408
FXUS02 KWBC 270656
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM 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 NEW 00Z
CMC ADDS A NEW WRINKLE OF SHOWING MORE EXTREME SOUTHWESTWARD
ELONGATION OF EASTERN TROUGH ENERGY, LEADING TO A DIFFERENT
SOUTHERN PLAINS PATTERN THAN OTHER GUIDANCE. MEANWHILE, THE MODEL
SPREAD IS REASONABLE FOR THE EASTERN PACIFIC UPPER LOW THAT MAY
EJECT INTO THE NORTHWEST BY THE FIRST HALF OF NEXT WEEK, THUS
FAVORING AN INTERMEDIATE SOLUTION FOR THAT PART OF THE FORECAST.
BASED ON THE ARRAY OF GUIDANCE, THE UPDATED FORECAST STARTED WITH
THE STANDARD 12Z/18Z OPERATIONAL MODEL COMPOSITE FOR THE FIRST HALF
OF THE PERIOD AND THEN TRANSITIONED TO A MODEL/ENSEMBLE MEAN MIX
(GEFS/ECENS/CMCENS) WITH OPERATIONAL INPUT CONSISTING OF MORE 12Z
ECMWF THAN 18Z GFS PLUS A SMALL 12Z CMC COMPONENT.
..WEATHER/HAZARDS HIGHLIGHTS
DURING THE FRIDAY-SATURDAY PERIOD COVERED BY THE DAYS 4-5
EXCESSIVE RAINFALL OUTLOOKS, THERE ARE CURRENTLY TWO REGIONS OF
FOCUS FOR MARGINAL RISK AREAS. ONE IS OVER AND JUST SOUTH OF THE
EASTERN GREAT LAKES (FRIDAY) EXTENDING SOUTH/EAST INTO THE
APPALACHIANS AND EAST COAST (SATURDAY) AS THE INITIAL
MIDWEST/PLAINS COLD FRONT CONTINUES EASTWARD. GUIDANCE SHOWS
SUFFICIENT MOISTURE AND INSTABILITY TO SUPPORT THE POTENTIAL FOR
SOME DECENT RAIN RATES, BUT FRONTAL PROGRESSION SHOULD TEMPER
OVERALL TOTALS SOMEWHAT. MODEL/ENSEMBLE SIGNALS FOR SPECIFICS ARE
STILL NOT AS AGREEABLE AS DESIRED THOUGH. THE OTHER REGION WITH A
MARGINAL RISK EACH DAY IS THE SOUTHERN ROCKIES AND IN SATURDAY'S
CASE EXTENDING INTO PARTS OF ARIZONA. EXPECT SOME EPISODES OF
CONVECTION TO OCCUR OVER SENSITIVE TERRAIN, WHILE ON SATURDAY SOME
SHORTWAVE ENERGY MAY APPROACH FROM THE WEST. ELSEWHERE, THE
AFOREMENTIONED FRONT MAY PRODUCE AREAS OF LOCALLY HEAVY RAINFALL
BETWEEN THE EAST AND SOUTHERN ROCKIES BUT WITHOUT ENOUGH FOCUS (OR
WET ENOUGH ANTECEDENT CONDITIONS) TO MERIT A RISK AREA SO FAR.
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. 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
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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