066
FXUS02 KWBC 292020
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
420 PM EDT TUE AUG 29 2023
VALID 12Z FRI SEP 01 2023 - 12Z TUE SEP 05 2023
...HEAT FORECAST TO INCREASE ACROSS THE PLAINS AND MIDWEST THIS
WEEKEND INTO EARLY NEXT WEEK...
..OVERVIEW
IDALIA IS FORECAST TO BE TRACKING AWAY FROM THE U.S. AS THE MEDIUM
RANGE PERIOD BEGINS FRIDAY, AND FOCUS WILL SHIFT TO AN AMPLIFYING
UPPER-LEVEL PATTERN COMPOSED OF AN UPPER TROUGH (WITH AN EMBEDDED
CLOSED LOW LATE WEEK) OVER THE WEST COAST AND UPPER RIDGE OVER THE
CENTRAL CONUS. THESE FEATURES SHOULD SHIFT GRADUALLY EASTWARD INTO
EARLY NEXT WEEK. THIS PATTERN WILL ALLOW FOR WELL ABOVE AVERAGE
TEMPERATURES TO BUILD ACROSS THE PLAINS AND UPPER MIDWEST AND RAIN
CHANCES FOR MUCH OF THE WEST AND PARTS OF THE GULF COAST AND
FLORIDA.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE SEEMS TO HAVE A GOOD HANDLE ON THE UPPER LEVEL
PATTERN WITH GOOD AGREEMENT THROUGH THE WEEKEND. DIFFERENCES IN
THE UPPER LEVEL PATTERN ARE MOST PRONOUNCED EARLY NEXT WEEK WHEN
ANOTHER CLOSED LOW IS FORECAST TO MOVE INTO THE PACIFIC NORTHWEST.
THE GFS BRINGS THIS LOW ONSHORE THE FASTEST AND QUICKLY COMBINES
THE ENERGY INTO THE TROUGH THAT WAS OVER THE WEST COAST AT THE
BEGINNING OF THE PERIOD. THE ECMWF BRINGS THE LOW ONSHORE SLOWER,
KEEPING IT FROM BEING SWEPT UP INTO THE ORIGINAL TROUGH. THESE
DIFFERENCES WILL HAVE CONSIDERABLE IMPACTS ON THE TIMING OF A
SURFACE FRONTAL SYSTEM THAT WILL MOVE ACROSS THE NORTHWEST EARLY
NEXT WEEK. FOR THE AFTERNOON FORECAST, A DETERMINISTIC BLEND WITH
NEARLY EQUAL WEIGHTS OF THE CMC/UKMET/ECMWF/GFS WAS USED FOR
FRIDAY AND SATURDAY. SUNDAY ONWARD, ENSEMBLE MEANS WERE BLENDED
WITH THE DETERMINISTIC CMC/GFS/ECMWF (ECMWF WEIGHTED SLIGHTLY
HEAVIER) TO SMOOTH OUT SMALLER SCALE DIFFERENCES.
SOMETHING WE ARE MONITORING, THOUGH CERTAINLY NOT BITING ON AT
THIS POINT, IS THE POTENTIAL FOR AT LEAST A PORTION OF IDALIA'S
ENERGY TO DRIFT SOUTH IN THE WESTERN ATLANTIC TO THE EAST OF
FLORIDA AND THEN ACTUALLY TRACK BACK WESTWARD INTO THE ATLANTIC
COAST OF FLORIDA. SINCE YESTERDAY, THE DETERMINISTIC GFS RUNS HAVE
ALL BEEN SHOWING SOME SEMBLANCE OF THIS. INTERESTINGLY, THE 00Z
CMC INDICATED THIS POTENTIAL AS WELL, BUT THE 12Z CMC BACKED OFF
OF IT A LITTLE BIT. A GOOD AMOUNT OF 12Z GEFS ENSEMBLE MEMBER ARE
SHOWING A SIMILAR TRACK, BUT ONLY A COUPLE MEMBERS OF THE 06Z EC
ENSEMBLE ARE SHOWING SOMETHING SIMILAR. THE NATIONAL HURRICANE
CENTER TRACK TAKES IDALIA EAST-SOUTHEAST AWAY FROM THE U.S. LATE
WEEK, DECELERATING WITH TIME. A LACK OF STEERING FLOW BETWEEN THE
CAROLINAS AND BERMUDA IS CONTRIBUTING TO A FAIR AMOUNT OF
UNCERTAINTY IN THE TRACK FORECAST BEYOND DAY 5, AND DEVELOPMENTS
IN THE FORECAST GUIDANCE WILL BE MONITORED CLOSELY IN THE COMING
DAYS.
