164
FXUS02 KWBC 172014
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
414 PM EDT THU AUG 17 2023
VALID 12Z SUN AUG 20 2023 - 12Z THU AUG 24 2023
...HILARY TO BRING NOTABLE EXCESSIVE RAINFALL THREAT FROM SOUTHERN
CALIFORNIA INTO THE GREAT BASIN SATURDAY THROUGH MONDAY...
...HEAT WAVE ACROSS THE CENTRAL TO SOUTHEAST U.S. PERSISTS THROUGH
THE MIDDLE OF NEXT WEEK...
..OVERVIEW
AN ANOMALOUSLY STRONG/MASSIVE UPPER RIDGE/CLOSED HIGH ACROSS MUCH
OF THE CENTRAL TO SOUTHEAST U.S. WILL CAUSE EXCESSIVE HEAT TO
PERSIST THROUGH MUCH OF NEXT WEEK. TROUGHING ALONG THE WEST COAST
WILL REMAIN INTACT AND IS FORECAST TO DIRECT HILARY AND A PLUME OF
TROPICAL MOISTURE IN ADVANCE FROM SOUTHERN CALIFORNIA THROUGH THE
GREAT BASIN AND THE NORTHERN ROCKIES WHICH WILL BRING A
SIGNIFICANT HEAVY RAINFALL/HIGH IMPACT EVENT TO PORTIONS OF
CALIFORNIA AND THE GREAT BASIN INTO MONDAY. OTHERWISE THE
EXPANSIVE RIDGE WILL KEEP SHORTWAVE ENERGY NORTH INTO SOUTHERN
CANADA WITH A COUPLE SYSTEMS MOVING THROUGH THE NORTHEAST AND ON
THE SOUTH SIDE A TROUGH OF TROPICAL ORIGIN OVER THE GULF OF MEXICO
EARLY NEXT WEEK PUSHES INTO TEXAS MIDWEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GOOD AGREEMENT ON THE LARGE SCALE REGARDING THE PATTERN OVER THE
CONUS PERSISTS FOR SUNDAY NIGHT-THURSDAY, PARTICULARLY AMONG THE
GFS AND ECMWF WITH THE GFS THE STEADIER MODEL SUITE FOR THE PAST
FEW DAYS FOR HILARY. THE CMC AND UKMET HAVE BEEN DIFFERENT WITH
THE CMC FASTER AND THE UKMET SLOWER WITH HILARY. THIS CONTINUES
WITH 12Z, BUT THE OVERALL FASTER TREND DOES BRING THE SLIGHTLY
SLOWER CMC INTO THE FOLD. OVERALL CONFIDENCE IN A CLOSED LOW
STALLING OFF THE WA COAST ON MONDAY HAS INCREASED. NHC TRACKS
HILARY ALONG THE NORTHERN BAJA CALIFORNIA COAST ON SUNDAY WITH
POTENTIAL FOR TROPICAL CHARACTERISTICS TO PERSIST INTO SOUTHERN CA
EARLY MONDAY. GIVEN THE STRONG TROPICAL PLUME OF MOISTURE SHIFTING
INTO THE SOUTHWEST AHEAD OF HILARY AND THE TERRAIN OF SOUTHERN CA
INTO THE GREAT BASIN, CONFIDENCE IS HIGH FOR SIGNIFICANT RAINFALL
TO IMPACT SOUTHERN CALIFORNIA INTO THE GREAT BASIN. THE WPC BLEND
FOR THE FORECAST PROGS TODAY USED MOSTLY THE DETERMINISTIC 00Z/06Z
GFS WITH THE 00Z ECMWF, SLOWLY TRANSITIONING TO THE 06Z GEFS/00Z
ECENS ENSEMBLE MEANS FOR DAYS 6/7. QPF FOR HILARY WAS BASED
LARGELY ON THE 13Z NATIONAL BLEND OF MODELS WITH FOCUSING FROM
INCLUSION OF THE SLOWER 06Z GFS AND FASTER 00Z ECMWF.
..WEATHER/HAZARDS HIGHLIGHTS
DEEP MONSOONAL/TROPICAL MOISTURE CHANNELING EAST OF WEST COAST
UPPER TROUGHING AND ON THE WESTERN PERIPHERY OF A DOWNSTREAM
CENTRAL U.S. UPPER RIDGE WILL KEEP MUCH OF SOUTHERN CA AND NORTH
THROUGH THE INTERIOR WEST THEN NORTHERN ROCKIES WET THIS PERIOD
WITH THE TRACK OF HILARY THROUGH HERE FURTHER ENHANCING THE
EXCESSIVE RAINFALL THREAT. HEAVIEST RAINFALL AMOUNTS ARE PROGGED
TO FOCUS OVER TERRAIN AND THE HIGH DESERTS OF SOUTHERN CALIFORNIA.
