582
FXUS02 KWBC 181857
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 PM EDT FRI AUG 18 2023
VALID 12Z MON AUG 21 2023 - 12Z FRI AUG 25 2023
...HILARY TO SPREAD RARE AND DANGEROUS LEAD EXCESSIVE RAINFALL
THREAT THROUGH CALIFORNIA AND THE GREAT BASIN EARLY NEXT WEEK...
...MAJOR HEAT WAVE FOR THE CENTRAL TO SOUTHEAST U.S. INTO
MID-LATER NEXT WEEK...
...HEAVY RAIN THREAT MAY EMERGE OVER SOUTHERN TEXAS BY NEXT
MIDWEEK...
..SYNOPTIC OVERVIEW
A HIGHLY ANOMALOUS LEAD SURGE OF DEEP TROPICAL MOISTURE ASSOCIATED
WITH THE INLAND TRACK OF HILARY WILL SPREAD NORTH ACROSS THE
SOUTHWEST AND CALIFORNIA, AND THEN NORTHWARD ACROSS MUCH OF THE
INTERMOUNTAIN WEST INTO THE ROCKIES, TO FUEL AN ELEVATED AND
WIDESPREAD HEAVY RAINFALL AND FLOODING POTENTIAL.
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 MAY START TO DECAY GOING INTO THE END OF NEXT WEEK AND
DISPLACED TOWARDS THE SOUTH AS SHORTWAVE ENERGY IMPINGES UPON IT
FROM THE NORTH, BUT THE HIGH HEAT AND HUMIDITY WILL LIKELY
CONTINUE FOR MUCH OF THE CENTRAL AND SOUTHERN STATES. ACROSS THE
NORTHEAST U.S., AND UPPER LEVEL TROUGH IN PLACE THROUGH AT LEAST
NEXT MIDWEEK WILL SHIELD THE REGION FROM THE EXCESSIVE HEAT.
AN INCREASE OF TROPICAL MOISTURE UNDERNEATH THE AFOREMENTIONED
STRONG UPPER RIDGE IS EXPECTED TO WORK WESTWARD ACROSS THE GULF OF
MEXICO AND INLAND ACROSS TEXAS TO FUEL WIDESPREAD COVERAGE OF
SHOWERS AND THUNDERSTORMS LIKELY TO FOCUS ACROSS SOUTHERN TEXAS
GOING INTO TUESDAY AND SPREAD FURTHER WESTWARD IN EARNEST INTO
WEDNESDAY/THURSDAY.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM A
COMPOSITE BLEND OF BEST CLUSTERED GUIDANCE FROM THE 06 UTC GFS AND
00 UTC ECMWF/UKMET ALONG WITH THE 13 UTC NATIONAL BLEND OF MODELS
AND WPC/WFO CONTINUITY MONDAY INTO WEDNESDAY. GRAVITATED TO THE
MORE COMPATABLE 06 UTC GEFS MEAN AND THE 00 UTC NAEFS/ECMWF
ENSEMBLE MEANS LATER WEDNESDAY INTO THURSDAY AMID GROWNING
FORECAST SPREAD, ESPECIALLY WITH THE SPLIT OF UPPER TROUGHING INTO
THE EASTERN PACIFIC/WEST COAST OR MORE INLAND TO SOUTHWEST CANADA.
THE ECMWF OFFERS LESS THAN STELLAR RUN TO RUN CONTINUITY FOR THESE
FEATURES. THE GFS WAS ALSO DISCOUNTED BY MID-LATER NEXT WEEK GIVEN
UNCERTAINTIES WITH TEMPERATURES FIELDS THAT AGAIN DO NOT SEEM IN
LINE WITH MOST OTHER GUIDANCE, ESPECIALLY OVER A HOT NORTH-CENTRAL
U.S. AND MIDWEST/GREAT LAKES.
..WEATHER/HAZARDS HIGHLIGHTS
A RARE AND DANGEROUS HEAVY RAINFALL EPISODE WILL CONTINUE TO BE A
THREAT ACROSS CENTRAL AND SOUTHERN PORTIONS OF CALIFORNIA INTO
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 2-3 INCH RAINFALL MAXIMA ARE POSSIBLE ACROSS
PORTIONS OF THE CENTRAL SOUTHERN SIERRA, AND 1-2 INCHES FOR THE
TRANSVERSE MOUNTAINS, ALBEIT WITH SOME RISK FOR HEAVIER TERRAIN
ENHANCED AMOUNTS LOCALLY. FARTHER TO THE NORTH, LEAD TROPICAL
MOISTURE WELL AHEAD OF HILARY 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, WITH
ADDITIONAL FUEL FOR ACTIVITY UP THROUGH THE GREAT BASIN WELL
DOWNSTREAM OF HILARY IN DEEPLY MOIST FLOW.
MEANWHILE, EXPECT AN EXPANDING RAINFALL AREA TO WORK INLAND FROM
THE GULF OF MEXICO THROUGH THE WESTERN GULF COAST STARTING TUESDAY
AS DEEPENED TROPICAL MOISTURE ADVECTS WESTWARD. POSSIBLE TROPICAL
CONNECTION AND INLAND SURGE FAVORED THE ISSUANCE OF A MARGINAL
RISK EXCESSIVE RAINFALL THREAT THROUGH MIDWEEK IN THE DAY 5 ERO
FOR SOUTHERN TEXAS. THIS THREAT AREA HAS ALSO BEEN EXPANDED
WESTARD AND AIMS AT THE BIG BEND/SW TEXAS INTO THURSDAY GIVEN
GUIDANCE AND NOW GREATER NHC WESTERN GULF LOW POTENTIAL.
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/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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