302
FXUS02 KWBC 011900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT TUE AUG 01 2023
VALID 12Z FRI AUG 04 2023 - 12Z TUE AUG 08 2023
..HEATWAVE TO PERSIST OVER THE SOUTH
...EXCESSIVE RAIN/FLASH FLOOD THREAT TO FOCUS OVER THE
NORTH-CENTRAL GREAT PLAINS FRIDAY INTO SATURDAY...
..OVERVIEW
DEEPENED MONSOONAL MOISTURE AND UPPER SYSTEM ENERGY WORKING
THROUGH THE ROCKIES ON THE WESTERN TO NORTHERN PERIPHERY OF AN
AMPLIFIED UPPER RIDGE WILL FAVOR A SLOW SHIFT OF A MAIN CONVECTIVE
RAINFALL FOCUS INTO THE NORTH-CENTRAL PLAINS. MEANWHILE, AMPLE
NORTHERN STREAM UPPER TROUGHING FROM THE CANADIAN PRAIRIES WILL
DIG SOUTHEAST TO THE GREAT LAKES FRIDAY THROUGH THIS WEEKEND WHICH
ALONG WITH UPPER ENERGIES WORKING FROM THE ROCKIES INTO THE PLAINS
WILL ACT TO SUPPRESS THE CENTRAL CONUS UPPER RIDGE AXIS SOUTH TO
THE SOUTHERN TIER OF STATES WHERE ABOVE NORMAL
TEMPERATURES/HEATWAVES PERSIST. ANOMALOUSLY HOT WEATHER WILL ALSO
SPREAD BACK INTO THE DESERT SOUTHWEST. DOWNSTREAM, SHORTWAVES
ROUNDING THE TOP OF THE RIDGE IN THE NORTHWEST FLOW WILL SPARK
STRONG TO SEVERE THUNDERSTORMS, AND LOCALLY HEAVY RAIN, INTO LATER
THIS WEEK FROM THE MID-SOUTH TO THE SOUTHERN
APPALACHIANS/SOUTHEAST. ACTIVITY SHOULD ALSO INCREASE UP TO THE
NORTHEAST AROUND FRIDAY AS THE NEW NORTHERN STREAM TROUGH ENERGY
COMBINES WITH AN ATLANTIC MOISTURE PLUME AND TERRAIN AHEAD OF AN
ORGANIZED EASTERN CANADIAN SURFACE LOW AND TRAILING FRONT TO BRING
DRIER AIR/MODERATED TEMPERATURES TO PARTS OF THE CENTRAL TO
EASTERN U.S. IN THE WAKE OF PASSAGE.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
OVERVIEW OF THE FORECAST MODEL GUIDANCE DURING THE CURRENT MEDIUM
RANGE PERIOD (FRIDAY AUG 4 - TUESDAY AUG 8) DEPICTS MEAN TROUGHING
OVER THE NORTHEAST SOUTH OF A HUDSON BAY UPPER-LOW, THE STAGNANT
UPPER-LEVEL HIGH OVER THE SOUTH-CENTRAL/SOUTHERN TIER OF THE U.S.,
AND GENERALLY WEAK WESTERLY UPPER-LEVEL FLOW TO THE NORTH. INITIAL
TRENDS THROUGH THE MIDDLE OF THE PERIOD SHOW THE TROUGH OVER THE
NORTHEAST BEGINNING TO LIFT TO THE NORTHEAST WITH SHORTWAVE ENERGY
IN THE NORTHERN STREAM DROPPING SOUTHWARD OVER CENTRAL PORTIONS OF
THE COUNTRY, HELPING TO AMPLIFY GENERAL MEAN TROUGHING ALONG AND
EAST OF THE ROCKIES INTO THE PLAINS/MIDWEST. THIS ALSO BEGINS TO
DISPLACE THE UPPER-HIGH AND GENERALLY HIGHER HEIGHTS OVER THE
SOUTHERN TIER FURTHER SOUTHWARD AND SHIFTING MEAN RIDGING TO THE
EAST AND WEST, ESPECIALLY OVER THE SOUTHWEST AND UP THE WEST COAST
INTO CANADA WHERE THE PATTERN AMPLIFIES IN RESPONSE TO THE
DEEPENING TROUGH OVER THE CENTRAL U.S. THE PATTERN LOOKS TO
AMPLIFY MORE BROADLY ACROSS THE COUNTRY AT THE END OF THE FORECAST
PERIOD AS THE TROUGH SHIFTS EASTWARD TOWARDS THE EAST COAST AND
HEIGHTS CONTINUE TO RISE OVER PORTIONS OF THE WEST, WITH THE
TROUGHING DEEPER IN THE 00Z/06Z GFS AND 00Z CMC COMPARED TO THE
00Z ECMWF, AND THE 00Z ECENS AND 00Z GEFS MEANS OFFERING AN
EXPECTED COMPROMISE BETWEEN THE DETERMINISTIC GUIDANCE. AN
UPPER-LEVEL WAVE DROPPING SOUTH FROM THE NORTHEASTERN PACIFIC MAY
IMPINGE ON THE MEAN UPPER-RIDGING OVER THE WEST COAST INTO THE
PACIFIC NORTHWEST AT THE END OF THE PERIOD, INDICATED MOST BY THE
00Z CMC AND LESS SO BY THE 06Z GFS, 00Z GFS, AND 00Z ECMWF,
RESPECTIVELY.
