506
FXUS02 KWBC 180720
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
320 AM EDT SUN JUN 18 2023
VALID 12Z WED JUN 21 2023 - 12Z SUN JUN 25 2023
...PROLONGED HEAT WAVE CONTINUES ACROSS THE SOUTHERN PLAINS
THROUGHOUT THIS WEEK...
...MULTIPLE DAYS OF HEAVY RAIN AND POTENTIAL FLOODING ARE POSSIBLE
IN THE SOUTHERN APPALACHIANS/SOUTHEAST...
..OVERVIEW
AN AMPLIFIED AND BLOCKY PATTERN WILL BE PRESENT OVER THE LOWER 48
THROUGH THE LATTER PART OF THE WEEK, INCLUDING (1) A DEEP
POSITIVELY-TILTED TROUGH ACROSS THE NORTHWESTERN U.S., (2) A WARM
RIDGE TO REMAIN ANCHORED OVER TEXAS/NORTHERN MEXICO, WITH SOME
EXTENSION OF THE RIDGE TRAVERSING THE GREAT LAKES REGION, AND (3)
AN UPPER LOW THAT WILL LIKELY BRING HEAVY RAIN AND POTENTIAL FOR
FLOODING INTO THE SOUTHEAST/SOUTHERN APPALACHIANS. MEANWHILE, THE
POTENTIAL FOR HEAVY RAIN EXISTS FROM THE CENTRAL HIGH PLAINS TO
THE NORTHERN PLAINS AS A FRONT IS FORECAST TO BECOME
QUASI-STATIONARY IN THE VICINITY MIDWEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW GENERAL AGREEMENT WITH THE
SYNOPTIC SCALE PATTERN THROUGH LATE THIS WORKWEEK. THE TROUGH OVER
THE NORTHWEST AND THE RIO GRANDE RIDGE ARE PARTICULARLY AGREEABLE,
WHILE THERE REMAIN SOME MODEL DIFFERENCES WITH THE POSITIONING OF
THE SOUTHEAST UPPER LOW. CMC RUNS FROM BOTH THE 12Z AND NEWER 00Z
SHOW A FARTHER NORTH POSITION OF THE LOW COMPARED TO OTHER MODELS,
IMPACTING ITS QPF THAT IS FARTHER NORTH AS WELL (WITH MORE RAIN
INTO THE MID-ATLANTIC). THEN THE 12/18Z GFS RUNS BECAME WEAKER
WITH THE FEATURE BY AROUND FRIDAY, WHICH MAY BE TOO FAST TO WEAKEN
GIVEN THE BLOCKY PATTERN. GENERALLY FAVORED THE ECMWF IN THE
FORECAST BLEND AS A GOOD PROXY MIDDLE GROUND OF POSITION (SIMILAR
TO THE ENSEMBLE MEANS) AND MAINTAINING STRENGTH FOR LONGER. THE
QPF AMOUNTS FOR THE WEEK ENDED UP STAYING SIMILAR TO THE PREVIOUS
FORECAST.
BY THE WEEKEND, THE OVERALL PATTERN MAY RELAX/DEAMPLIFY A BIT. THE
FUTURE OF THE SOUTHEAST UPPER LOW REMAINS SOMEWHAT UNCERTAIN AS TO
WHERE IT TRACKS AND HOW/WHEN IT MAY TRY TO PHASE WITH THE NORTHERN
STREAM. THE WPC FORECAST HAS THE ENERGY WEAKENING BUT MAINTAINS A
TROUGH ACROSS THE SOUTHEAST, WHICH SEEMED TO BE A REASONABLE
COMPROMISE. IN THE WEST, THERE IS SOME GENERAL SIGNAL FOR AN UPPER
LOW TO LIFT INTO THE NORTHERN PLAINS/PERHAPS SOUTH-CENTRAL CANADA
THAT COULD FINALLY PRESS THE FRONT/LOW PRESSURE SYSTEM IN THE
NORTHERN HALF OF THE PLAINS EASTWARD. THE DETAILS FOR THIS REMAIN
UNCERTAIN AT THIS POINT. BUT AT LEAST FARTHER WEST, THE 00Z MODEL
GUIDANCE APPEARS TO BE CONVERGING BETTER IN THE POSITION OF AN
UPPER LOW DIVING SOUTH THROUGH THE PACIFIC (FARTHER WEST/OFFSHORE
THAN SOME PREVIOUS MODELS). FOR THE LATTER PART OF THE FORECAST
PERIOD, MAINTAINED A FAIR PROPORTION OF THE ECMWF AND SMALL
PORTIONS OF OTHER DETERMINISTIC MODELS WHILE INCREASING THE
ENSEMBLE MEAN PROPORTION TO ABOUT HALF GIVEN THE INCREASING
SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER LOW MEANDERING OVER THE SOUTHEAST/TENNESSEE VALLEY OR SO
WILL PROVIDE SUPPORT FOR MULTIPLE ROUNDS OF RAIN AND THUNDERSTORMS
FOR THE SOUTHEASTERN QUADRANT OF THE COUNTRY THROUGH MUCH OF THIS
WEEK. INTO THE MEDIUM RANGE PERIOD STARTING WEDNESDAY, A BROAD
SLIGHT RISK IS IN PLACE FROM THE FLORIDA BIG BEND INTO GEORGIA AND
THE CAROLINAS AS TROPICAL MOISTURE FROM THE CARIBBEAN AND FROM THE
GULF FOCUSES NEAR AND OVERRUNS THE FRONTAL SYSTEM IN THE REGION.
