657
FXUS02 KWBC 041821
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
220 PM EDT FRI AUG 04 2023
VALID 12Z MON AUG 07 2023 - 12Z FRI AUG 11 2023
...HEAT WAVE TO PERSIST OVER THE SOUTH AND WORK BACK INTO THE
SOUTHWEST...
...EXCESSIVE RAIN/FLASH FLOOD THREAT FOR THE GREAT LAKES INTO THE
NORTHEAST EARLY NEXT WEEK...
..OVERVIEW
EXPECT A STRONG UPPER RIDGE TO REMAIN CENTERED OVER THE
U.S.-MEXICO BORDER, MAINTAINING A SOUTHERN TIER HEAT WAVE WITH
DAILY RECORD HIGH TEMPERATURES POSSIBLE MOST DAYS NEXT WEEK FROM
THE WESTERN GULF COAST REGION THROUGH THE SOUTHERN ROCKIES AND
POSSIBLY INTO ARIZONA. INITIAL UPPER RIDGING ACROSS THE FLORIDA
PENINSULA SHOULD WEAKEN TO THE SOUTH OF AN EASTERN NORTH AMERICA
TROUGH WHOSE EMBEDDED SHORTWAVES (FROM PACIFIC THROUGH NORTHERN
TIER U.S. FLOW ALONG WITH SOME INFLUENCE FROM SOUTHERN CANADA)
SHOULD SUPPORT AT LEAST A COUPLE FRONTAL SYSTEMS THAT MAY FOCUS
AREAS OF LOCALLY HEAVY RAINFALL OVER PORTIONS OF THE
CENTRAL/EASTERN U.S. MEANWHILE A NORTHEASTERN PACIFIC UPPER
TROUGH AND WEAKENING FRONTAL SYSTEM COULD BRING SOME RAINFALL TO
THE PACIFIC NORTHWEST AROUND THE MIDDLE OF NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GUIDANCE IS PRETTY DISPERSED WITH RESPECT TO THE 500MB PATTERN
ACROSS THE LOWER 48. THIS UNCERTAINTY IS REFLECTED IN THE MODEL
BLENDS THROUGHOUT THE PERIOD. A GENERAL MODEL BLEND CONSISTING OF
THE 00Z EC/UKMET/CMC AND 06Z GFS WAS UTILIZED ON DAY 3. MINIMAL
WEIGHTING WAS GIVEN TO THE UKMET BECAUSE IT IS WEAKER AND MORE
PROGRESSIVE THAN THE REST OF THE MODEL SUITE WITH RESPECT TO THE
UPPER TROUGH SWINGING THROUGH THE GREAT LAKES REGION. THE 06Z GFS
BECOMES AN OUTLIER ON DAY 4 AS IT IS MORE PROGRESSIVE WITH A
NORTHERN PLAINS SHORTWAVE AND SLOWER/WEAKER WITH AN EASTERN U.S.
TROUGH. THE 00Z ECE AND 06Z GEFS WERE INTRODUCED ON DAY 4 TO
MITIGATE SOME OF THE AFOREMENTIONED UNCERTAINTY IN THE
DETERMINISTIC GUIDANCE. THE MIX OF DETERMINISTIC AND ENSEMBLE MEAN
GUIDANCE IS CONTINUED THROUGH DAY 5, BUT IT IS WORTH NOTING THAT
THE 06Z GFS AND 00Z EC DIVERGE AGAIN FROM THE REST OF THE GUIDANCE
WITH RESPECT TO THE SHORTWAVE PROPAGATING THROUGH THE NORTHERN
PLAINS AND UPPER MIDWEST. THE BLEND IS REDUCED ON DAYS 6 AND 7 TO
JUST THE ENSEMBLE MEANS AND THE 00Z EC DUE TO GOOD CLUSTERING
BETWEEN THOSE MODELS.
..WEATHER/HAZARDS HIGHLIGHTS
THE UNSEASONABLY STRONG SYSTEM FORECAST TO TRACK OUT OF THE
CENTRAL GREAT LAKES DURING THE FIRST HALF OF THE WEEK WILL SPREAD
POTENTIALLY STRONG THUNDERSTORMS/HEAVY RAINFALL FROM THE GREAT
LAKES/OHIO VALLEY INTO THE NORTHEAST. THE DAYS 4-5 EXCESSIVE
RAINFALL OUTLOOKS COVERING THE MONDAY THROUGH TUESDAY NIGHT PERIOD
DEPICT A MARGINAL RISK AREA CORRESPONDING TO THIS RAINFALL THREAT.
