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FXUS66 KSTO [Sacramento/WFO CA] 250520 AFDSTO 

FXUS66 KSTO 250520
AFDSTO [HEAD]

National Weather Service Sacramento California 10:20pm PDT Fri Oct 24 2014 

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SACRAMENTO CA
1020 PM PDT FRI OCT 24 2014 [HEAD]

Synopsis. Wet and windy weather moves in later tonight into Saturday as an upper level trough and associated front move into the area. Thunderstorms with heavy rain, accumulating small hail and gusty winds and maybe even a weak tornado are possible on Saturday. Snow levels will lower below Sierra passes Saturday night. Decreasing winds are expected Saturday evening and precipitation will taper off Sunday. A brief break Monday will be followed by a chance of rain over areas north of Sacramento Tuesday and Wednesday. This precipitation threat will drop southward bringing rain or mountain snow over much of Northern California for the end of the next week. 

.SYNOPSIS...
WET AND WINDY WEATHER MOVES IN LATER TONIGHT INTO SATURDAY AS AN
UPPER LEVEL TROUGH AND ASSOCIATED FRONT MOVE INTO THE AREA.
THUNDERSTORMS WITH HEAVY RAIN, ACCUMULATING SMALL HAIL AND GUSTY
WINDS AND MAYBE EVEN A WEAK TORNADO ARE POSSIBLE ON SATURDAY. SNOW
LEVELS WILL LOWER BELOW SIERRA PASSES SATURDAY NIGHT. DECREASING
WINDS ARE EXPECTED SATURDAY EVENING AND PRECIPITATION WILL TAPER OFF
SUNDAY. A BRIEF BREAK MONDAY WILL BE FOLLOWED BY A CHANCE OF RAIN
OVER AREAS NORTH OF SACRAMENTO TUESDAY AND WEDNESDAY. THIS
PRECIPITATION THREAT WILL DROP SOUTHWARD BRINGING RAIN OR MOUNTAIN
SNOW OVER MUCH OF NORCAL FOR THE END OF THE NEXT WEEK. [DISC]

Discussion.  

.DISCUSSION... [DISC]

Occluded front looking very ominous offshore with definite/sharp backedge of front and area of enhanced clouds indicating band of convective precipitation along the front which is along 127°W [390 mi west of 18 Miles E Groveland]. WAA [Warm Air Advection - the advection (movement) of warm air into a region] clouds precede the front and will be overspreading interior Northern California overnite… but precipitation associated with the front not forecast to move into the coastal range until around 12Z [5am PDT] (5 am). Frontal precipitation moves into the Sierra Nevada late morning and afternoon with break in the valley until upper air dynamics and colder/unstable move into the area later in the afternoon and evening. 

OCCLUDED FRONT LOOKING VERY OMINOUS OFFSHORE WITH DEFINITE/SHARP
BACKEDGE OF FRONT AND AREA OF ENHANCED CLOUDS INDICATING BAND OF
CONVECTIVE PRECIP ALONG THE FRONT WHICH IS ALONG 127W. WAA CLOUDS
PRECEDE THE FRONT AND WILL BE OVERSPREADING INTERIOR NORCAL
OVERNITE...BUT PRECIP ASSOCIATED WITH THE FRONT NOT FORECAST TO MOVE
INTO THE COASTAL RANGE UNTIL AROUND 12Z (5 AM). FRONTAL PRECIP MOVES
INTO THE SIERNEV LATE MORNING AND AFTERNOON WITH BREAK IN THE VALLEY
UNTIL UPPER AIR DYNAMICS AND COLDER/UNSTABLE MOVE INTO THE AREA
LATER IN THE AFTERNOON AND EVENING. [DISC]

925 mbs winds indicate 40–45 kts [46–52 mph] southerly winds developing in the valley by and after 12Z [5am PDT] with MFR [Medford OR]-SAC [Sacramento CA] gradient at least topping 12 mbs. This is sufficient for Wind Advisory and AWW [Airport Weather Warning] for KRDD [Redding CA] as southerly gusts top out at 40+ MPH as front moves across the area and for a few hours behind the front. Strongly sheared environment expected to lead to isolated to scattered Cbs [cumulonimbus; towering cumulus] late morning and afternoon. 

