895
FXUS02 KWBC 160657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EDT WED AUG 16 2023
VALID 12Z SAT AUG 19 2023 - 12Z WED AUG 23 2023
...HEAT WAVE EXPANDING ACROSS THE CENTRAL TO EAST-CENTRAL U.S.
THIS WEEKEND AND EARLY NEXT WEEK...
...HEAVY RAINFALL POSSIBLE FOR PARTS OF THE SOUTHWEST ASSOCIATED
WITH POTENTIAL TROPICAL DEVELOPMENT...
..OVERVIEW
A VIGOROUS SHORTWAVE WITH EMBEDDED CLOSED LOW WILL SHIFT OUT OF
THE NEW ENGLAND THIS WEEKEND AS AN ANOMALOUSLY STRONG UPPER RIDGE
EXPANDS ACROSS THE CENTRAL TO EAST-CENTRAL U.S. BRINGING A
RESURGENCE OF EXCESSIVE HEAT THROUGH AT LEAST THE MIDDLE OF NEXT
WEEK. TROUGHING OVER THE WEST COAST LOOKS TO REMAIN IN TACT
THROUGH THE PERIOD RENEWED BY SHORTWAVES OUT OF THE GULF OF ALASKA
AND INTO SOUTHWEST CANADA. MEANWHILE, LIKELY TROPICAL DEVELOPMENT
IN THE EAST PACIFIC AND EVENTUALLY LIFTING NORTHWARD OFF THE BAJA
CALIFORNIA COAST MAY BRING A THREAT FOR HEAVY TO EXCESSIVE
RAINFALL INTO PARTS OF THE SOUTHWEST U.S. AND CALIFORNIA
SATURDAY-MONDAY.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODELS SHOW GOOD AGREEMENT WITH THE EXITING SYSTEM OUT OF THE
NORTHEAST ON SATURDAY AS WELL AS EXPANSION OF THE RIDGE OVER MUCH
OF THE EASTERN TWO THIRDS OF THE COUNTRY. WEST COAST TROUGH
EVOLUTION AND DETAILS REMAIN HIGHLY UNCERTAIN, EVEN AS EARLY AS
SUNDAY. THE 12Z/AUG 15 ECMWF AND UKMET ARE MUCH MORE AMPLIFIED
WITH A SHORTWAVE INTO THE NORTHWEST, RELATED TO A
STRONGER/BLOCKIER RIDGE ACROSS THE CENTRAL STATES. THE GFS AND CMC
(INCLUDING THE NEW 00Z RUNS) TRACK THIS SYSTEM EASTWARD ACROSS
SOUTHERN-CENTRAL CANADA, EVENTUALLY AMPLIFYING IT AGAIN THROUGH
THE GREAT LAKES AND NORTHEAST EARLY NEXT WEEK. GIVEN THIS SPLIT IN
THE GUIDANCE, OPTED TO STICK CLOSE TO CONTINUITY WHICH FALLS
CLOSER TO THE ECMWF, BUT NOT NEARLY AS STRONG, AS IS CONSISTENT
WITH THE ENSEMBLE MEANS. PLENTY OF ADDITIONAL LATE PERIOD
UNCERTAINTIES WITH THE FLOW UPSTREAM MOVING OUT OF THE GULF OF
ALASKA EARLY NEXT WEEK AS WELL. GIVEN THESE UNCERTAINTIES, THE WPC
BLEND FOR TONIGHT TRENDED STRONGLY TOWARDS THE ENSEMBLE MEANS FOR
THE LATTER HALF OF THE PERIOD, WHICH MAINTAINS GOOD CONTINUITY
WITH THE PREVIOUS WPC FORECAST.
