240
FXUS02 KWBC 301723
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
122 PM EDT WED AUG 30 2023
VALID 12Z SAT SEP 2 2023 - 12Z WED SEP 6 2023
...HEAT FORECAST TO INCREASE ACROSS THE PLAINS AND MIDWEST THIS
WEEKEND INTO EARLY NEXT WEEK...
..WET PATTERN ACROSS THE WEST WITH FLOODING POSSIBLE
18Z UPDATE: THE LATEST GUIDANCE AGREES WELL ON THE OVERALL
WEATHER PATTERN ACROSS THE UNITED STATES GOING INTO THE WEEKEND.
MODEL DIFFERENCES START BECOMING APPARENT ACROSS THE GULF OF
ALASKA BY SUNDAY REGARDING THE TIMING OF A SHORTWAVE TROUGH AND
POTENTIAL CLOSED LOW THAT WILL REACH THE PACIFIC NORTHWEST EARLY
NEXT WEEK. ADDITIONAL UNCERTAINTIES ARE INTRODUCED WHEN THIS
DISTURBANCE MAY PHASE WITH THE BROAD UPPER TROUGH THAT WILL
INITIALLY BE OVER THE INTERMOUNTAIN WEST, AND THIS WILL HAVE SOME
RAMIFICATIONS ON HOW THE TROUGH AND ASSOCIATED SURFACE LOW EVOLVES
ACROSS THE NORTHERN PLAINS BY NEXT TUESDAY-WEDNESDAY AND THE
EVENTUAL BREAK-DOWN OF THE UPPER RIDGE SITUATED OVER THE PLAINS.
IN TERMS OF THE FRONTS/PRESSURES FORECAST, THE 00Z CMC APPEARED TO
BE TOO FAR NORTH WITH THE SURFACE LOW BY THE FIRST HALF OF NEXT
WEEK WHEN COMPARED TO THE MODEL CONSENSUS, SO THE LATTER HALF OF
THE FORECAST PERIOD FOCUSED MORE ON A GFS/ECMWF/ENSEMBLE/WPC
CONTINUITY BLEND FOR THE EVOLVING LOW PRESSURE SYSTEM AND THE
ASSOCIATED FRONTS. IN TERMS OF TEMPERATURES, THE OUTPUT FROM THE
GFS IS PROBABLY TOO HIGH ACROSS MUCH OF THE MIDWEST STATES AND
EXTENDING EASTWARD TO THE MID-ATLANTIC, SO THE NBM AND ECMWF
SERVED AS A BETTER BASELINE FOR TEMPERATURES. THE PREVIOUS
FORECAST DISCUSSION IS APPENDED BELOW FOR REFERENCE. /HAMRICK
-------------------------
..OVERVIEW
AN AMPLIFIED UPPER-LEVEL PATTERN WILL BE IN PLACE THIS WEEKEND,
COMPOSED OF AN UPPER TROUGH (WITH AN EMBEDDED CLOSED LOW INTO
SATURDAY) OVER THE WEST COAST AND AN UPPER RIDGE OVER THE CENTRAL
CONUS. THESE FEATURES SHOULD SHIFT GRADUALLY EASTWARD NEXT WEEK.
THIS PATTERN WILL ALLOW FOR WELL ABOVE AVERAGE TEMPERATURES TO
BUILD ACROSS THE PLAINS AND UPPER MIDWEST, SHIFTING INTO THE EAST
BY MONDAY-WEDNESDAY, WHILE RAIN CHANCES ACROSS PARTS OF THE WEST
THIS HOLIDAY WEEKEND COULD CAUSE FLOODING CONCERNS. MEANWHILE
SCATTERED SHOWERS AND STORMS COULD PERSIST ACROSS THE GULF COAST
REGION INTO FLORIDA AS A SHORTWAVE ALOFT AND A FRONTAL BOUNDARY
LINGER THERE.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE AGREES QUITE WELL WITH THE SYNOPTIC MIDLATITUDE
PATTERN AT THE START OF THE PERIOD SATURDAY, WITH AN ANOMALOUSLY
STRONG UPPER HIGH ATOP THE CENTRAL U.S., A CLOSED UPPER LOW OVER
NORTHERN CALIFORNIA, AND A WEAK SHORTWAVE TROUGH MOVING THROUGH
THE GREAT LAKES TO NORTHEAST. THESE FEATURES ARE FORECAST TO MOVE
EASTWARD INTO EARLY NEXT WEEK, AS GREATER DIFFERENCES START TO
ARISE WITH AN ENERGETIC SHORTWAVE MOVING QUICKLY THROUGH THE
EASTERN PACIFIC UPSTREAM OF THE UPPER LOW. THE INTERACTION OF THIS
UPSTREAM ENERGY WITH THE PREEXISTING LOW COULD COMBINE WITH THE
TROUGH AND/OR HELP PUSH IT EASTWARD, WITH VARIATIONS AMONG MODELS.