925 MBS WINDS INDICATE 40-45 KTS SLY WINDS DEVELOPING IN THE VALLEY
BY AND AFTER 12Z WITH MFR-SAC GRADIENT AT LEAST TOPPING 12 MBS. THIS
IS SUFFICIENT FOR WIND ADVSRY AND AWW FOR KRDD AS SLY GUSTS TOP OUT
AT 40+ MPH AS FRONT MOVES ACROSS THE AREA AND FOR A FEW HOURS BEHIND
THE FRONT. STRONGLY SHEARED ENVIRONMENT EXPECTED TO LEAD TO ISOLATED
TO SCATTERED CBS LATE MORNING AND AFTERNOON. [DISC]

Locally heavy showers expected with deep convection as PW [precipitable water] over +2 standard deviations Saturday morning and at least one standard deviation behind the front Saturday afternoon. QPFs [Quantitative Precipitation Forecast] will vary widely due to the convection but the Shasta/Sierra Nevada Mountains with strong upslope flow should have significant QPF [Quantitative Precipitation Forecast]. (JHM [Lahaina/West Maui HI]) 

LOCALLY HEAVY SHOWERS EXPECTED WITH DEEP CONVECTION AS PW OVER +2
STANDARD DEVIATIONS SAT MORNING AND AT LEAST ONE STANDARD DEVIATION
BEHIND THE FRONT SAT AFTERNOON. QPFS WILL VARY WIDELY DUE TO THE
CONVECTION BUT THE SHASTA/SIERNEV MTNS WITH STRONG UPSLOPE FLOW
SHOULD HAVE SIGNIFICANT QPF.    JHM [DISC]

Previous discussion…  

.PREVIOUS DISCUSSION... [DISC]

Upper level trough offshore of the West Coast will move into Northern California tonight into Saturday bringing a change to wet and windy weather. Increasing clouds and precipitation moving into the coastal range and Shasta County around midnight and spread southeast across interior Northern California Saturday. Southerly winds will increase late tonight into Saturday as the cold front and trough move inland. This will bring breezy and gust winds with southerly winds 15–30 mph with gusts to around 40 mph in the valley… strongest in the northern Sacramento Valley and up to 70 mph over the ridges. Holiday decorations should be secured with the gusty winds and high profile vehicles should use caution. Winds should be decreasing Saturday evening. Shasta County Mountains could see up to 2 inches of precipitation by Sunday morning with good orographics with the southerly winds. Snow levels will be around 8000 ft Saturday morning and drop down to around 5000–6000 ft on Sunday morning. Snow accumulations up to 10 inches are possible on Mt Lassen and up to 4 inches possible over the higher Sierra passes. Motorists should be prepared for possible winter driving conditions in the mountains this weekend. Good instability and shear on Saturday will bring a threat of thunderstorms and possibly weak tornadoes. Precipitation will taper off on Sunday with showers mainly lingering over the Shasta County and down the Sierra. High temperatures this weekend will be in the mid 60s to low 70s in the valley and mid 30s to low 50s in the mountains. Monday will be a drier and warmer day with ridge building into the area as trough shifts east. 

