109
FXUS02 KWBC 241900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST SAT DEC 24 2022
VALID 12Z TUE DEC 27 2022 - 12Z SAT DEC 31 2022
...A STRONG ATMOSPHERIC RIVER COULD CAUSE NOTABLE HEAVY
RAIN/FLOODING FOR THE WEST COAST ALONG WITH HEAVY MOUNTAIN SNOW IN
THE WEST NEXT WEEK...
..OVERVIEW
GUIDANCE REMAINS AGREEABLE AND CONSISTENT IN SHOWING THE
ESTABLISHMENT OF A NEW LONG-TERM MEAN PATTERN ALOFT, WITH DYNAMICS
FROM A STRONG STORM REACHING WASHINGTON/VANCOUVER ISLAND ON
TUESDAY BEING THE FIRST OF MULTIPLE PIECES OF ENERGY TO FEED INTO
A DEVELOPING ROCKIES/PLAINS MEAN TROUGH WHILE A MEAN RIDGE BUILDS
OVER THE EAST COAST/WESTERN ATLANTIC AFTER MIDWEEK IN THE WAKE OF
A DEPARTING SHORTWAVE. FLOW AHEAD OF THE TUESDAY STORM WILL BRING
A STRONG ATMOSPHERIC RIVER WITH A LONG FETCH OF TROPICAL MOISTURE
INTO THE WEST DURING THE FIRST HALF OF NEXT WEEK. THIS EVENT WILL
LIKELY PRODUCE HEAVY RAIN, POTENTIALLY CAUSING FLOODING ESPECIALLY
FOR COASTAL REGIONS OF SOUTHERN OREGON AND CALIFORNIA, ALONG WITH
HEAVY SNOW POSSIBLE IN HIGHER ELEVATIONS OF THE WEST. TRAILING
PACIFIC FRONTAL SYSTEMS AND POSSIBLE WAVES WILL LIKELY BRING
ADDITIONAL ROUNDS OF PRECIPITATION INTO THE WEST THROUGH LATE NEXT
WEEK. FARTHER EAST, THE NEW MEAN PATTERN WILL PROMOTE 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 THE PATTERN
EVOLUTION WILL BRING A PRONOUNCED WARMING TREND TO ABOVE NORMAL
TEMPERATURES ACROSS THE CENTRAL-EASTERN U.S., LEADING TO ABOVE
NORMAL READINGS OVER MANY AREAS AFTER MIDWEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AS HAS BEEN THE CASE LATELY, THE MODELS AND ENSEMBLE MEANS AGREE
WELL FOR THE LARGE SCALE MEAN PATTERN BUT CONTINUE TO EXHIBIT
MEANINGFUL DETAIL DIFFERENCES FOR INDIVIDUAL FEATURES FROM THE
EASTERN PACIFIC, ULTIMATELY FEEDING INTO THE ROCKIES/PLAINS MEAN
TROUGH. GUIDANCE HAS ALSO HAD DIFFICULTY IN RESOLVING THE
SPECIFICS OF FLOW ALOFT AND SURFACE FEATURES ACROSS THE EASTERN
HALF OF SOUTHERN CANADA WITH SOME INFLUENCE ON NORTHERN PARTS OF
THE LOWER 48. A MODEL CONSENSUS APPROACH EARLY IN THE PERIOD
FOLLOWED BY A TRANSITION TOWARD A MODEL/MEAN MIX LATER IN THE
FORECAST MAINTAINS GOOD CONTINUITY FOR THE MEAN PATTERN AND SOME
INDIVIDUAL FEATURES, THOUGH LATEST RUNS PRODUCE SOME UNAVOIDABLE
ADJUSTMENTS FOR SPECIFICS FROM THE PLAINS INTO SOUTHERN CANADA
(WITH FUTURE CHANGES LIKELY AS WELL).
REGARDING THE RAPID SEQUENCE OF EASTERN PACIFIC SYSTEMS, THE
SPREAD HAS NARROWED A BIT FOR THE TUESDAY STORM REACHING THE
PACIFIC NORTHWEST/VANCOUVER ISLAND WITH MOST SOLUTIONS NOW IN THE
LOW 960S-LOW 970S MB. THE UKMET REMAINS THE WEAKEST SOLUTION,
WHILE THE PAST COUPLE ECMWF RUNS ARE STRONGEST BUT NOT AS DEEP AS
THE 12Z/23 RUN THAT WAS IN THE 950S. GUIDANCE IS SHOWING SIGNALS
FOR A POSSIBLE WAVE ALONG A WARM FRONT APPROACHING THE COAST BY
EARLY THURSDAY, BUT WITH A LOT OF SPREAD FOR TRACK/STRENGTH OF THE
WAVE. THE 00Z ECMWF/CMC WERE SIMILAR BUT STRONGER THAN OTHER
GUIDANCE AND NOT SURPRISINGLY THE NEW RUNS DIFFER WITH THE
DETAILS. AT LEAST MODEST WAVINESS FITS WITHIN THE THEME OF THE
ENSEMBLE MEANS, SO THE BLENDED APPROACH THAT YIELDED SUCH A WAVE
PROVIDED A GOOD STARTING POINT. THEN BY FRIDAY ANOTHER FEATURE MAY
APPROACH THE COAST, WITH THE NEW 12Z ECMWF/UKMET DEVELOPING A
FAIRLY STRONG SYSTEM BUT REMAINING GUIDANCE DEPICTING A MERE FRONT
WITH POSSIBLE TRIPLE POINT. THE FAVORED BLEND LEANED TO THE MORE
CONSERVATIVE LATTER SCENARIO FOR THE TIME BEING. YET ANOTHER
FEATURE MAY APPROACH ON SATURDAY, WITH DECENT MODEL/MEAN
CLUSTERING IN PRINCIPLE BUT AGAIN A LOT OF SPREAD FOR DETAILS.
