546
FXUS02 KWBC 150655
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
255 AM EDT TUE AUG 15 2023
VALID 12Z FRI AUG 18 2023 - 12Z TUE AUG 22 2023
...HEAT WAVE TO PREVAIL ACROSS THE SOUTHERN PLAINS AND BUILD INTO
THE CENTRAL U.S...
...HEAVY RAINFALL POSSIBLE FOR PARTS OF THE SOUTHWEST THIS WEEKEND
INTO EARLY NEXT WEEK...
..OVERVIEW
THE UPPER LEVEL PATTERN DURING THE MEDIUM RANGE PERIOD
(FRIDAY-TUESDAY) WILL STAY AMPLIFIED AS A VIGOROUS SHORTWAVE
SHIFTS ACROSS THE GREAT LAKES AND NORTHEAST LATE THIS WEEK, WITH A
POTENTIAL EMBEDDED CLOSED LOW TRACKING OVER OR NEAR NORTHERN NEW
ENGLAND AROUND SATURDAY. BEHIND THIS FEATURE AN UPPER LEVEL RIDGE
SHOULD GRADUALLY TREND STRONGER AS IT SHIFTS INTO THE
PLAINS/EAST-CENTRAL U.S., WHILE UPPER TROUGHING STEADILY DESCENDS
OVER THE WEST COAST. AN INITIAL UPPER LOW OFF THE CALIFORNIA COAST
MAY WEAKEN IN FAVOR OF ANOTHER POSSIBLY UPPER LOW OVER THE
NORTHWEST COAST EARLY NEXT WEEK. HAZARDOUS HEAT ACROSS THE SOUTH
WILL EXPAND NORTHWARD INTO THE CENTRAL PLAINS BY THIS WEEKEND,
WHILE THE WEST SHOULD TREND COOLER AND WETTER, POSSIBLY FURTHER
ENHANCED BY HIGHLY UNCERTAIN DETAILS INVOLVING EASTERN PACIFIC
TROPICAL DEVELOPMENT AND ASSOCIATED MOISTURE. FARTHER EAST, EXPECT
THE EVOLVING PATTERN TO CONFINE MOST RAINFALL TO A BRIEF EPISODE
OVER THE NORTHEAST THURSDAY-FRIDAY AS WELL AS THE FLORIDA
PENINSULA AND SPREADING WESTWARD ALONG THE GULF COAST BY THE
WEEKEND OR EARLY NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
AMPLIFIED SHORTWAVE/EMBEDDED CLOSED LOW OVER THE NORTHEAST EARLY
PERIOD CONTINUES TO SHOW GOOD AGREEMENT OVERALL, ALTHOUGH THE CMC
IS A BIT SLOWER ON SATURDAY. AFTER SUNDAY, SOME DIFFERENCES IN THE
DETAILS OF UPPER TROUGHING OVER THE WEST COAST ALONG WITH EMBEDDED
CLOSED LOW OVER THE PACIFIC NORTHWEST EARLY NEXT WEEK. ALSO SOME
QUESTION IN AMPLITUDE OF ADDITIONAL SHORTWAVE ENERGY SLIDING
THROUGH SOUTH-CENTRAL CANADA WHICH IMPACTS NORTHWARD EXTENT OF THE
HEAT OVER THE UPPER MIDWEST. FOR ALL THESE FEATURES, GENERALLY
FAVORED A GENERAL MODEL BLEND WITH SOME INCORPORATION OF THE
ENSEMBLE MEANS BY THE SECOND HALF OF THE PERIOD.
