369
FOUS30 KWBC 192041
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
341 PM EST FRI DEC 19 2025
DAY 1
VALID 16Z FRI DEC 19 2025 - 12Z SAT DEC 20 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL MAINLY OVER
PORTIONS OF NORTHWEST CALIFORNIA AND NEW ENGLAND...
THE AR CONTINUES TO SAG SOUTHWARD, REDUCE RESIDENCY TIME, AS WELL
AS GENERALLY WEAKENING IN FLUX INTENSITY. ALL FACTORS SUPPORT A
REDUCTION OF THE CATEGORY TO MARGINAL RISK FOR THE REMAINDER OF THE
DAY. ALSO, HAVE REMOVED THE NORTHERN PORTIONS WITHIN OREGON
WHERE EVEN THE NORTHERN EDGE OF THE AR PLUME HAS MOVED SOUTH
LIMITING RAINFALL TOTALS TO BELOW EXCESSIVE CONCERNS.
THINKING AND PLACEMENT OF THE MARGINAL RISK IN NEW ENGLAND REMAINS
ON TRACK; SO NO CHANGES WERE MADE FOR THIS UPDATE.
GALLINA
~~~~PRIOR DISCUSSIONS~~~~
..PACIFIC NORTHWEST
ONGOING ATMOSPHERIC RIVER WILL DRIFT SOUTHWARD WHILE SWIFT FLOW
ALOFT TRANSPORTS THE PLUME OF DEEP LAYER MOISTURE INTO PARTS OF
THE NORTHERN GREAT BASIN. IVT VALUES AT OR ABOVE 700 UNITS ARE
FORECAST TO COVER MUCH OF OREGON INITIALLY BEFORE IT WEAKENS AS
THE AXIS OF THE MOISTURE PLUME AND ASSOCIATED PRECIPITATION SETTLES
INTO NORTHERN CALIFORNIA. THE MARGINAL RISK WAS MAINTAINED FOR
SOUTHWEST OREGON AND NORTHERN CALIFORNIA, AS WELL AS THE SLIGHT
RISK OVER NORTHWEST CALIFORNIA. MINIMAL ADJUSTMENTS WERE MADE TO
ACCOUNT FOR THE LATEST QPF TRENDS AND WPC FORECAST.
..SOUTHERN NEW ENGLAND
LOCALLY HEAVY RAINFALL WILL BE POSSIBLE FOR PORTIONS OF THE
NORTHEAST DURING THIS PERIOD AS MOISTURE DRAWN NORTHWARD ENCOUNTERS
THE ADVANCING TROUGH OVER THE EASTERN STATES. WITH TIME THE
THE MOIST FLOW ALOFT BECOMES INCREASINGLY DIFFLUENT AND A SURFACE
WARM FRONT APPROACHES FROM THE SOUTH EARLY ON FRIDAY. WPC
DETERMINISTIC QPF IN THE 1.0 TO 1.4 INCH RANGE IS FORECAST ALONG
THE TRACK OF THE LOW. OFFSETTING THE POTENTIAL OF EXCESSIVE
RAINFALL BY LIMITING RAINFALL RATES/AMOUNT IS THE MEAGER
INSTABILITY EXPECTED TO BE IN PLACE. A MARGINAL RISK WAS MAINTAINED
FROM THE NEW YORK/CONNECTICUT/MASSACHUSETTS BORDER TO DOWNEAST
MAINE.
CAMPBELL/BANN
DAY 2
VALID 12Z SAT DEC 20 2025 - 12Z SUN DEC 21 2025
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF
NORTHERN CALIFORNIA|...
20Z UPDATE: CONTINUITY FROM THE PREVIOUS FORECAST WAS MAINTAINED AS
12Z MODEL SUITE CONTINUED TO PROJECT SIMILAR RESULTS FROM EARLIER
RUNS LEADING TO LITTLE/NO VARIABILITY IN THE FORECAST. EXPECTING
AREAS WITHIN THE NORTHERN SIERRA NEVADA TO RECEIVE THE MOST
APPRECIABLE PRECIPITATION GIVEN THE TRAJECTORY OF THE IVT PULSE AND
PERSISTENT 850-600MB MOISTURE FEED WITH ADDED UPSLOPE COMPONENTS
NEAR THE TERRAIN. GREATEST FLOOD CONCERNS LIKELY IN THE ADJACENT
FOOTHILLS ON THE WINDWARD SIDE OF THE SIERRA'S. HIGHEST PROBS FOR
>5" ARE ACROSS THE I-80 CORRIDOR AT THE CA/NV BORDER WITH THE
HIGHEST TERRAIN STILL LIKELY TO BE SNOW, LIMITING THE THREAT FOR
FLASH FLOODING FOR THE PERIOD TO MORE OF A MRGL STANCE. THUS, KEPT
THE PREVIOUS FORECAST WITH NO CHANGE.
