162
FXUS02 KWBC 110651
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
250 AM EDT FRI AUG 11 2023
VALID 12Z MON AUG 14 2023 - 12Z FRI AUG 18 2023
...SEVERE WEATHER AND FLASH FLOODING POSSIBLE ACROSS PARTS OF THE
UPPER MIDWEST TO EASTERN U.S. EARLY NEXT WEEK...
..HEAT WAVE TO PREVAIL ACROSS THE SOUTH INTO THE SOUTHWEST
..HAZARDOUS HEAT LIKELY OVER THE PACIFIC NORTHWEST NEXT WEEK
..OVERVIEW
UPPER RIDGING REMAINS LOCKED IN PLACE THROUGH THE PERIOD ACROSS
THE SOUTHEAST AND SOUTHERN PLAINS, WITH EXTENSION OF THE RIDGING
UP INTO THE NORTHWEST AS WELL. THIS SUPPORTS DUAL HEAT WAVES - THE
LONGSTANDING ONE ACROSS THE SOUTH AND A SECOND ONE BUILDING INTO
THE PACIFIC NORTHWEST THIS WEEKEND AND LASTING THROUGH MUCH OF
NEXT WEEK. TO THE NORTH, BROAD CYCLONIC MEAN FLOW WILL PERSIST
OVER THE NORTHERN HALF OR SO OF THE LOWER 48 EAST OF THE ROCKIES,
INCLUDING TWO INDIVIDUAL SYSTEMS TO MOVE THROUGH THE FLOW. THE
FIRST ONE SHOULD BE THE MOST ROBUST, MOVING FROM THE UPPER
MIDWEST, ACROSS THE GREAT LAKES, AND INTO THE NORTHEAST THE FIRST
HALF OF NEXT WEEK, POSSIBLY GENERATING EPISODES OF STRONG TO
SEVERE CONVECTION AND HEAVY RAINFALL. THE NEXT SYSTEM DROPS IN TO
THE NORTHERN PLAINS MID NEXT WEEK, WITH GREATER UNCERTAINTIES ON
STRENGTH AND HOW THIS MAY TRY TO BREAK DOWN THE RIDGE ACROSS THE
NORTHWEST.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MAIN AREA OF CONCERN REMAINS WITH THE VIGOROUS AND UNSEASONABLY
STRONG SHORTWAVE/EMBEDDED LOW TRACKING THROUGH THE UPPER MIDWEST,
GREAT LAKES, AND NORTHEAST. TIMING REMAINS UNCERTAIN, WITH THE GFS
CONTINUING TO BE FARTHER EAST/FASTER THAN THE NON-NCEP GUIDANCE
(WHICH IS A TYPICAL BIAS FOR THE GFS SUITE). CMC, ECMWF, AND UKMET
ALL SUPPORT SOMETHING QUITE A BIT SLOWER WHICH IS THE PREFERENCE
FROM WPC AT THIS TIME. THESE DIFFERENCES DO HAVE NOTABLE
IMPLICATIONS ON THE TIMING AND PLACEMENT OF QPF/SENSIBLE WEATHER.
ELSEWHERE, ADDITIONAL ENERGY MAY DROP IN FROM WEST-CENTRAL CANADA
LATER NEXT WEEK. GUIDANCE IS STILL VERY UNSURE WHETHER THIS IS A
WEAKER SHORTWAVE EMBEDDED WITHIN THE MAIN CYCLONIC FLOW ACROSS
CENTRAL-EASTERN CANADA (GFS/CMC), OR A MORE ROBUST SEPARATE
SHORTWAVE (ECMWF) WHICH MAY TRY TO BREAK DOWN THE WESTERN U.S.
RIDGING MORE. THERE IS NOT A WHOLE LOT OF SUPPORT AT THIS TIME FOR
THE STRONGER ECMWF FROM THE ENSEMBLE MEANS, ESPECIALLY WITH THE
NEW 00Z RUN WHICH CAME IN EVEN STRONGER AND WITH A COMPACT CLOSED
LOW OVER THE NORTHERN PLAINS NEXT THURSDAY. UPCOMING MODEL TRENDS
WILL NEED TO BE WATCHED WITH THIS FEATURE TO SEE IF THE OTHER
GUIDANCE STARTS TO TREND STRONGER.
THE WPC BLEND FOR DAYS 3 AND 4 USED A MULTI-MODEL DETERMINISTIC
MODEL BLEND, LEANING MOST HEAVILY ON THE 12Z/AUG 10 ECMWF FOR THE
INITIAL STRONG SYSTEM THROUGH THE GREAT LAKES/NORTHEAST WHICH WAS
MOST IN THE MIDDLE OF THE GUIDANCE SPREAD. BY DAY 5, REPLACED THE
UKMET WITH THE ENSEMBLE MEANS AS SPREAD BEGINS TO GROW WITH
REGARDS TO DETAILS WITHIN THE BROAD CYCLONIC FLOW. GRADUALLY
INCREASED THE ENSEMBLE MEANS (WITH LESS EMPHASIS ON THE ECMWF LATE
PERIOD) AMIDST GROWING UNCERTAINTIES WITH ADDITIONAL FEATURES.
