196
AWUS01 KWNH 252108
FFGMPD
CAZ000-260900-
MESOSCALE PRECIPITATION DISCUSSION 1276
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
405 PM EST THU DEC 25 2025
AREAS AFFECTED...PORTIONS OF CENTRAL AND NORTHERN CA
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY
VALID 252105Z - 260900Z
SUMMARY...NEW ROUNDS OF HEAVY SHOWERS AND THUNDERSTORMS ARE
EXPECTED THIS AFTERNOON THROUGH THIS EVENING FOR AREAS OF CENTRAL
AND NORTHERN CA, INCLUDING THE BAY AREA. GIVEN THE SATURATED SOIL
CONDITIONS AND HIGH STREAMFLOWS, ADDITIONAL AREAS OF FLOODING AND
FLASH FLOODING ARE LIKELY.
DISCUSSION...THE LATEST GOES-W IR/WV SUITE SHOWS A DEEP UPPER
TROUGH AND ASSOCIATED CLOSED LOW OFFSHORE OF THE WEST COAST.
SHORTWAVE IMPULSES CONTINUE TO ROUND THE BASE OF THE TROUGH AND
TAKE AIM ON CENTRAL AND NORTHERN CA, WITH EACH IMPULSE EFFECTIVELY
DRIVING RENEWED ATMOSPHERIC RIVER ACTIVITY INTO THE STATE.
SOLAR INSOLATION HAS FACILITATED SUFFICIENT LEVELS OF BOUNDARY
LAYER HEATING ACROSS THE NORTHERN PORTIONS OF THE CENTRAL VALLEY
TO RESULT IN SBCAPE VALUES AS HIGH AS 500 TO 750 J/KG. THIS
HEATING COUPLED WITH RELATIVELY DIVERGENT FLOW ALOFT DOWNSTREAM OF
THE DEEP OFFSHORE UPPER TROUGH/CLOSED 500 MB LOW CENTER WILL
FACILITATE AN INCREASE IN CONVECTIVE SHOWERS IN THE NEAR-TERM
ACROSS SIZABLE AREAS OF THE CENTRAL VALLEY. SOME LOCALLY ORGANIZED
LINE-SEGMENTS WILL BE POSSIBLE GIVEN THE ELEVATED BULK SHEAR
PARAMETERS (30 TO 50 KTS), AND THIS ACTUALLY MAY BECOME ALIGNED
WITH THE DEEP LAYER FLOW FOR SOME PERIODIC INSTANCES OF
CELL-TRAINING.
HOWEVER, OF GREATER SIGNIFICANCE WILL BE THE APPROACH OF A
WELL-ORGANIZED SHORTWAVE IMPULSE ROUNDING THE BASE OF THE UPPER
TROUGH HEADING INTO THE EARLY EVENING HOURS. THIS SHORTWAVE ENERGY
WHICH IS WELL-DEPICTED BY THE AXIS OF COLD CONVECTIVE CLOUD TOPS
IN IR SATELLITE IMAGERY ALONG 125W TO 130W WILL BE EJECTING
NORTHEAST TOWARD THE NORTHERN CA COASTAL RANGES IN THE 00Z TO 03Z
TIME FRAME, INCLUDING THE BAY AREA. THEREAFTER, THIS NEW SURGE OF
ENERGY AND RELATED PACIFIC-MOISTURE TRANSPORT SHOULD EJECT INTO
THE CENTRAL VALLEY AND SIERRA NEVADA FOOTHILLS. THE COASTAL RANGES
AND SACRAMENTO VALLEY IN PARTICULAR WILL SEE THE GREATEST IVT
MAGNITUDES. THE 12Z GFS/ECMWF SOLUTIONS SUGGEST IVT VALUES
REACHING INTO THE 500 TO 600 KG/M/S RANGE WITH THE AID OF STRONGER
LOW TO MID-LEVEL SOUTHWEST FLOW.
THE COMBINATION OF THIS ALONG WITH LARGE-SCALE FORCING/SHEAR AND
AT LEAST MODEST BOUNDARY LAYER INSTABILITY SHOULD SET THE STAGE
FOR MORE ORGANIZED BANDS OF CONVECTION TO APPROACH THE COASTAL
RANGES AND THEN ADVANCE INLAND GOING INTO THE EVENING HOURS. HEAVY
RAINFALL RATES OF 0.50" TO 1"+/HOUR WILL BE LIKELY WITH THESE NEW
ROUNDS OF HEAVY SHOWER AND THUNDERSTORM ACTIVITY.
ADDITIONAL RAINFALL TOTALS BY LATE THIS EVENING OF 2 TO 3 INCHES
WILL BE POSSIBLE LOCALLY FOR THE COASTAL RANGES AND ESPECIALLY THE
UPSLOPE AREAS OF THE NORTHERN SIERRA NEVADA FOOTHILLS. LESSER
TOTALS OF AS MUCH AS 1 TO 2 INCHES CANNOT RULED OUT IN AT LEAST
PARTS OF THE SACRAMENTO VALLEY. BY LATE THIS EVENING, THIS LATEST
SURGE OF ENERGY AND MOISTURE SHOULD SETTLE FARTHER SOUTH INTO
COASTAL RANGES SOUTH OF THE BAY AREA AND ADJACENT AREAS OF THE
CENTRAL VALLEY.
GIVEN THE SATURATED SOIL CONDITIONS AND HIGH STREAMFLOWS, THESE
ADDITIONAL RAINS ARE LIKELY TO RESULT IN MORE AREAS OF AREAL
FLOODING AND FLASH FLOODING, WHICH WILL INCLUDE URBAN FLOODING
IMPACTS (SAN FRANCISCO, SACRAMENTO, CHICO AND REDDING ALL AT RISK)
ALONG WITH LOCALIZED MUD AND LANDSLIDE ACTIVITY NEAR AREAS OF
HIGHER TERRAIN.
ORRISON
..PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT
ATTN...WFO...EKA...HNX...LOX...MFR...MTR...STO...
ATTN...RFC...RSA...NWC...
LAT...LON 41192196 40352163 39952136 39572073 39012031
38592000 37901945 37291920 36791979 35902040
35112065 35182118 35912190 37202260 37972308
39012405 39682433 40372428 40802375 40872295
41122238
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