POSSIBLY THE MOST SIGNIFICANT THREAT FOR SENSIBLE WEATHER HOWEVER
LIES WITH HIGHLY ANOMALOUS MONSOONAL MOISTURE INTO THE SOUTHWEST
U.S. STARTING THIS WEEKEND. MODELS CONTINUE TO SUGGEST THIS
MOISTURE SHOULD GET ENHANCED BY A POTENTIAL TROPICAL SYSTEM IN THE
EAST PACIFIC (WITH A HIGH PROBABILITY OF DEVELOPMENT IN THE NEXT
DAY OR TWO PER THE NATIONAL HURRICANE CENTER). THE GUIDANCE HAS
COME INTO MUCH BETTER AGREEMENT COMPARED TO THE LAST SEVERAL DAYS
ON TRACKING THIS SYSTEM NORTH-NORTHWESTWARD JUST OFF THE BAJA
CALIFORNIA COAST THIS WEEKEND INTO EARLY NEXT WEEK. DETERMINISTIC
GUIDANCE SUGGESTS IT COULD COME CLOSE TO THE SOUTHWEST U.S.,
ALTHOUGH THE ENSEMBLE MEANS ARE FARTHER WEST LOWING CONFIDENCE IN
ANY ONE INDIVIDUAL SOLUTION. A FURTHER EAST TRACK WOULD PROVIDE A
MORE DIRECT FEED OF MOISTURE INTO THE SOUTHWEST, SO THIS WILL NEED
TO BE CLOSELY MONITORED. THE WPC QPF THIS CYCLE STUCK CLOSE TO THE
NBM AND LATEST MULTI-MODEL ENSEMBLE BIAS CORRECTED QPF, WHICH
LARGELY TRENDED WELL WITH CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
MONSOONAL MOISTURE STREAMING NORTHWARD AHEAD OF WEST COAST
TROUGHING WILL KEEP MUCH OF THE INTERIOR WEST WET THROUGH THE
PERIOD. RAINFALL AMOUNTS OVER ESPECIALLY THE SOUTHWEST REMAIN
HIGHLY UNCERTAIN AND DEPENDENT ON POTENTIAL TROPICAL DEVELOPMENT
AND TRACK IN THE EASTERN PACIFIC. HOWEVER, MODELS CONTINUE TO
TREND TOWARDS A HEAVY TO EXCESSIVE THREAT AND CONFIDENCE/MODEL
AGREEMENT WAS HIGH ENOUGH TONIGHT TO INCLUDE A SLIGHT RISK ACROSS
ACROSS PORTIONS OF SOUTHEAST CALIFORNIA, SOUTHERN NEVADA, AND
WESTERN ARIZONA ON BOTH THE DAYS 4 AND 5 (SATURDAY AND SUNDAY)
EXCESSIVE RAINFALL OUTLOOKS. SHOULD THE FORECAST HOLD, IT IS
LIKELY AT LEAST A MODERATE RISK WILL BE NEEDED EVENTUALLY, BUT
OPTED TO HOLD OFF GIVEN THE SYSTEM IN QUESTION HAS NOT FORMED YET
AND EXACTLY PLACEMENT OF THE HEAVIEST QPF AXIS WILL BE HIGHLY
DEPENDENT ON EVENTUAL TRACK OF THIS SYSTEM. HEAVY RAINFALL MAY
ALSO LIFT ACROSS SOUTH-CENTRAL CALIFORNIA AS WELL EARLY NEXT WEEK.
TO THE NORTH, MOISTURE WILL INTERACT WITH A FRONTAL BOUNDARY
ACROSS THE NORTHWEST/NORTHERN ROCKIES PROVIDING SOME ENHANCEMENT
OF RAINFALL TO THAT REGION. BROAD MARGINAL RISKS ARE DEPICTED ON
THE EXCESSIVE RAINFALL OUTLOOKS. MUCH OF THE REST OF THE COUNTRY
LOOKS TO REMAIN MOSTLY DRY UNDER THE EXPANSIVE UPPER RIDGE, WITH
THE EXCEPTION OF FLORIDA AND THE GULF COAST AS TROPICAL MOISTURE
SHIFTS WESTWARD ACROSS THE GULF OF MEXICO. THIS COULD ALSO BRING
SOME RAINFALL INTO SOUTHERN TEXAS NEXT TUESDAY.
A HEAT WAVE WILL BUILD OVER THE SOUTHERN AND CENTRAL U.S. INTO THE
MIDWEST WITH DAYTIME HIGHS 10-20 DEGREES ABOVE NORMAL IN SOME
PLACES. THIS EQUATES TO DAYTIME HIGHS NEAR 100 DEGREES POSSIBLE AS
FAR NORTH AS THE UPPER MIDWEST STATES, WITH DAILY RECORD HIGHS
POSSIBLE, AND SOME LOCATIONS IN THE MIDWEST SEEING THEIR HOTTEST
DAY OF THE YEAR SO FAR. HUMIDITY LEVELS WILL SEND HEAT INDICES
INTO THE 100S FOR MANY. SEE KEY MESSAGES ISSUED BY WPC FOR
ADDITIONAL DETAILS AND GRAPHICS RELATED TO THIS HEAT WAVE. HEAT
MAY ALSO BUILD BACK INTO THE SOUTHEAST BY THE MIDDLE OF NEXT WEEK,
AND POSSIBLY BEYOND PER THE LATEST FORECASTS FROM THE CLIMATE
PREDICTION CENTER. ELSEWHERE, UPPER TROUGHING OUT WEST SHOULD KEEP
HIGHS GENERALLY 5-15 DEGREES BELOW NORMAL WITH PARTS OF THE
SOUTHWEST 15-20 DEGREES BELOW NORMAL ESPECIALLY SUNDAY AND MONDAY
AS A RESULT OF THE INCREASED CLOUD COVER AND THREAT OF HEAVY
PRECIPITATION.
SANTORELLI
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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