Hot weather through mid-week, then cooler by the end of the week. Heat Advisory from 11 AM Monday to 11 PM. Thursday Isolated mountain thunderstorm chances will persist through early this week, with some potential (10-20% chance) for these storms to impact the foothills into Monday.
Discussion
As of 12pm, satellite is currently showing mid and high level clouds continuing to stream across Tuolumne and Calaveras counties this afternoon. Temperatures across the region compared to 24 hours ago are running between 2 and 7 degrees warmer. More notably, temperatures are running 8 to 11 degrees warmer than 24 hours ago in the Redding/Red Bluff area of the northern Sacramento Valley.
Given the current temperature trends, KRDD has a decent chance of breaking their daily record high for the day, which is currently 112 set back in 2012. As we continue to be influenced by the Rex blocking pattern (a strong upper-level ridge to our northwest with an upper-level low trapped underneath it to our southwest), we will continue to see daily isolated chances of showers and thunderstorms across the mountains.
The upper low to our southwest will slowly shift northward late tonight through Monday. As this occurs, waves of energy will circulate around the low, allowing for mostly isolated mountain chances (15-40%) of thunderstorms to spread northward through Monday across NorCal.
Instability will be meager or capped off (CIN of 100-200 J/kg) outside of the mountains, so even though we will have east to southeasterly flow, anything that drifts off the mountains will likely dissipate quickly or be mostly virga. Given the addition of energy/vorticity lobes that are progged to move northward late tonight/early Monday, isolated showers will linger overnight across mainly the northern San Joaquin Valley. These showers look very light, and will likely be mostly virga, but there is a non- zero potential for a lightning strike.
As the low continues to retrograde northward Monday, shower and thunderstorm chances will also spread northward. Main impacts with any thunderstorms in the mountains will be brief heavy rain, small hail, lightning, and possible fire starts from lightning outside of main rain cores.
As showers drift off the mountains tomorrow, the main impact will be gusty winds, given the dry low- levels, though, a few lightning strikes will be possible across the northern Sacramento Valley tomorrow afternoon if enough CIN diminishes. Most hires models are depicting outflow winds from stronger mountain thunderstorms potentially producing gusts of 30-40 knots tomorrow afternoon. Also of note, the HREF is highlighting an area in western Plumas/Sierra counties where stronger updrafts may be possible tomorrow as well, bringing a higher risk of heavy rain and gusty winds.
As the low drifts further offshore, a more typical isolated diurnal mountain thunderstorm threat is expected Tuesday through of the week as steering flow becomes more southeasterly.
Outside of shower and thunderstorm chances, temperatures will continue to increase as heights to our north increase and influence NorCal. Today’s hot temperatures in the northern Sacramento Valley will be suppressed some Monday returning to ‘just’ 5-8 degrees above seasonal normals as the low off the coast shifts northward.
Elsewhere, temperatures will continue the upward trend with hottest temperatures Tuesday and Wednesday, where widespread triple digits will be likely (60-100%) across the Valley. These temperatures will result in widespread Moderate HeatRisk across the Valley and Foothills through mid week, with areas of Major HeatRisk on Tuesday in the Valley.
Given the higher end Moderate Risk, decided to issue a Heat Advisory for the Valley and portions of the lower foothills (mainly for elevations below 2000 feet) which will be in effect Monday at 11am through 11pm Thursday.
Outside of any shower or thunderstorm influence, winds will generally remain light each day across much of the region, except for some breezy southerly winds across the northern Sacramento Valley with gusts around 20-25 mph and breezy westerly winds with gusts to 25-30 mph in the Delta region.
Extended Discussion (Thursday through Sunday)
Ensembles show the Pacific NW ridge weakening and its remnants tracking northwestward, this will allow the closed low offshore to deepen as it merges with an open wave from the north causing strong troughing over our area by end of week.
Remaining influence from the high will result in Moderate HeatRisk on Thursday with high temperatures 3-7 degrees above normal, however we will begin a gradual cooling trend heading into the weekend as the troughing ramps up.
The National Blend of Models (NBM) is advertising weekend high temperatures 5-10 degrees below normal with widespread Minor level HeatRisk. Increasing moisture and instability from the trough will also promote chances for afternoon/evening mountain showers/thunderstorms through Sunday.
Additionally, locally breezy and gusty winds are expected through the period.