602
FXUS02 KWBC 190656
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
156 AM EST THU DEC 19 2024
VALID 12Z SUN DEC 22 2024 - 12Z THU DEC 26 2024
...HEAVY PRECIPITATION/WINDY PATTERN FOCUS FOR THE PACIFIC
NORTHWEST/NORTHERN CALIFORNIA THIS WEEKEND INTO THE HOLIDAYS...
..OVERVIEW
AN OVERALL AMPLIFIED PATTERN IS EXPECTED TO BE IN PLACE BY THE
START OF THE MEDIUM RANGE PERIOD ON SUNDAY WITH DEEP TROUGHING OVER
THE EAST AND MEAN RIDGING OVER THE WEST TO CENTRAL STATES. A
SERIES OF DYNAMIC SHORTWAVES ORIGINATING FROM AMPLIFIED EAST
PACIFIC TROUGHING WILL PUSH INTO/THROUGH THE MEAN RIDGE OUT WEST,
BRINGING A COUPLE OF ATMOSPHERIC RIVERS AND HEAVY RAIN/MOUNTAIN
SNOWS TO PARTS OF THE WEST COAST WELL INTO THE UPCOMING HOLIDAY
WEEK. CURRENT FORECASTS SHOW EASTWARD PROGRESS OF A LEADING
SHORTWAVE THROUGH THE NORTHERN PLAINS AND ONWARD BY EARLY NEXT WEEK
TO HELP PUSH THE EASTERN U.S. UPPER TROUGH EASTWARD WITH A RETURN
TO A MORE PROGRESSIVE (BUT STILL POTENTIALLY ACTIVE) OVERALL
PATTERN.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST MODELS AND ENSEMBLES CONTINUE TO SHOW GOOD CLUSTERING AND
PREDICTABILITY ON THE LARGE SCALE, BUT THERE ARE SOME LINGERING
UNCERTAINTIES ON TIMING AND DETAILS OF INDIVIDUAL SYSTEMS, WHICH
MAY HAVE SIGNIFICANT IMPACTS ON SENSIBLE WEATHER. THE EASTERN
TROUGH SHOULD BEGIN TO PUSH OFF THE COAST SUNDAY INTO MONDAY AHEAD
OF A LEADING SHORTWAVE INTO THE NORTHERN PLAINS. THE GUIDANCE HAS
SHOWN BETTER CLUSTERING ON THIS FEATURE BUT THERE IS SOME
UNCERTAINTY IN THE EVOLUTION OF THIS INTO NEXT WEEK WHICH SOME
MODELS INDICATING ENERGY MAY HANG BACK OVER THE GREAT LAKES RATHER
THAN PROGRESS QUICKLY THROUGH THE NORTHEAST. A GENERAL MODEL BLEND
SEEMED TO SERVE AS A GOOD STARTING POINT HERE.
THE NEXT SHORTWAVE WILL ENTER THE WEST BY MONDAY, WITH
AMPLIFICATION EXPECTED OVER THE CENTRAL PLAINS BEFORE IT BEGINS TO
LIFT OUT BY WEDNESDAY. THE MODELS SHOW GOOD AGREEMENT ON THIS
SYSTEM AS WELL, BUT SOME QUESTION ON HOW QUICKLY IT WEAKENS AS IT
LIFTS INTO THE MIDWEST. THE NEXT SYSTEM BEHIND THIS SEEMS TO BE
MUCH STRONGER THAN ITS PREDECESSORS WITH PLENTY OF AGREEMENT THAT
THIS MAY FORM A BRIEF CLOSED LOW OVER THE SOUTHERN PLAINS AROUND
NEXT THURSDAY. BUT THERE IS STILL NOTABLE UNCERTAINTY ON ITS TIMING
AND SPECIFICS SO THE LATE PERIOD WPC BLEND USED A MODEST AMOUNT OF
THE ENSEMBLE MEANS TO HELP SMOOTH THESE DIFFERENCES.
..WEATHER/HAZARDS HIGHLIGHTS
A SERIES OF MOSTLY WEAK ATMOSPHERIC RIVERS WILL BE ONGOING BY THIS
WEEKEND OVER THE WEST BRINGING A PROLONGED PERIOD OF WINDY AND
GENERALLY UNSETTLED CONDITIONS ALONG THE WEST COAST. FOR SUNDAY,
GUIDANCE CONTINUES TO SHOW SOME RAINFALL BUT RAINFALL RATES MAY NOT
BE HIGH ENOUGH TO CAUSE ANY WIDESPREAD HAZARDS, SO THE PREVIOUS
MARGINAL RISK WE HAD ON THE EXCESSIVE RAINFALL OUTLOOK HAS BEEN
DROPPED THIS CYCLE. BY MONDAY, HOWEVER, THE CORE AXIS OF THE
EASTERN PACIFIC TROUGH SHOULD PUSH EAST WITH AN INCREASE IN THE
MOISTURE AND HEAVY RAIN/MOUNTAIN SNOW POTENTIAL. MOISTURE AND
INSTABILITY WITH THIS AR LOOKS MORE SUFFICIENT FOR AT LEAST A
MARGINAL RISK ON THE MONDAY ERO PERIOD, THOUGH THE ACTIVITY WILL
CONTINUE INTO TUESDAY AS WELL. MODEL QPFS CONTINUE TO SUGGEST
SEVERAL INCHES OF RAINFALL POSSIBLE, ESPECIALLY FOR PARTS OF
NORTHWEST CALIFORNIA, WITH ACCUMULATING AND POSSIBLY HEAVY SNOWS IN
THE HIGHER TERRAIN OF THE CASCADES AND THE SIERRA.
FARTHER EAST, THE LEADING SHORTWAVE INTO THE NORTHERN PLAINS WILL
SPREAD SOME WEAK RAIN OR SNOW ACROSS PORTIONS OF THE UPPER MIDWEST
AND THE GREAT LAKES INTO MONDAY. THE SOUTHERN STREAM SYSTEM WILL
ALLOW PRECIPITATION TO DEVELOP ACROSS THE PLAINS INTO THE MIDWEST
NEXT WEEK, BUT WITH LINGERING UNCERTAINTY IN THE SPECIFICS.
A DEPARTING SURFACE HIGH OVER THE EAST WILL CONTINUE MUCH BELOW
NORMAL TEMPERATURES THROUGH MONDAY, WITH MODERATION THEREAFTER.
ABOVE NORMAL TEMPERATURES WILL EXPAND FROM THE WEST INTO THE PLAINS
AND MIDWEST THROUGH THE PERIOD. DAYTIME HIGHS IN SOME LOCATIONS
COULD BE 10 TO 20 DEGREES ABOVE NORMAL.
SANTORELLI
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 FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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