NORMAL TEMPERATURES TO DRIFT FROM THE PACIFIC NORTHWEST
INTO THE NORTHERN HIGH PLAINS FROM LATE THIS WEEK INTO EARLY NEXT
WEEK, CORRESPONDING TO MOVEMENT OF AN UPPER RIDGE.
RAUSCH/TATE
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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