Yesterday was mostly cloudy in the San Francisco. The night remained a little warmer than I had expected, due to the cloud cover and its ability to limit radiative cooling.
The National Weather Service (NWS) in San Francisco forecasts a mostly sunny day today, with a high temperature of 68 F. The winds will be light from the west-northwest, increasing to 5-9 mph by this afternoon. This evening will be partly cloudy, with a low temperature of 59 F. Winds will be from the west at 5-9 mph, becoming light and variable after midnight.
The visible satellite imagery shows a few clouds, though the Bay Area is becoming clearer this morning.
The enhanced infrared satellite imagery show no thick clouds over the state this morning. These images have been excluded from today’s post.
The water vapor imagery shows a few disturbances in the jetstream, one of which has already passed over the Bay Area this morning. Also notice the sharp line of moisture in Oregon; this marks the location of an approaching cold front.
The 12Z upper air sounding from San Francisco shows several damp layers- one near the surface and another near the 600 mb level. There was 0.77 inches of precipitable water. There was no Convective Available Potential Energy (CAPE) and no Convective Inhibition (CINH) present in the column this morning. There was no strong thermal inversion and the 0-3 km average lapse rate was 4.4 C/km.
The hodograph shows 13 kts of low-level shear and 36 kts of deep-layer shear this morning. The low-level shear was due largely due to directional changes, and the deep-layer shear was due almost entirely due to speed changes.
The surface observations (from the SPC Mesoscale Analysis Map) show cool temperatures, moderate humidity (as shown by moderate dewpoint depressions), light winds, and mostly clear skies across the state so far this morning. There are no major frontal boundaries over the state, though there is a cold front to the north and west of the Bay Area.
The surface pressure map shows that there are no strong pressure systems or gradients over the state so far this morning. The RAP shows that none are expected to develop over the next six hours.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows only weak flow over the Bay Area today. There is a trough, but it is more pronounced farther south.
The 500 mb NAM chart shows some weak Positive Vorticity Advection (PVA) associated with the trough. The vorticity is not very strong, but the winds do blow across the vorticity gradient.
The 700 mb NAM chart shows no rapidly rising air over the state today. This chart has been excluded from today’s post.
The 850 mb NAM chart shows no strong thermal advection over the Bay Area today. This chart has been excluded from today’s post.
The Precipitation chart shows that there is little chance of precipitation today, statewide. This chart has been excluded from today’s post.
Overall, today will be a mild and still day over the Bay Area. I am torn on cloud cover- I think the PVA will help mix the atmosphere, bringing moisture and cooler air into contact (and thus generating more clouds). However, the NWS and the temperature predictions are indicating that the cloud cover will be less. Also, there is virtually no rapidly-rising air at the 700 mb level. At this time, I’ll say partly cloudy skies this evening.
Thank you for reading my forecast.
The upper air soundings and mesoscale analysis plots are from the Storm Prediction Center website.
The forecasted upper air soundings are from TwisterData.com.
The surface observation and upper level charts are from Unisys Weather.
The satellite data is from NASA – MSFC