FXUS66 KSTO 151722 AAA
AFDSTO
AREA FORECAST DISCUSSION...UPDATED AVIATION DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
920 AM PST TUE DEC 15 2009
.DISCUSSION...
PACIFIC FRONTAL SYSTEM NOW JUST REACHING THE NORTH COAST WITH WARM
SECTOR FAIRLY LIGHT PRECIP NOW FALLING ACROSS THE NORTHWEST CORNER
OF THE STATE. CLOUDS COVER THE REMAINDER OF THE CWA AND TENDING TO
BREAK UP FOG WHICH FORMED IN THE DELTA OVERNIGHT. SURFACE
OBSERVATION REPORTS OVER WESTERN SHASTA COUNTY AVERAGING ABOUT 1/2
INCH OVER THE LAST 6 HOURS. AS MENTIONED IN PREVIOUS AFD...FRONTAL
BAND HAS BEEN ORIENTING MORE NORTH TO SOUTH OVER THE LAST 24 HOURS
AND HAS SLOWED AS IT HAS APPROACHED THE COAST. BIGGEST IMPACT ON CWA
FORECAST BY MODELS TO BE BETWEEN 00Z TONIGHT AND 12Z WEDNESDAY
MORNING WITH MAXIMUM UPWARD MOTION FORECAST OVER THE COASTAL RANGE
AROUND 06Z. WILL CONSIDER THE NEED FOR A SNOW ADVISORY IN THIS AREA
FOR TONIGHT. AT THIS TIME...MODELS IN FAIRLY GOOD AGREEMENT WITH
SNOW LEVELS AROUND 5000 FEET BUT THREAT STILL REMAINS OF COLD
TRAPPED AIR IN THE SHASTA COUNTY VALLEYS ALLOWING FOR LOCALLY LOWER
SNOW LEVELS. BLENDED TPW STILL SHOWS GOOD PLUME OF MOISTURE WITH
VALUES ASSOCIATED WITH THE FRONT OF 1.25 INCHES OR GREATER EXTENDING
WELL OUT BEYOND 140 WEST. AT THIS POINT...SEEMS TO BE NO SIGNIFICANT
TAP TO THE SUBTROPICS SO NOT EXPECTING FLOODING PROBLEMS AT THIS
TIME. 06Z GFS AND 12Z NAM BOTH GIVING QPF NORTH OF REDDING AT ABOUT
1 INCH OVERNIGHT. MODERATE 850 MB WINDS RIGHT OUT OF THE SOUTH WILL
PROVIDE GOOD OROGRAPHICS OVER THE SHASTA COUNTY MOUNTAINS FOR HIGHER
QPF THERE. ALL MODELS GOOD AGREEMENT IN RAPID WEAKENING OF FRONT AS
UPPER SHORTWAVE SHIFTS TO THE EAST. WILL RECONSIDER NEED FOR WINTER
STORM WARNING OVER THE SOUTHERN CASCADES AS SNOW AMOUNTS MAY NOT BE
SUFFICIENT. FRONTAL BAND FORECAST TO LINGER OVER NORCAL THROUGH
WEDNESDAY FOR CONTINUED BUT LIGHTER PRECIP. UPPER RIDGE REBUILDS
THURSDAY FOR END OF PRECIP THREAT. ECMWF MORE AMPLIFIED THAN GFS
WHICH ALLOWS SHORTWAVE TO BRING LIGHT PRECIP TO NORTHERN MOST CWA.
&&
.EXTENDED DISCUSSION (SATURDAY THROUGH TUESDAY)...
MODELS ARE INDICATING A RIDGE AMPLIFYING OVER THE INTERIOR WITH
SEVERAL SHORTWAVES PASSING TO THE NORTH THAT MAY CLIP THE FAR
NORTHERN SECTIONS OF THE STATE. FOG MAY DEVELOP IN THE CENTRAL
VALLEY AS A RESULT OF THE AMPLIFIED RIDGE IN PLACE SO THAT COULD BE
OUR BIGGEST CONCERN DURING MOST OF THIS PERIOD. MONDAY AND TUESDAY A
TROUGH BEGINS TO DEEPEN ALTHOUGH THE MODELS ARE OUT OF PHASE WITH
EACH OTHER ON THE TIMING AND DEEPNESS OF THE LOW...BUT AT LEAST FOR
NOW THAT WOULD APPEAR TO BE THE BEST CHANCE OF RAIN OVER MOST OF THE
INTERIOR.
&&
.AVIATION...PRECIPITATION WILL REMAIN PRIMARILY NORTH OF I-80
CORRIDOR TODAY...WITH A FEW -SHRA/SPRINKLES POSSIBLE SOUTH OF THE
INTERSTATE. AREAS OF IFR CIGS OVR THE FAR NORTH DUE TO RAIN WILL
PERSISTENT THROUGH THIS EVENING AND TONIGHT. FURTHER
SOUTH...CONDITIONS WILL REMAIN MOSTLY VFR OVER THE CENTRAL
VALLEY...WITH AREAS OF MVFR VSBYS DEVELOPING LATER THIS MORNING.
SOME LOCAL IFR CONDITIONS WILL BE POSSIBLE IN THE DELTA REGION
THROUGH 20Z. OVER THE NORTHERN MOUNTAINS...IFR CIGS AND VSBYS WILL
BE PERSISTENT...WHILE FURTHER SOUTH IN THE SIERRA NEVADA CONDITIONS
WILL DEGRADE THIS AFTERNOON INTO THE IFR CATEGORY.
&&
.STO WATCHES/WARNINGS/ADVISORIES...
...WINTER STORM WARNING UNTIL 4 PM PST WEDNESDAY FOR WESTERN PLUMAS
COUNTY AND THE LASSEN PARK AREA.
...WINTER WEATHER ADVISORY UNTIL 4 AM PST WEDNESDAY FOR THE
MOUNTAINS OF NORTHERN AND EASTERN SHASTA COUNTY AND THE BURNEY BASIN.
&&
$$
