693
FXUS02 KWBC 141901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM 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, WITH A POTENTIAL EMBEDDED CLOSED LOW TRACKING OVER OR
NEAR NORTHERN NEW ENGLAND AROUND SATURDAY. BEHIND THIS FEATURE AN
UPPER LEVEL RIDGE INITIALLY OVER THE WEST SHOULD GRADUALLY TREND
STRONGER AS IT SHIFTS INTO THE PLAINS/EAST-CENTRAL U.S., WHILE
UPPER TROUGHING STEADILY DESCENDS OVER THE WEST COAST. THIS TROUGH
MAY EJECT AN UPPER LOW THAT WILL BE OFFSHORE CALIFORNIA THROUGH
LATE THIS 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
THE MOST NOTICEABLE TREND IN MOST GUIDANCE OVER THE PAST DAY OR SO
IS TOWARD GREATER AMPLITUDE OVERALL. THIS INCLUDES A SLOWER AND
MORE CLOSED UPPER TROUGH CROSSING NEW ENGLAND/SOUTHEASTERN CANADA,
AS WELL AS STRONGER/MORE AMPLIFIED PLAINS/MISSISSIPPI VALLEY
RIDGING DURING THE WEEKEND AND EARLY NEXT WEEK DOWNSTREAM FROM
DEEPER TROUGHING THAT SETTLES OVER THE WEST COAST. THERE ARE STILL
INDIVIDUAL SOLUTIONS THAT DIFFER SIGNIFICANTLY FROM THIS MAJORITY
TREND. THE 00Z UKMET/CMC BOTH STRAYED A LOT FASTER WITH NORTH
PACIFIC/ALEUTIANS FLOW THAT IMPACTED THE DOWNSTREAM PATTERN, BUT
THE 00Z CMCENS MEAN LOOKED MUCH CLOSER TO THE GFS/ECMWF AND
GEFS/ECENS TO ADD SUPPORT TO THE LATTER MAJORITY. THE NEW 12Z
UKMET IS QUITE PROGRESSIVE WITH ENERGY CROSSING SOUTH-CENTRAL
CANADA (INTERESTINGLY, HINTED AT IN THE NEW 12Z ECMWF) BUT HAS
ADJUSTED CLOSER TO THE GFS/ECMWF IN PRINCIPLE FARTHER WEST. ON THE
OTHER HAND, THE 12Z CMC IS STILL SOMEWHAT PROGRESSIVE OVER THE
PACIFIC BUT IS GETTING CLOSER TO THE MAJORITY FARTHER EAST. THE
06Z COMPARED BETTER TO LATEST CLUSTERING THAN THE FASTER 12Z GFS
FOR THE NEW ENGLAND/SOUTHEASTERN CANADA UPPER LOW.
SPECIFICS OF THE WEST COAST PATTERN WILL BE IMPORTANT FOR
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 COMING 7 DAYS. THE 00Z
ECMWF/ECMWF MEAN AND 06Z GFS/GEFS MEAN FORM A MAJORITY TRACK THAT
IS FAIRLY WELL OFFSHORE THE WESTERN BAJA CALIFORNIA COAST BUT CMC
RUNS AND THE 00Z GFS (WITH THE NEW 12Z GFS BETWEEN THE TWO) ARE
FARTHER EAST AND WOULD PROVIDE A MORE DIRECT FEED OF SIGNIFICANT
MOISTURE INTO THE SOUTHWEST. AT THE VERY LEAST CMC RUNS ARE IN THE
MINORITY FOR EXACT UPPER LEVEL DETAILS ALONG THE CALIFORNIA COAST,
TEMPERING CONFIDENCE IN ITS TROPICAL SYSTEM TRACK.
