764
FXUS01 KWBC 202001
PMDSPD
SHORT RANGE FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EST WED DEC 20 2023
VALID 00Z THU DEC 21 2023 - 00Z SAT DEC 23 2023
...HEAVY TO EXCESSIVE RAINFALL IS EXPECTED TONIGHT AND THURSDAY
ACROSS SOUTHERN CALIFORNIA WHERE THERE WILL BE SIGNIFICANT
CONCERNS FOR FLASH FLOODING...
...VERY MILD TEMPERATURES FOR THIS TIME OF THE YEAR WILL CONTINUE
THROUGH THE REMAINDER OF THE WEEK FOR LARGE AREAS OF THE COUNTRY...
...UNSETTLED WEATHER TO ARRIVE ACROSS THE PACIFIC NORTHWEST BY
FRIDAY...
A SLOW-MOVING UPPER LOW AND ASSOCIATED FRONTAL SYSTEM CURRENTLY
OFFSHORE OF CALIFORNIA WILL BE STEADILY ADVANCING SOUTHEASTWARD
THROUGH THURSDAY AND WILL THEN ADVANCE INLAND ACROSS THE SOUTHWEST
ON FRIDAY. STRONG FORCING ASSOCIATED WITH THIS COUPLED WITH A
NOTABLE INCREASE IN PACIFIC MOISTURE OUT AHEAD OF IT WILL LEAD TO
BANDS OF HEAVY SHOWERS AND THUNDERSTORMS THROUGH THIS EVENING AND
INTO THE OVERNIGHT HOURS ACROSS PORTIONS OF CENTRAL AND ESPECIALLY
SOUTHERN CALIFORNIA. THE HEAVIEST RAINS WILL BE SITUATED ACROSS
THE COASTAL RANGES AND ESPECIALLY THE TRANSVERSE RANGE TONIGHT
WHERE THE BETTER MOISTURE TRANSPORT AND HEAVIER RAINFALL RATES ARE
EXPECTED TO SET UP. THE HEAVY RAINFALL THREAT WILL PERSIST THROUGH
THURSDAY ACROSS SOUTHERN CALIFORNIA, AND OVER THE NEXT 24 TO 36
HOURS, THE RAINFALL TOTALS ACROSS THE TRANSVERSE RANGE MAY
APPROACH OR EVEN LOCALLY EXCEED 4 TO 8 INCHES.
THESE RAINS ARE LIKELY TO RESULT IN SIGNIFICANT CONCERNS FOR FLASH
FLOODING WITH A NOTABLE THREAT TO THE AREA BURN SCAR LOCATIONS
WHERE DEBRIS FLOWS AND MUDSLIDES WILL BE POSSIBLE. URBAN FLASH
FLOODING WILL ALSO BE POSSIBLE IN THE LOWER ELEVATIONS CLOSER INTO
THE LOS ANGELES BASIN. SOME RAINFALL RATES WITH THIS EVENT MAY
APPROACH OR LOCALLY EXCEED 1 INCH PER HOUR WITH THE HEAVIER
SHOWERS AND THUNDERSTORMS, AND THE PERSISTENCE OF THESE RATES WILL
LEAD TO MUCH OF THE BROADER THREAT OF POTENTIALLY SIGNIFICANT
FLASH FLOODING. THE WEATHER PREDICTION CENTER HAS HIGHLIGHTED A
MODERATE RISK OF EXCESSIVE RAINFALL TO ADDRESS THESE CONCERNS
GOING THROUGH THURSDAY.
BY FRIDAY, THIS STORM SYSTEM WILL BE ACCELERATING OFF TO THE EAST
AND WILL BEGIN TO OVERSPREAD THE SOUTHWEST WHERE AREAS OF LOCALLY
HEAVY RAIN AND AT LEAST SOME ISOLATED CONCERNS FOR FLASH FLOODING
WILL CONTINUE. AS THE HEAVY RAINFALL THREAT BEGINS TO EXIT
SOUTHERN CALIFORNIA, AREAS OF CENTRAL AND SOUTHERN ARIZONA WILL
BEGIN TO SEE THESE IMPACTS. HOWEVER, THE RAINFALL TOTALS SHOULD BE
MUCH MORE BENEFICIAL AND MODEST BY COMPARISON.
IN GENERAL, THE THREAT OF WINTER WEATHER OVER THE NEXT COUPLE OF
DAYS WILL BE RATHER LIMITED DUE TO THE LACK OF COLD AIR ACROSS
MUCH OF THE COUNTRY, BUT THE STORM SYSTEM COMING INTO CALIFORNIA
AND THE BROADER SOUTHWEST WILL FOSTER SOME LOCALLY HEAVY
ACCUMULATING SNOWFALL TONIGHT AND THURSDAY FOR THE HIGH TERRAIN OF
THE SOUTHERN SIERRA NEVADA. GRADUALLY THE THREAT OF ACCUMULATING
SNOWFALL WILL INCREASE ALSO INTO THE SAN JUAN MOUNTAINS OF
SOUTHWEST COLORADO, WITH A MUCH HEAVIER THREAT OF SNOW HERE
EXPECTED BY THIS WEEKEND.
GIVEN THE PRESENCE OF A LOT OF PACIFIC AIR ACROSS THE COUNTRY, AND
GENERAL LACK OF SOUTHWARD ADVANCING ARCTIC AIR FROM CANADA, THE
TEMPERATURES ACROSS LARGE AREAS OF THE COUNTRY WILL BE WELL ABOVE
NORMAL FOR THIS TIME OF THE YEAR. THIS WILL ESPECIALLY BE THE CASE
ACROSS AREAS OF THE CENTRAL AND NORTHERN PLAINS, AND THE MIDWEST,
WHERE HIGH TEMPERATURES WILL BE AS MUCH AS 15 TO 25 DEGREE ABOVE
NORMAL FOR THE REMAINDER OF THE WEEK.
RELATIVELY COOLER AND MORE UNSETTLED WEATHER SHOULD BEGIN TO
ARRIVE ACROSS THE PACIFIC NORTHWEST BY FRIDAY AS A NEW COLD FRONT
ARRIVES AND ADVANCES INLAND. LOCALLY HEAVY SHOWERS WITH THE FRONT
CAN BE EXPECTED GIVEN THE INFLUX OF PACIFIC MOISTURE, AND THIS
WILL TRANSLATE INTO AREAS OF HEAVY SNOWFALL FOR THE HIGHER
ELEVATIONS INCLUDING THE CASCADES AND EVENTUALLY THE NORTHERN
ROCKIES.
ORRISON
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