Area Forecast Discussion
Issued by The National Weather Service
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FXUS66 KSTO 222329

Area Forecast Discussion
National Weather Service Sacramento CA
329 PM PST Thu Feb 22 2018

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  Vort max that was along the CA/OR coast dropping SSEwd, mainly along the Wrn side of the CWA. Clouds and scattered showers will spread SEwd over the CWA into this evening. Very low WBZs will result in low snow levels dropping into the foothills. AT press time, the snow level had dropped to Applegate around 2 kft. SPC SREF plumes suggest the Srn portion of the CWA will be more unstable, roughly from SMF Swd into the Nrn SJV where isolated storms are possible through late afternoon, than the Nrn portion of the CWA. This is 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. Can`t rule out a possible funnel cloud, but less likely there would be an actual touchdown. Looks as if the HRRR suggests some isolated cells in the extreme Srn Sac, Solano Co, Nrn SJV areas this afternoon. The 12z NAM 3 km keeps the precip in frozen form and basically out of the Valley. Timing of the snow advisory looks good with precip winding down around 06z. 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. Don`t expect normal snow water equivalents with these next few systems in the short term. After the wx system/trof moves through the area tonite, dry Nly winds will increase over the CWA, weakening Fri afternoon. Winds may continue to mix the Nrn and Wrn zones overnite, while decoupling will occur on the E side of the Valley. With clearing skies overnite, it will be another cold nite, with a brisk Nly wind on the W side of the Vly/lee-side of the Coastal Range. A couple more wx systems are forecast to drop out of the GOA in the highly amplified pattern along the W Coast of NOAM in the short term. The next system on Sat, followed by another late Sun into Mon. These systems will be similar, cold with relatively low QPF. The WR US West Coast AR Landfall tool (Probability of IVT) is virtually "bone dry" for the next 16 days. What moisture there is will be spilling over the Pac Ridge and eroding as it hits the Pac NW and spreads inland into our CWA. Even though Norcal will have the continuation of a cold, showery pattern, the QPF will be relatively low, albeit with high snow ratios. Some modest modification in temps is expected later in the weekend and into early next week. JHM
.EXTENDED DISCUSSION  (Monday THROUGH Thursday)   Models show a continuation of the wet pattern as several shortwaves dive southward from the Gulf of Alaska. Compared to this week`s systems, the upcoming storms have the potential to pull in some moisture from the Pacific before spreading precipitation inland. Given the cold nature of these systems, snow levels will remain low. Models are showing better agreement compared to previous forecast runs, though differences remain in terms of timing and precipitation amounts. The first system moves in Monday into Tuesday. Early precipitation estimates suggest 0.10-0.30 inches across the Valley, and around 0.25-0.65 inches over the mountains, though these amounts will likely change as we get closer to the event. A brief break is expected late Tuesday into Wednesday as weak upper ridging builds in. Then, a stronger and wetter storm will move across the area late Wednesday into Friday. Precipitation will initially impact the northern mountains on Wednesday before spreading across the forecast area on Thursday. Stay tuned to see how these systems develop as model uncertainty remains. Temperatures will remain below average as cold air continues to filter in.
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.
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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