FXUS66 KSTO 101723
AFDSTO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
920 AM PST THU DEC 10 2009
.DISCUSSION...
FREEZE WARNING WORKED OUT VERY WELL FOR THE NRN/CNTRL SAC VLY AS
MORNING LOWS WERE NEAR 19 AT RDD...21 AT RBL...25 AT MYV AND BAB AND
26 AT OVE. SMF FELL TO 26 BUT CLOUDS THICKENED BY EARLY MORNING AND
TEMPS BEGAN TO RISE.
NORCAL WILL BE TRANSITIONING FROM AN ARCTIC AIR MASS TO A MOIST
MARITIME AIR MASS AS WLY FLOW UNDERCUTS THE ERN PAC RIDGE WITH THE
HIGH PRESSURE CELL OVER AK. THIS TRANSITION WILL PRESENT SOME UNIQUE
CONCERNS AS THE ARCTIC AIR MAY BE SLOW TO SCOUR OUT IN SOME OF THE
COLDER BASINS/VALLEYS OF THE LOWER FOOTHILLS SURROUNDING THE
INTERIOR VALLEY THROUGH TONITE. E.G BUFKIT FORECAST SOUNDINGS FOR
RDD/RBL MAINTAIN THE WET BULB ZERO RIGHT ALONG THE ZERO DEG C
ISOTHERM AS THE AIR MASS MOISTENS TONITE WITH PCPN BEGINNING AFTER
MIDNITE. THIS RAISES THE FLAGS FOR THE POTENTIAL FOR PCPN TO BE OF
THE FROZEN TYPE IN THE NRN VALLEY...AND THE SURROUNDING FOOTHILLS.
HOWEVER...GRASS VALLEY PROFILE SHOWS THE WET BULB WARMING ABOVE ZERO
DEG C...PERHAPS DUE TO THE RELEASE OF LATENT HEAT AS PCPN IS
FORECAST TO BEGIN SEVERAL HRS SOONER THAN IN RDD/RBL.
IN ANY EVENT...NORCAL IS IN FOR A ONE/TWO PUNCH FOR FRI AND SAT...AS
THE FIRST SYSTEM MOISTENS THE AIR MASS TODAY. THIS IS EVIDENT IN THE
KOAK SOUNDING AS THE AIR MASS MOISTENED RAPIDLY IN THE 7-14 KFT
LEVELS. THIS IS LEADING TO SOME WEAK RETURNS...MAINLY AFFECTING THE
SRN SFO BAY AREA...AS SEEN ON THE SUPER-MOSAIC RADAR LOOP OF OUR 7
AREA RADARS. SATELLITE IMAGERY INDICATES THIS SYSTEM IS SPLITTING AS
THE TROF BECOMES MORE NEGATIVELY TILTED AS JET ENERGY FROM THE
CENTRAL PAC IS DROPPING SEWD TOWARDS THE ENTRANCE REGION OF THE
SUBTROPICAL JET. WITH A SPLITTING SYSTEM AND THE UPPER JET PROGGED
TO BE WELL S OF OUR AREA ON FRI...PREFER TO HILITE THE FIRST SYSTEM
WITH A SNOW ADVSRY...AND GOING WITH A WATCH TO WARNING SCENARIO FOR
THE 2ND STRONGER/WETTER SYSTEM ON SAT. ALTHOUGH THERE MAY NOT BE ANY
BREAK IN THE PCPN ESPECIALLY FOR THE SIERNEV...THERE APPEARS TO BE A
SHORT BREAK /A FEW HRS/ FOR THE VALLEY FROM THE TWO SYSTEMS. SINCE
THESE ARE TWO DIFFERENT SYSTEMS WITH THE LATTER DEFINITELY HAVING A
MORE ADVERSE IMPACT ON NORCAL WX...WE WILL ATTEMPT TO ADVERTISE THAT
IN OUR HEADLINES COMING OUT IN THE AFTERNOON PACKAGE. FOR EXAMPLE...
