March 21, 2018 – Wet weather continues into the weekend. Snow will remain above pass levels through most of tonight, then will lower with significant accumulations possible at the higher elevations.
Initial wave moved through late Tuesday and earlier in the night with most of the region receiving between 1/4 and 3/4’s of an inch of rain. Areas of mostly light to moderate precipitation continue in the warm air advection environment. Snow level radars at Chico and Colfax indicate the snow level remains above 8000 feet. Current temperatures are very mild ranging from the upper 30s and 40s in the mountains to the upper 40s to mid 50s across the Central Valley.
Off and on light precipitation will continue across the valley today, though there will be conditional threat for some thunderstorms by mid-afternoon that will be accompanied by heavy rain and gusty winds. Forecast soundings indicate a lower-level cap which may be overcome with enough surface heating (if there are any breaks long enough to allow for it), but sufficient elevated CAPE (250-500 j/kg).
Precipitation expected to increase across the region tonight into early Thursday as the surface front approaches and moisture transport from the south increases. Much of the valley expected to see around an inch of rain with 2-4 inches over the mountains where orographic enhancement will increase. Have decided to let the Winter Storm Watch continue for now as it’s looking more likely that snow levels will remain high into Thursday. Winds will also increase across the area.
Strong cold front will sweep southeast around midday with snow levels lowering rapidly in its wake during the afternoon. As mentioned above, confidence in the exact timing of frontal passage and lowering snow levels is not high at this time and some changes to details are likely. Travel is likely to deteriorate rapidly later in the day as this change over occurs. Stay tuned to the latest forecasts updates.
Thunderstorms possible again Thursday in the dry slot behind the front. Brief heavy rain and small hail likely the primary concerns. Over the northern Sierra, a relatively quick shot of heavy snow will be possible into the evening before precipitation tapers.
A brief break in the wet weather looks possible late Thursday night and Friday morning before a colder system brings potential heavier snow to the mountains Friday into Saturday.
Extended discussion (Sunday through Wednesday)
Main upper trough shifts east of the state during the day on Sunday bringing the last of the precipitation for a while as upper ridging builds over the eastern Pacific. Precipitation chances will mainly be restricted to east of the central valley. Precipitation amounts will be quite light but snow levels will be abnormally low at around 1500 to 2500 feet. A few lingering showers may still be possible Sunday night but building high pressure will bring an end to any precipitation threat by Monday. Temperatures warm by several degrees on Monday but are still forecast to run slightly below normal. Upper level and surface gradients indicate breezy north winds which will help to start drying things out. Temperatures continue to warm on Tuesday as upper ridge pushes inland with highs expected to warm to several degrees above normal. Even more warming is expected on Wednesday with highs throughout the valley expected to reach the mid 70s in the northern Sacramento valley. Breezy north winds will continue through the period between an upper low over the southwest U.S and building high pressure over the eastern Pacific.