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000
FXUS66 KSTO 201605
AFDSTO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
905 AM PDT MON MAY 20 2013

.SHORT TERM DISCUSSION...

UPPER RIDGE HAS BUILT OVER NORCAL BETWEEN PLAINS TROUGH AND
WESTERN CANADIAN TROUGH. THIS RIDGE WILL NOT REMAIN OVER THE AREA
FOR VERY LONG...AS UPPER LOW DROPS SOUTH AND DEEPENS OVER
WASHINGTON AND OREGON TOMORROW. WE WILL HAVE ONE LAST DAY OF MUCH
ABOVE NORMAL TEMPERATURES TODAY WITH READINGS GENERALLY IN THE
LOW 90`S FOR VALLEYS. THESE VALUES ARE ABOUT 10 DEGREES ABOVE
NORMAL. WINDS CONTINUE TO DIMINISH THIS MORNING AS PRESSURE
GRADIENT WEAKENS AND SPEEDS WILL GENERALLY BE LESS THAN 15 MPH FOR
THE REST OF THE AFTERNOON. CURRENT FORECAST IS ON TRACK AND NO
MORNING UPDATE WILL BE NEEDED.

PREVIOUS DISCUSSION...

MILDER TEMPERATURES ACROSS THE INTERIOR OF NORCAL EARLY THIS
MORNING AS UPPER RIDGE APPROACHES AND NORTHERLY WINDS CONTINUE IN
THE CENTRAL VALLEY WITH EASTERLY WINDS IN THE NORTHERN SIERRA
NEVADA. MOST AREAS ARE RUNNING AROUND 3 TO 8 DEGREES WARMER
COMPARED TO 24 HOURS AGO...BUT TRAVIS AFB AND VACAVILLE ALONG THE
WEST SIDE OF THE VALLEY ARE ABOUT 20 DEGREES WARMER THAN EARLY
SUNDAY AS BREEZY NORTH WINDS AS BREEZY NORTH WINDS CONTRIBUTE TO
ADIABATIC WARMING AS FLOW DESCENDS FROM THE COASTAL HILLS.

UPPER RIDGE SHIFTS OVER NORCAL TODAY WHICH WILL RESULT IN THE
WARMEST TEMPERATURES OF THE WEEK AS NORTH AND EAST WINDS DROP OFF
AND THE OVERALL AIRMASS CONTINUES TO WARM. HIGHS IN THE VALLEY ARE
EXPECTED TO CLIMB INTO THE LOWER 90S WHILE THE MOUNTAINS WILL BE
MAINLY IN THE UPPER 60S AND 70S.

A MAJOR CHANGE TOWARD COOLER AND BREEZIER WEATHER IS FORECAST TO
BEGIN TUESDAY AS THE UPPER LOW PRESENTLY CENTERED OVER THE
SOUTHERN YUKON TERRITORY DROPS SOUTHEASTWARD INTO THE PACNW AND
CLOSES OFF. THE COLD UPPER LOW IS FORECAST TO BE JUST OFFSHORE THE
MOUTH OF THE COLUMBIA RIVER BY LATER TUESDAY...THEN ONLY SHIFTING
SLIGHTLY EAST BY WEDNESDAY. APPEARS AT THIS TIME THAT THIS SYSTEM
WILL BE RELATIVELY DRY FOR NORCAL WITH SHOWER CHANCES MAINLY
LIMITED TO THE MOUNTAINS NORTH OF REDDING WHILE THE MAIN IMPACTS
WILL BE MUCH COOLER TEMPERATURES AND BREEZY CONDITIONS FOR MID-
WEEK.

BY NEXT WEEKEND...THE CLOSED LOW WEAKENS INTO AN OPEN WAVE ALONG
THE WEST COAST. ADDITIONAL WEAK SHORTWAVES LOOK TO MOVE ONSHORE TO
NORCAL BRINGING THE POSSIBILITY OF ISOLATED SHOWERS TO THE
MOUNTAINS AROUND THE NORTHERN SAC VALLEY. TEMPERATURES ARE
FORECAST TO WARM SOMEWHAT...BUT WILL REMAIN A LITTLE BELOW NORMAL.

&&

.EXTENDED DISCUSSION (FRIDAY THROUGH MONDAY)

CLOSED UPPER LOW OVER THE PACIFIC NW WILL IMPACT OUR AREA FROM
MIDWEEK INTO THE WEEKEND AS SECONDARY VORTMAXES ROTATE AROUND THE
PARENT LOW AND BRUSH THE NORTHERN REACHES OF OUR FORECAST AREA.
ALTHOUGH MODELS ARE HIGHLY CONFIDENT IN PLACEMENT OF PRIMARY UPPER
LOW...CONFIDENCE IS MUCH LESS IN INDIVIDUAL VORTMAXES PIVOTING
AROUND IT. THE ECMWF CONTINUES TO INDICATE ONE STRONG VORTMAX
SWINGING THROUGH OUR AREA THURSDAY AFTERNOON...WITH ENOUGH
DYNAMICS TO GENERATE A FEW SHOWERS OVER THE SIERRA FROM AROUND
TAHOE NORTHWARD. THE GFS MEANWHILE IS MUCH WEAKER WITH THIS
FEATURE...AND LIMITS ANY CHANCES OF SHOWERS MOUNTAINS FROM AROUND
SHASTA COUNTY NORTHWARD. HAVE EXTENDED A SLIGHT CHANCE OF RAIN
SHOWERS THURS NIGHT FARTHER SOUTH ALONG THE SIERRA SOUTH TO AROUND
BLUE CANYON SINCE THE ECMWF HAS BEEN MORE CONSISTENT. EITHER
WAY...THE MAIN MESSAGE IS STILL COOLER TEMPERATURES THROUGH THE
END OF THE WORKWEEK.

BY NEXT WEEKEND...THE CLOSED LOW WEAKENS INTO AN OPEN WAVE ALONG
THE WEST COAST. ADDITIONAL WEAK SHORTWAVES LOOK TO MOVE ONSHORE TO
NORCAL...BRINGING THE POSSIBILITY OF ISOLATED SHOWERS TO THE
MOUNTAINS AROUND THE NORTHERN SAC VALLEY. TEMPERATURES SHOULD
MODERATE TO AROUND NORMAL...WITH POSSIBLE DELTA BREEZE
MODIFICATION IN THE SAC METRO AREA GIVEN ONSHORE GRADIENT. -DVC

&&

.AVIATION...

VFR CONDITIONS CONTINUE. NORTH WINDS PERSIST THIS MORNING FOR THE
SACRAMENTO VALLEY BUT NOT AS STRONG AS YESTERDAY. GENERALLY AROUND
10 KTS WITH OCCASIONAL GUSTS UP TO 15 KTS FOR KRDD, KRBL, KSMF,
KSAC, KMHR. WIND DIRECTION WILL SWITCH THIS EVENING (BETWEEN 00-03Z) TO
SOUTHERLY FLOW RANGING 5-10 KTS FOR THOSE TAF SITES. FOR THE
NORTHERN SAN JOAQUIN VALLEY: NORTHWEST TO WEST WINDS AT 6-12 KTS.

OVER MOUNTAIN RIDGETOPS...LIGHT TO MODERATE NORTH WINDS WILL
TRANSITION TO WESTERLY TONIGHT. JBB


&&

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

$$