SIGNIFICANT EXCESSIVE RAINFALL THREAT AND CONFIDENCE/MODEL
AGREEMENT REMAINS GREAT ENOUGH FOR A MODERATE RISK ON THE DAY
4/SUNDAY EXCESSIVE RAINFALL OUTLOOK FOR THE PENINSULAR RANGE
THROUGH THE SAN BERNARDINOS IN SOUTHERN CALIFORNIA WHERE GUIDANCE
ALSO HAS HEAVY RAINFALL SATURDAY IN THE PREDECESSOR TROPICAL
MOISTURE PLUME. A SLIGHT RISK SURROUNDS THIS REGION FROM FAR
WESTERN ARIZONA INTO CENTRAL NEVADA AND SOUTH-CENTRAL CALIFORNIA
WITH MINOR ADJUSTMENTS FOR TERRAIN TODAY. FOR DAY 5/MONDAY, A
SLIGHT RISK IS MAINTAINED ACROSS MUCH OF SOUTHERN/CENTRAL
CALIFORNIA INTO SOUTHERN/CENTRAL NEVADA. THE FASTER SOLUTIONS TO
HILARY SUGGEST A MODERATE RISK MAY NOT BE NEEDED AS OF YET FOR
TRANSVERSE RANGES/VICINITY, BUT THIS WILL BE MONITORED. TO THE
NORTH, TROPICAL MOISTURE WILL INTERACT WITH A FRONTAL BOUNDARY
ACROSS THE NORTHWEST/NORTHERN ROCKIES PROVIDING ENHANCEMENT OF
RAINFALL AND GIVEN RAINFALL IN SIMILAR AREAS ON SUNDAY, A SLIGHT
RISK IS RAISED FOR PORTIONS OF EASTERN OR TO THE BITTERROOTS ALONG
THE ID/MT BORDER. MUCH OF THE REST OF THE COUNTRY LOOKS TO REMAIN
MOSTLY DRY UNDER THE EXPANSIVE UPPER RIDGE, WITH THE EXCEPTION OF
SOUTHERN FLORIDA AND THE GULF COAST AS TROPICAL MOISTURE SHIFTS
WESTWARD FROM FLORIDA SUNDAY (WITH A MARGINAL RISK ON DAY 4 FOR
SOUTHERN FL PENINSULA) AND POSSIBLY IMPACTING THE CENTRAL GULF
COAST MONDAY WITH A MARGINAL RISK ON DAY 5. THIS WAVE SHOULD PUSH
INTO OR THROUGH MUCH OF SOUTHERN TEXAS TUESDAY/WEDNESDAY, SO THAT
AREA WILL CONTINUE TO BE MONITORED.
A HEAT WAVE WILL PERSIST OVER THE CENTRAL THROUGH SOUTHEAST U.S.
WITH DAYTIME HIGHS 10-20+ DEGREES ABOVE NORMAL, PARTICULARLY OVER
THE MID-MISSOURI RIVER VALLEY (NEB/IA/SD) UNDER THE EXTREMELY DEEP
RIDGE. THIS EQUATES TO DAYTIME HIGHS IN EXCESS OF 100 DEGREES OVER
THE NORTH-CENTRAL PLAINS, WITH SEVERAL DAILY RECORD HIGHS
FORECAST, AND SEVERAL LOCATIONS IN THE MIDLANDS SEEING THEIR
HOTTEST DAY OF THE YEAR SO FAR AND PERHAPS THE HOTTEST AUGUST DAY
OF THE PAST SEVERAL DECADES. HUMIDITY LEVELS WILL SEND HEAT
INDICES EVEN HIGHER IN THE 100S FOR MANY. SEE KEY MESSAGES ISSUED
BY WPC FOR ADDITIONAL DETAILS AND GRAPHICS RELATED TO THIS HEAT
WAVE. HEAT WILL ALSO EXPAND TO THE LOWER MISSISSIPPI VALLEY BY THE
MIDDLE OF NEXT WEEK WITH RECORD HIGHS FORECAST TUESDAY/WEDNESDAY
IN FROM THE CENTRAL GULF COAST. ELSEWHERE, UPPER TROUGHING ALONG
THE WEST COAST AND RAINFALL OVER SOUTHERN CA AND THE GREAT BASIN
SHOULD KEEP HIGHS GENERALLY 15-25 DEGREES BELOW NORMAL MONDAY AND
5-10 DEGREES BELOW NORMAL DURING MIDWEEK.
JACKSON
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
HAZARDS:
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL GREAT BASIN, SOUTHERN
CALIFORNIA, AND THE SOUTHWEST,
SUN-MON, AUG 20-AUG 21.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE PLAINS, THE MISSISSIPPI
VALLEY, THE TENNESSEE VALLEY, THE
UPPER GREAT LAKES, THE UPPER MISSISSIPPI VALLEY, THE SOUTHEAST,
AND THE OHIO VALLEY, SUN-THU, AUG
20-AUG 24.
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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