I’ll have an additional post tomorrow when the storm has finally begun. There’s still some uncertainty, but it does appear that there will be a rather impressive frontal passage later tomorrow across nearly all of the state. Details below.
 *Precipitation*
Copious subtropical moisture has been entrained by the rapidly developing cyclone. A powerful 180 knot jet is inducing rapid cyclogenesis of this now clearly-defined low pressure area. The low is expected to generate a great deal of lift over NorCal tomorrow–from both warm air advection initially and, closer to frontal passage, from significant baroclinic instability. Moderate to heavy steady rains of the warm advection type will develop over the Bay Area early in the day, before intensifying even more as the front approaches during the late evening. Very heavy rainfall rates are possible around frontal passage as a result of intense convective activity (see below). The front will be a little less powerful as it moves south past Santa Barbara, but lift will still be strong and convective activity will still be possible. Rainfall totals north of the Bay Area could reach as high as 6-9 inches locally, the wettest portions of the Bay Area could see 4-6 inches (in only a 24 hour period!) and the favored mountain areas of SoCal could see as much as 2-3.5 inches. Some flash flooding is possible, especially near the front where actual rainfall rates could approach .75 in/hr. Minor river flooding could occur in the Eureka area. Burn areas of SoCal will have an enhanced threat of flash flooding. 2-4 feet of snow could fall on the highest peaks of the Sierras.
*Thunderstorms*
It appears that the prefrontal atmosphere on Tuesday will be somewhat favorable for deep convection, especially inland over NorCal. Could see some lines of hvy shws/tstms develop well ahead of the main cold frontal band as early as tomorrow morning, espcially if the sun can peek through the mid-level clouds. Scattered thunderstorms will likely be embedded in the cold front itself, given the copious moisture and powerful dynamics involved, and these could be rather strong. Very intense rainfall, strong winds (see below) and possibly some very localized hail could fall with the front. Even more impressive, however, are the convective parameters on Wednesday. The post-frontal atmosphere will be very unstable as cold air aloft filters in, a strong cyclonically curved jet streak remains just offshore, and additional vorticity maxima move over NorCal. Showers with scattered strong thunderstorms will be likely over the entire state, with the possibility for some severe activity, mainly in the Central Valley and possibly over the SoCal Bight as well. Hail and funnel clouds/waterspouts would be the main threats, but a small tornado threat may also exist.
*Wind*
The surface low, which will undergo significant cyclogenesis over the next 12-24 hours, is expected to reach 994 mb by the time it reaches the OR/CA border. This should produce a pressure gradient sufficient for 50-55 mph wind gusts in the hills as far south as the Bay Area. If the system deepens more than forecast, which is a possibility, 60+ mph gusts could occur. Also, strong convective elements embedded in the frontal band may potentially produce strong to locally damaging wind gusts. Some high winds could also affect SoCal, esp. mountain areas.Â
It will be the strongest storm of the California wet season so far… Â