THE QPFS FOR THE SECOND SYSTEM ARE EXPECTED TO DOUBLED/TRIPLE THE
FIRST SYSTEM. WE ALSO LOOK FOR WINDIER CONDITIONS WITH THE 2ND
SYSTEM AS A DEEPER/ALMOST VERTICALLY STACKED LOW PRESSURE SYSTEM
DEVELOPS OFF THE N COAST ON SAT. NAM 925 MBS SLY WINDS ARE FORECAST
TO REACH 40+ KTS IN THE NRN SAN JOAQUIN/SRN SAC VLY LEADING TO
POSSIBLE WIND ADVSRY CRITERIA AS WELL. SAT AFTERNOON AND EARLY
EVENING COULD TURN MORE UNSTABLE AS THE FORECAST SOUNDINGS FOR THE
VALLEY INDICATE A COUPLE OF HUNDRED J/KG OF CAPE AFTER COLD FROPA
AND MAY HAVE TO INCLUDE ISOLD THUNDER IN THE VALLEY AS WELL.
PCPN AND WIND IS FORECAST TO WIND DOWN ON SUN AS THE UPPER TROF AXIS
MOVES E...AND CROSS-COASTAL RANGE MOUNTAIN NWLY FLOW DEVELOPS.
OROGRAPHICS WILL WEAKEN IN THE SIERNEV THEN AND SNOWFALL WILL
DECREASE SUN AFTERNOON/EVENING. JHM
&&
.EXTENDED DISCUSSION (MONDAY THROUGH THURSDAY)...
COOL, UNSETTLED WEATHER WILL CONTINUE TO BRING PERIODS OF
PRECIPITATION IN THE EXTENDED. WEAK RIDGING SHOULD LESSEN SHOWER
ACTIVITY ON MONDAY. HOWEVER...MODELS SHOW SOME MOISTURE RIDING OVER
THE RIDGE WHICH COULD STILL BRING A SLIGHT CHANCE TO CHANCE OF
RAIN/SNOW NORTH OF INTERSTATE 80 AND IN THE SIERRA MTNS. RIDGING
OVER SOCAL WILL PERSIST THROUGH THE WEEK BUT MODELS VARY IN TERMS OF
HOW MUCH THAT HIGH TO THE SOUTH WILL KEEP US DRY OR HOW WET WE'LL BE
WITH MOISTURE OVERRIDING THE RIDGE. GFS LEANS TOWARDS THE DRIER SIDE
WITH STRONGER RIDGING NORTHWARD...ECMWF AND GEM MODELS HAVE WETTER
SOLUTIONS. KEPT MENTION OF SLIGHT CHANCE TO CHANCE OF RAIN/SNOW IN
EXTENDED FORECAST FOR MOST OF OUR CWA WITH SNOW LEVELS GENERALLY
4000-6500 FEET. JBB
&&
.AVIATION...
MORNING VISIBILITIES ARE REDUCED IN THE CENTRAL VALLEY FROM SAC
SOUTHWARD DUE TO HAZE AND THESE CONDITIONS MAY LINGER MOST OF THE
DAY AS AN INCREASING CLOUD COVER WILL RETARD THE HEATING NECESSARY
TO HELP DIMINISH THE HAZE AND WINDS WILL REMAIN LIGHT.
AN INCOMING VERY WEAK COLD FRONT WILL MOVE THROUGH THE REGION TODAY
AND WILL BRING A THICKENING CLOUD COVER AND A CHANCE OF LIGHT
PRECIPITATION. THE BEST CHANCE FOR SHOWERS IS SOUTH OF SACRAMENTO
BUT ALL AREAS IN INTERIOR NORTHERN CALIFORNIA HAVE A CHANCE FOR AT
LEAST SPRINKLES WITH THIS SYSTEM. DUE TO THE RESIDUAL COLD AIRMASS
FROM THE RECENT FREEZING CONDITIONS THE INITIAL PRECIPITATION COULD
BE SNOW AT LOWER THAN NORMAL ELEVATIONS AND MAY REACH THE VALLEY
FLOOR IN THE FAR NORTHERN SACRAMENTO VALLEY.
CLOUDS ARE EXPECTED TO INCREASE OVER THE SIERRA DURING THE DAY WITH
WIDESPREAD IFR/LIFR CONDITIONS EXPECTED DURING SHOWERS LATER THIS
AFTERNOON/EVENING.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
...FREEZE WARNING UNTIL 10 AM FOR THE NORTHERN AND CENTRAL
SACRAMENTO VALLEY.
...WINTER STORM WATCH FOR WESTERN PLUMAS COUNTY/LASSEN PARK AREA AND
WEST SLOPES OF THE NORTHERN SIERRA NEVADA FROM LATER TONIGHT
THROUGH SUNDAY MORNING.
&&
$$
