843
FXUS02 KWBC 221901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST THU DEC 22 2022
VALID 12Z SUN DEC 25 2022 - 12Z THU DEC 29 2022
***UNSETTLED CONDITIONS FOR THE WEST COAST WITH HEAVY RAIN AND
HEAVY MOUNTAIN SNOW WITH AN ATMOSPHERIC RIVER EVENT EARLY NEXT
WEEK***
..SYNOPTIC OVERVIEW
THE OVERALL WEATHER PATTERN IMPROVES GOING INTO THE WEEKEND ACROSS
THE EASTERN HALF OF THE NATION AS THE CORE OF THE INTENSE LOW
LIFTS FARTHER NORTHWARD ACROSS CENTRAL CANADA AND WEAKENS WITH
TIME. HOWEVER, THERE WILL STILL BE SOME LINGERING LAKE EFFECT
SNOW DOWNWIND OF LAKES ERIE AND ONTARIO AS WELL AS A CONTINUATION
OF MUCH BELOW NORMAL TEMPERATURES AT THE START OF THE PERIOD. THE
IMPRESSIVELY COLD AIRMASS THAT HAS PLAGUED THE EASTERN 2/3RDS OF
THE COUNTRY FOR THE PRIOR DAYS WILL BEGIN MODIFYING AS THE SURFACE
HIGH SETTLES ACROSS THE SOUTHERN TIER STATES, WITH COLDER
TEMPERATURES LINGERING LONGEST FOR THE MIDWEST. LOOKING AHEAD TO
EARLY NEXT WEEK, MULTIPLE SHORTWAVE PASSAGES NEAR THE PACIFIC
NORTHWEST WILL KEEP THINGS UNSETTLED FROM NORTHERN CALIFORNIA TO
WASHINGTON STATE WITH AN ATMOSPHERIC RIVER PATTERN SETTING UP
LEADING TO MULTIPLE DAYS OF HEAVY PRECIPITATION FOR THE REGION.
THE EASTERN HALF OF THE COUNTRY WILL SEE MUCH CLOSER TO NORMAL
TEMPERATURES AND GENERALLY DRY WEATHER.
..GUIDANCE/PREDICTABILITY ASSESSMENT
BOTH THE DETERMINISTIC AND ENSEMBLE MEAN MODEL GUIDANCE WERE VERY
SIMILAR AT THE START OF THE PERIOD (SUNDAY) WITH THE MAIN
UPPER-LEVEL FEATURES CONSISTING OF THE CLOSED LOW ASSOCIATED WITH
THIS WEEK'S POWERFUL STORM SYSTEM SHIFTING NORTHEASTWARD TOWARDS
HUDSON BAY AND MEAN TROUGHING OVER THE EASTERN U.S., A COUPLE
SHORTWAVES IN THE MEAN FLOW SLIDING SOUTHEASTWARD ACROSS THE
SOUTHERN PLAINS AND NORTHERN ROCKIES, AND TROUGHING UPSTREAM OVER
THE NORTHEASTERN PACIFIC. THE WPC FORECAST BEGAN WITH AN INITIAL
EVEN BLEND OF THE DETERMINISTIC GUIDANCE (00Z ECMWF/UKMET/CMC AND
06Z GFS) TO ACCENTUATE THE SMALLER SCALE DETAILS OF THE MORE
SUBTLE SHORTWAVES COMPARED TO THE ENSEMBLE MEANS. MORE NOTICEABLE
DIFFERENCES BETWEEN THE GUIDANCE AND INDIVIDUALLY RUN-TO-RUN WERE
RAPIDLY APPARENT BY 12Z MONDAY IN TERMS OF THE TIMING OF THE
SHORTWAVES ROTATING THROUGH THE MEAN TROUGHING IN THE EASTERN U.S.