GENERALLY GOOD AGREEMENT IN THE GUIDANCE EARLY IN THE PERIOD LEADS
TO AN UPDATED WPC FORECAST OF A COMPOSITE GUIDANCE BLEND, WITH A
LESSER CONTRIBUTION FROM THE 00Z UKMET WHOSE DETAILS DIVERGED A
BIT OVERALL COMPARED TO THE CLOSER CLUSTERING OF THE OTHER
GUIDANCE, PARTICULARLY WITH RESPECT TO THE EXTENT OF THE
UPPER-HIGH OVER THE SOUTHERN TIER. A CONTRIBUTION FROM THE 00Z
ECENS AND GEFS MEANS IS ADDED MID-PERIOD AS DIFFERENCES IN
SMALLER-SCALE SHORTWAVE ENERGY OVER THE CENTRAL U.S. INCREASE AND
THE FORECAST-TIME LIMITED UKMET IS RAMPED DOWN. THE BLEND TRENDS
TOWARDS A SLIGHT EMPHASIS ON THE 00Z ECMWF AND 00Z ECENS WHICH
SHOW THE GREATEST COMPROMISE IN THE OVERALL AMPLIFICATION OF THE
PATTERN, AND SPECIFICALLY TROUGHING OVER THE EASTERN HALF OF THE
U.S., TRENDS TOWARDS THE EC GUIDANCE AS WELL AS THE 00Z GEFS MEAN.
THE CONTRIBUTION FROM THE 00Z CMC IS REDUCED THE MOST AS IT SHOWS
THE GREATEST AMPLIFICATION IN THE PATTERN.
..WEATHER/HAZARDS HIGHLIGHTS
THE NORTH-CENTRAL PLAINS WILL SEE A FOCUS FOR DIURNALLY DRIVEN
CONVECTION FRIDAY INTO SATURDAY AS THE UPPER RIDGE AXIS CENTERED
OVER THE HIGH PLAINS WEAKENS/SUPPRESSES SOUTH AND NORTHERN TIER
FLOW INITIALLY BECOMES MORE ZONAL BEFORE TRANSITIONING TO
NORTHWESTERLY. DEEP MONSOONAL MOISTURE AND SLOW MOVING/ORGANIZED
EMBEDDED ENERGIES ROUNDING THIS RIDGE WILL PRODUCE HEAVY RAIN.
RECENT GUIDANCE HAD SUGGESTED AN EASTWARD SHIFT TO THE HEAVY RAIN
FOCUS ONTO THE PLAINS, AND A DAY 4 ERO SLIGHT RISK FOR EXCESSIVE
RAINFALL HAS BEEN OUTLINED FROM EASTERN MT INTO THE CENTRAL
DAKOTAS WHERE THE HIGHEST ENSEMBLE PROBABILITIES INDICATE THE
POTENTIAL FOR LOCALLY HEAVY RAINFALL DESPITE LINGERING
UNCERTAINTIES IN THE DETERMINISTIC GUIDANCE ON QPF PLACEMENT.
