Severe Thunderstorm Warning 6, NWS Sacramento, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
400 PM PDT THU MAY 20 2004

.DISCUSSION...
UPPER LOW CONTINUES TO GENERATE ACTIVE WEATHER OVER OUR FORECAST
AREA WITH NUMEROUS THUNDERSTORMS OVER OUR MOUNTAINS...MAINLY NORTH
OF I-80. WIND PROFILES ARE NOT FAVORABLE FOR ORGANIZED CONVECTION
UNDER THE UPPER LOW...AND WE/VE SEEN STORMS DISSIPATING AS THEY ROLL
OFF THE TERRAIN INTO THE VALLEY. IN ADDITION...DECENT ONSHORE
SURFACE GRADIENT HAS MANIFESTED ITSELF IN PERSISTENT MODERATE TO
STRONG FLOW THROUGH THE DELTA. MECHANICAL DIVERGENCE OF THE SURFACE
WIND FIELD THROUGH THE SOUTHERN SACRAMENTO/NORTHERN SAN JOAQUIN
VALLEYS IS ALSO NOT SUPPORTIVE OF ORGANIZED CONVECTION...WITH CELLS
MOVING OUT OF LAKE COUNTY RAPIDLY DISSIPATING AS THEY ENCOUNTER THIS
SURFACE WIND FIELD.

WATER VAPOR IMAGERY SHOWS A COUPLE OF WEAK SHORT WAVES ROTATING
AROUND THE UPPER LOW...SO EXPECT CONVECTION TO CONTINUE WELL INTO
THE EVENING. MODELS SHOW VERY LITTLE DYNAMIC SUPPORT OVERNIGHT
HOWEVER...SO WOULD EXPECT THE TYPICAL DIURNAL STABILIZATION OF THE
AIR MASS TO SHUT DOWN MUCH OF THE THUNDER THREAT OVERNIGHT.

UPPER TROUGH BEGINS DRIFTING EAST ON FRIDAY...BUT RELATIVE CLOSE
PROXIMITY AND ASSOCIATED CYCLONIC CONVERGENCE LOOKS STRONG ENOUGH TO
SUPPORT AT LEAST ISOLATED SHOWERS/THUNDER OVER THE SIERRA. MODELS
SHOW A SECONDARY INSTABILITY MAX OVER SHASTA COUNTY...SO LINGERED
SOME LOW POPS THERE AS WELL.

THE WEEKEND LOOKS DRY FOR MOST AREAS WITH NEXT...INSIDE SLIDER TYPE
OF SHORT WAVE DROPPING OUT OF BC INTO THE PACIFIC NORTHWEST. MODELS
HAVE A TOUGH TIME WITH THIS FEATURE TENDING TO KICK THE BULK OF THE
SHORT WAVE DYNAMICS STRAIGHT EAST RATHER THAN DIGGING INTO THE GREAT
BASIN (AS EARLIER MODELS SUGGESTED). AS IT KICKS EAST...A BROAD
TROUGH ELONGATES OVER OUR AREA EVENTUALLY CLOSING OFF AN UPPER LOW
JUST OFF THE COAST. PATTERN LOOKS RATHER FISHY...BUT EVEN THE EURO
MODELS SUPPORT A SIMILAR IDEA. FOR NOW...CONTINUED PREVIOUS FORECAST
IDEA BY MAINTAINING VERY LOW CHANCE POPS OVER OUR HIGHER TERRAIN
THROUGH MUCH OF THE WEEK...WHILE KEEPING MUCH OF THE VALLEY DRY.

JJ
&&

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

&&

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE SACRAMENTO CA
700 AM PDT THU MAY 20 2004

CAZ013>019-063-064-066>069-211400-
SHASTA LAKE AREA/NORTHERN SHASTA COUNTY-
BURNEY BASIN/EASTERN SHASTA COUNTY-NORTHERN SACRAMENTO VALLEY-
CENTRAL SACRAMENTO VALLEY-SOUTHERN SACRAMENTO VALLEY-
CARQUINEZ STRAIT AND DELTA-NORTHERN SAN JOAQUIN VALLEY-
MOUNTAINS SOUTHWESTERN SHASTA TO NORTHERN LAKE COUNTIES-CLEAR
LAKE/SOUTHERN LAKE COUNTY-EAST FOOTHILLS/NORTH SACRAMENTO
VALLEY-MOTHERLODE-WESTERN PLUMAS COUNTY/LASSEN PARK-WEST SLOPE
NORTHERN SIERRA NEVADA-
700 AM PDT THU MAY 20 2004

THIS HAZARDOUS WEATHER OUTLOOK IS FOR INTERIOR NORTHERN CALIFORNIA.

.DAY ONE...TODAY

ISOLATED TO SCATTERED THUNDERSTORMS ARE POSSIBLE OVER MOST OF THE
INTERIOR OF NORTHERN CALIFORNIA TODAY AND THIS EVENING.

.DAYS TWO THROUGH SEVEN...

ISOLATED TO SCATTERED THUNDERSTORMS ARE POSSIBLE OVER THE INTERIOR
MOUNTAINS AND THE NORTHERN SACRAMENTO VALLEY ON FRIDAY. ISOLATED
THUNDERSTORMS ARE POSSIBLE OVER THE COASTAL RANGE MOUNTAINS ON
MONDAY.

THE LATEST WEATHER INFORMATION IS AVAILABLE ON THE INTERNET
AT (LOWER CASE): HTTP://WWW.WRH.NOAA.GOV/SACRAMENTO

JHM