440
FXUS02 KWBC 260700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST MON DEC 26 2022
VALID 12Z THU DEC 29 2022 - 12Z MON JAN 02 2023
...MULTIPLE ROUNDS OF MODERATE TO HEAVY PRECIPITATION ARE LIKELY
ACROSS THE WEST THIS WEEK...
...HEAVY RAINFALL AND FLOODING ARE POSSIBLE ACROSS THE
SOUTHEASTERN QUADRANT OF THE COUNTRY LATE THIS WEEK...
..OVERVIEW
AN ACTIVE PATTERN IS LIKELY ACROSS MUCH OF THE LOWER 48 LATE THIS
WEEK INTO EARLY NEXT WEEK. A RAPID SEQUENCE OF PACIFIC SYSTEMS
WILL BRING MULTIPLE EPISODES OF PRECIPITATION TO MUCH OF THE WEST,
WITH SOME AREAS OF POTENTIALLY HEAVY RAIN/SNOW DEPENDING ON
ELEVATION. THERE WILL LIKELY BE A TENDENCY FOR GREATER MOISTURE
FOCUS OVER SOUTHERN PARTS OF THE REGION BY THE START OF THE
WEEKEND. THE ENERGY FROM THESE SYSTEMS WILL FEED INTO A LONG-TERM
UPPER-LEVEL MEAN TROUGH BECOMING ESTABLISHED OVER THE
ROCKIES/PLAINS THURSDAY-SATURDAY WHILE AN UPPER RIDGE BUILDS OVER
THE EAST COAST/WESTERN ATLANTIC. THE FLOW BETWEEN THE TROUGH AND
DOWNSTREAM RIDGE ALOFT, AND BETWEEN ONE OR MORE CENTRAL U.S. LOW
PRESSURE SYSTEMS AND HIGH PRESSURE TO THE EAST AT THE SURFACE,
WILL SPREAD PRECIPITATION ACROSS AN INCREASING PORTION OF THE
EASTERN HALF OF THE COUNTRY WHILE PROMOTING UNSEASONABLY WARM
TEMPERATURES THAT SHOULD LEAD TO A MAJORITY OF THIS PRECIPITATION
FALLING AS RAIN. THE BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS
SHOULD EXTEND FROM THE GULF COAST NORTHWARD THROUGH THE SOUTHERN
HALF OF THE MISSISSIPPI VALLEY AND EASTWARD INTO THE FAR SOUTHERN
APPALACHIANS. BY EARLY NEXT WEEK THIS TROUGH IS EXPECTED TO LIFT
INTO THE MIDWEST WHILE ANOTHER LONGWAVE TROUGH DEEPENS OVER THE
WEST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE HAS BEEN PERSISTENT IN SHOWING REASONABLY GOOD
AGREEMENT ON THE LARGE SCALE BUT WITH SOME SPREAD FOR INDIVIDUAL
FEATURES. DIFFERENCES IN SHORTWAVES WITHIN THE MAIN TROUGH AS
EARLY AS DAY 3/THURSDAY ARE NOT ANOMALOUSLY LARGE FOR THE MEDIUM
RANGE, BUT DO TRANSLATE INTO SOME SPREAD AT THE SURFACE FOR
SURFACE LOW TRACKS AND FRONTAL POSITIONS IN A REASONABLY COMPLEX
PATTERN WITHIN MEAN LOW PRESSURE IN THE CENTRAL CONUS. THE MAIN
OUTLIER INITIALLY THURSDAY-FRIDAY AT THE SURFACE SEEMED TO BE THE
12Z ECWMF WITH A FAST LOW TRACK ACROSS THE MIDWEST INTO EASTERN
CANADA WHERE OTHER MODELS WERE MAINTAINING A HIGH. BUT OTHER THAN
THAT, THERE DID NOT SEEM TO BE MAJOR OUTLIERS IN THE GUIDANCE,
JUST GENERAL VARIATIONS. THERE HAS BEEN SOME SIGNAL FOR A SURFACE
LOW TO LIFT ACROSS THE MIDWEST/GREAT LAKES OVER THE WEEKEND,
THOUGH THE NEWER 00Z MODEL CYCLE SHOWS THIS TO BE GENERALLY
WEAKER.