CONTINUE TO MONITOR FORECASTS AS SHORTER-RANGE GUIDANCE MAY
EVENTUALLY PROVIDE ENOUGH REFINEMENT TO SUPPORT A SLIGHT RISK
AREA, ESPECIALLY OVER AREAS WITH WET GROUND CONDITIONS. GENERAL
PROGRESSION OF THIS SYSTEM MAY BE ONE MITIGATING FACTOR THAT COULD
KEEP RAINFALL TOTALS FROM BECOMING TOO EXTREME. BY DAY 5 TUESDAY
THE TRAILING FRONT MAY TAKE ON A MORE WEST-EAST ORIENTATION AS IT
DROPS INTO THE LOWER MISSISSIPPI VALLEY/SOUTHEAST, WHICH COMBINED
WITH SUFFICIENT MOISTURE/INSTABILITY, COULD LEAD TO AN AXIS OF
HEAVY RAINFALL OVER THIS REGION. GUIDANCE SIGNALS ARE NOT
EXTREMELY WELL CLUSTERED YET THOUGH, MERITING A MARGINAL RISK AREA
AT THIS TIME. FINALLY, THERE IS A MARGINAL RISK AREA PROPOSED FOR
PARTS OF THE CENTRAL PLAINS ON MONDAY WHEN A NORTHWEST-SOUTHEAST
BAND OF CONVECTION MAY DEVELOP.
THE SOUTHERN TIER RAINFALL SHOULD PERSIST INTO MID-LATE WEEK AS
THE FRONT LINGERS OVER THE AREA AND THEN GRADUALLY LIFTS NORTH AS
A WARM FRONT. DURING THE LATTER HALF OF THE WEEK, THE INTERACTION
OF GULF MOISTURE WITH THIS FRONT AS WELL AS FRONT DROPPING SOUTH
FROM THE NORTHERN TIER SHOULD LEAD TO THE BEST SHOWER/THUNDERSTORM
FOCUS EXTENDING FROM THE EASTERN PLAINS INTO THE OHIO/TENNESSEE
VALLEYS WITH SOME HEAVY RAINFALL POSSIBLE. MEANWHILE THE PACIFIC
NORTHWEST SHOULD SEE SOME RAINFALL TUESDAY-WEDNESDAY AND PERHAPS
LINGERING A BIT INTO THURSDAY AS A WEAKENING FRONT APPROACHES.
ORGANIZED RAINFALL OVER THE NORTHERN ROCKIES DURING THE
SHORT-RANGE TIME FRAME (THROUGH SUNDAY) SHOULD GRADUALLY BECOME
LIGHTER AND MORE SCATTERED OVER THE COURSE OF NEXT WEEK.
THE ONGOING HEAT WAVE OVER THE SOUTHERN TIER SHOULD CONTINUE
THROUGH NEXT WEEK, WITH GREATEST EMPHASIS OVER THE SOUTHERN HIGH
PLAINS WHERE HIGHS COULD REACH 10-15F ABOVE NORMAL. TEMPERATURES
MAY REACH OR EXCEED DAILY RECORD HIGHS MOST DAYS NEXT WEEK FROM
THE WESTERN GULF COAST INTO ARIZONA, WITH THE DESERT SOUTHWEST
SEEING HIGHS REACHING THE 110S ONCE AGAIN. THE FLORIDA PENINSULA
COULD ALSO SEE A FEW RECORD HIGHS EARLY IN THE WEEK BEFORE UPPER
HEIGHTS DECLINE A BIT. RECORD WARM LOWS WILL BE POSSIBLE ACROSS A
BROADER PORTION OF THE SOUTHERN TIER. HIGH HUMIDITY SHOULD LEAD
TO MAXIMUM HEAT INDICES AT LEAST IN THE 110-115F RANGE ALONG AND
NEAR THE WESTERN HALF OF THE GULF COAST. SOME ABOVE AVERAGE
TEMPERATURES WILL ALSO BRIEFLY SPREAD UP THE WEST COAST FOLLOWING
A BIT OF UPPER-RIDGE AMPLIFICATION, WITH HIGHS IN THE CENTRAL
CALIFORNIA VALLEYS CLIMBING AS HIGH AS 105F OR SO NEXT MONDAY.
THE NORTHERN HALF OF THE ROCKIES INTO PARTS OF THE
NORTHERN-CENTRAL PLAINS WILL BE ON THE COOL SIDE NEXT WEEK, WITH
ROCKIES/HIGH PLAINS LOCATIONS SEEING THE BEST POTENTIAL FOR HIGHS
10-15F BELOW NORMAL NEXT MONDAY-TUESDAY.
KEBEDE/RAUSCH
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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