611
FXUS02 KWBC 061901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST TUE DEC 06 2022
VALID 12Z FRI DEC 09 2022 - 12Z TUE DEC 13 2022
..OVERVIEW
MOST GUIDANCE SHOWS A MEAN TROUGH AXIS NEAR THE WEST COAST
PERSISTING THROUGH THE WEEKEND, WITH THE DEEPEST REINFORCING
ENERGY ARRIVING BY SUNDAY. THEN A DEVELOPING CONSENSUS CARRIES
THE OVERALL TROUGH TOWARD THE PLAINS AS A POTENTIAL EMBEDDED UPPER
LOW CLOSES OFF. THIS PATTERN WOULD SUPPORT A PERIOD OF
SIGNIFICANT TERRAIN-ENHANCED RAIN AND MOUNTAIN SNOW OVER THE WEST
FROM LATE THIS WEEK THROUGH THE WEEKEND, FOLLOWED BY
EXPANDING/INTENSIFYING RAINFALL OVER THE MISSISSIPPI VALLEY AND
VICINITY ALONG WITH POSSIBLY SIGNIFICANT SNOW OVER THE NORTHERN
TIER EARLY NEXT WEEK IN ASSOCIATION WITH DEEPENING LOW PRESSURE
AND CORRESPONDING FRONTS. MEANWHILE A LEADING WAVY FRONT ACROSS
THE SOUTHERN TIER MAY PROMOTE SOME LOCALLY HEAVY RAINFALL LATE
THIS WEEK INTO THE WEEKEND. EXPECT MOSTLY BELOW NORMAL HIGH
TEMPERATURES OVER THE WEST AND NEAR TO ABOVE NORMAL READINGS OVER
THE EASTERN HALF OF THE COUNTRY.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MOST SIGNIFICANT LARGE-SCALE ASPECT OF THE FORECAST INVOLVES
RECENT PRONOUNCED TRENDS IN GUIDANCE OVER THE PACIFIC INTO BERING
SEA/ALASKA REGION LEADING TO A SIMILARLY DRAMATIC CHANGE FOR UPPER
DYNAMICS REACHING THE LOWER 48 MID-LATE PERIOD. THIS ADJUSTMENT
(STARTING IN SOME OF YESTERDAY'S 12Z MODELS AND
REINFORCED/EXTENDED IN THE 00Z-06Z RUNS) HAS LED TO MORE
PRONOUNCED DIGGING OF TROUGH ENERGY INTO THE WEST COAST BY SUNDAY
AND THEN A SLOWER AND MORE AMPLIFIED DEPICTION OF THE TROUGH AS IT
MOVES EASTWARD. THE ENSEMBLE MEANS ARE ALSO TRENDING TOWARD THE
LATEST OPERATIONAL MODEL RUNS THAT CLOSE OFF AN UPPER LOW BY NEXT
TUESDAY, WHICH WOULD SUPPORT SIGNIFICANT CENTRAL U.S. LOW
PRESSURE/FRONTAL SYSTEM. THE 00Z ECMWF MEAN HAS THIS UPPER LOW
AND THE GEFS MEAN HAS ACHIEVED A CLOSED LOW IN ITS NEW 12Z RUN.
PRESENT CLUSTERING OF THE GFS RUNS/00Z ECMWF AND THE ENSEMBLE
MEANS OFFERS SOME DEGREE OF CONFIDENCE, BUT RECENT VOLATILITY AND
FULL ENSEMBLE SPREAD (AS WELL AS A 12Z CMC RUN THAT EVENTUALLY
STRAYS ABOUT A DAY SLOWER WITH THE WESTERN TROUGH, WITH A SLOWER
TREND IN THE NEW 12Z ECMWF TOO) SUGGEST THERE MAY BE SOME
REFINEMENTS YET TO COME.
AS FOR OTHER FEATURES, THE ENSEMBLE MEANS OFFER REASONABLE SUPPORT
FOR LATEST GFS/ECMWF RUNS FOR A TRAILING UPPER TROUGH/SURFACE
SYSTEM NEARING THE WEST COAST BY NEXT TUESDAY. ON THE LEADING
SIDE OF THE INITIAL WEST COAST MEAN TROUGH, A COMPACT MIDWEST
SYSTEM AS OF EARLY FRIDAY SHOULD SHEAR OUT AS IT CONTINUES
EASTWARD WHILE TRAILING ENERGY THAT EJECTS FROM THE TROUGH SHOULD
PROGRESS EASTWARD/NORTHEASTWARD WITH SOME UNCERTAINTY OVER THE
PRECISE DISTRIBUTION OF ENERGY. CMC RUNS ARE AT ODDS WITH
CONSENSUS AS THEY BECOME MUCH DEEPER AND MORE CLOSED WITH THIS
ENERGY NEAR THE EAST COAST BY SUNDAY-MONDAY.
THE UPDATED FORECAST STARTED WITH A 00Z/06Z OPERATIONAL MODEL
COMPOSITE, TRADED THE MINIMAL CMC COMPONENT FOR THE 06Z GEFS/00Z
ECMWF MEANS BY MID-PERIOD, AND THEN GRADUALLY INCREASED TOTAL
ENSEMBLE MEAN WEIGHT TO 40 PERCENT BY DAY 7 TUESDAY TO ACCOUNT FOR
TYPICAL DETAIL UNCERTAINTY IN THE OTHERWISE AGREEABLE 06Z GFS/00Z
ECMWF RUNS.
