May 23, 2019 – Scattered light showers and isolated thunderstorms over the Sierra today and all interior NorCal mountains Friday as one low pressure system moves east and another drops in to take its place. Scattered showers and thunderstorms most areas over the weekend with below normal daytime temperatures. Afternoon mountain showers continuing through mid week.
Discussion
Mainly dry this morning as NorCal sits under northerly upper flow between high pressure over the eastern Pacific and a low pressure system now centered near southern Nevada. Some cloud cover associated with the low is spilling over mainly the eastern half of the forecast area this morning and some scattered northern Sierra showers/thunderstorms are not out of the question later today. Elsewhere, a little more sun should bring up daytime highs today to close to normal for this time of year.
An upper low, now over western Canada, is forecast to drop into the Pacific Northwest tonight and early Friday. Brief upper ridging over the state between this low and the Great Basin low should bring a short period of clearing most areas. This period of clearing will be brief, however, with afternoon heating, divergent flow aloft and orographics bringing afternoon showers and thunderstorms to the mountains Friday afternoon with some threat of showers to the northern Sacramento valley as well. The low over the Pacific Northwest now looks to be an inside slider type dropping into the Great Basin on Saturday. Therefore, the greatest risk of any shower activity will be over the eastern CWA with enough instability for thunderstorms over the Sierra Cascade range.
Another cold upper low is now forecast to drop out of the Pacific Northwest on Sunday but this system should take a more westerly track. As a result, scattered showers will be a possibility over the entire CWA and north state with a threat of thunderstorms over the Sierra Cascades.
Daytime highs will be taking a drop Saturday and Sunday as these two cold systems drop through the north state especially on Sunday when daytime highs are expected to come in from 10 to 15 degrees below normal. For the most part, snow levels should be above pass levels so mountain travel impacts should be minimal this weekend although snow levels may drop to pass levels at times causing periods of slick roads.
Sunday’s cold upper low is forecast to move east of the state Sunday night but will likely continue to have impacts on the CWA weather going in to the early part of next week.
Extended Discussion (Monday through Thursday)
Models have come into better agreement with progression of upper low into the Desert SW Monday, but then diverge significantly beyond this time. Although ensemble spread increases during the latter part of the extended, general consensus is that long wave troughing remains along 120W. Under broad scale weak synoptic lift, best chances for showers will be terrain induced forcing over the mountains in the afternoons.
Below normal temperatures progged into Monday with high temps returning to near normal Tuesday, then above normal Wednesday and Thursday.