422
FXUS02 KWBC 020714
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
313 AM EDT TUE MAY 02 2023
VALID 12Z FRI MAY 05 2023 - 12Z TUE MAY 09 2023
...COOL/WET PATTERN FOR THE PACIFIC NORTHWEST/GREAT BASIN/ROCKIES
INTO THE WEEKEND WITH LOCALIZED FLOODING CONCERNS...
..OVERVIEW
THE UPPER LEVEL PATTERN IS FORECAST TO EVOLVE FROM THE STUBBORN
OMEGA BLOCK CENTERED OVER THE CONUS THIS PAST WEEK/WEEKEND AND
TOWARDS MEAN RIDGING BUILDING OVER THE CENTRAL/EASTERN CONUS AND
MEAN TROUGHING OVER THE CENTRAL/WESTERN CONUS. CONDITIONS WILL BE
COOL AND WET IN THE WEST UNDER THE INFLUENCE OF THE UPPER-LEVEL
TROUGHING, WITH HEAVY RAINFALL ON TOP OF MOUNTAIN SNOW MELT
LEADING TO LOCALIZED FLOODING CONCERNS IN THE GREAT BASIN/NORTHERN
ROCKIES. MOISTURE SOURCED FROM THE GULF AND PACIFIC WILL BRING THE
POTENTIAL FOR MULTIPLE REGIONS OF LOCALLY HEAVY RAIN TO THE
PLAINS/MISSISSIPPI VALLEY THROUGH THE WEEKEND AND INTO NEXT WEEK
DEPENDING ON THE EVOLUTION OF SHORTWAVE ENERGY ALOFT.
..GUIDANCE/PREDICTABILITY ASSESSMENT
UPDATED MODEL GUIDANCE SHOWS A TRANSITION IN THE UPPER-LEVEL
PATTERN AWAY FROM THE STUBBORN OMEGA BLOCK CENTERED OVER THE CONUS
TO MEAN RIDGING IN THE CENTRAL-EAST CONUS AND MEAN TROUGHING OVER
THE CENTRAL-WEST CONUS LATE THIS WEEK AND INTO THE WEEKEND. AN
INITIAL WAVE OF ENERGY IN THE MEAN FLOW WILL BE EJECTING NORTHWARD
OVER THE GREAT BASIN/NORTHERN ROCKIES INTO CANADA AS ADDITIONAL
SHORTWAVE ENERGY ROUNDS THE WESTERN TROUGH, WITH THE RIDGE
BUILDING TO THE EAST. THE BUILDING RIDGE REPLACES LINGERING
TROUGHING OVER THE NORTHEAST FOLLOWING THE DEPARTURE OF A STUBBORN
CLOSED LOW OVER THE REGION. THE 12Z/18Z GFS AND 12Z CMC/ECMWF ARE
IN GENERALLY GOOD AGREEMENT ON THE LARGE SCALE DETAILS THROUGH THE
EARLY FORECAST PERIOD. THE 12Z UKMET DIVERGES FROM THE OTHER
SOLUTIONS RATHER QUICKLY IN THE EARLY TO MID-FORECAST PERIOD WITH
A DEEPER CLOSED-LOW/TROUGH LINGERING OFF THE CALIFORNIA COAST, A
BIT OUT OF PHASE WITH THE OTHER GUIDANCE. IN ADDITION, THE 12Z
UKMET IS ALSO MORE AGGRESSIVE WITH SHORTWAVE ENERGY ROUNDING THE
TROUGH OVER CALIFORNIA AND THE GREAT BASIN/SOUTHWEST.
