389
FXUS02 KWBC 180754
PMDEPD
EXTENDED FORECAST DISCUSSION...AMENDED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
353 AM EDT FRI AUG 18 2023
VALID 12Z MON AUG 21 2023 - 12Z FRI AUG 25 2023
***HILARY TO BRING CONTINUED EXCESSIVE RAINFALL THREAT FROM
SOUTHERN CALIFORNIA INTO THE GREAT BASIN ON MONDAY***
***MAJOR HEAT WAVE EXPECTED ACROSS THE CENTRAL TO SOUTHEAST U.S.
THROUGH THE MIDDLE OF NEXT WEEK***
..SYNOPTIC OVERVIEW
AN IMPRESSIVE 600DM UPPER RIDGE WILL BE ANCHORED IN PLACE ACROSS
THE MIDWEST AND CENTRAL PLAINS FOR THE BEGINNING TO MIDDLE OF NEXT
WEEK WITH WIDESPREAD EXCESSIVE HEAT AND HUMIDITY ACCOMPANYING IT.
THIS RIDGE WILL LIKELY START TO DECAY SOME GOING INTO THE END OF
THE WEEK AND THEN GETS DISPLACED TOWARDS THE SOUTH AS SHORTWAVE
ENERGY IMPINGES UPON IT FROM THE NORTH, BUT THE HEAT AND HUMIDITY
WILL LIKELY CONTINUE FOR MUCH OF THE CENTRAL AND SOUTHERN STATES.
THE OTHER STORY MAKING WEATHER DEADLINES WILL BE THE ANOMALOUS
SURGE OF DEEP TROPICAL MOISTURE ASSOCIATED WITH THE TRACK OF
HILARY MOVING NORTH ACROSS THE DESERT SOUTHWEST AND EXTENDING TO
THE CALIFORNIA COASTAL AREAS, AND THEN NORTHWARD ACROSS MUCH OF
THE INTERMOUNTAIN WEST, WITH AN ELEVATED POTENTIAL FOR FLOODING.
ACROSS THE NORTHEAST U.S., AND UPPER LEVEL TROUGH IS EXPECTED TO
BE IN PLACE THROUGH AT LEAST WEDNESDAY AND THIS WILL SHIELD THE
REGION FROM THE EXCESSIVE HEAT THAT IS EXPECTED ACROSS THE CENTRAL
U.S., AND AN INCREASE OF MOISTURE ACROSS THE NORTHERN GULF WILL
FUEL MORE WIDESPREAD COVERAGE OF SHOWERS AND STORMS ACROSS
SOUTHERN TEXAS GOING INTO TUESDAY.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE 00Z MODEL GUIDANCE SUITE INDICATES GOOD OVERALL SYNOPTIC SCALE
AGREEMENT ACROSS THE FORECAST DOMAIN, WITH THE HIGHEST OVERALL
CONFIDENCE WITH THE HUGE UPPER RIDGE OVER THE CENTRAL U.S. EARLY
IN THE WEEK, WITH MULTIPLE MODELS INDICATING A RARE 600DM CONTOUR
OVER THE MIDWEST. THE MAIN DIFFERENCE EARLY ON IS THE MUCH FASTER
CMC SOLUTION WITH THE TRACK OF HILARY ACROSS SOUTHERN CALIFORNIA,
BUT OTHERWISE IT FITS WELL ELSEWHERE ACROSS THE NATION. LOOKING
AHEAD TO THE MIDDLE OF THE WEEK, THE CMC IS DISPLACED TO THE WEST
OF THE MODEL CONSENSUS ACROSS THE PACIFIC NORTHWEST WITH THE
POSITION OF THE CLOSED LOW, AND IT ALSO EXTENDS THE RIDGE AXIS
FROM THE PLAINS NORTHWARD INTO SOUTH-CENTRAL CANADA. THE GFS
TEMPERATURES WITH THE UPCOMING HEATWAVE ARE LIKELY TOO HIGH ACROSS
MUCH OF THE MIDWEST AND EVEN ACROSS PARTS OF THE GREAT LAKES AND
OHIO VALLEY, WITH THE ECMWF AND ENSEMBLE MEAN TEMPERATURES CLOSER
TO WHAT IS EXPECTED TO EVOLVE. THE FORECAST TRACK FOR HILARY
BRINGS A WEAKENING TROPICAL STORM TO SOUTHERN CALIFORNIA BY EARLY
MONDAY MORNING, AND QUICKLY BECOMES A REMNANT LOW BY MONDAY NIGHT.
