514
FXUS02 KWBC 250730
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
230 AM EST SUN DEC 25 2022
VALID 12Z WED DEC 28 2022 - 12Z SUN JAN 01 2023
...MULTIPLE ROUNDS OF MODERATE TO HEAVY PRECIPITATION ARE LIKELY
ACROSS THE WEST THIS WEEK...
...RAINFALL COULD BE HEAVY ACROSS THE SOUTHEASTERN QUADRANT OF THE
COUNTRY LATE THIS WEEK...
..OVERVIEW
AN AMPLIFYING TROUGH TRACKING ACROSS PARTS OF THE WESTERN AND
CENTRAL LOWER 48 WILL BE THE MAIN UPPER-LEVEL FEATURE OF CONCERN
FOR THE LATTER HALF OF THIS WEEK INTO THE WEEKEND. THE TROUGH AND
UPSTREAM ROUNDS OF PACIFIC FRONTAL SYSTEMS AND ENERGY IS FORECAST
TO LEAD TO MULTIPLE ROUNDS OF PRECIPITATION WITH POTENTIALLY HEAVY
RAIN ALONG THE WEST COAST AND HEAVY SNOW IN HIGHER ELEVATIONS.
JUST AHEAD OF THE TROUGH, A COUPLE OF FRONTAL SYSTEMS SHOULD
DEVELOP IN THE CENTRAL U.S. WHILE THE PATTERN PROMOTES MULTIPLE
DAYS OF DEEP-LAYER SOUTHERLY FLOW FROM THE GULF, SPREADING
PRECIPITATION OVER AN INCREASING PORTION OF THE EAST-CENTRAL U.S.
LATE IN THE WEEK WITH BEST POTENTIAL FOR HEAVY RAINFALL OVER THE
GULF COAST/LOWER MISSISSIPPI VALLEY REGION. MEANWHILE PRONOUNCED
RIDGING DEVELOPING AHEAD OF THE TROUGH WILL BRING A NOTABLE
WARMING TREND TO THE CENTRAL-EASTERN U.S. FOR WIDESPREAD ABOVE
NORMAL TEMPERATURES.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW REASONABLY GOOD AGREEMENT WITH
THE LARGE SCALE PATTERN OF THE WESTERN TO CENTRAL U.S. UPPER
TROUGH AND DOWNSTREAM RIDGING, BUT WITH TYPICAL DIFFERENCES IN THE
DETAILS OF INDIVIDUAL FEATURES, INCLUDING SHORTWAVES AS WELL AS
SURFACE LOW AND FRONTAL POSITIONS. THIS IS TRUE BOTH WITHIN THE
BROAD AREA OF MEAN LOW PRESSURE THAT DEVELOPS OVER THE CENTRAL
U.S. AND FOR THE SPECIFICS OF SURFACE LOWS IN THE PACIFIC. WHILE
THERE IS SOME INCREASING SIGNAL FOR A CONSOLIDATED SURFACE LOW TO
LIFT NORTHEAST INTO THE MIDWEST THIS WEEKEND, DIFFERENCES IN THE
TRACK AND DEPTH OF THE LOW ARE SEEN ALONG WITH DIFFERENCES IN THE
TROUGH ALOFT (GFS RUNS HAVE BEEN CONSISTENT WITH KEEPING THE DEPTH
OF THE TROUGH DEEPER THROUGH NEXT SUNDAY WHILE THE CMC IS MUCH
WEAKER; THE 12Z ECMWF WEAKENED THE TROUGH MORE QUICKLY THAN THE
NEWER 00Z EC). THE DETAILS OF SURFACE LOWS AND FRONTS BEFORE THAT
THIS WEEK ARE ACTUALLY EVEN MORE NEBULOUS GIVEN THE EVEN SMALLER
SCALE SHORTWAVE DIFFERENCES AND UNCERTAINTIES WITH FEATURES'
INTERACTIONS. BUT AT LEAST THERE IS AGREEMENT FOR LOWER PRESSURES
IN GENERAL IN THE CENTRAL U.S. AHEAD OF THE TROUGH LEADING TO
INCREASED MOIST FLOW FROM THE GULF. MEANWHILE, RECENT GUIDANCE HAS
SHOWN BETTER AGREEMENT FOR A SURFACE LOW OFF THE PACIFIC NORTHWEST
AROUND DAY 5/FRIDAY, BUT THEN WHAT APPEARED TO BE FAIRLY GOOD
MODEL AGREEMENT FOR ANOTHER REASONABLY STRONG PACIFIC SURFACE LOW
COMING INTO CALIFORNIA NEXT WEEKEND ENDED UP SHOWING A WEAKENING
TREND IN THE NEWER 00Z GUIDANCE. ADDITIONAL CHANGES WILL LIKELY BE
NEEDED WITH ALL THESE FEATURES WITH TIME.