MORE SIGNIFICANT DIFFERENCES APPEAR RATHER QUICKLY DURING THE
MID-FORECAST PERIOD. THE FIRST CONCERN IS THE HANDLING OF A
POTENTIAL SHORTWAVE DISRUPTING THE RIDGE OVER THE CENTRAL/EASTERN
CONUS. THE 12Z ECMWF AS WELL AS THE 12Z ECENS MEAN ARE THE MOST
AGGRESSIVE WITH THIS FEATURE, WHICH IS APPARENT BUT TO A MUCH
LESSER EXTENT IN THE 12Z/18Z GFS AND 12Z CMC, AND NOT PRESENT IN
THE 18Z GEFS MEAN. THE DEGREE TO WHICH THIS DISRUPTS THE RIDGE IN
THE LATE PERIOD IS ALSO A SOURCE OF DISAGREEMENT IN THE
DETERMINISTIC GUIDANCE, WHICH MAKES THE ENSEMBLE MEANS LIKELY MORE
RELIABLE IN CAPTURING THE OVERALL PATTERN AT THIS POINT. THIS
FEATURE WILL HAVE DOWNSTREAM IMPACTS ON THE
ORGANIZATION/PROGRESSION OF FRONTAL SYSTEMS OVER THE
PLAINS/MISSISSIPPI VALLEY. THE HANDLING OF THIS FEATURE IS RATHER
POOR RUN-TO-RUN IN BOTH THE GFS AND ECMWF SO TOO MUCH EMPHASIS ON
ANY ONE SOLUTION WOULD NOT BE EXPECTED AT THIS POINT. UPDATED
GUIDANCE FROM THE 00Z GFS WAS MORE AGGRESSIVE WITH THIS SHORTWAVE,
SIMILAR TO THE 12Z ECMWF, BUT THE TIMING REMAINS DIFFERENT. THERE
IS ALSO SIGNIFICANT DISAGREEMENT WITH RESPECT TO THE EVOLUTION OF
THE PATTERN OVER THE WEST LATE PERIOD BETWEEN BOTH THE
DETERMINISTIC AND ENSEMBLE GUIDANCE. THE ECMWF HAS WAFFLED
RUN-TO-RUN WITH DEPICTING A CLOSED LOW MOVING INTO THE PACIFIC
NORTHWEST. THE 18Z GFS IS COMPLETELY OUT OF PHASE WITH THE 12Z
ECMWF, WHILE THE 12Z GFS, WITHOUT THE CLOSED LOW, AT LEAST STILL
RETAINS MEAN TROUGHING/RIDGING IN GENERALLY THE SAME PHASE. THE
00Z GFS FOLLOWED A SIMILAR EVOLUTION TO THE 18Z GFS, LEADING TO
EVEN LESS CONFIDENCE IN THE PATTERN LATE PERIOD. THE AMPLIFICATION
OF MEAN TROUGHING OVER THE WEST IS ALSO LOWER IN THE 18Z GEFS MEAN
COMPARED TO THE 12Z ECENS MEAN AND THE 12Z ECMWF/GFS/CMC.
THE WPC FORECAST PACKAGE BEGINS WITH A BLEND OF THE 12Z ECMWF/CMC
AND 12Z/18Z GFS. THE 12Z UKMET WAS NOT INCLUDED GIVEN THE NOTABLE
DIFFERENCES IN THE EVOLUTION OF THE PATTERN OVER THE WEST. THE
CONTRIBUTION FROM THE 18Z GFS IS REMOVED MID-PERIOD AS IT DIVERGES
FROM THE OTHER SOLUTIONS IN HANDLING THE CENTRAL/EASTERN CONUS
SHORTWAVE. THE UPDATED BLEND HAS LED TO A BIT WEAKER/MORE
NORTHERLY TRACK OF THIS SHORTWAVE, BUT RETAINS IT COMPARED TO THE
12Z AND ESPECIALLY 18Z GFS, WHICH ACCOUNTS FOR TRENDS IN THE
UPDATED GUIDANCE WHILE NOT SHIFTING TOO FAR AWAY FROM THE PRIOR
FORECAST PACKAGE UNTIL THERE IS MORE CONFIDENCE IN HOW THIS
PATTERN EVOLVES. THE BLEND IN THE LATE FORECAST PERIOD IS SPLIT
BETWEEN A CONTRIBUTION FROM THE 12Z CMC/ECMWF/GFS AND THE 12Z
ECENS/18Z GEFS MEANS GIVEN THE NOTED LOW CONFIDENCE IN THE PATTERN
OVER THE WEST. THE CONTRIBUTION FROM THE 12Z ECMWF WAS REDUCED
GIVEN THE EMPHASIS ON A CLOSED LOW IN THE LATEST RUN. THE
COMBINATION OF THIS SPECIFIC GUIDANCE RELIES ON SIMILAR OVERALL
PHASING OF MEAN TROUGHING OVER THE WEST, EVEN WITH NOTABLE SMALL
AND LARGE-SCALE DETAILS YET TO BE RESOLVED.
..WEATHER/HAZARDS HIGHLIGHTS
PRECIPITATION CHANCES WILL REMAIN HIGH AND WIDESPREAD FOR THE
PACIFIC NORTHWEST/GREAT BASIN/NORTHERN ROCKIES LATE THIS
WEEK/WEEKEND AS AN UPPER-LEVEL WAVE AND MULTIPLE FRONTAL SYSTEMS
IMPACT THE REGION. RAINFALL MAY BE LOCALLY HEAVY AT TIMES, WITH
SOME SCATTERED THUNDERSTORM CHANCES. THE RAINFALL ON TOP OF
ONGOING SNOW MELT MAY LEAD TO SOME FLOODING CONCERNS IN TERRAIN
SENSITIVE AREAS AROUND THE FOOTHILLS OF MOUNTAIN RANGES AS WELL AS
ALONG LOCAL STREAMS/RIVERS FRIDAY, WITH A BROAD "MARGINAL RISK"
EXCESSIVE RAINFALL OUTLOOK IN PLACE. SOME SNOW SHOWERS WILL BE
POSSIBLE AT HIGHER MOUNTAIN ELEVATIONS. TEMPERATURES WILL REMAIN A
BIT COOL MORE BROADLY ACROSS THE WEST, WITH HIGHS RUNNING 5-10
DEGREES BELOW AVERAGE THROUGH THE PERIOD.