GIVEN THE DIFFERENCES THAT APPEARED IN THE 00Z UKMET/CMC RATHER
EARLY (AFTER DAY 3 THURSDAY), THE UPDATED FORECAST STARTED WITH AN
AVERAGE OF THE 06Z GFS/00Z ECMWF EARLY IN THE PERIOD AND THEN
TRANSITIONED TO 50-60 PERCENT TOTAL GEFS/ECENS INPUT BY DAYS 6-7
SUNDAY-MONDAY GIVEN THE TYPICAL INCREASE IN DETAIL UNCERTAINTY AT
THAT TIME. REGARDING MOISTURE/RAINFALL OVER THE SOUTHWEST DURING
THE LATTER HALF OF THE PERIOD, LATEST TRENDS HAVE DRAMATICALLY
INCREASED NBM GUIDANCE OVER THE REGION. THE MANUAL FORECAST
REFLECTS A COMPROMISE BETWEEN CONTINUITY AND THE NBM AS A MORE
MEASURED INCREASE FOR THE DAY 6 PERIOD, WHILE THE NBM WAS CLOSER
TO A REASONABLE INTERMEDIATE SOLUTION BY DAY 7.
..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 VIGOROUS SHORTWAVE/POSSIBLE CLOSING UPPER LOW
AND SURFACE FRONTAL SYSTEM THROUGH THE GREAT LAKES INTO THE
NORTHEAST THURSDAY-FRIDAY. RECENT TRENDS TOWARD SLOWER SYSTEM
PROGRESSION (MINUS THE 12Z GFS) ALONG WITH ABOVE AVERAGE SOIL
MOISTURE OVER THE NORTHEAST HAVE LED TO A PLANNED INTRODUCTION OF
A MARGINAL RISK AREA OVER PARTS OF THE NORTHEAST FOR THE DAY
5/FRIDAY OUTLOOK. 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
SETTLES NEAR THE WEST COAST. THE DAY 4 ERO SHOWS A MARGINAL RISK
AREA OVER PARTS OF THE FOUR CORNERS REGION EXPANDING OVER MORE OF
UTAH/ARIZONA AND WESTERN COLORADO INTO THE DAY 5 PERIOD. MEANWHILE
THE COMBINATION OF ANOMALOUSLY HIGH MOISTURE AND SHORTWAVE
ENERGY/UPPER HEIGHT FALLS MAY ALSO HELP TO FOCUS LOCALLY HEAVY
CONVECTION OVER AND EAST OF THE SIERRA NEVADA. THUS THE PLAN IS TO
INTRODUCE ASSOCIATED MARGINAL RISK AREAS FOR THIS ACTIVITY DURING
DAYS 4-5, WITH THE DAY 5 OUTLOOK CONFINED MORE TO JUST THE SIERRA
NEVADA AS SOME OF THE MOISTURE TO THE EAST LIFTS NORTHWARD. 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. THE UPDATED WPC QPF
HAS TRENDED SOMEWHAT WETTER IN LIGHT OF THE GUIDANCE AVERAGE AND
THE POTENTIAL THAT MOISTURE FLOW INTO THE SOUTHWEST COULD BE
UNDERDONE IN THE 06Z GFS/00Z ECMWF EVEN IF THEIR MORE WESTERN
TROPICAL SYSTEM TRACKS VERIFY, BUT THE UNCERTAINTY SEEMED TO MERIT
LOWER AMOUNTS THAN THE 13Z NBM ESPECIALLY AROUND DAY 6/SUNDAY.
CONTINUE TO MONITOR UPCOMING FORECASTS AS THIS APPEARS TO BE A
VERY SENSITIVE EVOLUTION AND RAINFALL IN THE WETTER TWO-THIRDS OF
THE SPREAD COULD HAVE SIGNIFICANT EFFECTS ON PARTS OF THE
SOUTHWEST.
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 FOR
DAYTIME HIGHS. DAILY RECORDS FOR WARM OVERNIGHT LOWS ARE POSSIBLE
TOO. 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, WITH A BROAD AREA OF HIGHS 5-15
DEGREES BELOW NORMAL BY NEXT SUNDAY-MONDAY. THE EASTWARD SHIFT OF
THE UPPER RIDGE WILL HELP 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. THE COOLING TREND OVER THE WEST SHOULD LEAD TO A BROAD
AREA OF HIGHS 5-15 DEGREES BELOW NORMAL BY NEXT SUNDAY-MONDAY.
RAUSCH/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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