FXUS66 KSTO 100430
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
830 PM PST WED DEC 9 2009
.DISCUSSION...WATER VAPOR IMAGERY CONTINUES TO SHOW A COUPLE OF
SYSTEMS TRYING TO UNDERCUT A HIGH AMPLITUDE RIDGE OFF THE WEST
COAST. THE FIRST OF WHICH IS APPROACHING THE CENTRAL CALIFORNIA
COAST THIS EVENING...SPREADING PLENTY OF MID AND HIGH LEVEL CLOUD
ACROSS NORTHERN CALIFORNIA. THIS FIRST SYSTEM IS NOT THAT
IMPRESSIVE...AND WILL MOST LIKELY MOVE INTO CENTRAL CALIFORNIA LATE
TONIGHT AND ON THURSDAY. ALTHOUGH THE COMPOSITE RADAR IMAGERY IS
SHOWING A FEW LIGHT RETURNS JUST OFF THE COAST AND MOVING INTO THE
MONTERREY PORTION OF THE COASTLINE...BELIEVE THAT IT IS EVAPORATING
BEFORE REACHING THE SURFACE AS VERY DRY AIR REMAINS IN PLACE BELOW
600 MB. HOWEVER...IT MAY BE ABLE TO BRING A FEW SPRINKLES OR LIGHT
SHOWERS TO THE REGION TOMORROW. THE SECOND STRONGER SYSTEM IS
JUST MOVING INSIDE OF 140 WEST...AND WILL APPROACH NORTHERN
CALIFORNIA THURSDAY NIGHT AND FRIDAY. THIS SYSTEM LOOKS LIKE IT
WILL BRING A BETTER SHOT OF PRECIPITATION TO THE REGION THAN THIS
FIRST ONE.
IN THE MEANTIME THOUGH...EXPECT ANOTHER CHILLY NIGHT ACROSS THE
NORTHERN INTERIOR...EVEN IF IT IS A COUPLE OF DEGREES WARMER THAN
THE PAST COUPLE OF NIGHTS. REDDING AND RED BLUFF ARE ALREADY IN THE
UPPER 20S...AND MARYSVILLE IS AT 30 DEGREE. THEREFORE...BELIEVE THE
FREEZE WARNING LOOKS GOOD FOR THE NORTHERN AND CENTRAL SACRAMENTO
VALLEY...EVEN WITH INCREASING CLOUD COVER TONIGHT. AM GOING TO DO A
QUICK UPDATE TO INCREASE CLOUD COVER TONIGHT...BUT THE REST OF THE
FORECAST LOOKS ON TRACK. PALMER
.PREVIOUS DISCUSSION (ISSUED AT 335 PM)...NRN CAL IS UNDER A LESS
AMPLIFIED PORTION OF THE WEST COAST RIDGE AND THIS WILL ENABLE THE
UPPER WAVE TO PROGRESS INTO NRN CAL THURSDAY NIGHT...ALBEIT A
WEAKENING WAVE. THIS WILL GENERATE WARM FRONTAL TYPE PRECIP THURSDAY
AFTERNOON WITH CLOUDY SKIES AND A CHANCE OF MAINLY DRIZZLE TO LIGHT
RAIN SOUTHWARD OF AN ALDER SPRINGS-QUINCY LINE...AND SRN SAC VALLEY
SOUTHWARD. SEVERAL WAVES WILL MOVE THROUGH NRN CAL BRINGING PERIODS
OF RAIN AND SNOW WITH THE STRONGEST COMING SATURDAY. SNOW LEVELS
WILL START OUT AROUND 3000 FT ON THU AND RISE TO 5000 FT ON SAT.