AS WELL AS THE AMPLIFICATION OF THE PATTERN UPSTREAM. THE
INFLUENCE OF THE 00Z CMC WAS DECREASED IN THE FORECAST AS IT WAS
MORE OF AN OUTLIER COMPARED TO THE OTHER GUIDANCE IN TERMS OF THE
PHASING OF THE SHORTWAVES, WHICH HAD SPECIFIC RAMIFICATIONS ON
THE DEVELOPMENT OF LOW PRESSURE AT THE SURFACE OVER THE
PLAINS/MIDWEST, AND THE AMPLIFICATION OF THE PATTERN UPSTREAM.
FURTHER INTO THE PERIOD, THE GUIDANCE CONTINUED TO DIFFER IN THE
PHASING OF SHORTWAVES IN THE SOUTHERN AND NORTHERN STREAM OVER THE
EASTERN U.S., SPECIFICALLY THE ENERGY OF A SHORTWAVE OVER THE
SOUTH, AND A SHORTWAVE MOVING EAST THROUGH THE ROCKIES AS A
CLOSED LOW QUICKLY REPLACES THIS ENERGY UPSTREAM OVER THE
NORTHEASTERN PACIFIC. THE 00Z CMC WAS AGAIN AN OUTLIER WITH A
LOWER DEGREE OF MEAN TROUGHING OVER THE SOUTH. THERE WAS AT LEAST
AN ENCOURAGING CONVERGENCE TOWARDS MORE SIMILAR SOLUTIONS
RUN-TO-RUN BETWEEN THE GFS AND ECMWF TO KEEP THE SAME BLEND WITH
DECREASED INFLUENCE OF THE CMC. THE DETERMINISTIC GUIDANCE
CONTINUED TO BE EMPHASIZED DESPITE THE DIFFERENCES BECAUSE THE
ENSEMBLE MEANS WERE NOTABLY LESS AMPLIFIED. THE TREND COMPARED TO
THE PRIOR WPC FORECAST WAS FOR DEEPER MEAN TROUGHING OVER THE
EASTERN U.S. AND A CLOSED LOW UPSTREAM OVER THE NORTHEASTERN
PACIFIC.
LATE IN THE PERIOD, DIVERGENCE AMONGST THE MODEL SOLUTIONS AND
RUN-TO-RUN DIFFERENCES INCREASES WITH THE PHASING OF THE SHORTWAVE
ENERGY OVER THE EASTERN U.S. BETWEEN THE NORTHERN AND SOUTHERN
STREAM AND HOW THE CLOSED LOW OVER THE PACIFIC EVOLVES DOWNSTREAM
OVER THE WEST. THE INFLUENCE OF THE 00Z ECENS AND GEFS MEANS WAS
INCREASED AS THE MEANS STILL CAPTURED SHORTWAVES OVER THE
SOUTHEASTERN AND WESTERN U.S. WITH THE DIFFERENCES BETWEEN THE
DETERMINISTIC GUIDANCE BECOMING MORE DIFFICULT TO RESOLVE. THIS
PATTERN LASTED THROUGH THE END OF THE PERIOD ON THURSDAY WITH A
DIFFERENCE BETWEEN THE PHASING OF SHORTWAVE ENERGY MOVING OFF THE
EAST COAST AND THE DEGREE OF ENERGY WITH THE SHORTWAVE TO THE WEST
DIGGING SOUTHWARD INTO THE SOUTHERN ROCKIES AND PLAINS. A BLEND OF
THE DETERMINISTIC 00Z ECMWF AND 06Z GFS AND THE 00Z ECENS/GEFS
MEANS WAS ABLE TO CAPTURE THE OVERALL PATTERN RELATIVELY WELL
TOWARDS THE WEST WITH THE DETERMINISTIC GUIDANCE HELPING TO
CAPTURE THE SHORTWAVE ENERGY OFF THE EAST COAST.