MOST IMPACTFUL FOR SENSIBLE WEATHER HOWEVER, WILL BE RESOLVING HOW
MUCH MOISTURE REACHES THE SOUTHWEST BY THE WEEKEND AND EARLY NEXT
WEEK, WITH A POTENTIAL CONNECTION TO AN EASTERN PACIFIC TROPICAL
SYSTEM FOR WHICH THE NATIONAL HURRICANE CENTER SHOWS A HIGH CHANCE
OF FORMATION WITHIN THE NEXT SEVERAL DAYS. THE GEFS/ECENS MEANS
FORM A MOST REASONABLE TRACK FOR NOW THAT IS FAIRLY WELL OFFSHORE
THE WESTERN BAJA CALIFORNIA COAST BUT CMC AND GFS RUN HAVE
CONSISTENTLY BEEN FARTHER EAST AND WOULD PROVIDE A MORE DIRECT
FEED OF SIGNIFICANT MOISTURE INTO THE SOUTHWEST. THE ECMWF AND
ESPECIALLY THE UKMET ARE MORE WEST WITH THIS SYSTEM, BUT THE ECMWF
HAS TRENDED CLOSER TO THE MEANS/GFS COMPARED TO EVEN JUST A DAY
AGO. REGARDING THE RAINFALL FORECAST OVER THE SOUTHWEST DURING THE
LATTER HALF OF THE PERIOD, STAYED CLOSEST TO THE NBM FOR DAYS 5-7
BUT WITH SOMEWHAT TEMPERED AMOUNTS GIVEN THE LINGERING AND STILL
QUITE SIGNIFICANT UNCERTAINTY. COMPARED TO WPC CONTINUITY, DID
TREND SLIGHTLY WETTER.
..WEATHER/HAZARDS HIGHLIGHTS
MONSOONAL MOISTURE STREAMING NORTHWARD AHEAD OF WEST COAST
TROUGHING WILL KEEP MUCH OF THE INTERIOR WEST WET THROUGH THE
PERIOD. RAINFALL AMOUNTS OVER ESPECIALLY THE SOUTHWEST REMAIN
HIGHLY UNCERTAIN AND DEPENDENT ON POTENTIAL TROPICAL DEVELOPMENT
AND TRACK IN THE EASTERN PACIFIC. HOWEVER, MODELS CONTINUE TO
INDICATE POTENTIAL FOR HEAVY RAINFALL TO IMPACT PORTIONS OF THIS
REGION. GIVEN THE UNCERTAINTY THOUGH, THE DAYS 4 AND 5 EXCESSIVE
RAINFALL OUTLOOKS DEPICT ONLY A BROAD MARGINAL OVER THIS REGION
AND NORTHWARD INTO THE GREAT BASIN/ROCKIES. HOWEVER, SHOULD MODEL
CONSENSUS CONTINUE TO IMPROVE, IT'S LIKELY AT LEAST A SLIGHT RISK
WILL BE NEEDED IN FUTURE UPDATES TO THE ERO. TO THE NORTH,
MOISTURE WILL INTERACT WITH A FRONTAL BOUNDARY ACROSS THE
NORTHWEST WITH SOME ENHANCEMENT TO RAINFALL IN THAT REGION.
ELSEWHERE, SHOWERS AND STORMS WILL ACCOMPANY THE STRONG SHORTWAVE
THROUGH THE NORTHEAST, AND GIVEN SENSITIVITY IN THAT REGION TO
RAINFALL AND ANOMALOUS MOISTURE, A MARGINAL RISK WAS MAINTAINED ON
THE DAY 4/FRIDAY ERO. LINGERING MOISTURE WILL KEEP LOCALLY HEAVY
AND DIURNALLY DRIVEN RAINS ACROSS PARTS OF FLORIDA, AND TROPICAL
MOISTURE THROUGH THE GULF OF MEXICO LATE PERIOD MAY BEGIN TO
SPREAD RAINFALL ALONG THE GULF COAST.
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. HEAT WILL BEGIN TO
BUILD OVER THE CENTRAL PLAINS TO THE MIDWEST AS ANOMALOUS RIDGING
EXPANDS OVER THIS REGION. DAYTIME HIGHS NEAR 100 DEGREES ARE
POSSIBLE AS FAR NORTH AS THE UPPER MIDWEST, WHICH IS 15 TO 20
DEGREES ABOVE NORMAL. DAILY RECORD HIGHS ARE POSSIBLE INTO THE
CENTRAL PLAINS AND UPPER MISSISSIPPI VALLEY.HUMIDITY LEVELS WILL
SEND HEAT INDICES INTO THE 100S FOR MANY. SEE KEY MESSAGES ISSUED
BY WPC FOR ADDITIONAL DETAILS AND GRAPHICS RELATED TO THIS HEAT
WAVE. THE CURRENT HEAT WAVE OUT WEST SHOULD COME TO AN END BY THE
START OF THE MEDIUM RANGE PERIOD WITH UPPER TROUGHING KEEPING
HIGHS GENERALLY 5-15 DEGREES BELOW NORMAL. PARTS OF THE SOUTHWEST
COULD BE 15-20 DEGREES BELOW NORMAL ESPECIALLY SUNDAY AND MONDAY.
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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