FOR THE WEST, SIMILARLY THERE ARE SOME DIFFERENCES IN THE DETAILS,
BUT THE PATTERN FAVORING MULTIPLE ROUNDS OF PRECIPITATION AND A
SHIFT TOWARD FAVORING THE SOUTHERN HALF OF THE WEST BY EARLY NEXT
WEEK IS PERSISTENT AND AGREEABLE. MODELS HAVE WAFFLED WITH
INDICATING STRONGER SURFACE LOW FEATURES AT VARIOUS TIMES IN THE
EASTERN PACIFIC; FOR EXAMPLE THE 12/18Z GFS RUNS SHOWED A STRONG
LOW OFFSHORE OF THE PACIFIC NORTHWEST 12Z FRIDAY BUT THIS IS NOT
FAVORED IN THE NEWER MODEL RUNS. A MODEL COMPROMISE SEEMED TO DO A
FINE JOB TO MITIGATE THESE DIFFERENCES. THERE IS REASONABLY GOOD
AGREEMENT GIVEN THAT IT IS THE DAY 6-7 TIMEFRAME FOR ANOTHER
SYNOPTIC SCALE ROUND OF TROUGHING TO DIG ACROSS THE WEST AND TRACK
EAST WITH ITS AXIS AROUND THE ROCKIES BY NEXT MONDAY.
THE WPC FORECAST WAS ABLE TO USE A MULTI-MODEL DETERMINISTIC BLEND
OF GUIDANCE EARLY IN THE PERIOD GIVEN THE LARGE-SCALE MODEL
AGREEMENT, WITH THE BLENDING PROCESS SERVING TO CREATE A
COMPROMISE SOLUTION. GEFS AND EC ENSEMBLE MEANS WERE INTRODUCED
AND INCREASED IN THE BLEND BY DAYS 5-7 TO MITIGATE MODEL
VARIATIONS BUT WAS ABLE TO KEEP DETERMINISTIC MODELS AS THE
MAJORITY THROUGH THE PERIOD.
..WEATHER/HAZARD HIGHLIGHTS
A SERIES OF PACIFIC FRONTS/SYSTEMS SHOULD BRING A MULTI-DAY PERIOD
OF NOTABLE PRECIPITATION TO MUCH OF THE WEST. WHILE THE STRONGEST
ATMOSPHERIC RIVER EXPECTED IS NOW FULLY WITHIN THE SHORT RANGE
PERIOD, ADDITIONAL ROUNDS OF HEAVY RAINFALL ALONG COASTAL AREAS
MAY CAUSE FLOODING PROBLEMS IN AREAS THAT WILL BE HYDROLOGICALLY
SENSITIVE BY THAT TIME. SLIGHT RISK AREAS ARE SHOWN IN BOTH THE
DAYS 4 AND 5 EXPERIMENTAL EXCESSIVE RAINFALL OUTLOOKS ACROSS
SOUTHWESTERN OREGON/COASTAL CALIFORNIA DUE TO BOUTS OF HEAVY RAIN
MOVING IN DURING THOSE DAYS. ADDITIONALLY, SIGNIFICANT SNOW
AMOUNTS ARE LIKELY IN HIGHER ELEVATIONS OF THE WEST THROUGH THE
LATTER HALF OF THE WEEK AND INTO THE WEEKEND. EXACT TIMING AND
PLACEMENT DETAILS OF THE HEAVIEST PRECIPITATION STILL SHOW SOME
DIFFERENCES, SO CONTINUE TO MONITOR FORECASTS FOR SPECIFICS AS THE
DETAILS COME INTO BETTER FOCUS OVER COMING DAYS. HOWEVER, THERE IS
A GENERAL TREND OF PRECIPITATION LIGHTENING IN NORTHERN AREAS BY
THE WEEKEND WHILE THE BETTER FOCUS FOR PRECIPITATION BECOMES
ACROSS CALIFORNIA THROUGH THE CENTRAL/SOUTHERN GREAT BASIN AND
SOUTHWEST U.S.