..WEATHER/HAZARD HIGHLIGHTS
SATURDAY WILL FEATURE WIDESPREAD TEMPERATURE ANOMALIES 20-30
DEGREES BELOW AVERAGE FOR MORNING LOWS AND DAYTIME HIGHS ACROSS
MUCH OF THE CENTRAL AND EASTERN U.S. SUNDAY WILL STILL BE QUITE
COLD OVER THE EASTERN HALF OF THE COUNTRY WITH A BROAD AREA OF
READINGS 15-25 DEGREES BELOW NORMAL. THE THEME FOR NEXT WEEK WILL
BE A STEADY WARMING TREND AS THE UPPER PATTERN BECOMES LESS AND
LESS AMPLIFIED. MODERATELY ABOVE NORMAL TEMPERATURES WILL BECOME
ESTABLISHED OVER THE WEST WITH MEAN RIDGING ALOFT (LOWS TENDING TO
BE A LITTLE MORE ABOVE NORMAL THAN DAYTIME HIGHS) AND THEN SPREAD
WITH DOWNSLOPING WINDS INTO THE HIGH PLAINS BY NEXT
TUESDAY-WEDNESDAY WITH SOME AREAS REACHING 10-20F ABOVE NORMAL.
THESE WINDS MAY BE STRONG AND GUSTY AT TIMES. THE EASTERN HALF OF
THE COUNTRY WILL CONTINUE TO MODERATE, LEADING TO MOSTLY
SINGLE-DIGIT NEGATIVE ANOMALIES BY NEXT WEDNESDAY.
A SERIES OF NORTHEASTERN PACIFIC SYSTEMS SHOULD BRING A MULTI-DAY
PERIOD OF ATMOSPHERIC RIVER EVENTS TO THE WEST COAST, LEADING TO
SIGNIFICANT TOTALS OF RAIN AND HIGHER ELEVATION SNOW. SOME OF THIS
MOISTURE WILL ALSO EXTEND EASTWARD INTO THE GREAT BASIN AND THE
ROCKIES, THOUGH THERE IS LESS CERTAINTY IN TERMS OF TOTAL AMOUNTS
IN THESE LOCATIONS. THE PRIMARY FOCUS DURING THE WEEKEND IS
FORECAST TO BE OVER THE PACIFIC NORTHWEST. THEN THE PRECIPITATION
SHIELD WILL LIKELY PUSH SOUTHEASTWARD DURING THE FIRST HALF OF THE
WEEK AS A MORE AMPLIFIED UPPER TROUGH AND LEADING SURFACE FRONT
MOVE IN. AS THIS OCCURS, THE HEAVIEST RAIN/MOUNTAIN SNOW SHOULD
FOCUS ALONG THE CENTRAL WEST COAST INTO THE SIERRA NEVADA WHILE
SOME MOISTURE EXTENDS FARTHER SOUTH OVER CALIFORNIA AND EAST INTO
THE GREAT BASIN/ROCKIES, AND NOTEWORTHY PRECIPITATION PERSISTS
OVER THE NORTHWEST. THERE IS STILL SOME UNCERTAINTY OVER THE
PRECISE INTENSITY AND SOUTHWARD EXTENT OF PRECIPITATION AT ANY
PARTICULAR TIME, THOUGH AT THE MOMENT THE BEST SIGNAL FOR HIGHEST
AMOUNTS IS OVER THE OLYMPICS AND WASHINGTON CASCADES. THESE
SYSTEMS MAY ALSO BRING PERIODS OF HIGH WINDS TO SOME PORTIONS OF
THE WEST COAST AND GREAT BASIN. ELSEWHERE, A WEAK ALBERTA CLIPPER
MAY BRING AN AREA OF LIGHT SNOW FROM THE NORTHERN PLAINS INTO THE
MIDWEST DURING SUNDAY-MONDAY. GULF COAST SHORTWAVE ENERGY AND
ASSOCIATED STATIONARY FRONT SHOULD SPREAD SOME MOISTURE ACROSS THE
GULF OF MEXICO AND WESTERN ATLANTIC EARLY-MID WEEK, WITH FLORIDA
POSSIBLY SEEING SOME RAINFALL DURING THAT TIME. THERE MAY BE AN
INCREASE IN RAIN CHANCES FOR THE GULF COAST STATES AS WELL, BUT
THIS LOOKS TO BE MUCH MORE LIKELY JUST BEYOND THE CURRENT FORECAST
PERIOD.
PUTNAM/HAMRICK
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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