Area Forecast Discussion
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FXUS66 KSTO 212326

Area Forecast Discussion
National Weather Service Sacramento CA
326 PM PST Tue Nov 21 2017

Mainly dry and mild weather this week with rain chances limited to far northern CA. Cooler, wetter system possible early next week.
.DISCUSSION  Dry weather across the area today. Lingering low clouds have broken up, with some high clouds. Have kept just a slight chance of light rain showers over northwest Shasta County into the early evening. So far, radar is just showing a few weak returns in that area. Low cloud deck and northerly winds limited the development of fog this morning, but expect this will not be the case for late tonight through Wednesday morning, with dense fog developing in the Valley and in mountain valleys. While mild overall today, low clouds have kept down temperatures where they have persisted longer. A few spots may yet reach 70, such as Redding and Vacaville, but many Valley locations will remain in the 60s. Temperatures are tricky again tomorrow, with fog likely developing in the Valley. If this fog lingers for a while, it could keep some spots from reaching the low 70s that are forecast. Temperatures this time of year in the Valley are typically in the low 60s. For the mountains, 50s and 60s are expected, which is relatively mild. A weak weather system will make its way into NorCal for Thanksgiving, bringing the potential for some light rain to the area from I80 northward. Snow levels are highs, so even mountain travel will not be impacted by snow. Highs look mild again, similar to those of Wednesday. Friday will also be mild, and has trended dry as the weak system exits Thursday night/early Friday morning. EK
.EXTENDED DISCUSSION  (Saturday THROUGH Tuesday)   After a dry day on Fri, initial Holiday Weekend trof moves into/across Norcal shunted to the N by the seasonably strong Baja High Center. The frontal wave is forecast to remain offshore, so this will be a relatively warm, weak/glancing-blow system for our CWA. Shower chances will be mainly N of I-80 and the heavier amounts in the mountains (up to around 1/2 inch or so) with high snow levels. A deeper low and associated trough will approach the area Sunday and into Mon with the GFS 6-12 hrs faster than the slower and slightly colder ECMWF. There remains uncertainty with precipitation amounts, primarily due to the timing/speed of this system, with the slower/deeper ECMWF the wetter model. For now, amounts range from .10" south to around .50" north in the Valley with a little over an inch in the Nrn mtns/Sierra. Snow levels are forecast to drop below pass levels Sunday night, but precip could taper off as colder air moves in Mon morning as snow levels drop below 4000-5000 ft. Seasonably mild temperatures expected Sat, then cooling Sun/Mon to below normal mainly N of I-80 and near or slightly above normal south of I-80. Drier conditions Mon/Tue as high pressure rebuilds over Norcal leading to a warming trend on Tue. Nly winds should keep fog at bay Tue morning and local adiabatic warming effects from the katabatic winds. JHM

Areas of MVFR/IFR will be possible late tonite and again Wed morning due to patchy fog and low cigs. Conditions improve to VFR during the day. Winds remain under 10 knots.

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