454
FXUS02 KWBC 040657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 AM EDT THU MAY 04 2023
VALID 12Z SUN MAY 07 2023 - 12Z THU MAY 11 2023
..OVERVIEW
THE FORECAST FLOW PATTERN OVER NORTH AMERICA NEXT WEEK LOOKS TO
FEATURE MEAN RIDGING OVER THE CENTRAL-EASTERN CONUS AND MEAN
TROUGHING OVER THE CENTRAL-WESTERN CONUS. UPPER-LEVEL IMPULSES
THROUGH SOUTHWESTERLY FLOW WITH AN INFLUX OF GULF MOISTURE WILL
FAVOR BROAD AND WIDESPREAD SHOWER/STORM CHANCES FROM THE PLAINS TO
THE MISSISSIPPI VALLEY AND SOUTHEAST. CONDITIONS WILL REMAIN COOL
IN THE WEST UNDER THE INFLUENCE OF MEAN TROUGHING, WITH DAILY
PRECIPITATION CHANCES FOR THE NORTHERN TIER OF THE REGION.
..GUIDANCE/PREDICTABILITY ASSESSMENT
BOTH THE DETERMINISTIC AND ENSEMBLE GUIDANCE IS IN GOOD AGREEMENT
WITH THE OVERALL PATTERN OVER THE CONUS AT THE START OF THE
FORECAST PERIOD (12Z SUNDAY MAY 7), DEPICTING MEAN LONGWAVE
RIDGING OVER THE CENTRAL-EASTERN CONUS AND MEAN LONGWAVE TROUGHING
OVER THE CENTRAL-WESTERN CONUS. SHORTWAVE ENERGY EJECTING
NORTHEASTWARD FROM THE WESTERN TROUGH OVER THE PLAINS INTO THE
UPPER MIDWEST THAT HAS BEEN A MAINSTAY IN THE RECENT GUIDANCE IS
STILL APPARENT, THOUGH HAS TRENDED WEAKER, WITH THE RIDGE
AMPLIFYING A BIT MORE TO THE EAST. THIS FEATURE HAS AND WILL
CONTINUE TO HAVE DOWNSTREAM IMPACTS ON PRECIPITATION CHANCES OVER
THE NORTHERN TIER. ANOTHER CONSISTENT FEATURE IN THE GUIDANCE IS
THE DEVELOPMENT OF A CLOSED LOW OVER ATLANTIC CANADA MID-FORECAST
PERIOD. FLOW OVER THE EAST GRADUALLY BECOMES FLATTER/MORE ZONAL AS
THE CLOSED-LOW DEEPENS AND FLATTENS OUT THE RIDGE. THE 12Z CMC IS
MORE AGGRESSIVE WITH SHORTWAVE ENERGY BOTH IN THE SOUTHERN STREAM
AND AROUND THE CLOSED LOW TO THE NORTH, AMPLIFYING TROUGHING ALONG
THE EAST COAST LATE PERIOD.
TO THE WEST, SEVERAL NORTHERN STREAM IMPULSES DROPPING SOUTH FROM
THE NORTHEASTERN PACIFIC LEAD TO AN ENERGETIC AND LESS PREDICTABLE
PATTERN. ONE SUCH IMPULSE SHIFTS SOUTHWARD TOWARDS THE WEST COAST
AS A CLOSED LOW EARLY TO MID-FORECAST PERIOD. THE 12Z/18Z GFS AND
12Z ECMWF SHOW THIS FEATURE CONTINUING QUICKLY EASTWARD AS AN OPEN
WAVE IN THE MEAN FLOW. THE 12Z CMC IS MORE AGGRESSIVE THAN THE
OTHER GUIDANCE, WITH THE CLOSED LOW DEEPENING OVER THE WEST COAST
MID- TO LATE FORECAST PERIOD, BECOMING THE DOMINANT FEATURE IN THE
REGION. THIS SOLUTION DIVERGES MORE FUNDAMENTALLY FROM THE OTHER
GUIDANCE WHICH RETAIN A MORE STEADY PATTERN OF ADDITIONAL
IMPULSES. THE 12Z ECENS AND 18Z GEFS MEANS COMPLEMENT THEIR
DETERMINISTIC COUNTERPARTS IN SUGGESTING AN ACTIVE PATTERN
CONTINUING TO SUPPORT MEAN TROUGHING OVERALL ACROSS THE WEST, WITH
MORE SPECIFIC DETAILS IN THE TIMING OF INDIVIDUAL WAVES TO BE
RESOLVED OVER TIME. THIS SETS UP SOUTHWESTERLY FLOW OVER A GOOD
PORTION OF THE CONUS, SUPPORTIVE OF BROAD PRECIPITATION CHANCES
SEEN IN THE QPF FORECASTS. ONE IMPORTANT CAVEAT IS THE RUN-TO-RUN
CONSISTENCY DIMINISHES RAPIDLY BY THE END OF THE PERIOD, WITH THE
PHASE OF TROUGHING OVER THE WEST, AND POTENTIALLY INTO THE
CENTRAL/EASTERN CONUS, OUT OF SYNC BETWEEN PREVIOUS RUNS AND THE
CURRENT RUNS OF THE GUIDANCE. HOWEVER, THE 00Z GFS CLOSELY
FOLLOWED THE 18Z GFS/12Z ECMWF, ESPECIALLY WITH RESPECT TO THE
POTENTIAL FOR AN AMPLIFICATION OF THE TROUGH/RIDGE PATTERN
CENTERED OVER THE INTERMOUNTAIN WEST/PLAINS AT THE END OF THE
FORECAST PERIOD, WHICH INCREASES CONFIDENCE A BIT IN THE CURRENT
FORECAST PACKAGE.
