958
FXUS02 KWBC 200656
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EDT THU JUL 20 2023
VALID 12Z SUN JUL 23 2023 - 12Z THU JUL 27 2023
***DANGEROUS HEAT WAVE PERSISTS ACROSS THE SOUTHWESTERN U.S.
THROUGH THE MIDDLE OF NEXT WEEK, AND HEAT BUILDS FARTHER NORTH
ACROSS THE NORTHERN HIGH PLAINS AND EVENTUALLY EXTENDING INTO THE
MIDWEST***
..GENERAL OVERVIEW
THE UPPER LEVEL FLOW PATTERN IS EXPECTED TO BE RATHER AMPLIFIED
FOR LATE JULY ACROSS THE CONTINENTAL U.S. THROUGH AT LEAST MONDAY
WITH AN ANOMALOUS UPPER RIDGE/CLOSED HIGH CENTERED OVER THE
SOUTHERN ROCKIES, A TROUGH STRETCHING FROM THE GREAT LAKES TO THE
DEEP SOUTH, AND A CLOSED LOW OFF THE PACIFIC NORTHWEST COAST.
THIS WILL RESULT IN A CONTINUANCE OF THE ONGOING RECORD-BREAKING
HEAT WAVE ACROSS THE SOUTHWESTERN U.S. AND EXTENDING NORTHWARD TO
THE WESTERN HIGH PLAINS THROUGH MUCH OF NEXT WEEK, AND NEAR
AVERAGE TEMPERATURES OVER THE EASTERN STATES. THE PATTERN MAY
BREAK DOWN SOME ACROSS THE NORTHERN TIER STATES MID TO LATE WEEK
AS SHORTWAVE ENERGY CROSSES SOUTHERN CANADA, WHILE THE CLOSED
UPPER HIGH HOLDS FIRM OVER NEW MEXICO AND PORTIONS OF ADJACENT
STATES.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE MODEL GUIDANCE SUNDAY AND INTO MONDAY REMAINS IN VERY GOOD
OVERALL AGREEMENT ACROSS THE CONTINENTAL U.S., AND THIS HAS BEEN
THE CASE FOR THE PAST SEVERAL DAYS OF MODEL RUNS. HOWEVER, THERE
HAS BEEN AN INCREASE IN THE LEVEL OF MODEL UNCERTAINTY ACROSS
SOUTHERN CANADA AND ALSO ACROSS THE GREAT LAKES AND NORTHEAST
GOING INTO THE MIDDLE OF NEXT WEEK. THERE ARE DIFFERENCES IN HOW
QUICKLY SHORTWAVE ENERGY TRACKS EASTWARD FROM THE UPPER LEVEL
VORTEX OVER THE GULF OF ALASKA AND HOW THAT MAY SQUASH THE
NORTHERN PERIPHERY OF THE RIDGE AXIS NEAR THE U.S./CANADA BORDER.
THE GFS OVER THE PAST FEW RUNS HAS BEEN FASTER WITH THE CLOSED LOW
TRACKING EAST COMPARED TO THE CMC/ECMWF, BUT THE CMC HAS TRENDED
MORE IN LINE WITH THE MODEL CONSENSUS COMPARED TO ITS 12Z RUN.
THE WPC FRONT/PRESSURES FORECAST WAS PRIMARILY DERIVED FROM A
NEARLY MULTI-DETERMINISTIC MODEL BLEND THROUGH MONDAY, FOLLOWED BY
LESS OF THE 12Z CMC AND A GRADUAL INCREASE OF THE ENSEMBLE MEANS
TO ABOUT 50% BY NEXT THURSDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE WEAKENING FRONTAL BOUNDARY THAT STALLS OUT IN THE GENERAL
VICINITY OF THE INTERSTATE 20 CORRIDOR ACROSS THE DEEP SOUTH BY
SUNDAY IS EXPECTED TO BE A FOCUS FOR SCATTERED TO PERHAPS NUMEROUS
SHOWERS AND STORMS, PARTICULARLY NEAR ANY SEA BREEZE BOUNDARIES.
THERE IS A MARGINAL RISK AREA FOR FLOODING POTENTIAL ACROSS THE
COASTAL PLAIN OF THE GULF COAST REGION FROM LOUISIANA TO THE
EASTERN CAROLINAS ON DAY 4 WHERE THE BEST COMBINATION OF FORCING
AND INSTABILITY WILL EXIST. SOME SLOW MOVING STORMS COULD PRODUCE
A QUICK 1-2 INCHES OF LOCALIZED RAINFALL IN SOME OF THESE AREAS.