..WEATHER/HAZARD HIGHLIGHTS
A COMPACT UPPER SHORTWAVE EMERGING FROM THE MIDWEST ALONG WITH
LEAD FRONTAL MOISTURE MAY SPREAD SOME RAINFALL ACROSS THE LOWER
GREAT LAKES AND CENTRAL APPALACHIANS/MID-ATLANTIC ON FRIDAY. MOST
AMOUNTS SHOULD BE IN THE LIGHT TO MODERATE RANGE. SOME LOCALLY
HEAVIER ACTIVITY MAY BE POSSIBLE BUT CONFIDENCE IS NOT GREAT IN
LIGHT OF MODEL SPREAD AND VARIABILITY FOR MAGNITUDE AND LOCATION
OF HIGHEST TOTALS. THERE IS ALSO SOME POTENTIAL FOR A SWATH OF
SNOW/ICE IN THE COOLER NORTHERN PERIPHERY OF THIS PRECIPITATION
SHIELD.
INITIAL ENERGY DIGGING SOUTHEASTWARD FROM THE GULF OF ALASKA
SHOULD CONTINUE TO WORK ACROSS THE WEST COAST INTO THE WEST FRIDAY
AND RENEW MODERATELY HEAVY PRECIPITATION CHANCES INTO THE
NORTHWEST AND CALIFORNIA TO INCLUDE A THREAT FOR ENHANCED
TERRAIN/MOUNTAIN SNOWS THAT WILL SPREAD OVER AN UNSETTLED AND
COOLED INTERMOUNTAIN WEST/ROCKIES. SYSTEM EJECTION MAY SUPPORT
MODERATE TO HEAVY RAINFALL WITH SOME POTENTIAL FOR RUNOFF ISSUES
CENTERED OVER THE SOUTHERN PLAINS AND MID-LOWER
MISSISSIPPI/TENNESSEE VALLEYS DURING THE WEEKEND AS THE UPPER
ENERGY INTERACTS WITH GULF MOISTURE THAT COULD FEED INTO A WAVY
SOUTHERN TIER FRONTAL SYSTEM.
EXPECT RENEWED POTENTIAL FOR SOME HEAVY PRECIPITATION OVER THE
WEST LATE FRIDAY THROUGH THE WEEKEND, INCLUDING TERRAIN/MOUNTAIN
ENHANCED SNOWS. LATEST CONSENSUS SUGGESTS HIGHEST PRECIPITATION
TOTALS SHOULD BE ALONG THE CENTRAL WEST COAST INTO THE SIERRA
NEVADA. MOISTURE WILL LIKELY SPREAD INLAND AS WELL AND BRING A
PERIOD OF ENHANCED RAIN/MOUNTAIN SNOW TO THE GREAT BASIN/ROCKIES.
WITH LATEST GUIDANCE TRENDS SUPPORTING A SLOWER UPPER LOW REACHING
THE PLAINS BY NEXT TUESDAY AND ASSOCIATED DEEPENING LOW
PRESSURE/FRONTAL SYSTEM, CONFIDENCE IS GRADUALLY RISING (THOUGH IS
CERTAINLY NOT HIGH YET, GIVEN RECENT VARIABILITY IN THE GUIDANCE)
THAT GULF INFLOW MAY HELP TO ENHANCE THE NORTHWARD FLOW OF GULF
MOISTURE TO PRODUCE SIGNIFICANT RAINFALL OVER THE MISSISSIPPI
VALLEY AND VICINITY. SOME STRONG TO SEVERE CONVECTION IS ALSO
POSSIBLE, WITH THE STORM PREDICTION CENTER ALREADY MONITORING THIS
POTENTIAL. MOISTURE WRAPPING BACK INTO THE NORTHERN TIER MAY
PRODUCE SIGNIFICANT SNOW/ICE. PROBABILITIES FOR AT LEAST A
QUARTER-INCH OF LIQUID EQUIVALENT IN THE FORM OF SNOW OVER THE
NORTHERN PLAINS HAVE INCREASED SOMEWHAT IN THIS CYCLE. CONTINUE
TO MONITOR FORECASTS AS REFINEMENTS IN SPECIFICS ARE LIKELY.
THE WEST SHOULD SEE BELOW NORMAL HIGHS THROUGH THE PERIOD, WITH
THE MOST WIDESPREAD READINGS OF AT LEAST 10-15F BELOW NORMAL AS
THE DEEP UPPER TROUGH CROSSES THE REGION EARLY NEXT WEEK. SOME OF
THIS COLDER AIR SHOULD SPILL INTO THE HIGH PLAINS BY NEXT TUESDAY
BEHIND THE EXPECTED PLAINS STORM. ON THE OTHER HAND,
SOUTHERN/EASTERN AREAS WILL BE NEAR TO ABOVE NORMAL FOR
TEMPERATURES. ANOMALIES SHOULD BE GREATEST FOR MORNING LOWS, WITH
SOME PLUS 15-25F ANOMALIES ACROSS THE SOUTHERN TIER AND STRAYING
FARTHER NORTHWARD OVER SOME AREAS THIS FRIDAY AND AGAIN BY NEXT
TUESDAY.
RAUSCH/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, 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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