471
FXUS02 KWBC 140712
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
311 AM EDT MON AUG 14 2023
VALID 12Z THU AUG 17 2023 - 12Z MON AUG 21 2023
...HEAT WAVE TO PREVAIL ACROSS THE SOUTHERN PLAINS AND BUILD INTO
THE CENTRAL U.S. BY THIS COMING WEEKEND...
...HAZARDOUS HEAT ACROSS PARTS OF THE WEST TO WANE AFTER
THURSDAY...
..OVERVIEW
THE UPPER LEVEL PATTERN DURING THE MEDIUM RANGE PERIOD
(THURSDAY-MONDAY) WILL TREND MORE AMPLIFIED WITH TIME. A VIGOROUS
SHORTWAVE WILL SHIFT ACROSS THE GREAT LAKES AND NORTHEAST LATER
THIS WEEK AS STRONG UPPER LEVEL RIDGING HOLDS STRONG ACROSS THE
SOUTHERN TIER INTO THE NORTHWEST. SHORTWAVE ENERGY AND LOWERING
HEIGHTS SHOULD ERODE THE RIDGE OVER THE NORTHWEST AFTER THURSDAY
WITH RIDGING THEN BEGINNING TO BUILD ACROSS THE CENTRAL U.S. AND
TROUGHING OVERTAKING THE WEST. HAZARDOUS HEAT ACROSS THE SOUTH
WILL EXPAND NORTHWARD INTO THE CENTRAL PLAINS BY THIS WEEKEND,
WITH COOL AND WET CONDITIONS ACROSS THE WEST, POSSIBLY FURTHER
ENHANCED BY HIGHLY UNCERTAIN EASTERN PACIFIC TROPICAL DEVELOPMENT
AND ASSOCIATED MOISTURE.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL AGREEMENT CONTINUES TO IMPROVE COMPARED TO PREVIOUS DAYS
WITH REGARDS TO THE STRONG SHORTWAVE THROUGH THE GREAT LAKES AND
NORTHEAST AND FRIDAY. THERE REMAINS SOME DETAIL DIFFERENCES
INCLUDING STRENGTH AND TIMING THROUGH THE NORTHEAST, BUT A GENERAL
MODEL BLEND SEEMS TO MITIGATE THESE NOW MORE TYPICAL DIFFERENCES
FOR A DAY 3-4 LEAD TIME. MODEL GUIDANCE IS ALSO VERY AGREEABLE
WITH AN UPPER LOW 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 FLOW
DETAILS OVER CANADA, WHICH REMAIN RATHER UNCERTAIN. LAST FEW RUNS
OF THE ECMWF ARE STRONGER WITH TROUGHING THROUGH CENTRAL CANADA
ALLOWING IT TO QUICKLY BECOME OUT OF PHASE WITH OTHER GUIDANCE.
THIS AFFECTS ADDITIONAL ENERGY SLIDING THROUGH THE NORTHWEST,
STAYING SEPARATE FROM THE WEST COAST UPPER LOW (AS SUGGESTED BY
YESTERDAYS RUNS OF THE CMC AND GFS). THE ECMWF HOWEVER IS STRONGER
WITH THE NEXT ENERGY DROPPING OUT OF THE GULF OF ALASKA AND IS
MORE AGGRESSIVE WITH COMBINING WITH THE WEST COAST LOW LATE PERIOD
COMPARED WITH THE GFS AND CMC RUNS. THE ENSEMBLE MEANS ALSO
SUGGEST A SOLUTION MORE LIKE THE ECMWF (JUST MUCH WEAKER) AND IT'S
INTERESTING TO NOTE THAT THE NEW 00Z GFS RUN (AND TO SOME EXTENT
THE 00Z CMC AS WELL) HAVE TRENDED IN THIS DIRECTION AS WELL. THIS
IS COMPOUNDED IN THE GFS AND CMC HOWEVER BY A POTENTIAL TROPICAL
SYSTEM (WITH A HIGH CHANCE OF FORMATION WITHIN 7 DAYS PER LATEST
NHC FORECASTS). BOTH THE GFS AND CMC (INCLUDING THEIR NEW RUNS
TONIGHT) HAVE BEEN CONSISTENT LIFTING THIS SYSTEM NORTHWARD ALONG
THE BAJA CALIFORNIA COAST REACHING SOUTHWEST CALIFORNIA AROUND DAY
7/MONDAY IN A MUCH WEAKER STATE, WHEREAS THE ECMWF PULLS THIS
SYSTEM MORE WESTWARD OUT INTO THE EASTERN PACIFIC. THE NEW 00Z
ECMWF RUN THIS MORNING HOWEVER DID TREND IN A MORE NORTHERLY
DIRECTION, THOUGH STILL WEST AND MORE REMOVED FROM BAJA
CALIFORNIA/THE SOUTHWEST THAN THE GFS/CMC. ENSEMBLE GUIDANCE
SUGGEST A MORE WEST/NORTHWEST TRACK CLOSER TO THE ECMWF WHICH
GIVES LESS CONFIDENCE IN GFS/CMC SOLUTIONS AT THIS TIME. THE
EVENTUAL TRACK OF THIS SYSTEM MAY INFLUENCE EVOLUTION OF THE FLOW
PATTERN OVER THE WESTERN U.S. LATE PERIOD. IT'S OBVIOUSLY AT THIS
POINT A VERY VOLATILE AND UNCERTAIN FORECAST AND WILL DEFINITELY
NEED TO BE MONITORED IN THE COMING DAYS TO SEE WHAT, IF ANY,
IMPACTS THIS WILL HAVE ON SENSIBLE WEATHER ACROSS THE SOUTHWEST
U.S. THIS WEEKEND AND EARLY NEXT WEEK.