MOIST, UPSLOPE FLOW SETTLING IN ALONG PORTIONS OF THE
NORTHERN/CENTRAL HIGH PLAINS AHEAD OF A POTENTIAL BETTER ORGANIZED
SURFACE FRONTAL SYSTEM PASSING THROUGH THE PLAINS/UPPER MIDWEST
LOOK TO BRING MULTIPLE DAYS OF RAINFALL FRIDAY-SUNDAY. A "MARGINAL
RISK" HAS BEEN MAINTAINED FOR PORTIONS OF THE NORTHERN/CENTRAL
PLAINS FRIDAY WHERE MORE ORGANIZED CONVECTION MAY PRODUCE SOME
LOCALLY HEAVY RAINFALL TOTALS. FURTHER SOUTH, GULF MOISTURE
FLOWING NORTHWARD AS WELL AS ADDITIONAL MOISTURE SOURCED FROM THE
PACIFIC WILL SPREAD NORTHEASTWARD ACROSS THE CENTRAL/SOUTHERN
PLAINS AND MIDDLE/LOWER MISSISSIPPI VALLEY INTO THE SOUTHEAST LATE
WEEK LEADING TO A BROAD REGION OF SHOWERS AND THUNDERSTORMS. THE
EVOLUTION OF THE PATTERN HAS REMAINED RATHER NEBULOUS THE LAST FEW
DAYS AS THE GUIDANCE HAS VARIED ON THE TIMING AND EVOLUTION OF
EMBEDDED SHORTWAVE ENERGY ALOFT, LESSENING CONFIDENCE ON THE
SPECIFICS OF REGIONS OF HEAVIER RAINFALL AND POTENTIAL FLASH
FLOODING CONCERNS. HOWEVER, THE GUIDANCE HAS SHOWN THE HIGHEST
CONFIDENCE IN WIDESPREAD CONVECTION AND HIGH QPF AMOUNTS FROM THE
MIDDLE MISSISSIPPI TO TENNESSEE VALLEY FRIDAY, WITH THE POTENTIAL
FOR TRAINING APPEARING SUFFICIENT TO SUPPORT A "MARGINAL RISK"
AREA. ADDITIONAL HEAVY RAINFALL CHANCES LOOK POSSIBLE OVER THE
WEEKEND MORE BROADLY ACROSS THE CENTRAL/SOUTHEASTERN U.S.,
PARTICULARLY THE LOWER MISSISSIPPI VALLEY, BUT CONFIDENCE REMAINS
LOW GIVEN THE GUIDANCE SPREAD IN HOW THE SURFACE PATTERN AND
SUPPORTING SHORTWAVE DYNAMICS WILL EVOLVE. HIGH TEMPERATURES WILL
BE SEASONABLY WARM FOR EARLY MAY FROM THE PLAINS TO THE SOUTHEAST
UNDER THE INFLUENCE OF MEAN RIDGING ALOFT, WITH SOME HIGHS RUNNING
10-15 DEGREES ABOVE AVERAGE IN THE CENTRAL PLAINS.
THE PERSISTENT COOL/WET PATTERN OVER THE GREAT LAKES AND NORTHEAST
DUE TO A STUBBORN UPPER-LEVEL LOW THIS PAST WEEK/WEEKEND WILL
FINALLY MODERATE THIS UPCOMING WEEKEND, WITH HIGH TEMPERATURES
RETURNING TO NEAR TO SLIGHTLY ABOVE AVERAGE LEVELS INTO EARLY NEXT
WEEK. PRECIPITATION CHANCES WILL REMAIN LOW UNDER THE INFLUENCE OF
A BROAD AREA OF HIGH PRESSURE IN PLACE. SHOWER AND THUNDERSTORM
CHANCES WILL INCREASE FOR THE CAROLINAS SOUTH INTO FLORIDA THIS
WEEKEND, WITH AMOUNTS AND COVERAGE LIKELY DICTATED BY A WEAK WAVE
ALOFT AND SURFACE FRONTAL BOUNDARY INFLUENCE THAT REMAIN HARD TO
PIN DOWN AT THIS TIMEFRAME.
PUTNAM
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, EXPERIMENTAL EXCESSIVE RAINFALL
OUTLOOK, WINTER WEATHER OUTLOOK PROBABILITIES AND HEAT INDICES 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
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