105
FXUS02 KWBC 151758
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM 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
OVERALL, THE MODEL GUIDANCE THIS CYCLE SHOWED FAIRLY GOOD
AGREEMENT, AT LEAST THROUGH DAY 5, WITH THE LARGE SCALE PATTERN.
THE EXCEPTION IS WITH THE SHORTWAVE ENERGY ORIGINATING FROM THE
GULF OF ALASKA AND HOW THAT INTERACTS WITH THE TROUGHING OVER THE
WESTERN U.S. (OR LACK OF) THIS WEEKEND INTO EARLY NEXT WEEK. THE
00Z/06Z GFS WAS A FASTER AND LESS PHASED OUTLIER, TAKING THE MAIN
SHORTWAVE ENERGY THROUGH SOUTHERN CANADA AND AMPLIFYING A
DOWNSTREAM TROUGH OVER THE GREAT LAKES AND NORTHEAST BY DAYS 6-7.
THIS SOLUTION, WHILE LESS PREFERRED, DOES HAVE SOME SUPPORT FROM
PREVIOUS RUNS AND NOW ALSO THE 12Z MODEL CYCLE, AND WOULD BE A
NOTABLE CHANGE TO THE END OF THE FORECAST PERIOD.
ANOTHER POTENTIALLY SIGNIFICANT SENSIBLE WEATHER EVENT LIES WITH
THE MOISTURE ANOMALIES POSSIBLY LIFTING NORTHWARD THROUGH THE
SOUTHWEST AND GREAT BASIN/NORTHERN ROCKIES LATER THIS WEEKEND
THROUGH EARLY NEXT WEEK, WITH SOME CONNECTION TO THE TROPICAL
SYSTEM CURRENTLY BEING TRACKED BY THE NATIONAL HURRICANE CENTER,
WHICH DEPICTS A HIGH CHANCE OF DEVELOPMENT. THE LATEST RUN OF THE
GFS LIES ON THE RIGHT SIDE OF THE MODEL SPREAD AND WHILE NOT
COMPLETELY RULED OUT, WAS LESS PREFERRED THAN THE MODEL CONSENSUS.
GREATER MODEL DIFFERENCES ALSO LIE WITH THE POTENTIAL TROPICAL
SYSTEM IN THE EASTERN PACIFIC, AND ITS INTERACTION WITH THE
EXISTING TROUGHING AND HOW MUCH MOISTURE WILL BE ABLE TO BE PULLED
NORTHWARD. A BLEND OF THE ECMWF/GFS ENSEMBLE MEANS WAS THE MODEL
CONSENSUS, THOUGH ITS WORTH NOTING THAT THE TRACK HAS EDGED
FURTHER EAST THIS CYCLE, WITH THE LATEST GFS CONSISTENTLY BEING
THE FURTHER EAST SOLUTION COMPARED TO THE OTHER DETERMINISTIC
MODELS. THE FURTHER EAST SOLUTIONS WOULD PROVIDE A MORE DIRECT
FEED OF THE MOISTURE INTO THE SOUTHWEST U.S., SO SOMETHING THAT
WILL NEED TO BE CLOSELY MONITORED OVER THE NEXT SEVERAL DAYS. THE
NBM WAS INITIALIZED FOR THE QPF GRIDS THIS CYCLE, WITH SOME
INCLUSION OF THE LATEST MULTI-MODEL ENSEMBLE BIAS CORRECTED QPF,
WHICH LARGELY TRENDED WELL WITH CONTINUITY, WITH SOME MINOR
INCREASES ACROSS PORTIONS OF ARIZONA, SOUTHERN CALIFORNIA, AND
SOUTHERN UTAH.
..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, BY DAY 5
(SATURDAY INTO SATURDAY NIGHT) THERE'S A GROWING SIGNAL THAT A
SLIGHT RISK COULD BE NEEDED IN FUTURE UPDATES ACROSS PORTIONS OF
WESTERN/SOUTHERN ARIZONA, SOUTHERN NEVADA, AND POTENTIALLY
SOUTHWEST UTAH. 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 EXPECTED. 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
AS A RESULT OF THE INCREASED CLOUD COVER AND THREAT OF HEAVY
PRECIPITATION.
SANTORELLI/TAYLOR
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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