947
FXUS02 KWBC 221859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT SAT JUL 22 2023
VALID 12Z TUE JUL 25 2023 - 12Z SAT JUL 29 2023
...DANGEROUS HEAT WILL CONTINUE ACROSS THE SOUTHWEST AND BUILD
FARTHER INTO THE NORTH-CENTRAL AND NORTHEASTERN U.S. AS NEXT WEEK
PROGRESSES...
..OVERVIEW
A STRONG AND PERSISTENT UPPER RIDGE/HIGH ANCHORED OVER THE
SOUTHERN ROCKIES WILL KEEP HOTTER THAN USUAL TEMPERATURES ACROSS
THE SOUTHWEST NORTHEASTWARD INTO THE PLAINS AND MIDWEST UNDER THE
RIDGE AXIS. DOWNSTREAM, AN EAST-CENTRAL U.S. UPPER TROUGH ALREADY
STARTING TO WEAKEN AS OF TUESDAY SHOULD CONTINUE TO RETREAT
THEREAFTER WHILE AN ATLANTIC UPPER RIDGE BUILDS OVER FLORIDA AND
THE SOUTHEAST LATE IN THE WEEK. THIS EASTERN U.S. EVOLUTION SHOULD
RAISE TEMPERATURES TO WELL ABOVE NORMAL IN THE OHIO VALLEY/GREAT
LAKES AND NORTHEAST/MID-ATLANTIC FOR THE LATTER PART OF THE
WORKWEEK BEFORE A COLD FRONT MODERATES TEMPERATURES THERE BY NEXT
SATURDAY. MEANWHILE A SHORT-RANGE EASTERN PACIFIC UPPER LOW (PART
OF A REX BLOCK CONFIGURATION INCLUDING AN ALASKA/NORTHWESTERN
CANADA RIDGE) SHOULD EJECT EAST ACROSS SOUTHERN CANADA AND PUSH
FRONTS ACROSS THE NORTHERN U.S. TRAILING ENERGY ALOFT WILL LIKELY
REINFORCE THE MEAN TROUGH NEAR THE PACIFIC NORTHWEST BY THURSDAY,
WITH AN EMBEDDED LOW (WEAKER THAN THE ONE CROSSING BRITISH
COLUMBIA ON TUESDAY) POSSIBLY EJECTING BY NEXT SATURDAY. SOME
DIURNALLY FAVORED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE OVER
AND NEAR THE FOUR CORNERS STATES WHILE FRONTS/UPPER SHORTWAVES MAY
PRODUCE SOME EPISODES OF CONVECTION OVER THE EASTERN HALF OF THE
COUNTRY. COVERAGE OF HEAVY RAINFALL THREATS SHOULD BE LESS THAN
AVERAGE FOR THE MIDDLE OF SUMMER, THOUGH THE FORECAST PATTERN MAY
FAVOR SOME INCREASE OF RAINFALL COVERAGE AND INTENSITY FROM THE
MIDWEST/NORTHERN HALF OF THE MISSISSIPPI VALLEY EASTWARD TOWARD
THE END OF THE WEEK.
