Lingering showers in the Sierra, otherwise a break in the weather today. Precipitation returns to the region tonight and Saturday before another break Sunday. The unsettled pattern continues next week with the potential for more rain and mountain snow.
Discussion
Relative break in the weather across most of interior NorCal early this morning as yesterday’s trough has pushed off to the east. The exception is the continuing scattered light showers over the northern Sierra. The snow level continues to gradually rise as precip has changed over to light rain at KBLU, and radar indicates the level is currently around 6500 ft along I-80.
Today’s break will be brief as the next trough rapidly approaches. The next one will bring another round of significant precip to the region as another relatively weak AR transits, but the duration is expected to be considerably shorter compared to yesterday’s system with most precip forecast to occur tonight into midday Saturday.
QPF of an additional 1 to 2.5 inches will be possible over the mountains, with a quarter to half an inch possible in the Sacramento Valley. The northern San Joaquin Valley should see a tenth or two as the trough swings inland further south.
Snow amounts likely won’t be as impressive as the last system over the northern Sierra (and snow levels will remain around 5-6k ft). Nevertheless, significant travel impacts will likely develop again tonight and persist through much of Saturday, and Winter Weather Advisories have been posted.
We do see some CAPE build in Saturday afternoon mainly over the northern Sierra Foothills and the Sierra north of I-80. Forcing will be rather weak by the afternoon and the thunderstorm threat is looking low but not zero. An isolated cell or two will be possible. The environment on Saturday will be very similar to today and similar threats can be expected with any thunderstorms that end up developing.
Our active and unsettled weather will take a break on Easter Sunday as weak ridging builds in and we will see temperatures warm to near average. Another system is forecast to move in Monday afternoon.
Extended Discussion (Tuesday through Friday)
A cooler than normal wx pattern is forecast over Norcal with periods of unsettled wx. Currently, later in the day on Tue and into Wed morning appears to be the milder/dry wx period before the next AR/TPW plume moves onshore/inland.
The GEFS Probability of IVT/IVT Plume Forecasts indicate another “weak” AR/TPW plume will bring more rain and mountain snow to Norcal late on the 20th or by the 21st.
Deterministic models vary the onset of the precip frontal band moving into NorCal with the ECMWF faster than the GFS. Apparently these models have “flipped their scripts” in handling this wx system with the ECMWF now very progressive in sweeping this AR/Plume across NorCal followed by another short wave trof around the 23rd, while the GFS follows yesterday’s ECMWF in suggesting a prolonged showery pattern as the parent closed low from the NErn Pac drops SEwd into central CA/Srn CA on Fri. Although confidence is low at this time, this scenario would have a greater potential for “strong” CA convection, especially over SOCAL.
Because the Day 3-7 Cluster Prototype page is based on the 12Z Thu Apr 14th runs, day 7 ends at 00z Fri Apr 22nd so that leaves some speculation for the Fri forecast.
All the clusters except #4 follow yesterday’s ECMWF and this evening’s GFS in generally a longer wave trof and deep closed low forming over the Ern Pac continuing the showery, unsettled wx pattern over NorCal for Fri.