EAST OF THE ROCKIES, A WEAK LEADING WAVE LIFTING NORTHEASTWARD MAY
PRODUCE SOME PRECIPITATION (LIKELY AT LEAST SOME OF IT SNOW)
ACROSS THE NORTHERN PLAINS INTO THURSDAY. MORE NOTABLY AROUND
THURSDAY-SATURDAY, DEVELOPING MOISTURE INFLOW FROM THE GULF OF
MEXICO INTO THE MISSISSIPPI VALLEY AND VICINITY AHEAD OF ONE OR
MORE PLAINS LOW PRESSURE/FRONTAL SYSTEMS AND SUPPORTING UPPER
TROUGH WILL LEAD TO INCREASING COVERAGE AND INTENSITY OF
PRECIPITATION, MOST OF IT RAIN. WITHIN THIS EXPANDING MOISTURE
SHIELD, RAINFALL COULD BECOME HEAVY WITH HIGHEST RAINFALL TOTALS
CURRENTLY EXPECTED TO BE APPROXIMATELY FROM THE GULF COAST THROUGH
THE LOWER MISSISSIPPI VALLEY AND INTO THE TENNESSEE VALLEY AND
SOUTHEAST/SOUTHERN APPALACHIANS. SLIGHT RISKS OF EXCESSIVE
RAINFALL ARE ALSO IN PLACE FOR THESE AREAS AS INSTABILITY COULD
PRODUCE HIGH RAINFALL RATES THAT MAY CAUSE FLOODING/FLASH
FLOODING. PROGRESSION OF A POTENTIAL PLAINS INTO GREAT LAKES
SYSTEM NEXT WEEKEND SHOULD SPREAD MOISTURE THROUGH THE REST OF THE
EAST AS MOISTURE STREAMS NORTHWARD, BUT WITH MAXIMUM RAINFALL
TOTALS LIKELY TO TREND LOWER. EXPECT MOST WINTRY WEATHER FROM THIS
SYSTEM TO BE CONFINED TO THE NORTHERN PLAINS/UPPER MIDWEST THROUGH
LATE THIS WEEK. BETTER BUT STILL UNCERTAIN CHANCES FOR SNOW MAY BE
ACROSS THE CENTRAL HIGH PLAINS BY SUNDAY INTO NEXT MONDAY AS
ANOTHER UPPER TROUGH PROVIDES COLDER AIR THERE.
AFTER THE RECENT COLD SPELL, THE TEMPERATURE PATTERN IN THE MEDIUM
RANGE PERIOD WILL SIGNIFICANTLY CHANGE ACROSS MUCH OF THE NATION.
A WIDESPREAD AREA OF ABOVE NORMAL TEMPERATURES IN THE CENTRAL U.S.
WILL BE EXPANDING EASTWARD THROUGH THE LATTER PART OF THE WEEK AND
CONTINUING INTO EARLY NEXT WEEK. LOW TEMPERATURES LOOK TO BE
15-25F ABOVE AVERAGE OVER THE PLAINS/MISSISSIPPI VALLEY THURSDAY
WITH HIGHS AROUND 10-20F ABOVE AVERAGE, AND TEMPERATURES OF THIS
MAGNITUDE SHOULD SHIFT EAST UNDERNEATH THE UPPER RIDGE INTO THE
EAST-CENTRAL U.S. LATE WEEK AND THE EAST BY NEXT SUNDAY. DAILY
RECORDS FOR WARM LOWS MAY BE WIDESPREAD AS THE WARMEST AIR REACHES
THE EAST, WHILE A FEW ISOLATED DAILY RECORD HIGHS CANNOT BE RULED
OUT. THOUGH MEANWHILE IN THE WEST, THE WET/SNOWY PATTERN WILL TEND
TO KEEP DAYTIME TEMPERATURES WITHIN A FEW DEGREES OF NORMAL. LOWS
SHOULD GENERALLY BE A BIT ABOVE AVERAGE UNTIL EARLY NEXT WEEK AS
THE SECOND LARGE-SCALE UPPER TROUGH MOVES THROUGH, BRINGING HIGHS
TO AROUND 10F BELOW NORMAL AS WELL.
TATE/RAUSCH
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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