215
FXUS02 KWBC 250657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST WED DEC 25 2024
VALID 12Z SAT DEC 28 2024 - 12Z WED JAN 01 2025
...UNSETTLED WEATHER FOR THE WEST COAST WITH PERIODS OF HEAVY RAIN
AND HIGH ELEVATION MOUNTAIN SNOW...
..OVERVIEW
THE LARGE SCALE FLOW PATTERN WILL FEATURE A RAPID SUCCESSION OF
PACIFIC SYSTEMS MOVING INTO/THROUGH THE WEST AND FEEDING INTO A
MEAN UPPER TROUGH OF MODERATE DEPTH ALIGNED OVER THE
PLAINS/MISSISSIPPI VALLEY, WHILE AN UPPER RIDGE INITIALLY NEAR THE
EAST COAST DRIFTS INTO THE WESTERN ATLANTIC. THIS PATTERN WILL LEAD
TO AREAS OF HEAVY PRECIPITATION ALONG THE NORTHERN-CENTRAL WEST
COAST THROUGH THE WEEKEND AND SNOW OVER THE NORTHERN HALF OR SO OF
THE ROCKIES, ALONG WITH SOME AREAS OF STRONG WINDS. IN ADDITION A
BROAD AREA OF PRECIPITATION (MOSTLY RAIN) WILL SPREAD ACROSS THE
EASTERN HALF OF THE COUNTRY WITH SOME ENHANCED TOTALS POSSIBLE. FOR
THE MOST PART, THIS SHOULD BE A MILD PATTERN FOR THE LOWER 48 WITH
MUCH OF THE COUNTRY EXPERIENCING ABOVE NORMAL TEMPERATURES
INITIALLY, WHICH DROP BACK TOWARD CLIMATOLOGY BY MID NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE GUIDANCE IS SHOWING BETTER AGREEMENT THAN LAST NIGHT, BUT
DETAIL ISSUES REMAIN. A COMPOSITE OF 12Z/18Z OPERATIONAL MODEL
GUIDANCE PROVIDED A GOOD REPRESENTATION OF CONSENSUS OR
INTERMEDIATE IDEAS FOR THE FIRST HALF OF THE PERIOD, WITH SOME 12Z
NAEFS/12Z ECMWF MEAN GUIDANCE INCORPORATED THEREAFTER TO ACCOUNT
FOR INCREASING DETAIL UNCERTAINTIES.
..WEATHER/HAZARDS HIGHLIGHTS
MULTIPLE ROUNDS OF MODERATE TO HEAVY RAINFALL WILL LIKELY IMPACT
THE WEST COAST THROUGH THE REMAINDER OF THIS WEEK AND INTO THE
WEEKEND AS MULTIPLE FRONTAL SYSTEMS APPROACH THE COAST, WITH THE
SIGNAL FOR HEAVY RAINFALL BECOMING STRONGEST ON SATURDAY
CONCENTRATING MORE NEAR THE CALIFORNIA/OREGON BORDER WITH TIME.
FLOODING WILL BE POSSIBLE IN RIVERS AND STREAMS, WITH MORE FLASHY
RESPONSES POSSIBLE WHERE BURN SCARS EXIST AND POSSIBLY NEAR THE
STEEP TERRAIN OF THE COASTAL MOUNTAIN RANGES AND INTO THE FOOTHILLS
OF THE CASCADES. THE COMBINATION OF RAINFALL EXPECTED THROUGH
FRIDAY NIGHT AND A MORE CONCENTRATED HEAVY QPF SIGNAL IN SOME
GUIDANCE FOCUSED OVER FAR SOUTHWEST OR AND FAR NORTHWEST
CA ON SATURDAY CONTINUES TO FAVOR A DAY 4/SATURDAY SLIGHT RISK
AREA OVER THAT REGION. WHILE QPF IS LESS THE FOLLOWING DAY, WHAT
SHOULD BE INCREASED SENSITIVITY ALLOWED FOR THE INTRODUCTION OF A
DAY 5/SUNDAY MARGINAL RISK FOR WESTERN OR AND NORTHWEST CA.
