Increased chances of Thunderstorms for the SJV, with some cells possibly reaching SEVERE levels (especially NORTH VALLEY) There are 2 focal points of concern on radar and satellite imagery already showing up at 8-9am;
1) The largest feature is the development surrounding the Low Pressure circulation which is moving slowly SE along the NORCAL coastline with a -31°C cold core 540dm Low Pressure center west of the CA/OR border region. This is forming a new frontal boundary with a baroclinic leaf structure which is rapidly expanding and spreading east and south. This will be THE major factor for later this evening/tonight for CENCAL.
2) The more immediate concern lies closer, locally…in the infamous Merced Co Thunderstorm birth region where a convective line is forming, which is traveling east toward Merced and Chowchilla. This feature has the potential to develop into a line of Thunderstorms during the next several hours. Southward propagation of the line into Fresno Co along the trailing flank could bring Thunderstorms into the Fresno metro area by noon and then further south and east during the afternoon. This line should be closely monitored.
The Water Vapor Image below shows an upper Low Pressure circulation approaching NORCAL and the Bay region. Scattered showers and Thunderstorms are already occurring over NORCAL down to near Merced Co. This activity will continue to develop and spread across portions of CENCAL throughout today and this evening. The very cold temps aloft at -31°C combined with steepening lapse rates will promote Thunderstorms with hail, with larger sized hail within the stronger updrafts. Also a backed low level flow in orographically favored areas may result in FUNNEL CLOUD or a brief TORNADO or two, especially in the North Valley region.
Water Vapor Image showing cold core upper Low Pressure center west of Cape Mendocino which is sliding SE, with a developing vortmax near the center.
A 110kt Jet Stream is currently over Lodi aligned west to east and is sagging southward as the upper Low Pressure drops into CA. At the same time 500mb temps have dropped to -31°C over much of NORCAL, as depicted on the 500mb chart below. A tight thermal gradient exists over CENCAL between the -31°C cold core isotherm over SAC and the -14°C isotherm over Long Beach as is depicted on the chart below with the red-dashed lines concentrated over CENCAL, with a westerly directional flow up to 70kts over Kern Co. Current 500mb chart below;
The flow aloft at 300mb, 500mb, 700mb and 850mb is currently unidirectional from the west which would favor lineal lines of convection, not SEVERE for the SJV. However, winds aloft veer over the Delta and the SAC Valley which will favor rotation, with Thunderstorms possibly reaching SEVERE thresholds. Convective trigger temps are low due to the very cold air aloft, so it won’t take much surface heating to get things going.
As the current vortmax exits CENCAL later this afternoon, the newly developing vort rapidly approaches the coast at the same time with a short break in-between. It punches into the Central Coast near SLO and aims at the South Valley region with powerful top end energy plowing into the South Valley region overnight.
Convective forecast charts favor Thunderstorm development along the Westside SJV from the Delta southward by this afternoon, with very high values along the coast which diminish to the east. The highest probability for Supercells with large hail appears from Monterey Bay southward along the coast to near Morro Bay. These cells could rotate into the Westside SJV and produce some strongThunderstorms.
Forecast soundings for the SAC Valley this afternoon indicate CAPE values of 500-1000 joules/kg and the potential for rotating Thunderstorms. Any breaks in cloud cover will add to the instability. The primary threat will be downpours accompanied with 0.25 to 0.5 inch hail or larger, frequent lightning, and strong gusty outflow boundary winds. A brief tornado or two is possible.
Additional statements and quick memo’s for refinement regarding Thunderstormsor possible SEVERE wx are an option later today if warranted.
Storm Prediction Center Categorical Risk for Thunderstorms today for CA.
Everyone is urged to keep NOAA Weather Alert Radio’s on today and to keep an eye to the sky.