180
FXUS02 KWBC 170659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT THU AUG 17 2023
VALID 12Z SUN AUG 20 2023 - 12Z THU AUG 24 2023
...SIGNIFICANT SOUTHWEST U.S./CALIFORNIA EXCESSIVE RAINFALL/RUNOFF
THREAT FUELED BY DEEP TROPICAL MOISTURE IN ADVANCE OF HILARY...
...HEAT WAVE EXPANDING ACROSS THE CENTRAL TO EAST-CENTRAL U.S.
THIS WEEKEND THROUGH EARLY-MID NEXT WEEK...
..OVERVIEW
AN ANOMALOUSLY STRONG/MASSIVE UPPER RIDGE/CLOSED HIGH ACROSS MUCH
OF THE CENTRAL TO EAST-CENTRAL U.S. WILL BRING A RESURGENCE OF
EXCESSIVE HEAT THROUGH MUCH OF NEXT WEEK. THIS SHOULD ALSO KEEP
SHORTWAVE ENERGY NORTH INTO SOUTHERN CANADA WITH A COUPLE SYSTEMS
MOVING THROUGH THE NORTHEAST. A VIGOROUS UPPER LOW WILL BE EXITING
NEW ENGLAND ON SUNDAY, WITH THE NEXT ONE COMING THROUGH EARLY NEXT
WEEK. TROUGHING OVER THE WEST COAST LOOKS TO REMAIN INTACT THROUGH
THE PERIOD RENEWED BY SHORTWAVES OUT OF THE GULF OF ALASKA AND
INTO SOUTHWEST CANADA. CONFIDENCE CONTINUES TO INCREASE ON A
SIGNIFICANT HEAVY RAINFALL/HIGH IMPACT EVENT TO UNFOLD AND FOCUS
ACROSS PARTS OF THE SOUTHWEST AND CALIFORNIA BEGINNING SATURDAY TO
AT LEAST MONDAY WITH A DEEP LEAD MOISTURE PLUME AND FAVORABLE
INGREDIENTS ASSOCIATED WITH HILARY IN THE TROPICAL EAST PACIFIC AS
PER THE LATEST FROM THE NATIONAL HURRICANE CENTER.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THERE IS GOOD AGREEMENT ON THE LARGE SCALE REGARDING THE PATTERN
OVER THE CONUS FOR THE MEDIUM RANGE PERIOD (SUNDAY-THURSDAY) WITH
PLENTY OF LINGERING UNCERTAINTY IN THE DETAILS. SOME DISAGREEMENT
ON AMPLITUDE OF SOUTHERN CANADIAN SYSTEMS AND HOW MUCH SUCH
SYSTEMS MAY ERODE THE NORTH SIDE OF THE UPPER RIDGE OVER THE UPPER
MIDWEST/GREAT LAKES AS WELL AS SYSTEMS DROPPING INTO THE MEAN
UPPER TROUGHING OUT WEST. THE LATEST TRACK GUIDANCE FROM THE
NATIONAL HURRICANE CENTER TRACKS HILARY JUST OFF THE BAJA
CALIFORNIA COAST ON SUNDAY AND INTO SOUTHERN CALIFORNIA ON MONDAY
(LIKELY AS A POST-TROPICAL SYSTEM BY THEN). THERE IS STILL PLENTY
OF UNCERTAINTY IN THE GUIDANCE ON THE EXACT TRACK, BUT REGARDLESS,
CONFIDENCE IS HIGH FOR SIGNIFICANT RAINFALL TO IMPACT THE
SOUTHWEST AND INTO CALIFORNIA. THE WPC BLEND FOR THE FORECAST
PROGS TONIGHT USED THE DETERMINISTIC MODELS EARLY IN THE PERIOD
WHEN AGREEMENT WAS BETTER, TRANSITIONING TO THE ENSEMBLE MEANS
(WITH SOME GFS/ECMWF INCLUSION FOR ADDED DEFINITION). QPF FOR
HILARY WAS BASED LARGELY ON THE 01Z NATIONAL BLEND OF MODELS WITH
THE GFS WHICH WAS MOST CONSISTENT WITH NHCS TRACK FOR HILARY.
..WEATHER/HAZARDS HIGHLIGHTS
DEEP MONSOONAL/TROPICAL MOISTURE CHANNELING NORTHWARD AHEAD OF
WEST COAST UPPER TROUGHING AND ON THE WESTERN PERIPHERY OF A
DOWNSTREAM CENTRAL U.S. UPPER RIDGE WILL KEEP MUCH OF THE INTERIOR
WEST THEN NORTHERN ROCKIES WET THIS PERIOD. HEAVIEST RAINFALL
AMOUNTS WILL ESPECIALLY FOCUS OVER PARTS OF THE SOUTHWEST
U.S./SOUTHERN CALIFORNIA, BUT REMAIN LOCALLY UNCERTAIN AS
DEPENDENT ON THE ULTIMATE TRACK OF HILARY. HOWEVER, MODELS DO
CONTINUE TO TREND TOWARDS A SIGNIFICANT EXCESSIVE RAINFALL THREAT
AND CONFIDENCE/MODEL AGREEMENT IS HIGH ENOUGH FOR A MODERATE RISK
ON THE DAY 4/SUNDAY EXCESSIVE RAINFALL OUTLOOK FOR THE PENINSULAR
RANGE IN FAR SOUTHERN CALIFORNIA WHERE GUIDANCE ALSO HAS HEAVY
RAINFALL IN THE DAY 3/SATURDAY PERIOD. A SLIGHT RISK SURROUNDS
THIS REGION FROM FAR WESTERN ARIZONA INTO SOUTHERN/CENTRAL NEVADA
AND SOUTH-CENTRAL CALIFORNIA. FOR DAY 5/MONDAY, A SLIGHT RISK WAS
DRAWN ACROSS MUCH OF SOUTHERN/CENTRAL CALIFORNIA INTO
SOUTHERN/CENTRAL NEVADA. THERE IS A NOTABLE SIGNAL FOR
CONSIDERABLE RAINFALL ACROSS THE TRANSVERSE RANGES/VICINITY BUT
CONTINUED UNCERTAINTY IN HILARY'S EVENTUAL TRACK PRECLUDED THE
INCLUSION OF A MODERATE RISK AT THIS TIME. TO THE NORTH, MOISTURE
WILL INTERACT WITH A FRONTAL BOUNDARY ACROSS THE
NORTHWEST/NORTHERN ROCKIES PROVIDING SOME ENHANCEMENT OF RAINFALL
TO THAT REGION WITH BROAD MARGINAL RISKS 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 FLORIDA AND THE GULF OF MEXICO. THIS COULD ALSO BRING SOME
RAINFALL INTO SOUTHERN TEXAS AROUND THE MIDDLE OF NEXT WEEK.
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
SHOULD ALSO BUILD BACK INTO THE SOUTHEAST BY THE EARLY TO MIDDLE
OF NEXT WEEK. ELSEWHERE, UPPER TROUGHING OUT WEST SHOULD KEEP
HIGHS GENERALLY 5-15 DEGREES BELOW NORMAL WITH PARTS OF THE
SOUTHWEST 15-25 DEGREES BELOW NORMAL ESPECIALLY SUNDAY AND MONDAY
AS A RESULT OF THE INCREASED CLOUD COVER AND HEAVY PRECIPITATION
THREAT.
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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