Possible mountain travel delays midday through Saturday, wet pattern next week

March 22, 2019 – Another round of rain and snow Friday through Saturday with enough mountain snow to impact weekend travel. Dry weather Sunday then more rain and mountain snow early next week.

Discussion

Cloudiness over the Northern Sierra associated with the upper low vicinity of KLAS will erode/dissipate overnight as the cyclonic flow around the upper low weakens. A plethora of CU/SC cloudiness and a few light showers over the west slope Sierra will also dissipate after sunset as diurnal/differential heating ceases as well. Subsidence from the Ern Pac ridge axis moving inland will also bring clearing to the west slope Sierra overnight, or continue the clear conditions elsewhere.

But, by Fri afternoon and evening precip will once again spread over interior NorCal as Mar wx continues to roar like the proverbial lion. A couple of the higher resolution QPF/REF progs show the leading edge of the precip reaching the coast by 5 am Fri, (with some light showers ahead of it over Shasta Co) moving towards I-5 by 10 am Fri, and into the Sierra by early afternoon. The timing of the WSW looks “about right”, and will continue messaging some travel impacts developing by afternoon and into Sat. Snow levels forecast to be around 5000 ft over the Sierra to 4000 ft over Shasta Co Fri afternoon, and 4000-4500 ft Sat.

By late Sat afternoon, the main frontal precip band will be near the CA/NV border and cross mountain flow from the Coastal Range should bring some subsidence warming/drying and eroding of the cloud cover on the west side of the Vly. BUFKIT forecast soundings suggest differential heating could lead to surface-based convection Sat afternoon mainly from the Sac area Nwd towards RDD as the upper trof axis moves over the area. The higher probability for storms appears to be with the colder mid-tropospheric (5H) temps over the Nrn portion of the CWA, (i.e. CIC, RBL, RDD areas) where deeper instability is forecast than at locations farther south in our CWA.

The upper trof is forecast to move into NV Sat evening and precip will end or wind-down overnight in our CWA. Upper-level short-wave ridging moving in behind the trof on Sun will result in a dry day with warmer daytime highs than on Sat. Sun will be a good day to travel, but the dry wx will be brief, as the next wx system is forecast to spread another round of precip over interior NorCal beginning Sun night. And, the wet wx could persist through the middle of next week. It seems as if Mar wx will continue like the “proverbial lion” over NorCal until the end of the month.

Extended Discussion (Monday through Thursday)

Models in good agreement with Pacific frontal system moving through interior NorCal Monday bringing widespread precipitation. Precip turns showery Tuesday, mainly over the mountains.

A stronger Pacific storm follows with more widespread rain and mountain snow beginning Tuesday night and continuing into Thursday. Total QPF for Monday into Thursday attm looks to be around 1 to 2 inches for the Central Valley and 1 to 4 inches for the foothills and mountains. Mountain travel impacts due to periods of heavy snow and strong wind are likely next week.