Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sacramento California
325 PM PDT Mon Apr 20 2015
threat of afternoon and evening showers and thunderstorms...
mainly over the mountains...will continue through the week with
gradual cooling trend.
Omega block in place along the West Coast with strong upper ridging
over norcal and trapped low off the socal coast. Instability over
the mountains this afternoon has resulted in scattered showers and
thunderstorms with activity likely to subside after sunset. Delta
breeze will continue moderate to strong tonight with marine layer
currently around 2500 feet. Coastal stratus expected to push back
into the Delta later tonight and enough marine air may advect
into the southern SAC valley for some patchy stratus around the
Sacramento area Tuesday morning.
Omega block progresses next 24 hours as elongating upper
troughing in the epac approaches the West Coast. Synoptic cooling
will increase Tuesday along with onshore flow resulting in more
substantial drop in Max temps across interior norcal. Instability
again depicted over the mountains for showers and thunderstorms
tomorrow afternoon. Best instability progged by models over the
mountains from I-80 northward, and also suggest slight chance of
deep moist convection drifting into portions of the northern/central
Sacramento Valley, given proximity to higher terrain.
Upper trough stretches as it reaches the West Coast with southern
portion forming closed low. Models dig this low south of the
California coast Wed as minor upstream ridging begins to nose
inland over S Oregon. Limited instability still depicted over
portions of the higher terrain Wed, but less extensive than today
or Tuesday. Max temps continue to trend downward slightly Wednesday.
Closed low forecast to track into the desert SW Thursday as
another elongating upper trough approaches the West Coast.
Afternoon instability looks even more limited Thursday with slight
chance of afternoon development confined to the higher terrain of
the eastern mountains of our County Warning Area. High temperatures Thursday
expected to be similar to Wednesday.
Extended discussion (friday through monday)
A shortwave trough will have moved into Nevada by Friday with a
Flat Ridge overhead. However, enough instability and orographic
flow will remain to generate some showers mainly over the
Sierra/Cascades Friday/Saturday. The Flat Ridge will strengthen a
bit through the weekend, bringing warmer and drier weather.
Previous medium range model runs had hinted at a stronger trough
moving through northern California early next week. However,
latest runs have kept the bulk of moisture primarily to our north.
We've trended down our precipitation forecast chance accordingly.
Mainly VFR conditions for the next 24 hours. Isolated
thunderstorms will continue over the Sierra and coastal range
through 03z. MVFR stratus may reach Sacramento taf sites between
10-16z. Across the valley...south winds will increase to 7-15 kt
this afternoon and tonight. Near the Delta, southwest winds 15-25
kt will continue.