Cloudy skies today, atmospheric river arrives late tomorrow

April 4, 2018 – Dry and warm today. Wetter pattern returns beginning Thursday and continues into the weekend.

Discussion

Mostly cloudy skies cover interior NorCal early this morning as a weak short-wave approaches the West Coast. Current temperatures are very mild and range from the lower to mid 30s in the colder mountain valleys to the mid 50s to lower 60s across the Central Valley.

Dry and mild, albeit with mostly cloudy skies, weather will continue across the region today as high pressure gradually flattens. The next short-wave will move across the area tonight and may bring some sprinkles or light precipitation to the northern mountains.

The dry weather will end and light rain will overspread NorCal on Thursday and Thursday night. Then it will turn quite wet on Fri and into the weekend as a highly anomalous TPW plume becomes entrained into the WSWly flow ahead of an approaching Ern Pac trof. The Pacific Jet at the base of the trof will lift NEwd into the Pac NW and will move across Norcal on Sat focusing the TPW plume/AR into our CWA. Loops of the plume suggest a high likelihood of a landfalling AR from SW of HI, thus we can call it a “Pineapple Express”, using our local nomenclature.

It’s still looking like the heaviest precipitation will occur from midday Friday into midday Saturday. Latest QPF forecasts indicate potential for 1.5 to 2.5 inches of rain for the valley with 3 to 6 inches of rain across the foothills and mountains.

Indications are that the snow level will remain above the Sierra passes into the weekend which is typical with this type of system. They may not lower below the passes until Saturday night or Sunday when the precip decreases or tapers off. The high snow levels, the amount of precip and snow melt at the lower mountain elevations could lead to some potential hydro issues which we will monitor during the next few days, i.e. localized flooding, small river and stream flooding, and “muck” (mud/rock) slides.

Extended discussion (Sunday through Wednesday)

Few lingering showers possible early Sunday, otherwise upper ridging and increasing subsidence will result in drier weather into Monday. Next Pacific frontal system begins to spread precipitation over western/northern portions of the CWA Monday night and across most of the forecast area Tuesday and Wednesday. QPF looks less than system later this week. Snow levels Tue/Wed will be around 5000 to 6000 feet in the Shasta mountains and 6000 to 7000 feet over the Trans-Sierra passes.