906
FXUS02 KWBC 071930
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
230 PM EST WED DEC 07 2022
VALID 12Z SAT DEC 10 2022 - 12Z WED DEC 14 2022
...HEAVY COASTAL RAINS AND MOUNTAIN/INLAND HEAVY SNOW THREATS
ACROSS THE WEST/ROCKIES THIS WEEKEND INTO MONDAY...
...HEAVY RAINFALL AND SEVERE THREATS OVER THE MISSISSIPPI
VALLEY/VICINITY ALONG WITH HEAVY SNOW/ICE/FREEZING RAIN THREATS
OVER THE NORTH-CENTRAL U.S. MONDAY THROUGH WEDNESDAY...
..OVERVIEW
OVER THE WEEKEND, RAIN SHOWERS AND THUNDERSTORMS ARE POSSIBLE
ACROSS THE SOUTHEASTERN U.S. WITH A WAVY FRONT IN THE VICINITY,
WHILE FARTHER NORTH A CLIPPER SYSTEM THAT SEEMS STRONGER IN THE
UPDATED FORECAST MAY SPREAD RAIN/SNOW ACROSS THE GREAT LAKES
REGION INTO THE NORTHERN MID-ATLANTIC AND NORTHEAST. BUT THE MAIN
STORY DURING THE MEDIUM RANGE PERIOD WILL BE UPSTREAM WHERE
GUIDANCE AGREES UPON A MEAN TROUGH AXIS WORKING INLAND FROM THE
WEST COAST THROUGH THE WEEKEND INTO EARLY NEXT WEEK, WITH THE
DEEPEST REINFORCING ENERGY ARRIVING SUNDAY INTO MONDAY LEADING TO
CLOSED LOW/TROUGH FORMATION, WITH THE DYNAMIC SYSTEM LIFTING OVER
THE PLAINS TUESDAY/WEDNESDAY. THIS PATTERN WOULD SUPPORT A PERIOD
OF WIDESPREAD AND SIGNIFICANT TERRAIN-ENHANCED RAIN AND MOUNTAIN
SNOW OVER THE WEST FROM LATE THIS WEEK THROUGH THE WEEKEND. THIS
IS INCREASINGLY EXPECTED TO BE FOLLOWED BY EXPANDING/INTENSIFYING
RAINFALL OVER THE MISSISSIPPI VALLEY/VICINITY EASTWARD ACROSS THE
EAST-CENTRAL U.S. ALONG WITH POSSIBLY SIGNIFICANT WRAPBACK SNOW
OVER THE NORTH-CENTRAL U.S. EARLY NEXT WEEK IN ASSOCIATION WITH
DEEPENING LOW PRESSURE AND CORRESPONDING FRONTS. EXPECT MOSTLY
BELOW NORMAL HIGH TEMPERATURES OVER THE WEST AND NEAR TO ABOVE
NORMAL PRE-FRONTAL READINGS OVER MUCH OF THE EASTERN HALF OF THE
COUNTRY.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT MODEL GUIDANCE CONTINUES TO AGREE IN PRINCIPLE WITH THE
LARGE-SCALE ASPECTS OF THE FORECAST INCLUDING A TROUGH THAT IS
STILL PREDICTED TO DIG AND AMPLIFY AS IT COMES INTO THE WEST BY
SUNDAY AND TRACKS SLOWLY EASTWARD THROUGH MIDWEEK, ALONG WITH A
CONSOLIDATING SURFACE LOW PRESSURE SYSTEM. THIS SHOWS RELATIVELY
MINOR SPREAD COMPARED TO SOME MEDIUM RANGE FORECASTS AT TIMES. BUT
MINOR DIFFERENCES INCLUDE TIMING DIFFERENCES AS SHOWN BY SOME
WEST-EAST SPREAD IN MODELS/ENSEMBLE MEMBERS, BUT WITH A GENERAL
TREND OF SLIGHTLY SLOWER MOVEMENT EAST CONTINUING. 12Z MODELS SEEM
TO HAVE COME INTO SOMEWHAT BETTER AGREEMENT WITH TIMING OF THE
UPPER LOW THROUGH AROUND DAY 5/MONDAY. AS THE UPPER/SURFACE LOWS
EJECT THERE IS STILL A GOOD SIGNAL FOR A SIGNIFICANT CENTRAL U.S.
STORM SYSTEM.
REGARDING OTHER ASPECTS OF THE FORECAST, THERE HAS BEEN SOME MODEL
WAFFLING REGARDING THE STRENGTH OF A SHORTWAVE/WEAK SURFACE LOW
MOVING ACROSS THE GREAT LAKES AROUND SUNDAY. THE 00Z/06Z GUIDANCE
TRENDED STRONGER WITH THAT (THOUGH THE INCOMING 12Z MODEL CYCLE IS
GENERALLY WEAKER) TO SUPPORT MORE PRECIPITATION INCLUDING POSSIBLE
SNOW IN THE GREAT LAKES/NORTHERN MID-ATLANTIC/NORTHEAST COMPARED
TO THE NBM AND PREVIOUS FORECAST. THEN BEHIND THE MAIN UPPER
TROUGH/LOW THERE MAY BE SOME BRIEF WEST COAST RIDGING BY AROUND
DAY 7/WED BUT THE OVERALL FLOW ACROSS THE PACIFIC IS QUITE
UNCERTAIN BY THEN AND MAY POSE FORECAST CHALLENGES FOR THE CONUS
IN THE COMING DAYS.