HOWEVER, IT IS POSSIBLE GIVEN NOTABLE PW ANOMALIES 1.5-2 STANDARD
DEVIATIONS ABOVE THE MEAN ACROSS THE REGION AND A WESTWARD TREND
WITH SOME HIGHER QPF IN THE GUIDANCE THAT A LARGER SLIGHT RISK
AREA EXTENDING WESTWARD INTO MONTANA MAY BE NECESSARY IF
CONFIDENCE IN RAINFALL AMOUNTS INCREASES, BUT DRIER ANTECEDENT
CONDITIONS MAY ULTIMATELY KEEP ANY FLOOD RISK LOW. THE MORE ZONAL
FLOW FOR SATURDAY/DAY 5 ALLOWS THE HEAVY RAIN FOCUS TO EXPAND
EAST/SOUTHEAST ACROSS THE DAKOTAS INTO MN/IA WITH GENERAL LOCATION
UNCERTAINTY AND LESS CURRENT CONSENSUS ON HEAVIER RAINFALL AMOUNTS
LEADING TO A ERO MARGINAL RISK AREA FOR NOW.
FARTHER DOWNSTREAM AND AROUND THE UPPER RIDGE, EXPECT AREAS OF
THUNDERSTORMS TO SHIFT OUT FROM THE MID-SOUTH/TN VALLEY TO THE
SOUTHERN APPALACHIANS/SOUTHEAST ON FRIDAY. A HEAVY RAIN/FLASH
FLOODING THREAT FROM ORGANIZED CONVECTIVE SYSTEMS IN THE
NORTHWESTERLY FLOW OVER LINGERING FRONTS WARRANTS A MARGINAL RISK
FOR EXCESSIVE RAINFALL OVER MORE SENSITIVE AREAS OF PORTIONS OF
THE UPPER TENNESSEE VALLEY.
MEANWHILE, THE APPROACH OF THE MORE AMPLIFIED NORTHERN STREAM
UPPER TROUGH AND LEADING LOW/FRONTAL SYSTEM SHOULD TAP A DEEPENED
ATLANTIC MOISTURE PLUME OVER THE NORTHEAST LEADING TO WIDESPREAD
HEAVY RAINFALL ON FRIDAY. THIS HAS RESULTED IN A DAY 5 MARGINAL
RISK FOR EXCESSIVE RAINFALL OVER NEW ENGLAND, WITH THE BULK OF THE
HEAVIEST RAINFALL LOOKING TO STAY OVER LESS SENSITIVE COASTAL
REGIONS COMPARED TO THE MORE SENSITIVE INTERIOR.
THE ONGOING HEATWAVE SHOULD SETTLE A BIT SOUTH OVER THIS WEEKEND,
CLOSER TO THE SOUTHERN BORDER AND ALONG THE GULF COAST AS UPPER
TROUGHING SHIFTS SOUTHWARD AND THE UPPER-RIDGING FLATTENS OUT.
HIGH TEMPERATURES WILL BE 5 TO 10 DEGREES ABOVE NORMAL BUT HIGH
HUMIDITY WILL LEAD TO HEAT INDICES IN THE 110-115F IN SOME AREAS,
PARTICULARLY THE LOWER MISSISSIPPI RIVER VALLEY. RECORD HIGH
MAXIMUM AND ESPECIALLY MINIMUM TEMPERATURES ARE WIDELY FORECAST IN
THESE REGIONS. ADDITIONALLY, AFTER A BRIEF DROP IN TEMPERATURES
OVER THE DESERT SOUTHWEST, HIGHS WILL ONCE AGAIN WARM INTO THE
110S. SOME ABOVE AVERAGE TEMPERATURES WILL ALSO SPREAD UP THE WEST
COAST FOLLOWING A BIT OF RIDGE AMPLIFICATION, WITH HIGHS IN THE
CENTRAL CALIFORNIA VALLEYS CLIMBING INTO THE MID-100S INTO THE
WEEKEND AND EARLY NEXT WEEK. OTHERWISE, WELCOME POST-FRONTAL
COOLER AND DRIER AIR SPREAD DOWN OVER THE CENTRAL AND EASTERN
CONUS THIS WEEKEND.
PUTNAM/SCHICHTEL
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES 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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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