275
FXUS02 KWBC 291900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST WED JAN 29 2025
VALID 12Z SAT FEB 01 2025 - 12Z WED FEB 05 2025
***PROLONGED ATMOSPHERIC RIVER EVENT EXPECTED FOR THE PACIFIC
NORTHWEST THIS WEEKEND INTO NEXT WEEK***
..GENERAL OVERVIEW
THE OVERALL UPPER LEVEL FLOW PATTERN THROUGH THE MEDIUM RANGE
PERIOD IS EXPECTED TO BE DOMINATED BY A DEVELOPING REX BLOCK OVER
WESTERN NORTH AMERICA, FAST QUASI-ZONAL FLOW ACROSS MUCH OF THE
CONTINENTAL U.S., AND DEEP TROUGHING OVER CENTRAL AND EASTERN
CANADA. A SLOW MOVING, UPPER LEVEL, CUT-OFF LOW SITUATED NORTHWEST
OF THE WASHINGTON COAST, COMBINED WITH THE FAST MID-UPPER LEVEL
FLOW DOWNSTREAM, WILL SUSTAIN A PROLONGED ATMOSPHERIC RIVER EVENT
ACROSS NORTHERN CALIFORNIA AND SOUTHWESTERN OREGON. MEANWHILE, A
COLD FRONT IN BETWEEN BROAD CYCLONIC FLOW ALOFT OVER THE GREAT
LAKES AND NORTHEAST AND A DEVELOPING RIDGE OVER THE BAHAMAS AND THE
FAR SOUTHEAST U.S. WILL SEPARATE COLD ARCTIC AIR TO THE NORTH AND
ABOVE NORMAL TEMPERATURES TO THE SOUTH THROUGH THE MIDDLE OF NEXT
WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
OVERALL, THERE REMAINS GOOD SYNOPTIC SCALE AGREEMENT WITHIN THE
00Z/06Z/12Z DETERMINISTIC GUIDANCE TO BEGIN THE FORECAST PERIOD
SATURDAY. THIS IS PARTICULARLY THE CASE FOR THE STRENGTH AND
POSITION OF THE CUT-OFF UPPER LOW NORTHWEST OF WASHINGTON THIS
WEEKEND INTO EARLY NEXT WEEK. ELSEWHERE, AS THE FORECAST
PROGRESSES INTO NEXT WEEK, DIFFERENCES IN TIMING AND AMPLITUDE OF
SEVERAL SHORTWAVES WITHIN THE FAST MID AND UPPER LEVEL FLOW ALONG
THE U.S./CANADA BORDER CONTINUE TO RESULT IN UNCERTAINTY,
PARTICULARLY WITH REGARDS TO THE DEVELOPMENT OF A NORTHERN STREAM
MID-LATITUDE CYCLONE AND ITS ATTENDANT ARCTIC COLD FRONT. COMBINED
WITH DIFFERENCES IN STRENGTH AND AMPLITUDE OF A RIDGE OVER THE
BAHAMAS AND THE FAR SOUTHEAST U.S., QUESTIONS REMAIN WITH HOW FAST
AND HOW FAR SOUTH THE ARCTIC FRONT WILL PROGRESS FROM THE NORTHERN
PLAINS INTO THE CENTRAL AND EASTERN U.S. SIMILAR TO THE LAST
FORECAST PACKAGE, BLENDED MODEL PREFERENCES SKEWED MORE TO THE
ECMWF, THE GFS, AND THEIR RESPECTIVE ENSEMBLE MEANS FOR THE LATTER
HALF OF THE PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
THE MOST IMPACTFUL WEATHER EVENT DURING THIS FORECAST PERIOD WILL
BE THE PROLONGED ATMOSPHERIC RIVER THAT IS LIKELY TO BRING EXTENDED
PERIODS OF MODERATE TO HEAVY RAINFALL FROM SOUTHWESTERN OREGON INTO
THE NORTHERN THIRD OF CALIFORNIA. AN ANOMALOUS MOISTURE FLUX
ORIENTED NEARLY ORTHOGONAL TO THE TERRAIN AHEAD OF A STATIONARY
FRONT IS LIKELY TO PRODUCE WIDESPREAD 3-6 INCH RAINFALL TOTALS
BETWEEN SANTA ROSA AND THE OREGON STATE LINE DURING THE DAY 4
PERIOD SATURDAY. GIVEN GOOD MODEL AGREEMENT OUTSIDE OF THE GFS, THE
INHERITED SLIGHT RISK OF EXCESSIVE RAINFALL WAS NOT ONLY
MAINTAINED, BUT EXPANDED FARTHER SOUTH ALONG THE NORTHERN
CALIFORNIA COAST AND FARTHER EAST INTO THE HIGHER TERRAIN. THE RISK
OF HEAVY RAIN CONTINUES THROUGH DAY 5 ON SUNDAY WITH REASONABLE
MODEL AGREEMENT SO DECIDED TO LEAVE THE INHERITED MARGINAL RISK FOR
NORTHERN CALIFORNIA AND THE FOOTHILLS OF THE CENTRAL AND NORTHERN
SIERRA AS IS. ADDITIONAL HEAVY RAINFALL IS LIKELY ACROSS THE STATE
THROUGH THE MIDDLE OF NEXT WEEK AS THE MOISTURE MOVES VERY SLOWLY
SOUTHWARD.
ELSEWHERE ACROSS THE COUNTRY, HEAVY SNOW IS EXPECTED FOR THE HIGHER
TERRAIN OF THE INTERMOUNTAIN WEST AND THE NORTHERN ROCKIES AS THE
PACIFIC MOISTURE ADVECTS FARTHER INLAND, WITH SEVERAL FEET OF
ACCUMULATION LIKELY FOR THE HIGHEST ELEVATIONS OVER THE 5 DAY
PERIOD. LIGHTER SNOW IS LIKELY FROM THE NORTHERN GREAT LAKES TO
NORTHERN NEW ENGLAND AS A SURFACE LOW PASSES THROUGH THE REGION
MONDAY WITH SOME LAKE EFFECT SNOW BEHIND IT.
TEMPERATURES ARE EXPECTED TO BE MILD FOR EARLY FEBRUARY ACROSS MUCH
OF THE CONTINENTAL U.S., AND HIGHS REACHING 15 TO 25 DEGREES ABOVE
NORMAL ARE POSSIBLE FROM THE SOUTHERN PLAINS TO THE MIDWEST STATES
FOR THIS WEEKEND. IT SHOULD BE CLOSE TO SEASONAL AVERAGES FOR THE
MID-ATLANTIC AND SLIGHTLY BELOW AVERAGE FOR NEW ENGLAND, AND THE
ARCTIC FRONT STARTS TO BRING MUCH COLDER WEATHER STARTING IN
MONTANA ON SUNDAY. THE ARCTIC AIRMASS OOZES SOUTHEASTWARD ACROSS
THE DAKOTAS AND INTO MINNESOTA GOING INTO EARLY NEXT WEEK, BUT IS
PROBABLY GOING TO BE HALTED IN ITS SOUTHWARD PROGRESS BY THE TIME
IT REACHES THE CENTRAL PLAINS DUE TO A DEVELOPING RIDGE ACROSS THE
SOUTHERN PLAINS BY MIDWEEK. ABOVE AVERAGE READINGS ARE FORECAST FOR
THE SOUTHERN U.S. THROUGH THE ENTIRE PERIOD.
MILLER/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 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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