THE DETAILS OF THIS FLOW DO HAVE IMPACTS FOR POSITIONS OF SURFACE
FRONTS MOVING ACROSS THE WEST. IN TERMS OF THE 12/18Z GUIDANCE,
THE TIMING OF THE ECMWF AND GFS SEEMED MOST ALIGNED COMPARED TO
THE UKMET AND CMC. THE 12Z CMC WAS DEEPER THAN CONSENSUS WITH THE
TROUGH BY TUESDAY-WEDNESDAY AS IT SHIFTS INTO THE NORTH-CENTRAL
CONUS. ANOTHER SHORTWAVE MAY COME IN AROUND MIDWEEK TO FURTHER
COMPLICATE MATTERS. SO THE WPC FORECAST BLEND FAVORED THE EC AND
GFS WITH SMALLER PROPORTIONS OF THE UKMET AND CMC EARLY,
EVENTUALLY REMOVING THOSE WHILE INCLUDING SOME GEFS AND EC
ENSEMBLE MEANS REACHING AROUND HALF BY THE END OF THE PERIOD AMID
INCREASING UNCERTAINTY.
SOMETHING ELSE WE HAVE BEEN MONITORING IS THE EVENTUAL TRACK OF
IDALIA AFTER IT MOVES INTO THE WESTERN ATLANTIC. MANY GFS RUNS AND
SOME CMC RUNS, AS WELL AS SOME ENSEMBLE MEMBERS FROM BOTH SUITES,
DRIFTED THE REMAINING LOW FROM IDALIA SOUTHWARD EAST OF FLORIDA
WITH SOME EVEN TURNING IT WESTWARD INTO FLORIDA. ECMWF AND MOST OF
ITS ENSEMBLE MEMBERS INSTEAD TOOK THE SYSTEM EAST AWAY FROM THE
U.S. COAST. NOW THE NEWER SUITE OF 00Z GUIDANCE HAS TRANSITIONED
TOWARD THIS EASTWARD TRACK AND ARE EVEN FASTER TAKING IT FARTHER
OUT INTO THE ATLANTIC. SO WILL CONTINUE TO MONITOR BUT IT MAY BE
MOST CONCERNING TO BERMUDA AT THIS POINT.