OVERALL CONFIDENCE IS FAIRLY LOW FOR EXACT DETAILS WITHIN THE
BROAD AREA OF MEAN LOW PRESSURE EXPECTED TO DEVELOP OVER THE
CENTRAL U.S. DURING THE LATTER HALF OF THE WEEK, WITH SPECIFICS
BECOMING INCREASINGLY SENSITIVE TO SHORTWAVE DETAILS THAT HAVE LOW
PREDICTABILITY SEVERAL DAYS OUT IN TIME. THIS LACK OF CONFIDENCE
EXTENDS TO THE PATTERN ALONG AND EITHER SIDE OF THE EASTERN HALF
OF THE CANADIAN BORDER. HOWEVER CONFIDENCE IS MUCH HIGHER FOR THE
GENERAL PATTERN OF INCREASING FLOW FROM THE GULF BETWEEN PLAINS
LOW PRESSURE AND EAST COAST/ATLANTIC HIGH PRESSURE FROM WEDNESDAY
ONWARD.
..WEATHER/HAZARD HIGHLIGHTS
A SERIES OF PACIFIC FRONTS/SYSTEMS SHOULD BRING A MULTI-DAY PERIOD
OF PRECIPITATION TO MUCH OF THE WEST. A STRONG TO EXTREME
ATMOSPHERIC RIVER WITH A MOISTURE FETCH ALL THE WAY FROM THE
WESTERN PACIFIC SHOULD BE IN PROGRESS OVER THE WEST AT THE START
OF THE PERIOD EARLY TUESDAY AND SINK SOUTHWARD INTO MIDWEEK. THIS
EVENT WILL LIKELY PRODUCE HIGH RAINFALL RATES AND AMOUNTS
PARTICULARLY ALONG THE SOUTHERN OREGON AND CALIFORNIA COASTS WITH
HEAVY RAIN/SNOW ALONG THE FAVORED TERRAIN OF THE SIERRA NEVADA AS
WELL. A SLIGHT RISK OF EXCESSIVE RAINFALL HAS BEEN MAINTAINED IN
WPC'S EXPERIMENTAL DAY 4 EXCESSIVE RAINFALL OUTLOOK DUE TO
FLOODING CONCERNS WITH THIS RAIN. HIGH WINDS ARE ALSO POSSIBLE
ACROSS PORTIONS OF THE WEST COAST AND GREAT BASIN--ESPECIALLY WITH
THE STRONG STORM TRACKING INTO WASHINGTON OR VANCOUVER ISLAND ON
TUESDAY. BY DAY 5/WEDNESDAY, LOCALLY HEAVY RAINFALL COULD BECOME
HAZARDOUS FOR PARTS OF SOUTHERN CALIFORNIA INTO ARIZONA AS THE
MOISTURE AXIS SHIFTS SOUTH. GUIDANCE SUGGESTS A RAPID SUCCESSION
OF ADDITIONAL PACIFIC FRONTS/SYSTEMS MAY BRING SIGNIFICANT RAIN
AND MOUNTAIN SNOW TO PORTIONS OF THE WEST THROUGH THE LATTER HALF
OF THE WEEK INTO THE WEEKEND, BUT WITH LOW CONFIDENCE FOR THE
EVOLUTION OR TRACK FOR EACH SYSTEM. THESE SYSTEMS ARE NOT EXPECTED
TO BE AS INTENSE AS THE INITIAL ONE, BUT COULD STILL PRODUCE
SIGNIFICANT PRECIPITATION OVER SOME AREAS THAT ARE HYDROLOGICALLY
SENSITIVE SO MONITOR FORECASTS FOR SPECIFICS AS THE DETAILS COME
INTO BETTER FOCUS OVER COMING DAYS.
EAST OF THE ROCKIES, MOST AREAS SHOULD BE GENERALLY DRY
TUESDAY-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 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.
TEMPERATURES ACROSS THE EASTERN HALF OF THE LOWER 48 WILL STAY
10-15F BELOW AVERAGE ON TUESDAY (PERHAPS LOCALLY COLDER FOR LOWS
IN THE MIDWEST) BUT GRADUALLY MODERATE TOWARD NORMAL BY AROUND
MIDWEEK. ABOVE NORMAL TEMPERATURES BY 15-25F FOR LOWS AND 10-20F
IN TERMS OF HIGHS OVER THE WEST/HIGH PLAINS TUESDAY SHOULD MIGRATE
INTO THE CENTRAL AND EVENTUALLY THE EASTERN U.S. BY LATE WEEK AS
THE UPPER RIDGE REACHES THE EAST COAST. THE WET/SNOWY PATTERN OVER
THE WEST WILL TEND TO KEEP DAYTIME TEMPERATURES WITHIN A FEW
DEGREES OF NORMAL FROM WEDNESDAY ONWARD WHILE MORNING LOWS REMAIN
5-15F ABOVE NORMAL BY LATE IN THE WEEK.
RAUSCH/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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