NATIONAL WEATHER SERVICE FORECAST DISCUSSION
Area Forecast Discussion
Issued by The National Weather Service
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000
FXUS66 KSTO 221647
AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
850 AM PST Thu Feb 22 2018

.SYNOPSIS  
A cold system will bring low elevation snow to the mountains into early Friday, which could impact travelers. A few showers are possible in the Valley. Below normal temperatures will continue. Additional systems likely next week.
.DISCUSSION  Update: Vort max along the coast near the CA/OR border forecast to drop SSEwd, mainly along the Wrn side of the CWA this afternoon. Clouds and scattered showers will spread SEwd over the CWA with very low WBZs. SPC SREF plumes suggest the Srn portion of the CWA will be more unstable, roughly from SMF Swd into the Nrn SJV, than the Nrn portion of the CWA due to the timing of the vort max dropping Swd this afternoon. Effective Shear indicates the potential for some mid level rotation/rotating storms. The limiting factor is the elevated instability forecast in the Bufkit forecast soundings and not surface based convection. Central Valley rotating storms are rooted in the boundary layer with low LCLs. Dewpoints in the 30s makes this more problematic, thus a funnel cloud is possible, but less likely for an actual touchdown. Looks as if the HRRR suggests some isolated cells in the Nrn SJV this afternoon while the NAM 3 km keeps the precip in frozen form and basically out of the Valley. Precip amounts will be light with this system due to a dry air mass with PWs ranging from .13 at REV to .34 at MFR and .44 at OAK. These values are below average. AMSU data also indicate very low PW over the region. However, good dynamics and cold temps will result in some brief periods of heavy showers with high snow ratios. For example for BLU, using the max temp in the profile, a snow ratio of 16:1 to 18:1 is calculated for today. The average SLR is 9:1. So although the snow will look "good", the snow water equivalent won`t look so "good". JHM .Previous Discussion  
Still awaiting the first signs of precipitation on the radar as of 1 am. System has slowed but snow expected to begin over the northern Sierra within the next few hours, reaching the I-80 and Highway 50 corridors between 3 and 5 am. Snow levels will generally be in the 1500-2000 foot range. Snow will continue through the morning hours, impacting the AM commute and mountain travelers. Another burst of snow is likely this afternoon and evening during the PM commute, with lowering snow levels. Still looks like this secondary wave of snow could bring a dusting down to the lower foothills. Total accumulations remain generally intact, with 3 to 6 inches above 2000 feet, locally up to 10 inches near the Sierra crest. Winter Storm Advisory remains in effect through tonight. Travelers should check Caltrans road conditions before they go, carry chains, and prepare for delays. Visibilities may rapidly change over short distances especially in bursts of heavier snow. Meanwhile in the valley, latest hi-res guidance is indicating the possibility of some pop up showers this afternoon and evening as trough swings through NorCal. Hard to pinpoint where activity might be as most areas will likely remain dry. Of note, NAM showing CAPE values of 100-200 J/kg so also cannot rule out a thunderstorm or two. These may drop some small hail with colder air overhead. Breezy winds possible as system moves through. System pushes east tonight with snow ending between 10pm and midnight. Brief period of dry weather is expected Friday as shortwave ridging moves in. Depending on how fast cloud cover clears out, could be fairly chilly Friday morning with valley temperatures dropping below freezing again. Next in a series of shortwaves will drop south in NW flow Saturday. This will bring another chance for mountain snow, mainly north of Highway 50 corridor. Moisture looks fairly starved with this wave so only expecting light accumulations, but may be enough for some minor travel impacts. Dry weather again Sunday with additional systems likely next week. Temperatures will remain below normal. CEO
.EXTENDED DISCUSSION  (Monday THROUGH Thursday)  Active pattern is expected through the extended period as successive disturbances embedded in the upper flow drop down from the north. Guidance is still struggling with the progression of these systems and precipitation amounts. Regardless, these systems have the potential to bring precipitation, mountain snow, and breezy winds. Given the cold nature of these systems, snow levels will remain low.
.AVIATION 
VFR conditions expected next 24 hours except local MVFR vicinity scattered showers and thunderstorms 22Z-02Z and widespread IFR over the northern Sierra with snow showers thru about 03Z Friday.
.STO   WATCHES/WARNINGS/ADVISORIES    
Winter Weather Advisory until 10 PM PST this evening for Burney Basin / Eastern Shasta County-Motherlode-Northeast Foothills/Sacramento Valley-West Slope Northern Sierra Nevada- Western Plumas County/Lassen Park.
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