THE WPC FORECAST STUCK WITH A BLEND OF THE 12/18Z GFS AND 12Z
ECWMF AND CMC EARLY IN THE PERIOD, TRUSTING THE BLENDING PROCESS
TO SMOOTH OUT SMALLER DIFFERENCES AND COME TO A CONSENSUS. THE 12Z
UKMET WAS NOT USED GIVEN ITS DIFFERENCES FROM CONSENSUS EARLY IN
THE PERIOD FOR A SURFACE LOW POSITION ALONG THE U.S.-CANADA
BORDER. THE ENSEMBLE MEANS FROM THE EC AND GEFS WERE INTRODUCED
AND INCREASED IN THE MODEL BLEND FOR THE LATTER PART OF THE PERIOD
AS SPREAD INCREASED.
..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 HAS NOW SHIFTED INTO 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. A SLIGHT RISK WAS ADDED TO THE
EXPERIMENTAL DAY 5 ERO FOR SOUTHWESTERN OREGON/COASTAL CALIFORNIA
AS MOST MODEL GUIDANCE SHOWS ANOTHER BOUT OF HEAVY RAIN MOVING IN
BY THEN, THOUGH THERE REMAINS SOME UNCERTAINTY WITH THE TIMING.
ADDITIONALLY, SIGNIFICANT SNOW AMOUNTS ARE LIKELY IN HIGHER
ELEVATIONS OF THE WEST THROUGH MUCH OF THE WEEK. EXACT TIMING AND
PLACEMENT DETAILS OF THE HEAVIEST PRECIPITATION THROUGH LATE WEEK
AND THE WEEKEND STILL SHOW SOME DIFFERENCES, SO CONTINUE TO
MONITOR FORECASTS FOR SPECIFICS AS THE DETAILS COME INTO BETTER
FOCUS OVER COMING DAYS.
EAST OF THE ROCKIES, MOST AREAS SHOULD BE GENERALLY DRY WEDNESDAY
OTHER THAN SOME LIGHT PRECIPITATION CHANCES ACROSS THE NORTHERN
TIER. BY THE LATTER HALF OF THE WEEK, MOISTURE INFLOW FROM THE
GULF OF MEXICO IS FORECAST TO RETURN TO THE MISSISSIPPI VALLEY
AHEAD OF ONE OR MORE PLAINS LOW PRESSURE/FRONTAL SYSTEMS AND
SUPPORTING UPPER TROUGH. WITHIN THIS EXPANDING MOISTURE SHIELD,
RAINFALL COULD BECOME LOCALLY MODERATE TO HEAVY BY FRIDAY-SATURDAY
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.
AFTER THE RECENT COLD SPELL, THE TEMPERATURE PATTERN IN THE MEDIUM
RANGE PERIOD WILL SIGNIFICANTLY CHANGE ACROSS MUCH OF THE NATION.
THOUGH THE SOUTHEAST MAY STILL HAVE SOME BELOW NORMAL READINGS FOR
MIDWEEK, A WIDESPREAD AREA OF ABOVE NORMAL TEMPERATURES IN THE
CENTRAL U.S. WILL BE EXPANDING EASTWARD THROUGH THE LATTER PART OF
THE WEEK. LOW TEMPERATURES LOOK TO BE 15-30F ABOVE AVERAGE OVER
THE HIGH PLAINS WEDNESDAY 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. THOUGH MEANWHILE IN THE WEST, THE WET/SNOWY
PATTERN WILL TEND TO KEEP DAYTIME TEMPERATURES WITHIN A FEW
DEGREES OF NORMAL, THOUGH LOWS SHOULD GENERALLY BE A BIT ABOVE
AVERAGE.
TATE
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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