Severe Thunderstorm Warning 31, NWS Sacramento, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
330 PM PDT TUE JUL 28 2009

.DISCUSSION...
TEMPERATURES RUNNING A FULL 13 DEGREES COOLER AT SACRAMENTO THIS
AFTERNOON THAN YESTERDAY. WINDS AT TRAVIS AFB CONTINUE TO BLOW OVER
25 KTS. THE MARINE LAYER SHOULD RE-ESTABLISH ITSELF OVERNIGHT AS THE
UPPER RIDGE OVER OUR REGION HAS WEAKENED. A DECENT 3.8 MB ONSHORE
GRADIENT KSFO-KSAC SHOULD HELP BRING SOME STRATUS CLOUDS INTO THE
DELTA TONIGHT AND KEEP TEMPERATURES PLEASANTLY COOL IN SACRAMENTO.
THE COOLING HAS WORKED INTO STOCKTON AND MODESTO...BUT THE FOLKS IN
REDDING WILL HAVE TO WAIT ONE MORE DAY TO SEE THEIR COOLING TREND
KICK IN.

ISOLATED THUNDERSTORMS DEVELOPED NEAR THE SIERRA CREST LATE THIS
AFTERNOON. ANOTHER CELL DEVELOPED NORTH OF CHESTER WHILE IT APPEARS
SOME BUILDUPS ARE ALSO OCCURRING OVER THE COASTAL MOUNTAINS...BUT NO
LIGHTNING HAS BEEN OBSERVED THERE. MOST OF THE ACTIVITY NEAR THE
SIERRA WILL SOON SHIFT EAST OF THE CREST BUT WILL CONTINUE TO
MONITOR. THE BUILDUPS ARE DUE TO THE DESTABILIZING OF THE ATMOSPHERE
AS A WEAK UPPER LOW DEVELOPS ALONG THE CALIFORNIA COAST WHILE THE
UPPER RIDGE WEAKENS. MODEST DIVERGENCE ALOFT CONTINUES TONIGHT NEAR
THE CREST AS THE REMNANTS FROM AN EASTERLY WAVE LINGER NEAR THE
CREST PER THE 18Z MODELS. THIS MAY KEEP SOME CLOUDS GOING IN THE
SIERRA. WE DO NOT HAVE ANY SHOWERS IN OVERNIGHT BUT A STRAY ONE MAY
SHIFT WEST OF THE CREST OVER TUOLUMNE / ALPINE COUNTIES IN WEAK
EASTERLY 700-500MB FLOW.

THE RIDGE BREAKS DOWN MORE TOMORROW. WE WILL HAVE ISOLATED
THUNDERSTORMS IN ALL OF OUR MOUNTAIN AREAS. WEAK NORTHEAST 700-500MB
FLOW SOUTH OF I-80 MAY LET SOME CLOUDS AND SHOWERS DRIFT INTO THE
FOOTHILLS AS WELL. WITH MORE SYNOPTIC COOLING AND MORE MOISTURE IN
PLACE (MODEL PRECIPITABLE WATER FORECASTS IN THE 0.75 INCH
RANGE)...TOMORROW WILL BE OUR BETTER CHANCE OF CONVECTION AS THE
THERMAL TROF EXPANDS OVER CALIFORNIA ABOVE 925 MB. TEMPERATURES
SHOULD COOL A FEW MORE DEGREES. WE EXPECT HIGHS IN THE LOWER 90S IN
SACRAMENTO AND PERHAPS 100-102 AT REDDING/RED BLUFF. BY THURSDAY A
STRONGER SOUTHWEST FLOW ALOFT DEVELOPS WHICH DRIES AND STABILIZES
THE ATMOSPHERE. A NICE BREEZE SHOULD BLOW THROUGH THE
DELTA...KEEPING TEMPERATURES FROM GETTING TOO HOT IN SACRAMENTO. THE
LATEST GFS IS MORE PROGRESSIVE AGAIN WITH A STRONGER UPPER LOW
APPROACHING CALIFORNIA FROM THE PACIFIC. THE ECMWF IS SLOWER AND
THE UKMET IS FARTHER SOUTH AND WEAKER. THE CURRENT FORECAST IS BASED
ON THE SLOWER PROGRESSION OF THE LOW...WITH A SHORT WAVE RIDGE
OVER THE REGION FRIDAY. THIS TRANSLATES TO SEASONALLY WARM
TEMPERATURES AND DRY WEATHER. WE DID LINGER JUST A SLIGHT CHANCE OF
A THUNDERSTORM NEAR THE CREST IN CASE THE LOW COMES IN EVEN SLOWER.
SG