..WEATHER/HAZARDS HIGHLIGHTS
RAINY CONDITIONS ARE LIKELY ACROSS MUCH OF THE WEST THROUGH THIS
WEEKEND WITH THE AMPLIFYING UPPER TROUGH/EMBEDDED CLOSED LOW
SETTLED OVER THE WEST COAST. THE UPPER LOW ITSELF WILL PROVIDE
FORCING FOR ASCENT AND HELP CAUSE STEEPENING LAPSE RATES FOR SOME
INSTABILITY INTO THE WEST COAST, WHILE EAST OF THE TROUGH STRONG
MEAN SOUTHERLY FLOW WILL CARRY MONSOONAL MOISTURE WITH
PRECIPITABLE WATER VALUES WELL ABOVE NORMAL AND INSTABILITY INTO
THE INTERIOR WEST. ON SATURDAY, A BROAD MARGINAL RISK IS PLANNED
FOR AREAS FROM OREGON AND NORTHERN CALIFORNIA INTO THE GREAT BASIN
AND SOUTHWEST, WITH AN EMBEDDED SLIGHT RISK WHERE THE HEAVIEST
RAINFALL LOOKS TO FOCUS IN THE CENTRAL GREAT BASIN. FOR THE 18Z
UPDATE, THE EXISTING SLIGHT RISK WAS EXTENDED SOME TO THE SOUTH TO
INCLUDE MORE OF NORTHWESTERN ARIZONA. THE SHIFT OF THE TROUGH
EASTWARD ON SUNDAY SHOULD POSITION FOCUSED MOISTURE ACROSS THE
INTERMOUNTAIN WEST, WITH A SLIGHT RISK PROPOSED A BIT FARTHER
NORTH COMPARED TO SATURDAY. THE 18Z UPDATE INCLUDED A SMALL
NORTHWARD EXTENSION OF THE SLIGHT RISK TOWARDS YELLOWSTONE. BURN
SCARS, SLOT CANYONS, AND URBAN AREAS WILL BE PARTICULARLY
VULNERABLE TO FLOODING. AS THE TROUGH CONTINUES TO SHIFT INLAND,
PRECIPITATION SHOULD EXPAND/PUSH INTO THE NORTHERN PLAINS BY
MONDAY AND POSSIBLY PROVIDE SOME SHOWERS IN THE MIDWEST BY TUESDAY.
SHOWERS AND THUNDERSTORMS WILL ALSO IMPACT PORTIONS OF THE GULF
COAST AND FLORIDA AS A STALLED FRONTAL BOUNDARY LINGERS ACROSS THE
REGION ALONG WITH A SHORTWAVE ALOFT. MOISTURE WILL POOL ALONG THE
FRONT, PRODUCING SCATTERED PRECIPITATION CHANCES THROUGH THE
WEEKEND. THERE IS SOME CHANCE OF FLASH FLOODING WITH THIS ACTIVITY
ESPECIALLY IF HIGH RAIN RATES OCCUR IN SENSITIVE URBAN AREAS OR
LOCATIONS THAT HAVE SEEN HEAVY RAINFALL RECENTLY. THUS A MARGINAL
RISK IS IN PLACE IN THE ERO FOR SATURDAY. WHILE RAINFALL
MAGNITUDES LOOK TO DECREASE INTO EARLY NEXT WEEK, SOME SCATTERED
SHOWERS AND STORMS COULD CONTINUE.
COOLER THAN AVERAGE HIGH TEMPERATURES ARE FORECAST FOR THE WEST
COAST INTO THE GREAT BASIN GIVEN THE TROUGHING ALOFT. SLIGHTLY
BELOW NORMAL TEMPERATURES ARE FORECAST ACROSS THE SOUTHEAST ON
SATURDAY BEFORE WARMING TO NEAR NORMAL ON SUNDAY. BUT IN THE
CENTRAL U.S., THE STRENGTHENING UPPER RIDGE WILL ALLOW
TEMPERATURES TO RISE CONSIDERABLY THROUGH THE HOLIDAY WEEKEND.
HIGHS WELL INTO THE 90S WILL EXPAND ALL THE WAY INTO THE NORTHERN
PLAINS AND EAST ACROSS THE MISSISSIPPI VALLEY, WHILE HIGHS
EXCEEDING 100 ARE LIKELY FOR THE SOUTHERN HALF OF THE PLAINS.
THESE FORECAST HIGH TEMPERATURES ARE AROUND 10-20 DEGREES ABOVE
AVERAGE, PERHAPS LOCALLY HIGHER IN THE UPPER MIDWEST. SOME RECORD
WARM MAXIMUM AND MINIMUM TEMPERATURES COULD BE SET THERE AS WELL.
AS THE UPPER RIDGE SHIFTS EAST WITH TIME, TEMPERATURES IN THE
MID-ATLANTIC AND NORTHEAST LOOK TO WARM TO ABOUT 10-15 DEGREES
ABOVE NORMAL BY MONDAY-WEDNESDAY, FOR HIGHS IN THE UPPER 80S AND
90S.
TATE/HAMRICK
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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