THE WPC BLEND FOR THE FRONTS/PRESSURES INCORPORATED A NEARLY
DETERMINISTIC MODEL BLEND THROUGH EARLY WEDNESDAY, AND THEN
REDUCING THE CMC AND SLOWLY TRANSITIONING TO ABOUT 40% OF THE 18Z
GEFS/12Z ECENS ENSEMBLE MEANS FOR THURSDAY AND FRIDAY.
..WEATHER/HAZARDS HIGHLIGHTS
HEAVY RAINFALL WILL CONTINUE TO BE A THREAT ACROSS CENTRAL AND
SOUTHERN PORTIONS OF CALIFORNIA ON MONDAY, AND ALSO INCLUDING
PORTIONS OF SOUTH-CENTRAL NEVADA, AS THE MOISTURE PLUME STEADILY
LIFTS NORTH ACROSS THIS REGION. THERE IS A SLIGHT RISK OF
EXCESSIVE RAINFALL FOR THE DAY 4 PERIOD MONDAY, WITH THE MAJORITY
OF THE RAIN EXPECTED FROM 12Z MONDAY TO 00Z TUESDAY. SOME
ISOLATED 2-3 INCH RAINFALL MAXIMA ARE POSSIBLE ACROSS PORTIONS OF
THE CENTRAL SOUTHERN SIERRA, AND 1-2 INCHES FOR THE TRANSVERSE
MOUNTAINS. IF THE SYSTEM SLOWS DOWN MORE THAN EXPECTED, A FUTURE
UPGRADE TO MODERATE RISK MAY EVENTUALLY BECOME NECESSARY IN LATER
UPDATES.
FARTHER TO THE NORTH, TROPICAL MOISTURE WILL INTERACT WITH A
FRONTAL BOUNDARY ACROSS THE NORTHWEST/NORTHERN ROCKIES PROVIDING
ENHANCEMENT OF RAINFALL, AND THEREFORE A SLIGHT RISK IS IN EFFECT
FOR DAY 4 FROM EASTERN OREGON TO NORTHWESTERN MONTANA. MOST AREAS
FARTHER EAST ARE FORECAST TO REMAIN MOSTLY DRY UNDER THE EXPANSIVE
UPPER RIDGE, WITH THE EXCEPTION OF THE WESTERN GULF COAST AS
TROPICAL MOISTURE ADVECTS WESTWARD (WITH A MARGINAL RISK ON DAY 5
FOR SOUTHERN TEXAS).
A MAJOR HEATWAVE IS EXPECTED ACROSS MUCH OF THE MIDWEST AND
EXTENDING ACROSS THE CENTRAL AND SOUTHERN PLAINS THROUGH ABOUT
THURSDAY WITH THE MASSIVE UPPER RIDGE IN PLACE AND SUPPRESSING
CLOUD DEVELOPMENT. WIDESPREAD 100+ DEGREE HIGH TEMPERATURES ARE
EXPECTED FROM SOUTHERN SOUTH DAKOTA AND POINTS SOUTHWARD, AND SOME
OVERNIGHT LOWS MAY STAY ABOVE 80 DEGREES, ESPECIALLY FROM CENTRAL
TEXAS TO THE FLORIDA COAST. THESE READINGS ACROSS THE MIDWEST ARE
ON THE ORDER OF 10-20 DEGREES ABOVE LATE AUGUST AVERAGES, WITH THE
CORE OF THE ANOMALIES CENTERED MAINLY OVER IOWA THROUGH MID-WEEK.
THERE WILL LIKELY BE SOME ABATEMENT TO THE LATER SUMMER HEATWAVE
ACROSS THE MIDWEST BY FRIDAY AS THE RIDGE IS SUPPRESSED TO THE
SOUTH. THE OPPOSITE HOLDS TRUE FROM CALIFORNIA TO NEVADA AND
WESTERN UTAH WITH TEMPERATURES RUNNING 15 TO 25 DEGREES BELOW
AVERAGE OWING TO EXPANSIVE CLOUD COVER AND WIDESPREAD RAINFALL,
PERHAPS SETTING SOME RECORD LOW MAX TEMPERATURES ACROSS THIS
REGION. TEMPERATURES SLOWLY MODERATE GOING INTO THE SECOND HALF
OF THE WEEK ACROSS THIS REGION, WHILST STILL REMAINING SLIGHTLY
BELOW AVERAGE.
HAMRICK
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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