468
FXUS02 KWBC 230708
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
307 AM EDT SUN JUL 23 2023
VALID 12Z WED JUL 26 2023 - 12Z SUN JUL 30 2023
...DANGEROUS HEAT WILL CONTINUE ACROSS THE SOUTHWEST WHILE HOT
TEMPERATURES ARE LIKELY ACROSS THE MIDWEST AS WELL...
..OVERVIEW
THE MAIN HALLMARK OF THE UPPER-LEVEL PATTERN CONTINUES TO BE THE
PERSISTENT UPPER RIDGE/HIGH ANCHORED ATOP THE SOUTHERN ROCKIES,
KEEPING EXTREMELY HOT TEMPERATURES ACROSS THE DESERT SOUTHWEST. AN
ATLANTIC UPPER RIDGE MAY ALSO BUILD WEST ACROSS FLORIDA AND THE
SOUTHEAST BY LATE WEEK. THE FLOW PATTERN FARTHER NORTH WILL BE
RELATIVELY LOW AMPLITUDE BUT INCLUDES SOME RIDGING STRETCHING INTO
THE NORTHERN HALF OF THE PLAINS AND MISSISSIPPI VALLEY, PROMOTING
MUCH ABOVE NORMAL TEMPERATURES THERE AND EAST TOWARD THE OHIO
VALLEY/GREAT LAKES AND THE MID-ATLANTIC/NORTHEAST ESPECIALLY
WEDNESDAY-FRIDAY. ENERGY TRACKING ACROSS SOUTHERN CANADA SHOULD
PUSH FRONTS INTO THE NORTHERN TIER OF THE U.S. AND PROVIDE A FOCUS
FOR ROUNDS OF CONVECTION, BUT WITH LOW CONFIDENCE IN THE DETAILS.
DIURNALLY DRIVEN SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE OVER
AND NEAR THE FOUR CORNERS STATES THROUGH LATE WEEK AND THE WEEKEND
WITH MONSOONAL MOISTURE INCOMING UNDER THE RIDGE. MEANWHILE, MEAN
UPPER TROUGHING IS LIKELY NEAR THE PACIFIC NORTHWEST, PRODUCING
THE ONLY AREA OF PERSISTENT BELOW NORMAL TEMPERATURES ACROSS THE
LOWER 48.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
AS HAS BEEN THE CASE FOR A WHILE, THE MOST CONTENTIOUS PART OF THE
MASS FIELD FORECAST CONTINUES TO INVOLVE EXACTLY HOW THE EARLY
WEEK BRITISH COLUMBIA UPPER LOW PROGRESSES EASTWARD ACROSS
SOUTHERN CANADA AND POSSIBLY SHEARS OUT AT SOME POINT, WITH SOME
POSSIBLE INFLUENCE FROM FLOW COMING AROUND THE ALASKA/NORTHERN
CANADA UPPER RIDGING. RECENT CYCLES OF GUIDANCE HAVE BEEN
FOLLOWING A CADENCE OF SLOWER 00Z RUNS AND FASTER 12Z RUNS, SO
AGAIN TRIED TO KEEP A MIDDLE GROUND FOR TIMING OF THE ASSOCIATED
FRONTS/LOWS. THE 12Z ECMWF SEEMED TO BE AN OUTLIER THOUGH IN ITS
HANDLING OF THIS UPPER LOW AND UPSTREAM ENERGY BEHIND, TENDING TO
COMBINE THEM AND LEADING TO A MUCH FASTER SECOND UPPER LOW
COMPARED TO OTHER GUIDANCE THAT LINGERS IT NEAR THE PACIFIC
NORTHWEST. THE NEWER 00Z ECMWF LEANS AWAY FROM THIS AND SEEMS TO
SHOW BETTER CONSENSUS WITH THE OTHER GUIDANCE. BUT THE NEWER 00Z
GFS IS STRONGER WITH THE FIRST LOW AS IT PRESSES EAST, DIGGING
TROUGHING MORE DEEPLY INTO THE EAST-CENTRAL U.S. AND IS LIKELY TOO
FAST/SOUTH WITH ITS FRONT AND QPF. SO SOME OF THESE DIFFERENCES
ARE FAR FROM RESOLVED.