.EXTENDED DISCUSSION (SATURDAY THROUGH TUESDAY)...
THE BIG QUESTION IN THE EXTENDED FORECAST REMAINS...WHAT PATH WILL
THE CUT-OFF LOW FOLLOW AS IT AFFECTS THE WEST COAST. MODELS START
DIVERGING THE STORM TRACK SATURDAY. BOTH THE GFS AND ECMWF SHOW A
569 LOW EARLY SATURDAY AT 12Z. THE ECMWF HAS THE LOW CENTERED NEAR
38N 130W BUT THE GFS HAS THE CENTER OF THE LOW EAST OF 130W AND
PROGRESSIVELY MOVES INTO THE WEST COAST BY EARLY SUNDAY MORNING. THE
GFS THEN TRACKS THE LOW NORTHEAST AFFECTING CENTRAL AND NORTHERN
CALIFORNIA UNTIL IT MOVES INTO OREGON LATE TUESDAY/EARLY WEDNESDAY.
THE ECMWF SOLUTION HAS THE LOW BRUSHING THE NORCAL COASTLINE ON
SUNDAY THEN RETROGRADING BACK OVER THE OCEAN AND MOVING NORTHWARD AS
HIGH PRESSURE RIDGING RE-DEVELOPS ACROSS CALIFORNIA. WITH EITHER
SOLUTION...TEMPERATURES WILL BE COOLER AND BELOW NORMAL OVER THE
WEEKEND AS THE LOW APPROACHES...DEEPENS THE MARINE LAYER AND
ENHANCES THE DELTA BREEZE. ON MONDAY TEMPERATURES WILL REMAIN BELOW
NORMAL INTO MIDWEEK IF THE GFS PANS OUT OR GRADUALLY WARM IF THE
ECMWF IS CORRECT. HAVE GONE WITH THE MIDDLE GROUND FOR TEMPS IN THE
FORECAST. OTHER VARIABLES THAT MAY NEED TO BE ADJUSTED IN THE
EXTENDED ARE THE POSSIBILITY OF GUSTY RIDGETOP WINDS AND
THUNDERSTORMS/SHOWERS ACROSS HIGHER TERRAIN. HAVE LEFT FORECAST DRY
UNTIL MODEL AGREEMENT IMPROVES AS MOST INSTABILITY LOOKS TO BE NORTH
AND EAST OF OUR REGION.  JBB

&&

.AVIATION...VFR CONDS FOR ALL TAF SITES IN THE VALLEY IN THE NEXT 24
HRS. ONSHORE WINDS STARTING TO PICK UP THIS AFTERNOON AND WILL
CONTINUE THAT TREND INTO TONIGHT. ONSHORE GRADIENT FROM
SFO-SAC IS NOW AT 3.6 MB...OVER 1 MB STRONGER THAN YESTERDAY. WIND
SPEEDS RANGING 8-15 MPH FOR THE TAF SITES...STRONGEST WINDS NEAR THE
DELTA AND AROUND SAC METRO AREA. BUILDING CUMULUS CLOUDS ACROSS
HIGHER TERRAIN INCLUDING THE COASTAL AND NORTHERN MTNS BUT MAINLY
OVER THE SIERRA WHERE THUNDERSTORMS ARE NOW AFFECTING THE CREST NEAR
LAKE TAHOE AND SOUTHWARD. THUNDERSTORMS MAY SPREAD NORTHWARD ALONG
THE CREST...RADAR DETECTS A CELL NEAR THE PLUMAS/SHASTA COUNTY LINE
THAT LOOKS PROMISING.  JBB

&&

.STO WATCHES/WARNINGS/ADVISORIES...NONE.

&&