THIS APPROACH ALSO WORKS FOR THE SOUTHERN U.S. UPPER RIDGE WHICH
SHOWS MUCH MORE CONSISTENCY AND AGREEMENT AND ALSO FITS WELL WITH
THE PREVIOUS WPC FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
THE MOST PROMINENT FOCUS FOR ACTIVE WEATHER DURING THE PERIOD WILL
BE FROM THE MIDWEST/GREAT LAKES INTO THE NORTHEAST NEXT
MONDAY-TUESDAY. STRONG COLD FRONT SWEEPING THROUGH THE REGION
SHOULD SUPPORT AREAS OF STRONG TO SEVERE THUNDERSTORMS AND HEAVY
RAINFALL ALONG AND AHEAD OF THE BOUNDARY. THIS IS REFLECTED IN THE
DAY 4 WPC EXCESSIVE RAINFALL OUTLOOK HIGHLIGHTING SLIGHT RISKS
ACROSS THE UPPER GREAT LAKES/MICHIGAN, NEAR AND ALONG THE TRACK OF
THE SURFACE LOW, WITH A SECOND RISK AREA ACROSS THE CENTRAL
APPALACHIANS. PLENTY OF UNCERTAINTY IN THE DETAILS STILL,
ESPECIALLY WITH REGARDS TO TIMING, SO THESE AREAS AREA LIKELY TO
BE MODIFIED WITH TIME. THE STORM PREDICTION CENTER ALSO HIGHLIGHTS
A RISK FOR SEVERE THUNDERSTORMS ACROSS PARTS OF THE OHIO
VALLEY/CENTRAL APPALACHIANS ON MONDAY AS WELL. THIS SYSTEM SHOULD
RACE QUICKLY THROUGH THE NORTHEAST ON TUESDAY, BUT GIVEN ANOMALOUS
MOISTURE AND SENSITIVITY IN SOME REGIONS OF THE NORTHEAST, HEAVY
RAINFALL AND/OR FLASH FLOODING CANNOT BE RULED OUT. A MARGINAL
RISK WAS ADDED ACROSS THIS REGION TO THE DAY 5 EXCESSIVE RAINFALL
OUTLOOK. SOME RISK FOR HEAVY RAINS NEAR AN AREA OF LOW PRESSURE
ALONG THE BOUNDARY FOR EASTERN NORTH CAROLINA AS WELL ON TUESDAY.
SHOWERS AND STORMS SHOULD ACCOMPANY THIS COLD FRONT INTO THE
SOUTHEAST/FLORIDA AS WELL. THE BROAD TROUGHING ACROSS THE GREAT
LAKES SHOULD CONTINUE UNSETTLED WEATHER INTO THE LATTER PART OF
NEXT WEEK AS WELL FOR THIS REGION. ELSEWHERE, MONSOONAL MOISTURE
WILL CONTINUE INTO THE SOUTHWEST/FOUR CORNERS NEXT WEEK, PEAKING
MONDAY INTO TUESDAY. AS SUCH, MARGINAL RISKS WERE CARRIED ON BOTH
DAYS IN THE EXCESSIVE RAINFALL OUTLOOKS.
OPPRESSIVE HEAT WILL CONTINUE THROUGH AT LEAST NEXT WEEK ACROSS
MUCH OF THE SOUTHERN TIER OF THE COUNTRY. IT WILL BE MOST
PERSISTENT FROM SOUTHERN AND SOUTHEAST TEXAS INTO LOUISIANA, WITH
SOME MODERATION EXPECTED ACROSS THE SOUTHEAST BY LATER NEXT WEEK.
NUMEROUS DAILY RECORD HIGHS AND WARM OVERNIGHT LOWS ARE LIKELY.
THE COMBINATION OF HEAT AND HUMIDITY WILL ALSO RESULT IN EXTREMELY
DANGEROUS HEAT INDICES IN EXCESS OF 110F IN MANY LOCATIONS. HEAT
SHOULD ALSO BUILD BACK INTO THE SOUTHWEST LATER NEXT WEEK AS WELL.
MEANWHILE, A SECOND AREA OF HEAT IS EXPECTED TO BUILD INTO PARTS
OF THE NORTHWEST THIS WEEKEND AND THROUGH MUCH OF NEXT WEEK, WITH
DAYTIME HIGHS 10 TO 20 DEGREES ABOVE NORMAL THROUGH THURSDAY. HIGH
TEMPERATURES IN THE 90S TO NEAR (OR ABOVE) 100 ARE LIKELY ACROSS
MUCH OF WASHINGTON, OREGON, AND THE INTERIOR VALLEY OF CALIFORNIA.
SHORTWAVE ENERGY INTO THE NORTHWEST AROUND FRIDAY OF NEXT WEEK MAY
BEGIN TO MODERATE TEMPERATURES SOME. SEE KEY MESSAGES ISSUED BY
WPC FOR ADDITIONAL INFORMATION AND GRAPHICS RELATED TO THESE TWO
HEAT WAVES.
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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