MOISTURE SPREADING INLAND ACROSS THE WEST WILL LIKELY PRODUCE
HEAVY SNOW OVER THE HIGHER/HIGHEST TERRAIN OF THE CASCADES AND
ROCKIES, AS WELL AS PORTIONS OF THE SIERRA NEVADA. EXPECT THE HEAVY
SNOW POTENTIAL TO CONTINUE THROUGH SUNDAY. PRECIPITATION OVER THE
PACIFIC NORTHWEST SHOULD TREND LIGHTER EARLY NEXT WEEK, WITH
ROCKIES SNOW TAPERING OFF BY TUESDAY. TIGHT PRESSURE GRADIENTS MAY
RESULT IN SOME PERIODICALLY GUSTY WINDS FOR SOME LOCATIONS OVER THE
PACIFIC NORTHWEST, INTERIOR WEST, PLUS THE NORTHERN/CENTRAL
ROCKIES AND HIGH PLAINS.
DOWNSTREAM, MULTIPLE FRONTAL SYSTEMS WILL BRING RAIN AND
THUNDERSTORM CHANCES TO THE PLAINS, MISSISSIPPI VALLEY, AND
EAST. SOME MODERATE TO HEAVY RAINFALL WILL BE POSSIBLE WITHIN A
GENERAL AREA FROM THE LOWER HALF OF THE MISSISSIPPI VALLEY INTO THE
MID-ATLANTIC AND NORTHEAST. THE WETNESS OF THE SOME MOST RECENT
GUIDANCE IN AND NEAR TN AND THE SOUTHERNMOST APPALACHIANS SHOWING
LOCAL AMOUNTS IN THE 3-5" RANGE LED TO THE INTRODUCTION OF A DAY
4/SATURDAY MARGINAL RISK AREA, WHICH SHIFTS INTO THE VICINITY OF
PENNSYLVANIA ON DAY 5/SUNDAY. ANY WINTRY WEATHER SHOULD BE LIMITED
TO LOCATIONS CLOSE TO THE CANADIAN BORDER.
WHILE DAY-TO-DAY TRENDS IN TEMPERATURE WERE NOTICEABLY DOWNWARD IN
PORTIONS OF THE MIDWEST, OHIO VALLEY, AND SOUTHERN MID-ATLANTIC
STATES, A MAJORITY OF THE CENTRAL/EASTERN U.S. SHOULD SEE WELL
ABOVE NORMAL TEMPERATURES FROM LATE THIS WEEK INTO EARLY NEXT WEEK.
EXPECT THE WARMEST ANOMALIES TO BE FOR MORNING LOWS ACROSS THE
EAST-CENTRAL PART OF THE COUNTRY FRIDAY-SUNDAY, IN THE PLUS 20-30F
RANGE AND POSSIBLY CHALLENGING DAILY RECORDS FOR WARM LOWS IN SOME
AREAS. WARMEST MIN TEMPERATURE ANOMALIES SHOULD BE MORE CONFINED
TO THE UPPER MISSISSIPPI VALLEY TO NEW ENGLAND EARLY NEXT WEEK.
ALSO THE SOUTHERN PLAINS COULD SEE HIGHS REACH 15-25F ABOVE NORMAL
ON MONDAY IN THE WARM SECTOR AHEAD OF A SYSTEM REACHING THE HIGH
PLAINS BY EARLY IN THE DAY. READINGS CLOSER TO CLIMATOLOGY WILL
BROADEN EASTWARD FROM THE WEST COAST EARLY NEXT WEEK, REACHING THE
PLAINS EARLY NEXT WEEK AND THE EAST BY MID-NEXT WEEK.
ROTH/RAUSCH
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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