THERE ARE ALSO SIGNS THAT SOME THUNDERSTORM COMPLEXES COULD
DEVELOP ACROSS PORTIONS OF THE MIDWEST IN THE NORTHWEST FLOW
PATTERN ALOFT ON SUNDAY, AND A MARGINAL RISK AREA HAS BEEN
INTRODUCED FOR THAT POSSIBILITY. ACROSS THE WESTERN U.S., A
MARGINAL RISK AREA REMAINS VALID FOR BOTH DAYS 4 AND 5 FROM
SOUTHERN COLORADO ACROSS MUCH OF NEW MEXICO AND INTO EASTERN
PORTIONS OF ARIZONA. WITH THE CORE OF THE UPPER HIGH EXPECTED TO
BE OVER THE FOUR CORNERS REGION, THIS WILL RESULT IN SOME
ADVECTION OF MONSOONAL MOISTURE FROM WESTERN MEXICO NORTHWARD
ACROSS THE SOUTHERN ROCKIES, RESULTING IN ISOLATED TO PERHAPS
SCATTERED COVERAGE OF SLOW MOVING THUNDERSTORMS THAT CAN BE
EXPECTED MAINLY DURING THE AFTERNOON AND EVENING HOURS. LOOKING
AHEAD TO MONDAY, SCATTERED SHOWERS AND STORMS WILL LIKELY DEVELOP
FROM THE SOUTHEAST COAST NORTHWARD TO INCLUDE THE CENTRAL
APPALACHIANS, AND A DAY 5 MARGINAL RISK AREA IS IN EFFECT FOR THIS
REGION WITH THE POTENTIAL FOR SOME SLOW MOVING CELLS. ELSEWHERE,
A FAIRLY UNEVENTFUL PATTERN IS EXPECTED WITH NO MAJOR AREAS OF
CONCERN AT THIS TIME.
THE LARGE UPPER RIDGE/CLOSED HIGH OVER THE INTERMOUNTAIN
WEST/DESERT SOUTHWEST WILL CONTINUE TO HAVE STAYING POWER THROUGH
MUCH OF NEXT WEEK AS THE OVERALL PATTERN HERE DOES NOT CHANGE THAT
MUCH. THIS WILL RESULT IN A CONTINUATION OF THE ONGOING EXTREME
HEAT GOING THROUGH NEXT WEEKEND AND INTO THE MIDDLE OF NEXT WEEK,
WITH TEMPERATURES GENERALLY RUNNING 5 TO 10 DEGREES ABOVE AVERAGE,
AND LOCALLY HIGHER, FOR BOTH HIGHS AND LOWS FROM THE DESERT
SOUTHWEST TO MONTANA AS THE HEAT BUILDS FARTHER TO THE NORTH, AND
ALSO EXTENDING ACROSS THE NORTH-CENTRAL U.S. BY TUESDAY AND
WEDNESDAY. THIS WILL UNFORTUNATELY EQUATE TO HIGHS INTO THE 110S
FOR THE LOWER ELEVATIONS OF EASTERN CALIFORNIA, SOUTHERN ARIZONA,
AND SOUTHERN NEVADA, WITH THE POTENTIAL FOR ADDITIONAL RECORD HIGH
TEMPERATURES BEING ESTABLISHED AND RECORD LONG STREAKS OF 110+
DEGREE READINGS FOR SOME ARIZONA LOCATIONS, AND WIDESPREAD 90S ARE
LIKELY FOR THE CENTRAL AND NORTHERN PLAINS. READINGS MAY BE A FEW
DEGREES LOWER BY TUESDAY-WEDNESDAY ACROSS MUCH OF THE
INTERMOUNTAIN WEST, BUT NOT SUBSTANTIAL RELIEF. IT WILL STILL BE
QUITE HOT AND HUMID FOR SOUTHERN TEXAS WHERE HEAT INDICES MAY
EXCEED 110 DEGREES DURING PEAK AFTERNOON HEATING WITH VERY HIGH
DEWPOINTS REMAINING IN PLACE. MUCH OF THE EASTERN U.S. SHOULD
GENERALLY BE WITHIN A FEW DEGREES OF LATE JULY AVERAGES WITH NO
MAJOR HEAT WAVES EXPECTED THROUGH NEXT WEEK.
HAMRICK
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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