876
FXUS02 KWBC 061857
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 PM EDT SUN AUG 06 2023
VALID 12Z WED AUG 09 2023 - 12Z SUN AUG 13 2023
..HEAT WAVE TO PERSIST OVER THE SOUTH INTO THE SOUTHWEST
...EXCESSIVE RAIN/FLASH FLOOD THREAT FROM THE MIDDLE MISSISSIPPI
TO TENNESSEE VALLEYS WEDNESDAY INTO THURSDAY...
..OVERVIEW
AN ELONGATED UPPER RIDGE EXTENDING FROM NORTHERN MEXICO/SOUTHERN
TEXAS ACROSS THE GULF OF MEXICO AND FLORIDA PENINSULA AS OF
WEDNESDAY SHOULD WEAKEN A BIT INTO THURSDAY BUT THEN STRENGTHEN
ONCE AGAIN OVER THE SOUTHERN ROCKIES/PLAINS AND NORTHERN MEXICO
LATE WEEK THROUGH THE WEEKEND. THIS RIDGE WILL MAINTAIN THE
ONGOING SOUTHERN TIER HEAT WAVE WITH DAILY RECORD HIGH
TEMPERATURES POSSIBLE EVERY DAY OF THE FORECAST PERIOD, WITH THE
MOST ANOMALOUS TEMPERATURES LIKELY TO BE OVER TEXAS. TO THE
NORTH, EXPECT BROADLY CYCLONIC FLOW WITH THE AXIS OF A BROAD
TROUGH CENTERED OVER THE EASTERN HALF OF THE LOWER 48. EMBEDDED
SHORTWAVES PROGRESSING FROM THE PACIFIC INTO/THROUGH THE TROUGH
WILL PRODUCE A SERIES OF CENTRAL-EASTERN U.S. WAVES/FRONTAL
SYSTEMS THAT ARE EXPECTED TO PRODUCE EPISODES OF LOCALLY HEAVY
RAINFALL. THE SCALE OF THE SHORTWAVES IS SMALL ENOUGH TO YIELD
LOWER PREDICTABILITY FOR DAY-TO-DAY SPECIFICS VERSUS THE LARGER
SCALE PATTERN BY THE LATTER HALF OF THE PERIOD. IN THE WEST,
UPPER LEVEL TROUGHING AND A WEAKENING FRONTAL SYSTEM WILL BRING
RAINFALL TO THE PACIFIC NORTHWEST INTO WEDNESDAY FOLLOWED BY
RISING UPPER HEIGHTS DOWNSTREAM FROM AN UPPER LOW SETTLING OVER OR
NEAR THE ALEUTIANS. MEANWHILE THE NATIONAL HURRICANE CENTER IS
ISSUING ADVISORIES FOR TROPICAL STORM EUGENE IN THE PACIFIC. THE
SURFACE REFLECTION SHOULD DISSIPATE WELL OFFSHORE AFTER WEDNESDAY
BUT THE SYSTEM'S UPPER LEVEL ENERGY AND POSSIBLY SOME MOISTURE
COULD GET INCORPORATED INTO AN UPPER TROUGH/LOW FORECAST TO EVOLVE
ALONG OR A LITTLE OFFSHORE THE CALIFORNIA COAST.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE INDICATES THE MEDIUM-RANGE PERIOD WILL BEGIN WITH
AN ENERGETIC NORTHERN STREAM THAT HAD BEEN POSITIONED SOUTHWARD
AND IMPINGING ON A STAGNANT UPPER-LEVEL HIGH OVER THE SOUTHERN
TIER OF THE COUNTRY, ESPECIALLY OVER CENTRAL/EASTERN PORTIONS OF
THE U.S., RETREATING BACK NORTHWARD A BIT THROUGH THE FORECAST
PERIOD. ALL OF THE GUIDANCE IS IN RELATIVELY GOOD AGREEMENT WITH
THE PLACEMENT OF THESE LARGE-SCALE FEATURES AS WELL AS THE GENERAL
FLOW PATTERN, WITH EXPECTED DIFFERENCES IN THE TIMING/PLACEMENT OF
EMBEDDED SHORTWAVES IN THE FLOW THAT WILL IMPACT A COUPLE EXPECTED
FRONTAL BOUNDARIES AT THE SURFACE AND SUBSEQUENT PRECIPITATION
CHANCES. BOTH THE DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE
INDICATE THAT THE PATTERN WILL ALSO BEGIN TO AMPLIFY A BIT THROUGH
THE PERIOD, WITH MEAN TROUGHING REMAINING IN PLACE OVER THE
MIDWEST/NORTHEAST AS UPPER-LEVEL RIDGING EXPANDS NORTHWESTWARD
OVER THE CENTRAL/WESTERN U.S. AS RIDGING ALSO BUILDS OVER THE
NORTHEASTERN PACIFIC ON AN EVEN BROADER SCALE. ALL OF THE GUIDANCE
IS ALSO IN AGREEMENT TOWARDS THE END OF THE PERIOD IN THE
EVOLUTION OF A LINGERING UPPER-LEVEL LOW ALONG THE CALIFORNIA
COAST FROM THE REMNANTS OF TROPICAL STORM EUGENE, LIKELY WITH SOME
TEMPERATURE AND POSSIBLE PRECIPITATION IMPLICATIONS FOR
CALIFORNIA. GOOD SIMILARITY RUN-TO-RUN BETWEEN THE GFS AND ECMWF
INDICATES GOOD PREDICTABILITY IN THE LARGE-SCALE PATTERN OVERALL.