..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. THERE WILL BE AMPLE FORCING AND
INSTABILITY IN PLACE TO ALLOW FOR SOME LOCALIZED FLOODING CONCERNS
IN THE INTERIOR NORTHWEST AND GREAT BASIN IN PARTS OF OREGON,
NORTHERN CALIFORNIA, AND NEVADA. MARGINAL RISKS OF EXCESSIVE
RAINFALL ARE IN PLACE FOR THIS THREAT FRIDAY AND SATURDAY, AND
BURN SCARS AND URBAN AREAS WILL BE PARTICULARLY VULNERABLE.
MARGINAL RISKS AREAS ARE ALSO IN PLACE FOR PARTS OF THE SOUTHWEST
AND INTERMOUNTAIN WEST ON FRIDAY AND SATURDAY AS A SURGE OF
MONSOONAL MOISTURE WORKS ITS WAY NORTH INTO THE REGION. MODEL
GUIDANCE IS SHOWING PWATS WELL ABOVE NORMAL FRIDAY AND SATURDAY
NEAR THE NEVADA/ARIZONA/UTAH BORDER, WHICH WILL LIKELY BE THE AREA
TO RECEIVE THE MOST RAINFALL AND HAVE THE HIGHEST RISK OF FLASH
FLOODING. AS THE TROUGH SHIFTS INLAND, COVERAGE OF PRECIPITATION
COULD FOCUS ACROSS THE INTERMOUNTAIN WEST EARLY NEXT WEEK AND
EVENTUALLY INTO THE NORTHERN PLAINS.
SHOWERS AND THUNDERSTORMS WILL ALSO IMPACT PORTIONS OF THE GULF
COAST AND FLORIDA AS A STALLED FRONTAL BOUNDARY LINGERS ACROSS THE
REGION. MOISTURE WILL POOL ALONG THE FRONT, PRODUCING SCATTERED
PRECIPITATION CHANCES THROUGH THE WEEKEND. THERE MAY BE A NONZERO
CHANCE OF FLASH FLOODING WITH THIS ACTIVITY, MAINLY ON FRIDAY,
ESPECIALLY IF HIGH RAIN RATES OCCUR IN SENSITIVE URBAN AREAS.
PRECIPITATION CHANCES ALONG THE GULF COAST WILL DECREASE EARLY
NEXT WEEK.
COOLER THAN AVERAGE HIGH TEMPERATURES ARE FORECAST FOR THE
NORTHWEST AND SHIFTING/EXPANDING INTO CALIFORNIA TO THE GREAT
BASIN GIVEN THE TROUGHING ALOFT. SLIGHTLY BELOW NORMAL
TEMPERATURES ARE FORECAST ACROSS THE EAST AS WELL ON FRIDAY BEFORE
WARMING TO NEAR NORMAL THIS WEEKEND. IN THE CENTRAL U.S., THE
STRENGTHENING UPPER RIDGE WILL ALLOW TEMPERATURES TO RISE
CONSIDERABLY IN BY LATE WEEK AND 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-TUESDAY.
DOLAN/TATE
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
HAZARDS:
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL GREAT BASIN, THE
NORTHERN GREAT BASIN, AND THE SOUTHWEST, FRI, SEP 1.
- FLOODING POSSIBLE ACROSS PORTIONS OF THE SOUTHEAST AND THE
MID-ATLANTIC.
- FLOODING OCCURRING OR IMMINENT ACROSS PORTIONS OF THE GREAT
LAKES.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY
AND THE SOUTHERN PLAINS, MON-TUE, SEP 4-SEP 5.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE CENTRAL PLAINS, THE GREAT
LAKES, THE UPPER MISSISSIPPI VALLEY, AND THE NORTHERN PLAINS,
SAT-MON, SEP 2-SEP 4.
- HIGH WINDS ACROSS PORTIONS OF COASTAL NORTH CAROLINA, FRI, SEP 1.
- HEAVY RAIN ACROSS PORTIONS OF MAINLAND ALASKA, FRI-MON, SEP
1-SEP 4.
- HEAVY RAIN ACROSS PORTIONS OF MAINLAND ALASKA, FRI-SUN, SEP
1-SEP 3.
- HIGH WINDS ACROSS PORTIONS OF SOUTHWEST MAINLAND ALASKA AND THE
ALASKA PANHANDLE, FRI-SAT, SEP 1-SEP 2.
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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