UPPER LEVEL TROUGH OFFSHORE OF THE WEST COAST WILL MOVE INTO
NORTHERN CALIFORNIA TONIGHT INTO SATURDAY BRINGING A CHANGE TO WET
AND WINDY WEATHER. INCREASING CLOUDS AND PRECIPITATION MOVING INTO
THE COASTAL RANGE AND SHASTA COUNTY AROUND MIDNIGHT AND SPREAD
SOUTHEAST ACROSS INTERIOR NORTHERN CALIFORNIA SATURDAY. SOUTHERLY
WINDS WILL INCREASE LATE TONIGHT INTO SATURDAY AS THE COLD FRONT AND
TROUGH MOVE INLAND. THIS WILL BRING BREEZY AND GUST WINDS WITH
SOUTHERLY WINDS 15 TO 30 MPH WITH GUSTS TO AROUND 40 MPH IN THE
VALLEY...STRONGEST IN THE NRN SACRAMENTO VALLEY AND UP TO 70 MPH
OVER THE RIDGES. HOLIDAY DECORATIONS SHOULD BE SECURED WITH THE GUSTY
WINDS AND HIGH PROFILE VEHICLES SHOULD USE CAUTION. WINDS SHOULD BE
DECREASING SATURDAY EVENING. SHASTA COUNTY MOUNTAINS COULD SEE UP TO
2 INCHES OF PRECIPITATION BY SUNDAY MORNING WITH GOOD OROGRAPHICS
WITH THE SOUTHERLY WINDS. SNOW LEVELS WILL BE AROUND 8000 FT
SATURDAY MORNING AND DROP DOWN TO AROUND 5000 TO 6000 FT ON SUNDAY
MORNING. SNOW ACCUMULATIONS UP TO 10 INCHES ARE POSSIBLE ON MT
LASSEN AND UP TO 4 INCHES POSSIBLE OVER THE HIGHER SIERRA PASSES.
MOTORISTS SHOULD BE PREPARED FOR POSSIBLE WINTER DRIVING CONDITIONS
IN THE MOUNTAINS THIS WEEKEND. GOOD INSTABILITY AND SHEAR ON
SATURDAY WILL BRING A THREAT OF THUNDERSTORMS AND POSSIBLY WEAK
TORNADOES. PRECIPITATION WILL TAPER OFF ON SUNDAY WITH SHOWERS
MAINLY LINGERING OVER THE SHASTA COUNTY AND DOWN THE SIERRA. HIGH
TEMPERATURES THIS WEEKEND WILL BE IN THE MID 60S TO LOW 70S IN THE
VALLEY AND MID 30S TO LOW 50S IN THE MOUNTAINS. MONDAY WILL BE A
DRIER AND WARMER DAY WITH RIDGE BUILDING INTO THE AREA AS TROUGH
SHIFTS EAST. [DISC]

Extended discussion (Tuesday through Friday) 

.EXTENDED DISCUSSION (TUESDAY THROUGH FRIDAY) [DISC]

Flat high pressure over the southwest US will keep at least the southern half of the forecast area dry Tuesday. The northern portions of the forecast area will see a continued threat of light rain. Daytime high temperatures under mainly cloudy skies are expected to come in a few to several degrees below normal. Another cool low pressure system dropping out of the Gulf of Alaska will bring precipitation chances southward Wednesday through Friday. Significant model differences exist in the Wednesday–Friday time frame. The European model is much deeper and brings deeper moisture into Northern California versus the drier GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.]. Have opted for a blend of solutions but forecast confidence not high during this time frame. 

FLAT HIGH PRESSURE OVER THE SOUTHWEST U.S WILL KEEP AT LEAST THE
SOUTHERN HALF OF THE FORECAST AREA DRY TUESDAY. THE NORTHERN
PORTIONS OF THE FORECAST AREA WILL SEE A CONTINUED THREAT OF LIGHT
RAIN. DAYTIME HIGH TEMPERATURES UNDER MAINLY CLOUDY SKIES ARE
EXPECTED TO COME IN A FEW TO SEVERAL DEGREES BELOW NORMAL. ANOTHER
COOL LOW PRESSURE SYSTEM DROPPING OUT OF THE GULF OF ALASKA WILL
BRING PRECIPITATION CHANCES SOUTHWARD WEDNESDAY THROUGH FRIDAY.
SIGNIFICANT MODEL DIFFERENCES EXIST IN THE WEDNESDAY-FRIDAY TIME
FRAME. THE EUROPEAN MODEL IS MUCH DEEPER AND BRINGS DEEPER
MOISTURE INTO NORCAL VERSUS THE DRIER GFS. HAVE OPTED FOR A BLEND
OF SOLUTIONS BUT FORECAST CONFIDENCE NOT HIGH DURING THIS TIME
FRAME. [DISC]

Aviation.  

