665
FXUS02 KWBC 201858
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 PM EST FRI DEC 20 2024
VALID 12Z MON DEC 23 2024 - 12Z FRI DEC 27 2024
...HEAVY RAIN/MOUNTAIN SNOW AND HIGH WIND THREAT PATTERN FROM
NORTH-CENTRAL CALIFORNIA TO THE NORTHWEST INTO THE HOLIDAYS...
..GENERAL OVERVIEW
A LESS AMPLIFIED/TRANSITIONAL UPPER LEVEL FLOW PATTERN WITH
MULTIPLE EMERGING STREAMS IS EXPECTED OVER THE CONTINENTAL U.S. BY
NEXT WEEK ONCE A DEEP UPPER TROUGH DEPARTS THE EAST COAST AND
AMPLIFIED PACIFIC SYSTEMS PROGRESS INLAND. AN UPPER RIDGE TRIES TO
DEVELOP ACROSS THE INTERMOUNTAIN WEST AND ROCKIES TUESDAY BEFORE A
POTENT PACIFIC SHORTWAVE MOVES IN AND PRODUCES AN ATMOSPHERIC
RIVER EVENT FOR THE WEST COAST, AND ANOTHER DISTURBANCE DEVELOPS
OVER THE SOUTHERN PLAINS MIDWEEK AND SPURS A NEW SURFACE LOW WITH
ENHANCED RAINFALL IN ADVANCE. LOOKING AHEAD TO THE END OF THE NEXT
WEEK, A CONTINUED PERIODIC AND ENERGETIC STORM TRACK FROM THE
PACIFIC LEADS TO AN ADDITIONAL STORM SYSTEM MOVING INLAND ACROSS
THE WESTERN U.S. WITH ANOTHER ROUND OF HEAVY RAIN AND WINDY
MOUNTAIN SNOW.
..GUIDANCE/PREDICTABILITY ASSESSMENT
FORECAST CLUSTERING AND UNCERTAINTY HAVE MARGINALLY INCREASED OVER
MEDIUM RANGE TIME SCALES WITH INTRODUCTION OF A MORE TRANSITIONAL
FLOW PATTERN NEXT WEEK, BUT IN PARTICULAR BY LATER NEXT HOLIDAY WEEK.
THE WPC PRODUCT SUITE AND COLLABORATED THREAT MESSAGING WAS
PRIMARILY DERIVED FROM A PREFERRED COMPOSITE OF BEST CLUSTERED
GUIDANCE FROM THE 06 UTC GFS/GEFS MEAN AND 00 UTC ECMWF/ECMWF
ENSEMBLE MEAN. THIS COMPOSITE PROVIDES MAX CONSISTENCY WITH WPC
CONTINUITY AND THE 13 UTC NATIONAL BLEND OF MODELS. THIS BLEND
PROCESS TENDS TO MITIGATE MODEL VARIANCES CONSISTENT WITH
PREDICTABILITY. A SIMILAR BLEND OF 12 UTC GUIDANCE SEEMS IN LINE.
..WEATHER/HAZARDS HIGHLIGHTS
IT STILL REMAINS GENERALLY THE CASE THAT MODERATE TO HEAVY
RAINFALL IS EXPECTED TO PERSIST ACROSS PORTIONS OF NORTHERN
CALIFORNIA AND INTO WESTERN OREGON ON MONDAY, WHEN A CORE AXIS OF
THE EASTERN PACIFIC TROUGH SHOULD PUSH EASTWARD WITH AN INCREASE IN
THE MOISTURE AND UPSLOPE FLOW ACROSS WINDWARD TERRAIN. MOISTURE
AND INSTABILITY WITH THIS ATMOSPHERIC RIVER CONTINUES TO LOOK
SUFFICIENT ENOUGH FOR A CONTINUED MARGINAL RISK ON THE WPC
EXCESSIVE RAINFALL OUTLOOK FOR DAY 4/MONDAY AND TO A LESSER EXTENT
FOR DAY 5/TUESDAY, AND SUBSEQUENT ROUNDS OF RAIN AND MOUNTAIN SNOW
APPEAR LIKELY THROUGH CHRISTMAS AND INTO THE END OF THE WEEK.
MODEL QPFS CONTINUE TO SUGGEST ENHANCED RAINFALL POSSIBLE OVER THE
PACIFIC NORTHWEST, BUT ESPECIALLY FOR PARTS OF NORTHWEST
CALIFORNIA, WITH ACCUMULATING AND POSSIBLE HEAVY SNOW FOCUS IN THE
HIGHER TERRAIN OF THE CASCADES AND THE SIERRA, THEN WORKING INLAND
ACROSS THE NORTHWEST AND NORTH-CENTRAL INTERMOUNTAIN WEST/ROCKIES.
FARTHER EAST, THE LEADING SHORTWAVE AND FRONTAL LOW INTO THE
NORTHERN PLAINS WILL SPREAD SOME MAINLY LIGHT RAIN AND SNOW ACROSS
PORTIONS OF THE UPPER MIDWEST AND THE GREAT LAKES INTO MONDAY. THE
SOUTHERN STREAM TROUGH IS EXPECTED TO DEVELOP FURTHER WITH
POTENTIAL CLOSED LOW DEVELOPMENT IN EMERGING SOUTHERN STREAM FLOW
AS DEEPER MOISTURE SURGES NORTHWARD FROM THE GULF OF MEXICO. GIVEN
THE POTENTIAL FOR HIGHER INSTABILITY AND RAINFALL RATES OF AN INCH
PER HOUR OR GREATER WITH THE CONVECTION THAT DEVELOPS, A MARGINAL
RISK IS IN PLACE FOR THE WPC DAY 5/TUESDAY EXCESSIVE RAINFALL
OUTLOOK FROM EASTERN TEXAS THROUGH ARKANSAS. SOME STRONG
THUNDERSTORMS MAY ALSO DEVELOP IN THIS SAME GENERAL AREA AS PER SPC.
A DEPARTING SURFACE HIGH OVER THE EAST WILL CONTINUE MUCH BELOW
NORMAL TEMPERATURES THROUGH MONDAY, WITH MODERATION EXPECTED GOING
INTO LATER NEXT WEEK. 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.
HAMRICK/SCHICHTEL
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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