THE WPC BLEND FOR TONIGHT USED A GENERAL MODEL BLEND OF THE GFS,
CMC, AND ECMWF FOR THE FIRST HALF OF THE PERIOD. GIVEN EXTREME
UNCERTAINTY LATE PERIOD OVER THE WESTERN U.S., QUICKLY TRENDED
TOWARDS A MAJORITY BLEND OF THE ENSEMBLE MEANS TO HELP MITIGATE
THESE DIFFERENCES UNTIL DETAILS BECOME MORE CLEAR.
..WEATHER/HAZARDS HIGHLIGHTS
A WEAKENING FRONTAL BOUNDARY ALONG THE SOUTHEAST COAST WILL ALLOW
ANOMALOUS MOISTURE TO LINGER OVER FLORIDA WHERE A MARGINAL RISK IS
IN PLACE OVER MUCH OF THE FLORIDA PENINSULA FOR THE DAY 4/THURSDAY
EXCESSIVE RAINFALL OUTLOOK. TO THE NORTH, SHOWERS AND STORMS
SHOULD ACCOMPANY THE SHORTWAVE AND SURFACE FRONTAL SYSTEM THROUGH
THE GREAT LAKES INTO THE NORTHEAST THURSDAY-FRIDAY. IT SEEMS ANY
HEAVY RAIN SHOULD BE FAIRLY LOCALIZED AND 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.
BEGINNING THURSDAY, BUT ESPECIALLY INTO THIS WEEKEND, MONSOONAL
MOISTURE WILL INCREASE ACROSS THE SOUTHWEST AND EXPAND NORTHWARD
INTO THE GREAT BASIN/ROCKIES AS UPPER TROUGHING BUILDS OVER THE
REGION. FOR THE SOUTHWEST LATER IN THE WEEKEND AND EARLY NEXT
WEEK, AMOUNTS ARE HIGHLY UNCERTAIN AND DEPENDENT ON POTENTIAL
TROPICAL DEVELOPMENT AND TRACK IN THE EASTERN PACIFIC. SHOULD THE
GUIDANCE TREND TOWARDS A MORE NORTHERLY TRACK AS SUGGESTED RIGHT
NOW BY THE GFS AND CMC, THIS WOULD BRING SIGNIFICANT MOISTURE AND
RAINFALL INTO PARTS OF OF THE SOUTHWEST. AT THIS POINT, THE WPC
QPF TRENDS WETTER WITH TIME OVER THE REGION (AND THE WEST AS A
WHOLE), BUT REMAINS ON THE MORE CONSERVATIVE SIDE REGARDING
RAINFALL AMOUNTS AND IMPACTS AS THIS VERY LATE PERIOD FORECAST IS
LIKELY TO CHANGE IN THE COMING DAYS.
MEANWHILE, OPPRESSIVE HEAT WILL CONTINUE THROUGH THE PERIOD ACROSS
THE SOUTHERN PLAINS TO CENTRAL GULF COAST WITH NUMEROUS DAILY
RECORD HIGHS AND WARM OVERNIGHT LOWS LIKELY. 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 NORTHWEST SHOULD CONTINUE INTO THURSDAY 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, WHICH COULD SET TEMPERATURE RECORDS (BOTH
DAYTIME HIGHS AND WARM OVERNIGHT LOWS). SEE KEY MESSAGES ISSUED BY
WPC FOR ADDITIONAL INFORMATION AND GRAPHICS RELATED TO THESE TWO
HEAT WAVES. THE HEAT WAVE ACROSS THE WEST SHOULD COME TO AN END ON
FRIDAY AS UPPER TROUGHING TAKES HOLD OVER THE REGION THIS WEEKEND.
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, WITH
HUMIDITY LEVELS BRINGING HEAT INDICES EVEN HIGHER. HAZARDOUS HEAT
IS BECOMING MORE AND MORE LIKELY ACROSS THIS REGION STARTING THIS
WEEKEND AND LASTING INTO NEXT WEEK.
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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