505
FXUS02 KWBC 211856
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 PM EDT FRI JUL 21 2023
VALID 12Z MON JUL 24 2023 - 12Z FRI JUL 28 2023
...DANGEROUS HEAT WILL CONTINUE ACROSS THE SOUTHWEST AND BUILD
FARTHER NORTH AND EAST INTO THE PLAINS/MIDWEST 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 UPPER TROUGH OVER THE EAST-CENTRAL U.S.
EARLY IN THE WEEK SHOULD GRADUALLY DEAMPLIFY AND ALLOW FOR HOTTER
THAN NORMAL TEMPERATURES TO SPREAD INTO THE OHIO VALLEY/GREAT
LAKES AND NORTHEAST/MID-ATLANTIC BY LATE NEXT WEEK. TOWARD THE END
OF NEXT WEEK AN ATLANTIC UPPER RIDGE POTENTIALLY BUILDING OVER TO
FLORIDA AND THE SOUTHEAST COAST COULD FURTHER AID THE TREND TO
HIGHER HEIGHTS OVER THE EAST. MEANWHILE AN EASTERN PACIFIC UPPER
LOW (PART OF A REX BLOCK CONFIGURATION INCLUDING AN
ALASKA/NORTHWESTERN CANADA RIDGE) MAY EJECT EAST ACROSS SOUTHERN
CANADA AND DROP FRONTS INTO THE NORTHWESTERN TO NORTH-CENTRAL U.S.
ENERGY SURROUNDING/TRAILING THE UPPER LOW MAY REINFORCE THE MEAN
TROUGH NEAR THE PACIFIC NORTHWEST COAST MID-LATE WEEK. SOME
DIURNALLY FAVORED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE OVER
THE FOUR CORNERS STATES WHILE FRONTS/UPPER SHORTWAVES MAY PRODUCE
SOME EPISODES OF CONVECTION OVER THE EASTERN HALF OF THE COUNTRY,
BUT OVERALL THE COVERAGE OF HEAVY RAINFALL THREATS SHOULD BE LESS
THAN AVERAGE FOR THE MIDDLE OF SUMMER.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
WITH THE MODEL/ENSEMBLE GUIDANCE AGREEING FAIRLY WELL FOR THE
ROCKIES/PLAINS UPPER HIGH AND WEAKENING EASTERN U.S. TROUGH, THE
MAIN MODEL DIFFERENCES CONTINUE TO INVOLVE THE EVENTUAL
TIMING/MOVEMENT OF THE UPPER LOW IN THE EASTERN PACIFIC AT THE
START OF THE PERIOD MONDAY AND INFLUENCE ON SURFACE FRONTS OVER
THE NORTHERN U.S. ONGOING SHUFFLING AMONG CONSECUTIVE RUNS OF
VARIOUS GUIDANCE KEEPS CONFIDENCE LOWER THAN DESIRED FOR THE
OPTIMAL SOLUTION. RECENT GFS RUNS HAD BEEN ON THE FASTER SIDE WITH
THE UPPER LOW AND ASSOCIATED SURFACE LOW/FRONT BUT THE 06Z/12Z
RUNS HAVE ADJUSTED SOMEWHAT SLOWER. THE 00Z ECMWF IS FASTER THAN
ITS 00Z RUN FROM 24 HOURS AGO BUT SLOWER THAN THE INTERVENING 12Z
RUN. MEANWHILE THE CMC HAD TENDED TO BE ON THE SLOWER SIDE BUT THE
NEW 12Z RUN HAS JUMPED SIGNIFICANTLY FASTER. GUIDANCE BEHAVIOR
SEEMS TO FAVOR A MODEL/ENSEMBLE AVERAGE APPROACH UNTIL CLUSTERING
IMPROVES/STABILIZES. BEHIND THIS SYSTEM THE MODELS/ENSEMBLES SHOW
BETTER THAN AVERAGE AGREEMENT FOR THE TROUGH AND POSSIBLE EMBEDDED
UPPER LOW NEAR THE PACIFIC NORTHWEST COAST INTO LATE WEEK, WITH AN
INTERMEDIATE SOLUTION LOOKING GOOD AT THIS TIME. FARTHER EAST
THERE ARE STILL SOME SMALLER SCALE DIFFERENCES WITHIN THE EASTERN
CANADA/NORTHEASTERN U.S. UPPER TROUGH WITH LOW PREDICTABILITY
EFFECTS ON THE FRONT FORECAST TO REACH THE NORTHEAST BY MIDWEEK.