EVEN WITH THE LARGE SCALE PATTERN AGREEMENT OVER THE EASTERN HALF
OF THE COUNTRY EARLY-MID PERIOD, THESE ABOVE DIFFERENCES IN
TIMING/UPPER FLOW AMPLITUDE DETAILS AND THEIR ASSOCIATED SURFACE
FRONT VARIATIONS ARE CAUSING AMPLE QPF DIFFERENCES AS THE NORTHERN
U.S. PATTERN EAST OF THE ROCKIES TRANSITIONS FROM A TYPICALLY
UNCERTAIN NORTHWEST FLOW REGIME TOWARD A ZONAL ALIGNMENT.
GFS/ECMWF/CMC RUNS AND THEIR ENSEMBLE PROBABILITIES CONTINUE TO
DISAGREE OVER WHERE THE BEST EMPHASIS FOR MERELY MEASURABLE OR
HEAVIER RAINFALL MAY OCCUR. THE WPC QPF CONTINUES TO SHOW SOMEWHAT
BROADER COVERAGE OF LIGHT AMOUNTS IN THE NORTH-CENTRAL U.S. AND
EASTERN U.S. COMPARED TO THE NBM USING A MULTI-MODEL AND ENSEMBLE
BLEND TO PROVIDE SOME DEPICTION OF THE RAINFALL POTENTIAL, BUT
THESE RAINFALL FORECASTS COULD CHANGE NOTABLY WITH TIME IF/AS
MODELS START TO CONVERGE. FOR THE MASS FIELDS THE WPC FORECAST
STARTED WITH A BLEND OF THE 12/18Z OPERATIONAL MODELS BUT AS THE
12Z ECMWF SEEMED TO BE A RELATIVE OUTLIER, GRADUALLY LESSENED ITS
PROPORTION AND REMOVED IT FROM THE BLEND WITH TIME, WHILE
INCREASING THE GEFS AND EC ENSEMBLE MEAN PROPORTIONS TO ABOUT HALF
BY THE END OF THE PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND THUNDERSTORMS LOOK LIKELY ACROSS PARTS OF THE CENTRAL
AND EASTERN U.S. THROUGH THE LATTER PART OF THE WORKWEEK, BUT
UNFORTUNATELY MOST FOCI FOR ORGANIZED CONVECTION ARE STILL QUITE
UNCERTAIN. FOR WEDNESDAY/WEDNESDAY NIGHT AND THUS THE DAY 4 ERO,
LOW PRESSURE AND FRONTS TRACKING THROUGH SOUTHERN CANADA
APPROACHING THE MIDWEST MAY PROVIDE ONE FOCUS THOUGH, SO A
MARGINAL RISK IS IN PLACE FOR THIS ACTIVITY. AREAS FARTHER SOUTH
ACROSS THE MID-MISSISSIPPI AND OHIO VALLEYS INTO THE APPALACHIANS
COULD ALSO CONVECTION WITH HEAVY RAINFALL AMOUNTS GIVEN THE
INSTABILITY IN PLACE, BUT EROS CONTINUE TO DEPICT NO EXCESSIVE
RAINFALL AREAS AT THIS TIME DUE TO THE UNCERTAINTY IN THE
PLACEMENT OF STORMS. FOR DAY 5/THURSDAY, LIKEWISE THERE IS
CONSIDERABLE UNCERTAINTY WITH PLACEMENT OF HEAVY RAINFALL WITHIN
THE GENERAL AREA OF THE NORTHERN TWO-THIRDS OF THE PLAINS AND
MISSISSIPPI VALLEY INTO THE APPALACHIANS AND
MID-ATLANTIC/NORTHEAST AHEAD OF FRONTAL BOUNDARIES. FOR NOW, HAVE
NO ERO RISKS IN PLACE GIVEN THE AMPLE MODEL SPREAD, BUT IF MODEL
GUIDANCE STARTS TO CONVERGE ON AN AREA OF FOCUS RATHER THAN
SHOWING QPF STATES APART BETWEEN MODELS, RISK AREAS MAY NEED TO BE
ADDED. BY LATER IN THE WEEK THE MAIN COLD FRONT AND LOW PRESSURE
SYSTEM PUSHING SOUTHEAST LOOK TO PROVIDE A CLEARER FOCUS FOR