THE UPDATED WPC FORECAST PACKAGE USED A GENERAL BLEND OF THE
DETERMINISTIC GUIDANCE FOR THE EARLY FORECAST PERIOD GIVEN GOOD
OVERALL AGREEMENT ON LARGE-SCALE DETAILS. THE CONTRIBUTION FROM
THE 12Z CMC WAS REMOVED BY THE MID-FORECAST PERIOD WHEN ITS
SOLUTION BEGINS TO DIVERGE FROM THE OTHER GUIDANCE NOTED OVER BOTH
THE EAST AND WEST COASTS. THE BLEND ALSO LEANS ON THE 18Z GFS OVER
THE 12Z IN THE MID- TO LATE PERIOD AS THE 12Z IS MORE AGGRESSIVE
ON SHORTWAVE DETAILS HARD TO PREDICT AT THIS RANGE. A CONTRIBUTION
FROM THE ENSEMBLE MEANS IS INTRODUCED MID- TO LATE PERIOD TO
ACCOUNT FOR AREAS OF LOWER CONFIDENCE IN FINER-SCALE DETAILS, BUT
RETENTION OF THE 18Z GFS AND 12 ECMWF, WHICH GENERALLY FALL IN
LINE WITH THE MEANS, PROVIDES A BIT OF ADDED DETAIL DESPITE NOTED
UNCERTAINTY WITH HIGH RUN-TO-RUN VARIABILITY.
..WEATHER/HAZARDS HIGHLIGHTS
SOUTHWESTERLY FLOW WITH A STEADY SUPPLY OF GULF MOISTURE OVER THE
CENTRAL/EASTERN CONUS WILL FAVOR A STORMY AND WET PATTERN NEXT
WEEK. EMBEDDED SHORTWAVE ENERGY ALOFT AS WELL AS GENERAL FRONT
PROGRESSION WILL DICTATE THE CHANCES FOR LOCALLY HEAVIER RAINFALL.
WHILE NO EXCESSIVE RAINFALL OUTLOOKS HAVE BEEN INTRODUCED IN THE
DAY 4/5 TIMEFRAME, THE ENVIRONMENT WOULD SUGGEST THERE WILL LIKELY
EVENTUALLY BE AT LEAST MARGINAL RISK AREAS, IF NOT HIGHER. MODEL
QPF HAS BEEN RATHER NEBULOUS BUT THE FOCUS LOOKS TO BE ON THE
MISSISSIPPI VALLEY, PERHAPS EXTENDING EASTWARD TO THE
MID-ATLANTIC/CAROLINAS, AND THEN WEST INTO THE PLAINS MID- TO LATE
WEEK. HIGH TEMPERATURES WILL BE RUNNING ABOVE AVERAGE BROADLY FROM
THE PLAINS TO THE SOUTHEAST/MID-ATLANTIC UNDER THE INFLUENCE OF
MEAN RIDGING ALOFT, WITH SOME UNSEASONABLY WARM HIGHS 15+ DEGREES
ABOVE AVERAGE CENTERED ON THE CENTRAL PLAINS/MIDDLE MISSISSIPPI
VALLEY.
SEASONABLE TEMPERATURES AND LOW PRECIPITATION CHANCES ARE EXPECTED
ACROSS THE NORTHEAST WITH HIGH PRESSURE IN THE VICINITY THROUGH
THE FORECAST PERIOD. SOME TEMPERATURE FLUCTUATION AS WELL AS
BREEZY WINDS MAY EFFECT NEW ENGLAND DEPENDENT ON THE PROXIMITY OF
AN UPPER-LEVEL CLOSED LOW OVER CANADA AND SUBSEQUENT COLD FRONT
PASSAGE. ENERGETIC UPPER-LEVEL PATTERN WITH MEAN TROUGHING ALOFT
WILL FAVOR PERIODIC PRECIPITATION CHANCES IN THE WEST NEXT WEEK,
ESPECIALLY FOR THE PACIFIC NORTHWEST/GREAT BASIN/NORTHERN ROCKIES.
SOME SNOW SHOWERS WILL BE POSSIBLE FOR HIGHER MOUNTAIN ELEVATIONS,
BUT SEASONABLY RISING SNOW LEVELS WILL KEEP THESE CHANCES
ISOLATED. TEMPERATURES WILL REMAIN COOL AND BELOW AVERAGE,
ESPECIALLY ACROSS THE WEST COAST STATES.
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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