PARTS OF THIS REGION COULD BE RATHER SENSITIVE TO ADDITIONAL
RAINFALL BY WEDNESDAY AFTER A WET PATTERN IN THE SHORT RANGE
PERIOD. IT STILL APPEARS THAT THE BIGGEST THREAT OF THE HEAVY RAIN
IN THE SOUTHEAST WILL BE THROUGH MIDWEEK, BUT WITH SOME
CONTINUATION OF RAIN CHANCES THROUGH THE LATTER PART OF THE WEEK
AS WELL. BY DAY 5/THURSDAY, A BROAD MARGINAL RISK IS IN PLACE
EXCEPT FOR AN EMBEDDED SLIGHT FOR THE SOUTHERN APPALACHIANS, WHERE
UPSLOPE FLOW COULD CAUSE RAIN TOTALS POTENTIALLY NEARING 10 INCHES
OVER THE NEXT 5 DAYS OR SO. FUTURE MODERATE RISKS ARE CERTAINLY
POSSIBLE FOR THE SOUTHERN APPALACHIANS IN FUTURE FORECAST CYCLES,
BUT MAY TAKE UNTIL THE SHORT RANGE ONCE IT BECOMES CLEARER
IF/HOW/WHERE THE EARLY WEEK RAINFALL CAUSES FLOODING. BY LATE WEEK
INTO NEXT WEEKEND, RAIN AND THUNDERSTORMS COULD SPREAD FARTHER
NORTH ACROSS THE EAST COAST, BUT WITH UNCERTAINTY IN PLACEMENT AND
AMOUNTS DEPENDENT ON THE UPPER LOW TRACK.
UPPER TROUGHING WITH EMBEDDED UPPER LOWS IN THE NORTHWEST WILL
CAUSE AN INITIALLY EASTWARD-MOVING COLD FRONT TO THEN STALL AROUND
MID-LATE WEEK, PROVIDING ANOTHER FOCUS FOR THUNDERSTORMS IN THE
NORTHERN/CENTRAL PLAINS. THERE SHOULD BE AMPLE INSTABILITY IN THIS
AREA AND THUS HIGH RAIN RATES ESPECIALLY WITH EVENING/OVERNIGHT
CONVECTION ON WEDNESDAY NIGHT WHEN THE LOW-LEVEL JET IS ENHANCED.
A SLIGHT RISK HAS BEEN MAINTAINED IN THE WEDNESDAY-WEDNESDAY NIGHT
ERO WITH THIS ACTIVITY NEAR THE FRONT, WITHIN THE BROADER MARGINAL
COVERING THE NORTHERN/CENTRAL PLAINS WEDNESDAY AND THURSDAY.
CONSIDERED A SLIGHT RISK FOR THURSDAY AS WELL, BUT AT THIS POINT
IT APPEARS SUPPORT FOR HEAVY RAIN MAY WANE SLIGHTLY AS THE UPPER
PATTERN PIVOTS TOWARD MORE RIDGING BY THEN, BUT A SLIGHT MAY
EVENTUALLY BE NEEDED, ESPECIALLY IF THE QPF LOOKS TO FOCUS OVER
COLORADO GIVEN THE WET ANTECEDENT CONDITIONS THERE. COOLER THAN
NORMAL TEMPERATURES ARE FORECAST FOR THE WEST WITH THE UPPER
TROUGH OVERHEAD, BUT WITH A SLOW MODERATING TREND INTO NEXT
WEEKEND.
MEANWHILE, THE EXCESSIVE HEAT WILL CONTINUE THROUGH THIS WEEK
ACROSS MUCH OF TEXAS AND INTO PORTIONS OF LOUISIANA AND NEW
MEXICO, WITH WIDESPREAD 100S AND EVEN SOME 110S ACROSS CENTRAL AND
SOUTHERN PORTIONS OF TEXAS, AND 90S WITH HEAT INDICES WELL OVER
100F ELSEWHERE. MANY DAILY RECORD HIGH TEMPERATURES ARE POSSIBLE,
AND SOME MONTHLY RECORDS MAY EVEN BE CHALLENGED. HEAT INDICES AS
HIGH AS 115-120 DEGREES ARE POSSIBLE FOR PARTS OF SOUTHERN AND
SOUTHEASTERN TEXAS WHERE THE GREATEST COMBINATION OF HIGH
DEWPOINTS AND TEMPERATURES WILL LIKELY EXIST. AS MEAN UPPER
RIDGING EXPANDS FARTHER NORTH, HIGHS IN THE 90S ARE LIKELY ACROSS
MUCH OF THE PLAINS AND MISSISSIPPI VALLEY, WHILE 80S AND LOW 90S
IN THE UPPER MIDWEST/GREAT LAKES REGION WILL BE 10-15F ABOVE
NORMAL.
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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES ARE
AT:
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
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