..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 INITIAL
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 SETTLED INTO A ROUTINE OF
OSCILLATING IN EACH 12-HOURLY CYCLE, WITH 00Z RUNS TENDING TO BE
SLOWER THAN THE 12Z ONES. THE NEW 12Z CYCLE GENERALLY MAINTAINS
THIS TENDENCY, WHILE ALSO TENDING TO SHEAR OUT THE UPPER LOW
EARLIER. THE END RESULT IS A SOMEWHAT FASTER EASTWARD PROGRESSION
OF THE FRONT, ANCHORED BY A WEAKER AND FASTER SURFACE LOW, BUT
WITH PORTIONS OF THE FRONT POSSIBLY NOT REACHING QUITE AS FAR
SOUTH BY LATE IN THE PERIOD. FOR THE RELOADING PACIFIC NORTHWEST
UPPER TROUGH, MOST GUIDANCE CLUSTERS FAIRLY WELL INTO DAY 6
FRIDAY, THOUGH THE 12Z GFS STARTS TO EJECT THE EMBEDDED UPPER LOW
A LITTLE MORE QUICKLY AND THE NEW 12Z CMC LEANS A BIT TO THE
SOUTHWEST SIDE OF THE SPREAD (BUT CLOSER TO CONSENSUS THAN THE 00Z
RUN'S DAY 7 SATURDAY UPPER LOW). BY THE END OF THE PERIOD THE 00Z
ECMWF MEAN AND 00Z GFS WERE CLOSEST TO THE GUIDANCE AVERAGE,
FASTER THAN THE GEFS MEAN BUT SLOWER THAN THE 00Z ECMWF AND
06Z/12Z GFS. THE NEW 12Z ECMWF HAS CAST NEW DOUBT ON THIS TROUGH'S
FORECAST THOUGH, WITH A NOTICEABLY FASTER TREND FOR MUCH OF THE
PERIOD.
EVEN WITH THE LARGE SCALE PATTERN AGREEMENT OVER THE EASTERN HALF
OF THE COUNTRY EARLY-MID PERIOD, THE MODELS/ENSEMBLES CONTINUE TO
SHOW AMPLE DIFFERENCES IN TERMS OF QPF 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. A PROMINENT EXAMPLE IS OVER THE APPALACHIANS/MID-ATLANTIC
AROUND MIDWEEK WITH THE GFS RUNS ON THE HEAVY/EXPANSIVE SIDE
VERSUS MOST OTHER GUIDANCE (THOUGH ODDLY WITH THE MOST RELATIVE
SUPPORT FROM CMC ENSEMBLES). ADDITIONAL QPF DIFFERENCES ARISE ONCE
SOUTHERN CANADA LOW PRESSURE AND TRAILING FRONTS BECOME MORE
DOMINANT FEATURES, WITH TIMING/UPPER FLOW AMPLITUDE DETAILS
AFFECTING QPF PLACEMENT AND AMOUNTS. THE WPC QPF CONTINUES TO SHOW
SOMEWHAT BROADER COVERAGE OF LIGHT AMOUNTS IN THE NORTH-CENTRAL
U.S. AND EVENTUALLY THE 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 00Z/06Z
OPERATIONAL MODELS EARLY AND THEN INCREASED 06Z GEFS/00Z ECENS
TOTAL WEIGHT TO 50 PERCENT BY THE END OF THE PERIOD. OSCILLATING
TRENDS WITH THE NORTHERN TIER SYSTEM LED TO SOMEWHAT SLOWER
FRONTAL TIMING, BUT TEMPERED BY KEEPING PROGRESSION A LITTLE AHEAD
OF WHAT THE LITERAL BLEND WOULD HAVE SUGGESTED BY THE LATTER HALF
OF THE PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
AT THE BEGINNING OF THE PERIOD TUESDAY, MOIST INFLOW INTO THE MEAN
BUT ERODING UPPER TROUGH ATOP THE NORTH-CENTRAL TO NORTHEASTERN
U.S., ALONG WITH FRONTS AT THE SURFACE, WILL LEAD TO THE
POSSIBILITY OF CONVECTION FROM THE NORTHWEST FLOW PATTERN IN THE
MID-MISSISSIPPI VALLEY EASTWARD INTO THE OHIO VALLEY AND INTO THE
NORTHEAST AHEAD OF THE TROUGH AXIS. MODEL GUIDANCE IS STILL QUITE
VARIABLE WITH THE PLACEMENT OF STORMS AND RAINFALL AMOUNTS.
