784
FXUS02 KWBC 180657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 AM EDT SUN AUG 18 2024
VALID 12Z WED AUG 21 2024 - 12Z SUN AUG 25 2024
..HAZARDOUS HEAT TO PERSIST FOR THE SOUTHERN PLAINS THIS WEEK
..OVERVIEW
AN AMPLIFIED UPPER PATTERN IS OVERALL FORECAST TO PREVAIL ACROSS
MUCH OF OUR COUNTRY THIS WEEK. THIS WILL BE HIGHLIGHTED BY AN
UPPER HIGH CENTERED OVER THE SOUTHERN PLAINS THAT WILL FOCUS
HAZARDOUS HEAT. THE NORTHWARD BUILDING RIDGE OVER TIME WILL BE
SANDWICHED BY AN AMPLIFIED UPPER TROUGH UPSTREAM NEAR THE WEST
COAST AND AN AMPLIFIED UPPER TROUGH DOWNSTREAM OVER THE EAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
FORECAST SPREAD HAS INCREASED OVER THE PAST FEW RUN CYCLES OVER
MEDIUM RANGE TIME SCALES, PRIMARILY AS ATTRIBUTED TO DIFFERENCES
WITH THE HANDLING OF DEEP EASTERN PACIFIC UPPER TROUGH/LOW ENERGIES
INLAND INTO THE WEST/NORTHWEST STARTING LATE THIS WEEK. THE 12 UTC
ECMWF/UKMET TRENDED MUCH MORE PROGRESSIVE AND LESS AMPLIFIED WHILE
THE 12/18 UTC GFS/CANADIAN CONTINUED TO HOLD THE BULK OF SYSTEM
ENERGY MUCH LONGER OFFSHORE. MULTI-CENTER ENSEMBLES OFFERED SOME
PROGRESSIVE SOLUTIONS WITHIN THE FORECAST ENVELOPE, BUT MANY
FAVORED A SLOWER TRANSLATION THAT SEEMED MORE REASONABLE GIVEN THE
BLOCKY LEAD FLOW OVER NORTH AMERICA AS WELL AS UPSTREAM FLOW WHOSE
AMPLITUDE IS BEING ENERGIZED BY THE ONGOING DEEP EXTRATROPICAL LOW
TRACK TOWARD ALASKA OF FORMER WEST PACIFIC TYPHOON AMPIL.
AN 18 UTC GFS/GEFS MEAN AND 12 UTC CANADIAN AND ECMWF/CANADIAN
ENSEMBLE MEAN COMPOSITE BLEND WAS USED EARLIER TO DERIVE THE WPC
MEDIUM RANGE PRODUCT SUITE IN CONJUCTURE WITH THE 01 UTC NATIONAL
BLEND OF MODELS. THE 00 UTC GFS/CANADIAN STAYED THE COURSE. THE 00
UTC ECMWF AND ESPECIALLY THE 00 UTC UKMET MADE PRONOUNCED REVERSALS
BACK TOWARD THE GFS/CANADIAN AND ENSEMBLE MEANS, BOLSTERING CONFIDENCE.
..WEATHER/HAZARDS HIGHLIGHTS
AN EASTERN U.S. UPPER TROUGH OFFERS SOME LINGERING UNSETTLED
CONDITIONS GIVEN SLOW TRANSLATION, BUT RAINFALL WILL TAPER OFF
MIDWEEK AS SURFACE HIGH PRESSURE BUILDS IN BEHIND A QUITE
REFRESHING COLD FRONTAL PUSH IN THE WAKE OF ERNESTO LIFTING WELL
OFF INTO THE NORTH ATLANTIC. HOWEVER, TRAILING FRONTS SLATED TO
STALL FARTHER SOUTH ACROSS FLORIDA AND VICINITY WILL MAINTAIN
PERIODIC THUNDERSTORM CHANCES THERE OVER A MULTI-DAY PERIOD LATER
WEEK INTO NEXT WEEKEND. FLOODING ISSUES ARE A POSSIBILITY IF HEAVY
RAIN FALLS IN URBAN AREAS. THIS FRONT MOVES SLOWLY AS IT BENDS BACK
TO THE WEST ATOP THE CENTRAL GULF COAST AND BACK NORTH INTO THE
PLAINS. SCATTERED STORMS ARE POSSIBLE NEAR THE FRONT AS IMPULSES
RIDE AND DIG TO THE LEE OF THE UPSTREAM RIDGE ALOFT, BUT WITH
GENERALLY LOW CONFIDENCE IN THE PREDICTED PLACEMENT.
MEANWHILE, A MONSOONAL CONVECTIVE PATTERN WILL CONTINUE TO AFFECT
THE SOUTHWEST/FOUR CORNERS STATES THIS WEEK WITH MOIST SOUTHERLY
FLOW WEST OF THE UPPER RIDGE AXIS. THE WPC DAY 4/5 EXCESSIVE
RAINFALL OUTLOOKS FOR WEDNESDAY/THURSDAY SHOW A MARGINAL RISK FOR
ARIZONA AND PORTIONS OF UTAH, SOUTHEAST CALIFORNIA, SOUTHERN NEVADA
AND WESTERN COLORADO. LOCAL STORMS WITH EMBEDDED HEAVY RAIN COULD
CAUSE ISOLATED FLOODING, PARTICULARLY IN VULNERABLE AREAS.
MEANWHILE, UPPER TROUGH AMPLIFICATION PROXIMITY ALONG/OFF THE WEST
COAST PORTENDS A COOLER AND WETTER PATTERN FOR THE NORTHWEST. SOME
RAINS ARE ALSO FORECAST TO BROADLY FOCUS DOWNSTREAM ACROSS THE U.S.
NORTHERN TIER AS THE WEEK PROGRESSES. SOME CONVECTION MAY BREAK OUT
ALONG/AHEAD OF RELATIVELY SLOW MOVING FRONTAL SYSTEMS ACROSS THE
NORTH-CENTRAL U.S. AS THE WEEK PROGRESSES, BUT THE AFOREMENTIONED
FORECAST PLAN DOWNPLAYS FASTER/ORGANIZED ECMWF/UKMET POTENTIAL.
UPPER RIDGING OVER THE SOUTH-CENTRAL U.S. WILL SUPPORT 5-15 DEGREE
ABOVE NORMAL TEMPERATURES. HIGHS WELL INTO THE 100S IN MUCH OF
TEXAS COULD SET DAILY RECORDS AND HIGH DEWPOINTS SHOULD PRODUCE
HEAT INDICES LIKELY ABOVE 110 DEGREES. LOW TEMPERATURES IN THE
UPPER 70S TO LOW 80S WILL EXACERBATE HEAT STRESS AND COULD SET
RECORDS FOR WARM MINIMUM TEMPERATURES. EXPERIMENTAL HEATRISK
CONTINUES TO INDICATE MAJOR TO EXTREME IMPACTS FROM THIS HEAT. THIS
WILL BE IN CONTRAST TO COOLING CONDITIONS TO CREEP INTO THE
WEST/NORTHWEST AND OVER THE EAST TO INCLUDE SOME RECORD VALUES.
SCHICHTEL
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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