GIVEN THE GOOD CLUSTERING OF THE GUIDANCE A GENERAL MODEL BLEND OF
THE 00Z ECMWF/CMC/UKMET AND 06Z GFS IS USED FOR THE FIRST PART OF
THE UPDATED WPC FORECAST. A CONTRIBUTION FROM THE 00Z ECENS AND
06Z GEFS MEANS IS ADDED IN THE MID- TO LATE PERIOD WHILE
DECREASING THE DETERMINISTIC GUIDANCE A BIT (INCLUDING THE
FORECAST TIME-LIMITED UKMET) AS NATURAL DIFFERENCES GROW AT
INCREASING TIME SCALES, SPECIFICALLY WITH REGARD TO SHORTWAVE
PLACEMENT IN THE UPPER-LEVEL FLOW. THIS GENERALLY FOLLOWS THE
PRIOR WPC FORECAST BLEND AND SUBSEQUENTLY DID NOT LEAD TO MANY
SIGNIFICANT DIFFERENCES OVERALL EXCEPT FOR EXPECTED ADJUSTMENTS OF
SURFACE SYSTEM POSITIONS AND QPF LOCATION/AMOUNTS AT THE
MESOSCALE.
..WEATHER/HAZARDS HIGHLIGHTS
DURING THE WEDNESDAY-THURSDAY NIGHT PERIOD COVERED BY THE DAYS 4-5
EXCESSIVE RAINFALL OUTLOOKS, THE MOST PROMINENT AREA OF FOCUS FOR
HEAVY RAINFALL/FLASH FLOOD POTENTIAL WILL EXTEND FROM THE
EAST-CENTRAL PLAINS INTO THE APPALACHIANS AND MID-ATLANTIC. THE
DAY 4 ERO DEPICTS A SLIGHT RISK AREA EXTENDING FROM EASTERN KANSAS
INTO PARTS OF THE LOWER OHIO VALLEY AND TENNESSEE VALLEY AS A
SOUTHERN PLAINS WAVE SHOULD INCREASE THE FLOW OF MOISTURE
INTERACTING WITH A LEADING WARM FRONT WHILE A SEPARATE FRONT DROPS
FROM THE NORTHERN TIER INTO THE MIDWEST. QPF IN THE GUIDANCE HAS
CONTINUED TO TREND UPWARD, BOTH IN TERMS OF DETERMINISTIC AMOUNTS
AND ENSEMBLE PROBABILITIES FOR AREAL AVERAGE TOTALS OF 1"+ OVER
THIS GENERAL AREA. AN EMBEDDED UPGRADE CERTAINLY LOOKS POSSIBLE
IF THERE IS FURTHER CONSOLIDATION OF LOCALLY MORE SIGNIFICANT
RAINFALL TOTALS, LIKELY DEPENDENT ON HOW SHORTER-TERM GUIDANCE
RESOLVES MESOSCALE CONVECTIVE EVOLUTION. THERE IS ALSO THE
POTENTIAL SOME OF THE HEAVIER AMOUNTS MAY OVERLAP AREAS OF THE
MIDDLE MISSISSIPPI AND LOWER OHIO VALLEYS MORE SENSITIVE DUE TO
RECENT HEAVY RAINFALL EVENTS. BY DAY 5, THE BEST MODEL/ENSEMBLE
CLUSTERING BECOMES MORE CONFINED OVER THE UPPER TENNESSEE VALLEY
AND SOUTHERN APPALACHIANS WHERE THE OUTLOOK SHOWS A SLIGHT RISK
AREA. SIGNALS ARE MORE DIFFUSE WITHIN THE SURROUNDING MARGINAL
RISK AREA. MEANWHILE MOST GUIDANCE IS SHOWING AN AREA OF LOCALLY
HEAVY RAINFALL POTENTIAL OVER PARTS OF THE NORTHERN PLAINS/UPPER
MIDWEST IN THE DAY 5 PERIOD DUE TO THE APPROACHING NORTHERN TIER
SYSTEM. ALTHOUGH EXPECTED TOTALS AND PLACEMENT WILL POTENTIALLY
BE TIED TO ORGANIZED CONVECTION WHICH MAY REMAIN PROGRESSIVE,
AREAL COVERAGE OF LOCALLY HEAVIER RAINFALL STILL WARRANTS A
MARGINAL RISK.
