250
FXUS02 KWBC 131859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT SUN AUG 13 2023
VALID 12Z WED AUG 16 2023 - 12Z SUN AUG 20 2023
...HEAT WAVE TO PREVAIL ACROSS THE SOUTH-CENTRAL U.S. THROUGH THE
PERIOD...
..HAZARDOUS HEAT ACROSS THE WEST TO WANE BY LATE WEEK
..OVERVIEW
THE UPPER FLOW PATTERN SHOULD BE FAIRLY AMPLIFIED ACROSS THE
NORTHERN TIER AND INTO PARTS OF THE EAST THIS WEEK AS AN INITIAL
SHORTWAVE EXITING THE NORTHEAST ON WEDNESDAY GIVES WAY TO ANOTHER
VIGOROUS SYSTEM MOVING THROUGH THE UPPER MIDWEST, GREAT LAKES, AND
THE NORTHEAST LATER THIS WEEK. MEANWHILE, HAZARDOUS HEAT WILL
CONTINUE ACROSS THE WEST INTO WEDNESDAY OR THURSDAY BEFORE AN
AMPLIFIED TROUGH COMING IN BY NEXT WEEKEND SHOULD QUICKLY BRING
TEMPERATURES BACK TO NORMAL OR BELOW NORMAL. HEAT OVER THE
SOUTHERN PLAINS AND CENTRAL GULF COAST LOOKS TO CONTINUE AND
EXPAND NORTHWARD INTO THE CENTRAL STATES THROUGH THE PERIOD.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
AS THE MEDIUM RANGE PERIOD BEGINS WEDNESDAY, MODEL GUIDANCE IS IN
GOOD AGREEMENT REGARDING INITIAL ENERGY/TROUGHING EXITING THE EAST
AND STRONG RIDGING FROM THE SOUTHERN PLAINS TO THE NORTHWEST, AS
WELL AS WITH A LINGERING BROAD CLOSED LOW OFF THE CALIFORNIA
COAST. THEN BY THURSDAY-FRIDAY, FORTUNATELY MODEL GUIDANCE HAS
COME INTO NOTABLY BETTER AGREEMENT WITH THE TIMING AND STRENGTH OF
THE NEXT SHORTWAVE THAT LOOKS TO TRACK ACROSS THE MIDWEST TO
NORTHEAST, AFTER SEVERAL GFS RUNS IN A ROW WERE SIGNIFICANTLY
FLATTER AND FASTER WITH THE SYSTEM. STARTING WITH ITS 00Z RUN AND
EVEN MORE SO IN THE 06Z AND 12Z RUNS, THE GFS STARTED TO TEND
TOWARD THE NON-NCEP CAMP WITH A MORE AMPLIFIED/ENERGETIC SLOWER
SHORTWAVE. THERE ARE STILL SOME TIMING DIFFERENCES BUT AT LEAST
THE SPREAD IS MORE REASONABLE/TYPICAL FOR THE DAY 4-5 LEAD TIME.
THE ECMWF/CMC/UKMET SPED UP JUST A BIT WITH ITS TRACK AS WELL,
ADDING TO THE BETTER AGREEMENT AMONG RECENT MODEL CYCLES BUT
LEADING TO A FASTER NUDGE FROM CONTINUITY. THUS THE WPC FORECAST
WAS ABLE TO DO A MULTI-MODEL DETERMINISTIC BLEND EARLY IN THE
FORECAST PERIOD.
MODEL GUIDANCE IS GENERALLY AGREEABLE WITH AN UPPER LOW IN THE
EASTERN PACIFIC MEANDERING GRADUALLY CLOSER TO AND PERHAPS OVER
THE WEST COAST BY NEXT WEEKEND. ITS POTENTIAL TO JOIN WITH THE
NORTHERN STREAM FLOW LATER IN THE PERIOD WILL DEPEND ON SHORTWAVE
DETAILS OF THAT FLOW...WHICH ARE RATHER UNCERTAIN INTO THE
NORTHWEST AND EASTWARD NEAR THE U.S./CANADA BORDER. ONE SUCH ISSUE
IS THE 00Z ECMWF AND NOW ALSO THE 12Z ECMWF (THOUGH TO A BIT
LESSER EXTENT) DEVELOPING A STRONG TROUGH ACROSS SOUTH-CENTRAL
CANADA AND THE NORTH-CENTRAL U.S. LATE THIS WEEK THAT PUTS IT OUT
OF PHASE WITH OTHER GUIDANCE AND AFFECTS TEMPERATURES
CONSIDERABLY. AS THE FORECAST PERIOD PROGRESSED, LESSENED AND
REMOVED THE ECMWF FROM THE BLEND WHILE REDUCING DETERMINISTIC
MODEL CONTRIBUTIONS OVERALL IN FAVOR OF THE PRETTY AGREEABLE GEFS
AND EC ENSEMBLE MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