.AVIATION... [AVIA]

Pacific cold front moves across the area on Saturday bringing chances of rain and increasing winds. Wind gusts to 45 mph will be possible after 12Z [5am PDT] especially over the northern valley. 

PACIFIC COLD FRONT MOVES ACROSS THE AREA ON SAT BRINGING CHANCES OF
RAIN AND INCREASING WINDS. WIND GUSTS TO 45 MPH WILL BE POSSIBLE
AFTER 12Z ESPECIALLY OVER THE NORTHERN VALLEY. [AVI2]

STO [Sacramento/WFO CA] Watches/Warnings/Advisories

  • Wind Advisory from 5am to 8pm PDT Saturday central Sacramento Valley…northern Sacramento Valley…southern Sacramento Valley. 
.STO WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY FROM 5 AM TO 8 PM PDT SATURDAY CENTRAL SACRAMENTO
VALLEY...NORTHERN SACRAMENTO VALLEY...SOUTHERN SACRAMENTO VALLEY. [WARN]

Point Forecast

18 Miles E Groveland CA

9:40am PDT Oct 24 2014

Overnight. Partly cloudy, with a low around 50°. SSE wind 8 to 10 mph.


Saturday. A 30% chance of showers, mainly after 11am. Partly sunny, with a high near 61°. SSW wind 11 to 21 mph, with gusts as high as 31 mph. New precipitation amounts of less than a tenth of an inch possible.

Saturday Night. A 30% chance of showers. Mostly cloudy, with a low around 45°. South wind around 9 mph. New precipitation amounts of less than a tenth of an inch possible.


Sunday. Mostly sunny, with a high near 58°. SSW wind around 7 mph becoming west in the morning.

Sunday Night. Mostly clear, with a low around 42°. West wind 7 to 9 mph becoming ENE in the evening.


Monday. Sunny, with a high near 59°.

Monday Night. Mostly clear, with a low around 45°.


Tuesday. Mostly sunny, with a high near 63°.

Tuesday Night. Partly cloudy, with a low around 47°.


Wednesday. Mostly sunny, with a high near 66°.

Wednesday Night. Partly cloudy, with a low around 49°.


Thursday. Mostly sunny, with a high near 66°.

Thursday Night. A slight chance of rain. Partly cloudy, with a low around 49°.


Friday. A slight chance of showers. Mostly sunny, with a high near 65°.

  • Overnight Partly cloudy, with a low around 50. South southeast wind 8 to 10 mph.
  • Saturday A 30 percent chance of showers, mainly after 11am. Partly sunny, with a high near 61. South southwest wind 11 to 21 mph, with gusts as high as 31 mph. New precipitation amounts of less than a tenth of an inch possible.
  • Saturday Night A 30 percent chance of showers. Mostly cloudy, with a low around 45. South wind around 9 mph. New precipitation amounts of less than a tenth of an inch possible.
  • Sunday Mostly sunny, with a high near 58. South southwest wind around 7 mph becoming west in the morning.
  • Sunday Night Mostly clear, with a low around 42. West wind 7 to 9 mph becoming east northeast in the evening.
  • Monday Sunny, with a high near 59.
  • Monday Night Mostly clear, with a low around 45.
  • Tuesday Mostly sunny, with a high near 63.
  • Tuesday Night Partly cloudy, with a low around 47.
  • Wednesday Mostly sunny, with a high near 66.
  • Wednesday Night Partly cloudy, with a low around 49.
  • Thursday Mostly sunny, with a high near 66.
  • Thursday Night A slight chance of rain. Partly cloudy, with a low around 49.
  • Friday A slight chance of showers. Mostly sunny, with a high near 65.

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