MODERATE TO HEAVY SNOW ACCUMULATIONS ARE POSSIBLE ABOVE 5000 FT FRI
AND SAT...ESPECIALLY OVER THE SIERRA. FRI MORNING COULD BE TRICKY
IN THE NRN SAC VALLEY NEAR REDDING. RIGHT NOW MIN TEMPS IN THE
REGION ARE JUST ABOVE FREEZING AS THE INITIAL WARM FRONTAL PRECIP
IMPACTS THE AREA. PRECIP COMES IN DURING THE DIURNAL MIN AND IF
COLD AIR GET TRAPPED IN THE AREA IT COULD MEAN SNOW INSTEAD OF
RAIN. IT ALL DEPENDS ON THE STRENGTH OF THE SOUTHERLY WINDS TO
SCOUR OUT THE COLD DENSE AIR AND HOW MUCH RADIATIONAL COOLING
HAPPENS THU NIGHT. RIGHT NOW THE WINDS DO NOT APPEAR STRONG
ENOUGH. JMC
.EXTENDED DISCUSSION (SUNDAY THROUGH WEDNESDAY)...
THE WETTER AND MILDER SYSTEM ON SATURDAY WILL BE WINDING DOWN ON
SUNDAY AS IT MOVES EWD ACROSS THE GREAT BASIN. MODELS INDICATE
WIDESPREAD BUT MUCH LIGHTER QPFS WILL BE LINGERING OVER INTERIOR
NORCAL DURING THE DAY ON SUN IN THE NWLY FLOW. NOT TOO CERTAIN THAT
THE NWLY FLOW WILL BE AS MOIST AS PROGGED...BUT FOR NOW THE GFS
CROSS SECTIONS SHOW THE MID LEVELS DRYING DURING THE DAY...BUT THE
LOWER LEVELS BELOW 10 KFT REMAINING VERY MOIST. THUS...THERE WILL
LIKELY BE SOME LINGERING LIGHT SHOWERS THROUGH MUCH OF THE
DAY...ESPECIALLY OVER THE W SLOPE SIERNEV WHERE UPSLOPE FLOW WILL
CONTINUE UNTIL SUN EVENING. SNOW LEVELS DURING THIS TIME LOOK TO BE
AROUND 4500 TO 5000 FT. RIDGING IN THE WAKE OF THIS SYSTEM WILL GIVE
NORCAL A BREAK IN THE PCPN ON MON.
AFTER MON...THE GFS DIVERGES FROM THE ECMWF AND CANADIAN MODELS FOR
TUE AND WED. WHILE THE GFS AMPLIFIES THE RIDGE ALONG THE W COAST AND
RETURNS DRY WX TO NORCAL...THE ECMWF AND CANADIAN MODELS MAINTAIN A
FLAT RIDGE OVER NORCAL WITH A RATHER WET SWLY FLOW INTO NORCAL
EMANATING FROM THE DEEP TROF ALONG 140-150W. NOT TAKING INTO ACCOUNT
OROGRAPHICS...QPFS COULD RANGE FROM 1-2 INCHES ON TUE...WITH ABOUT
HALF OF THAT ON WED. THIS WOULD BE A MILDER PATTERN WITH SNOW LEVELS
PROBABLY MORE IN THE 5000 TO 6000 FOOT RANGE. JHM
&&
.AVIATION...GENERALLY VFR CONDITIONS EXPECTED ACROSS THE AREA
TONIGHT INTO THURSDAY WITH HIGH CLOUDS AND LIGHT WINDS. CLOUDS WILL
INCREASE DURING THE DAY ON THURSDAY AS A WEAK FRONT MOVES OVER THE
AREA. PRECIPITATION MAY BEGIN OVER THE SACRAMENTO AND NORTHERN SAN
JOAQUIN VALLEY AREAS BY THURSDAY AFTERNOON AND SPREADING OVER
INTERIOR NORTHERN CALIFORNIA OVERNIGHT. AREAS OF MVFR CONDITIONS AND
LOCAL IFR CONDITIONS ARE POSSIBLE WITH THE PRECIPITATION IN THE
VALLEY THURSDAY NIGHT INTO FRIDAY.
CLOUDS ARE EXPECTED TO INCREASE OVER THE SIERRA DURING THE DAY
TOMORROW WITH WIDESPREAD IFR/LIFR CONDITIONS EXPECTED WITH
PRECIPITATION BY AROUND 03Z FRIDAY.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
...FREEZE WARNING FROM 10 PM TONIGHT TO 9 AM PST THURSDAY FOR THE
NORTHERN AND CENTRAL SACRAMENTO VALLEY
&&
$$