HOWEVER THE COMBINATION OF FORECAST MOISTURE/INSTABILITY ALONG
WITH MODEL/ENSEMBLE SIGNALS FROM THE GFS/ECMWF AND
GEFS/ECENS/CMCENS FAVOR MAINTAINING THE MARGINAL RISK IN THE
EXCESSIVE RAINFALL OUTLOOK FOR DAY 4/TUESDAY FROM THE
MID-MISSISSIPPI VALLEY (WHERE POSSIBLE TUESDAY MORNING ACTIVITY
MAY LINGER FROM DAY 3 PERIOD) THROUGH PARTS OF THE GREAT LAKES AND
NORTHEAST. PORTIONS OF THE NORTHEAST REMAIN SUSCEPTIBLE TO
FLOODING WITH ANY ADDITIONAL RAINFALL DUE TO VERY WET ANTECEDENT
CONDITIONS, SO FORECAST RAINFALL AMOUNTS WILL BE CLOSELY MONITORED
FOR THAT AREA IN CASE THE RISK LEVEL MAY NEED TO BE UPGRADED. BY
DAY 5/WEDNESDAY THERE IS EVEN MORE SPREAD IN ANY SIGNAL FOR
FOCUSED RAINFALL ACROSS THE EAST. EROS CONTINUE TO DEPICT NO
EXCESSIVE RAINFALL AREAS AT THIS TIME DUE TO THE UNCERTAINTY IN
THE PLACEMENT OF STORMS, BUT CONVECTION THAT DOES FORM COULD
CONTAIN HEAVY RAINFALL AMOUNTS GIVEN THE INSTABILITY IN PLACE.
MEANWHILE A LOW PRESSURE SYSTEM TRACKING THROUGH SOUTHERN CANADA
COULD HELP FOCUS RAIN/STORMS IN THE UPPER MIDWEST BY
WEDNESDAY-WEDNESDAY NIGHT. LATEST MODELS ARE NOT PARTICULARLY
AGREEABLE WITH RAINFALL FORECASTS, BUT THERE SEEMS TO BE BETTER
CONSENSUS TOWARD AN ASSOCIATED AREA OF ENHANCED MOISTURE THAT
SUPPORTS A LOCALIZED HEAVY RAINFALL THREAT AND MAINTAINING THE
MARGINAL RISK THERE. BY LATER IN THE WEEK THE COLD FRONT
ASSOCIATED WITH THIS LOW PRESSURE SYSTEM LOOKS 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 NEXT 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 MAY SEE SOME SCATTERED SHOWERS AND
THUNDERSTORMS AS WELL.
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 WILL CONTINUE TO
BE HAZARDOUS INTO NEXT WEEK. HIGHS OVER 100F ARE FORECAST TO
STRETCH INTO MUCH OF TEXAS AS WELL, AND EASTERN TEXAS NEAR THE
GULF COAST CAN EXPECT HEAT INDICES ABOVE 110F DURING PEAK
AFTERNOON HEATING AS HIGH DEWPOINTS COMBINE WITH THE HOT
TEMPERATURES, ESPECIALLY ON TUESDAY BEFORE SOME MINOR MODERATING
OF THE TEMPERATURES. A FEW DAILY RECORD HIGHS WILL BE POSSIBLE
MAINLY OVER AND NEAR THE SOUTHERN HIGH PLAINS INTO MIDWEEK, WITH
SOME DAILY RECORD WARM LOWS ALSO POSSIBLE OVER SOME OF THE ABOVE
REGIONS. UPPER RIDGING EXTENDING TO THE NORTH WILL ALSO ALLOW FOR
THE PLAINS AND MIDWEST TO WARM TO GENERALLY 10-15F ABOVE NORMAL
THROUGH MUCH OF NEXT WEEK, REACHING THE 100F MARK AT SOME
LOCATIONS. WARMER THAN NORMAL TEMPERATURES (BY 5-15F) ARE LIKELY
TO EXPAND EAST INTO THE OHIO VALLEY, GREAT LAKES, AND
MID-ATLANTIC/NORTHEAST REGIONS BY THE LATTER PART OF NEXT WEEK
AFTER THE UPPER TROUGHING FILLS. 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.
RAUSCH/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
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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