THE LOWER 48 LOOKS FAIRLY QUIET FOR AUGUST IN TERMS OF
PRECIPITATION THROUGH AT LEAST MID TO LATE WEEK, THOUGH WITH SOME
AREAS OF CONCERN. AMPLE MOISTURE WITH PRECIPITABLE WATER ANOMALIES
LIKELY OVER THE 90TH PERCENTILE SHOULD POOL ALONG AND AHEAD OF A
LINGERING FRONTAL BOUNDARY OVER THE SOUTHEAST, WHICH COULD RESULT
IN POCKETS OF HEAVY TO EXCESSIVE RAINFALL ALONG THE SOUTHEAST
COAST AND OVER THE FLORIDA PENINSULA. AS SUCH, A MARGINAL RISKS
ARE IN PLACE FOR THIS REGION ON BOTH THE DAY 4/WEDNESDAY AND DAY
5/THURSDAY EXCESSIVE RAINFALL OUTLOOKS. FLORIDA LOOKS TO REMAIN
WET INTO NEXT WEEKEND TOO. TO THE NORTH, SHOWERS AND STORMS SHOULD
ACCOMPANY THE SHORTWAVE AND SURFACE FRONTAL SYSTEM THROUGH THE
GREAT LAKES INTO THE NORTHEAST WEDNESDAY-FRIDAY, WITH SOME
REMAINING UNCERTAINTY REGARDING RAINFALL AMOUNTS. AT THIS POINT IT
SEEMS ANY HEAVY RAIN WILL MOVE PRETTY PROGRESSIVELY TO PRECLUDE
ANY EXCESSIVE RAINFALL RISK, BUT THE RISK COULD BE NONZERO ACROSS
THE NORTHEASTERN U.S. THURSDAY-THURSDAY NIGHT ESPECIALLY IF
PREFRONTAL CONVECTION FORMS AND IS SLOWER MOVING THAN THE RAIN
ACCOMPANYING THE FRONT. ACROSS THE WEST, MONSOONAL MOISTURE COULD
CAUSE SCATTERED SHOWERS AND STORMS ACROSS THE FOUR CORNERS STATES
THROUGH WEDNESDAY-THURSDAY, AND RAINFALL IS LIKELY TO EXPAND IN
COVERAGE ACROSS THE WEST BY LATE WEEK INTO NEXT WEEKEND AS AN
AMPLIFIED TROUGH SHIFTS EASTWARD A BIT TOWARD/INTO THE WEST COAST
AND DRAWS IN PACIFIC MOISTURE THAT IS PERHAPS ENHANCED BY EASTERN
PACIFIC TROPICAL SYSTEMS.
THE BIG STORY DURING THE MEDIUM RANGE PERIOD THOUGH WILL CONTINUE
TO BE OPPRESSIVE HEAT ACROSS THE SOUTHERN PLAINS TO CENTRAL GULF
COAST. THERE MAY BE SOME SLIGHT MODERATION OF THE
TEMPERATURES/DEWPOINTS AROUND WEDNESDAY, BUT HEAT INDICES LOOK TO
RISE AGAIN THURSDAY AND BEYOND ACROSS THE SOUTH-CENTRAL CONUS.
NUMEROUS DAILY RECORD HIGHS AND WARM OVERNIGHT LOWS ARE LIKELY,
AND THE COMBINATION OF HEAT AND HUMIDITY WILL ALSO RESULT IN
EXTREMELY DANGEROUS HEAT INDICES IN EXCESS OF 110F IN MANY
LOCATIONS. THE HEAT WAVE ACROSS THE WEST SHOULD CONTINUE INTO AT
LEAST WEDNESDAY WITH TEMPERATURES AROUND 10-20 DEGREES ABOVE
AVERAGE ACROSS THE NORTHWEST AND AROUND 5-15 DEGREES ABOVE AVERAGE
IN THE CENTRAL VALLEY OF CALIFORNIA INTO THE DESERT SOUTHWEST.
THIS EQUATES TO HIGHS IN THE 90S/100S AND EVEN LOCALIZED 110S AS
WELL AS WARM LOWS, BOTH OF WHICH COULD SET TEMPERATURE RECORDS.
SEE KEY MESSAGES ISSUED BY WPC FOR ADDITIONAL INFORMATION AND
GRAPHICS RELATED TO THESE TWO HEAT WAVES. WESTERN AREAS'
TEMPERATURES COULD MODERATE SOMEWHAT BY THURSDAY BUT MORE
SIGNIFICANTLY ON FRIDAY AND BEYOND AS UPPER TROUGHING TAKES HOLD.
THIS WILL ALSO PUSH AN UPPER RIDGE EAST, SERVING TO WARM UP THE
CENTRAL U.S. BY LATE THIS WEEK. DAYTIME HIGHS IN THE 90S TO NEAR
100 LOOK TO BUILD INTO THE CENTRAL PLAINS TO MIDWEST, THOUGH WITH
SOME UNCERTAINTY IN HOW FAR NORTH THE ABOVE NORMAL TEMPERATURES
REACH.
TATE/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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