FROM FRIDAY ONWARD EXPECT ADDITIONAL AREAS OF RAIN AND
THUNDERSTORMS ACROSS THE CENTRAL/EASTERN U.S. IN AN OVERALL
ENERGETIC AND WET PATTERN BUT WITH STEADILY DECREASING CONFIDENCE
FOR SPECIFICS DUE TO UNCERTAINTY WITH DETAILS/TIMING OF INDIVIDUAL
WAVES/FRONTAL SYSTEMS. SHOWER AND THUNDERSTORM CHANCES SHOULD
REMAIN IN PLACE MORE BROADLY NEXT WEEK INTO THE WEEKEND ACROSS THE
MIDWEST/SOUTHEAST/EAST COAST WITH LINGERING FRONTAL BOUNDARIES IN
THE AREA. ANOTHER AREA OF SHOWERS/STORMS MAY DEVELOP OVER THE
PLAINS DURING THE WEEKEND. THE PACIFIC NORTHWEST SHOULD SEE SOME
RAINFALL CONTINUE INTO WEDNESDAY AS A WEAKENING FRONT REACHES THE
AREA. SCATTERED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE OVER
THE FOUR CORNERS STATES AND THE SIERRA NEVADA. SOME UPPER LEVEL
MOISTURE FROM TROPICAL STORM EUGENE MAY ENTER THE WEST BUT AT THIS
TIME IT DOES NOT LOOK TO HAVE MUCH INFLUENCE ON PRECIPITATION
CHANCES.
THE ONGOING HEAT WAVE OVER THE SOUTHERN TIER SHOULD CONTINUE
THROUGH THE WEEK AND NEXT WEEKEND, WITH GREATEST EMPHASIS OVER THE
SOUTHERN HIGH PLAINS WHERE HIGHS COULD REACH 10-15F ABOVE NORMAL
ESPECIALLY MID-LATE WEEK. TEMPERATURES MAY REACH OR EXCEED DAILY
RECORD HIGHS MOST DAYS DURING THE PERIOD FROM THE CENTRAL/WESTERN
GULF COAST INTO THE SOUTHERN ROCKIES. THE DESERT SOUTHWEST SHOULD
SEE A BRIEF LULL IN THE 110F+ HIGHS MID-LATE WEEK BUT TEMPERATURES
MAY REACH INTO THE LOW 110S ONCE AGAIN BY NEXT SUNDAY AS THE UPPER
RIDGE STRENGTHENS OVER THE WEST. THE FLORIDA PENINSULA COULD ALSO
SEE A FEW RECORD HIGHS. RECORD WARM LOWS WILL BE POSSIBLE ACROSS A
BROADER PORTION OF THE SOUTHERN TIER EACH NIGHT. 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. IN CONTRAST,
THE FORECAST PATTERN WILL TEND TO FAVOR NEAR TO BELOW NORMAL HIGH
TEMPERATURES FROM THE NORTHWESTERN U.S. INTO THE MIDWEST/OHIO
VALLEY. THE UPPER TROUGH/LOW DEVELOPING NEAR CALIFORNIA SHOULD
KEEP HIGHS OVER MOST OF THE STATE NEAR OR SLIGHTLY BELOW NORMAL AS
WELL. AREAS NEAR THE PACIFIC NORTHWEST COAST MAY BE AN EXCEPTION
WITH A BUILDING UPPER RIDGE HELPING TO PUSH READINGS ABOVE NORMAL
BY LATE WEEK AND NEXT WEEKEND, WITH THE EXPECTATION THIS WILL
LIKELY CONTINUE BEYOND THE CURRENT FORECAST PERIOD.
PUTNAM/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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