THE WPC FORECAST WAS BASED ON A BLEND OF THE 00Z/06Z DETERMINISTIC
GUIDANCE LED BY THE ECMWF AND GFS EARLY IN THE PERIOD, WITH
INCORPORATION AND GRADUAL INCREASE OF THE ENSEMBLE MEANS THROUGH
EARLY-MID NEXT WEEK AMID INCREASING (BUT STILL NOT TOO NOTABLE)
MODEL SPREAD.
..WEATHER/HAZARD HIGHLIGHTS
FOR THE WEEKEND, GULF OF MEXICO MOISTURE INFLOW COMBINED WITH A
WAVY SURFACE FRONT AND WEAK UPPER ENERGY MAY SUPPORT MODERATE TO
HEAVY RAINFALL ACROSS THE SOUTHEASTERN PART OF THE COUNTRY.
LOCALIZED FLOODING/RUNOFF ISSUES MAY BE A THREAT DEPENDING ON
WHERE THE RAINFALL SETS UP. FARTHER NORTH, PRECIPITATION LIKELY
INCLUDING SOME SNOW IS FORECAST TO SPREAD FROM THE GREAT LAKES
REGION INTO THE NORTHERN MID-ATLANTIC INTO SOUTHERN PARTS OF THE
NORTHEAST SATURDAY NIGHT INTO SUNDAY.
AS THE MAIN UPPER TROUGH DIGS ACROSS THE WEST THIS WEEKEND AND
SHIFTS EASTWARD THROUGH THE FIRST HALF OF NEXT WEEK, MULTIPLE
HAZARDS ARE LIKELY ACROSS THE COUNTRY. HEAVY PRECIPITATION IS
INITIALLY LIKELY ACROSS THE WEST THIS WEEKEND, INCLUDING HEAVY
TERRAIN-ENHANCED SNOWS ACROSS THE SIERRA NEVADA AND FARTHER EAST
INTO HIGHER ELEVATIONS OF THE GREAT BASIN AND ROCKIES. LOWER
ELEVATION HEAVY RAIN IS ALSO POSSIBLE THIS WEEKEND PARTICULARLY
ACROSS COASTAL AND INTERIOR PARTS OF CALIFORNIA IN CONJUNCTION
WITH A WEAK-MODERATE ATMOSPHERIC RIVER. THEN AS THE UPPER
TROUGH/LOW APPROACHES THE PLAINS AND A SURFACE LOW CONSOLIDATES
AND DEEPENS, AMPLE MOISTURE WILL SPREAD AHEAD OF A COLD FRONT TO
PRODUCE SIGNIFICANT RAINFALL ACROSS THE LOWER/MIDDLE MISSISSIPPI
VALLEY AND VICINITY BEGINNING MONDAY AND SHIFTING EAST WITH TIME.
SOME STRONG TO SEVERE CONVECTION IS ALSO POSSIBLE, WITH THE STORM
PREDICTION CENTER ALREADY MONITORING THIS POTENTIAL. ON THE
BACKSIDE OF THE LOW, COLD TEMPERATURES AND WRAPBACK MOISTURE WILL
LIKELY SUPPORT SIGNIFICANT SNOW IN THE NORTHERN PLAINS/UPPER
MIDWEST AS WELL AS AREAS OF ICE IN A TRANSITION ZONE.
PROBABILITIES FOR AT LEAST A QUARTER INCH OF LIQUID EQUIVALENT IN
THE FORM OF SNOW OVER THE NORTHERN PLAINS CONTINUE TO INCREASE.
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 FOR THE FIRST HALF OF
NEXT WEEK. SOME OF THIS COLDER AIR SHOULD SPILL INTO THE HIGH
PLAINS BY NEXT TUESDAY/WEDNESDAY BEHIND THE EXPECTED PLAINS STORM.
ON THE OTHER HAND, SOUTHERN/EASTERN AREAS WILL BE NEAR TO ABOVE
NORMAL FOR TEMPERATURES. ANOMALIES WILL BE GREATEST FOR MORNING
LOWS, WITH SOME PLUS 15-25F ANOMALIES ACROSS THE SOUTHERN TIER
THROUGH MUCH OF THE PERIOD EXPANDING NORTHWARD AT TIMES ACROSS THE
MISSISSIPPI VALLEY. HIGHS ARE FORECAST TO BE AROUND 10-15F ABOVE
NORMAL FOR THE CENTRAL AND THEN EAST-CENTRAL U.S., LOCALLY HIGHER.
TATE/SCHICHTEL
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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