KLEEBAUER
..PREVIOUS DISCUSSION..
A RELATIVE LULL IN MOISTURE SURGES EXPECTED DURING THE MAJORITY OF
THIS PERIOD BEFORE THE NEXT STRONGER ATMOSPHERIC RIVER FROM THE
PACIFIC REACHES NORTHERN CALIFORNIA TOWARDS THE END OF DAY 2.
ALTHOUGH THERE MAY BE SOME OVERLAP IN HEAVY RAINFALL, THE AXIS OF
THE STRONGEST ON-SHORE WINDS AND THE GREATEST ON-SHORE MOISTURE
TRANSPORT LOOKS TO BE SOUTH OF THE AXIS OF THE PREVIOUS EVENT.
PRECIPITABLE WATER VALUES INCREASE TO BETWEEN 1.2 AND 1.4 INCHES
ALONG THE NORTHERN CALIFORNIA COAST NORTH OF SAN FRANCISCO SOUTH OF
THE OREGON/CALIFORNIA BORDER BY EARLY SATURDAY EVENING. GIVEN SUCH
HIGH SNOW LEVELS...THERE IS NOT A LOT OF CONCERN ABOUT
PRECIPITATION TYPE UNTIL THE MOISTURE LADEN FLOW REACHES THE
NORTHERN END OF THE SIERRA NEVADA RANGE (SAVE FOR A FEW MOUNTAIN
TOPS ALONG THE TRAJECTORY INLAND FROM THE COAST). THE URBAN CENTERS
BETWEEN SACRAMENTO TO SAN FRANCISCO WILL BE MORE AT PLAY THIS
PERIOD LEAD
CAMPBELL/BANN
DAY 3
VALID 12Z SUN DEC 21 2025 - 12Z MON DEC 22 2025
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
NORTHERN CALIFORNIA...
20Z UPDATE: A MORE SUBSTANTIAL IVT PULSE WILL IMPACT THE NORTHERN
HALF OF CALIFORNIA COME SUNDAY LEADING TO OVERLAP OF IMPACTED
AREAS FROM D2. MULTI-DAY ACCUMULATIONS COUPLED WITH HIGHER RATES
WILL INDUCE GREATER FLASH FLOOD POTENTIAL OVER A LARGE AREA WITH
THE GREATEST RISK OVER THE NORTHERN SIERRA NEVADA FOOTHILLS.
MULTI-DAY TOTALS OF 5-10" ARE STILL FORECAST IN THAT REGION OF THE
STATE, ENOUGH TO WARRANT A CONTINUATION OF A SLGT RISK WITH
PROBABILITIES APPROACHING THE HIGHER END OF THE RISK THRESHOLD AS
OF THIS TIME. THERE IS AN OPPORTUNITY FOR AN UPGRADE IN THE
FOLLOWING UPDATES, SO PLEASE STAY TUNED FOR THE LATEST IN THIS
EVOLVING THREAT.
KLEEBAUER
..PREVIOUS DISCUSSION..
THE ATMOSPHERIC RIVER WILL STRENGTHEN OVER THE NORTHERN HALF OF
CALIFORNIA RESULTING IN SCATTERED TO WIDESPREAD AREAS OF ENHANCED
RAINFALL OVERLAPPING MUCH OF THE SAME FOOTPRINT FROM THE DAY 2
PERIOD. THE LATEST GUIDANCE IS SUGGESTING DAILY ACCUMULATIONS OF 3
TO 7 INCHES, WITH 48 HOUR TOTALS NEARING 5 TO 9 INCHES. A SLIGHT
RISK IS IN EFFECT FROM THE WEST COAST TO MUCH OF THE SIERRA NEVADA
RANGE. THERE MAY BE A NEED FOR AN UPGRADE FOR PORTIONS OF THE
SIERRA NEVADA SHOULD GUIDANCE CONTINUE TO INCREASE OVER THIS AREA.
CAMPBELL
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
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