CONVECTION GENERALLY ACROSS THE MIDWEST TO GREAT LAKES REGION AND
THE NORTHEAST. TRAINING/REPEAT ACTIVITY COULD EVENTUALLY BECOME
MORE OF A FACTOR FOR A TIME IF FLATTENING FLOW ALOFT ALLOWS SOME
OF THE FRONT TO TAKE ON A MORE EAST-WEST ORIENTATION. MEANWHILE
OVER THE WEST, THE FOUR CORNERS STATES ARE LIKELY TO SEE DIURNALLY
DRIVEN CONVECTION EACH DAY THIS WEEK AS MONSOONAL MOISTURE COMES
IN UNDERNEATH THE SOUTHERN ROCKIES UPPER RIDGE. THE EROS DEPICT A
MARGINAL RISK AREA ON BOTH DAYS. FLORIDA AND PARTS OF THE GULF
COAST SHOULD SEE SOME SCATTERED SHOWERS AND THUNDERSTORMS AS WELL,
AND A MARGINAL RISK OF FLASH FLOODING IS IN PLACE FOR SOUTH
FLORIDA FOR THURSDAY AS PRECIPITABLE WATER VALUES WILL BE WELL
ABOVE AN ALREADY HIGH AVERAGE AND CONVECTION COULD PRODUCE HEAVY
RAINFALL RATES, WITH URBAN AREAS PARTICULARLY SENSITIVE.
THE RELENTLESS UPPER HIGH CENTERED OVER THE SOUTHERN ROCKIES WILL
KEEP TEMPERATURES IN THE DESERT SOUTHWEST AROUND 5-10F ABOVE
AVERAGE FOR HIGHS IN THE 110S. THOUGH THESE ANOMALIES ARE A LITTLE
LESS EXTREME THAN IN THE SHORTER TERM, THE HEAT COULD CONTINUE TO
BE HAZARDOUS THROUGH THE WEEK, BUT WITH PERHAPS SOME POSSIBLE
EASING BY NEXT WEEKEND. HIGHS AROUND 100F ARE FORECAST TO STRETCH
INTO MUCH OF TEXAS AND FARTHER NORTH INTO PARTS OF THE PLAINS AND
MISSISSIPPI VALLEY. THE LARGEST ANOMALIES FOR HIGHS OF 10-20F
ABOVE NORMAL WILL BE LOCATED ACROSS THE NORTHERN PLAINS TO UPPER
MIDWEST/GREAT LAKES FOR WEDNESDAY-THURSDAY, WHILE PLUS 10-15F
ANOMALIES ARE LIKELY ACROSS THE OHIO VALLEY AND
MID-ATLANTIC/NORTHEAST PEAKING THURSDAY-FRIDAY. A HANDFUL OF
RECORD HIGHS MAY BE POSSIBLE IN THE SOUTH-CENTRAL U.S. INTO
WEDNESDAY AND OVER FLORIDA UNDERNEATH RIDGING, BUT THE HEAT IN THE
EAST MAY TIE/SET MORE WIDESPREAD RECORDS FOR HIGHS AND WARM
MINIMUM TEMPERATURES. THE COLD FRONT FORECAST TO PROGRESS FROM THE
NORTHERN PLAINS AROUND MIDWEEK INTO THE NORTHEAST BY SATURDAY
SHOULD ERODE THE NORTHERN TIER HEAT AREA FROM WEST TO EAST,
KEEPING THE EVENT FAIRLY SHORT-LIVED OVER THE NORTHERN THIRD OF
THE COUNTRY. THE HEAT MAY PERSIST LONGER TO THE SOUTH. DETAILS OF
CONVECTION AND RESIDUAL CLOUD COVER WILL ADD AN EXTRA ELEMENT OF
UNCERTAINTY TO THE FORECAST OF DAYTIME HIGHS OVER SOME AREAS.
MEANWHILE THE ONLY NOTABLE AREA OF POTENTIALLY BELOW NORMAL
TEMPERATURES WILL BE THE PACIFIC NORTHWEST GIVEN THE ROUNDS OF
TROUGHING ALOFT IN THE VICINITY.
TATE/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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