WHILE MODELS AGREE FAIRLY WELL ON THE LARGER SCALE, THEY CONTINUE
TO SHOW AMPLE DIFFERENCES IN TERMS OF QPF ESPECIALLY IN THE
UNCERTAIN NORTHWEST FLOW PATTERN (LIKELY TRENDING MORE ZONAL WITH
TIME) IN THE NORTH-CENTRAL U.S. THE WPC QPF CONTINUES TO SHOW
SOMEWHAT BROADER COVERAGE OF LIGHT AMOUNTS IN THE NORTH-CENTRAL
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 00Z/06Z OPERATIONAL MODEL BLEND EARLY AND THEN TRANSITIONED
TO ABOUT HALF MODELS AND HALF ENSEMBLE MEANS BY LATE IN THE
PERIOD. THE BLEND SPLIT THE GFS COMPONENT BETWEEN THE 00Z/06Z RUNS
DUE TO VARIED PREFERENCES OF ONE OR THE OTHER DEPENDING ON THE
REGION, WHILE MINIMIZING CMC INFLUENCE ALONG THE U.S.-CANADIAN
BORDER BY MID-LATE PERIOD. LATEST CONSENSUS AMONG 00Z/06Z GUIDANCE
ULTIMATELY SLOWED PROGRESSION OF THE NORTHERN TIER SYSTEM BY
MID-LATE WEEK AND MAINTAINED DEFINITION OF THE NORTHEAST FRONT A
LITTLE LONGER. GUIDANCE BEHAVIOR SUGGESTS ADDITIONAL ADJUSTMENTS
ONE WAY OR THE OTHER ARE LIKELY.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND STORMS ARE LIKELY IN THE EAST EARLY IN THE WORKWEEK
WITH MOIST INFLOW AHEAD OF THE UPPER TROUGH AXIS. WITH SCATTERED
HEAVY AMOUNTS POSSIBLE, A MARGINAL RISK IS MAINTAINED FOR MONDAY'S
(DAY 4) EXCESSIVE RAINFALL OUTLOOK FOR GENERALLY THE APPALACHIANS
REGION INTO THE CAROLINAS AND FLORIDA. ALSO DURING THE DAY 4 TIME
FRAME THE LATEST GUIDANCE IS STARTING TO SHOW SOME LOOSE
CLUSTERING WITH THE IDEA OF CONVECTIVE POTENTIAL OVER PARTS OF THE
MIDWEST, SOUTH OF A FRONT DROPPING INTO THE UPPER MIDWEST/GREAT
LAKES. THUS THE PLAN IS TO INTRODUCE A MARGINAL RISK AREA COVERING
MOST OF IOWA AND PARTS OF ILLINOIS/MISSOURI. ON TUESDAY THE
EASTWARD PROGRESS OF THE GREAT LAKES FRONT AND SUPPORTING UPPER
SHORTWAVE SHOULD PROMOTE HEAVIEST RAIN TOTALS ACROSS THE
NORTHEASTERN U.S. A MARGINAL RISK REMAINS IN PLACE FOR THE
NORTHERN MID-ATLANTIC INTO THE INTERIOR NORTHEAST FOR THIS
ACTIVITY. VERY WET ANTECEDENT CONDITIONS WILL MAKE THIS AREA
SENSITIVE TO ANY ADDITIONAL HEAVY RAINFALL. 12Z GUIDANCE THUS FAR
HAS NOT INCREASED THE OVERALL SIGNAL FOR HEAVY ACTIVITY VERSUS THE
PRIOR CYCLE, BUT THIS AREA WILL REQUIRE MONITORING FOR ANY
POSSIBLE HIGHER TRENDS IN GUIDANCE THAT WOULD FAVOR AN UPGRADE IN
RISK LEVEL. FARTHER SOUTH, A LINGERING FRONT ACROSS THE GULF COAST
COULD SPARK SOME CONVECTION PARTICULARLY ON MONDAY-TUESDAY BEFORE
THE FRONT DISSIPATES. UNCERTAINTY WHETHER THE RAIN WILL FOCUS ON-
OR OFFSHORE PRECLUDES A MARGINAL RISK FARTHER WEST OF FLORIDA AT
THIS TIME (SOUTHEASTERN LOUISIANA MAY BE THE MOST LIKELY PLACE FOR
HEAVY RAIN TO END UP ONSHORE). BEYOND TUESDAY, EXPECT ADDITIONAL
EPISODES OF CONVECTION FROM ABOUT THE NORTHERN HALF OF THE PLAINS
EASTWARD, WITH ONE OR MORE FRONTAL BOUNDARIES HELPING TO FOCUS
THIS ACTIVITY THAT COULD BE LOCALLY HEAVY. IN GENERAL THERE MAY BE
A TENDENCY TOWARD HIGHER RAINFALL TOTALS AROUND THE END OF NEXT
WEEK WITH UPPER FLOW ACROSS THE NORTHERN TIER BECOMING MORE ZONAL
AND BETTER POTENTIAL FOR FRONTS TO CONTAIN EMBEDDED WAVES. 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. THE PACIFIC NORTHWEST SHOULD SEE
SOME RAINFALL EARLY IN THE WEEK AS THE UPPER LOW OVER THE EASTERN
PACIFIC COMES ASHORE INTO BRITISH COLUMBIA.
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 WELL OVER 110F DURING PEAK
AFTERNOON HEATING AS HIGH DEWPOINTS COMBINE WITH THE HOT
TEMPERATURES. A FEW DAILY RECORD HIGHS WILL BE POSSIBLE MAINLY
OVER AND NEAR THE SOUTHERN HIGH PLAINS EARLY-MID WEEK, 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 5-10F) 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 ERODES. 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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