Isolated to scattered showers and thunderstorms possible over the next week, primarily in the mountains. More widespread thunderstorms possible over the weekend and Monday, including isolated Valley thunderstorms. Continued near or below normal temperatures for the foreseeable future.
Discussion
Latest satellite imagery and observations shows a weak low pressure system broadly circulating across northern NV/southeastern Oregon/southwestern Idaho. Meanwhile, a weak short-wave ridge is squished between another area of low pressure starting to take shape near 40N/130W. Additionally, the GOES-18 Night Fog RGB is showing some low stratus starting to creep eastward into the Delta region this morning. Hires models indicate about a 25-40% of the low stratus (with ceilings of <3Kft) may creep into portions of Sacramento County Friday morning, between 11-15Z.
Throughout the day today the area of low pressure to our west (off the coast) will continue to organize and dig southeast, reaching the south-central CA coast by Saturday morning. This low will eventually become a broad closed low just to our south, continuing to keep anomalous moisture (PWATs 100-200%) across northern CA and thunderstorm chances in the forecast through at least Monday.
However, given lack of forcing and limited instability today, thunderstorm chances will be fairly limited, with only some isolated chances (15-25%) along the Coastal Range and slightly better chances along the Sierra crest south of Highway 50 with 20-40% of showers and thunderstorm development.
Mean wind will be from the south- southwest around 10 knots today, so showers/storms along the Sierra will be moving eastward. Impacts with thunderstorms today look minimal, with brief heavy rain, lightning and gusty winds possible.
As the next low approaches CA Saturday morning, a few weak pieces of energy will circulate around the low. Some convective allowing models (CAMs) are indicating some weak reflectivity returns with virga/light rain may accompany some of these pieces of energy and move northward across the eastern San Joaquin Valley and into the southern Sacramento Valley and surrounding foothills early Saturday morning (11-15Z).
Throughout Saturday morning, instability will continue to build as the low continues to circulate to our south. With added forcing and southeast flow, scattered showers and thunderstorms will once again form along the mountains and drift northwest into the foothills and portions of the Sacramento Valley.
Similar or slightly better shower and thunderstorm chances (30-60+%) are expected again on Sunday. Better bulk shear (35-50 knots) will be in place over the weekend as the low circulates to our south, allowing for more organized storms, especially across the mountains and foothills along and south of the I-80 corridor Sunday. Even so, impacts expected with any thunderstorms will be similar to what we’ve already seen with this same pattern over the last week or so, with periods of heavy rain, gusty winds, and hail possible.
By Monday, the low will have weakened and shifted southeast, keeping east-northeast flow overhead, which will continue to keep isolated to scattered showers and thunderstorms in the forecast for the mountains, foothills, and northern portions of the Sacramento Valley. With continued low-pressure influencing us, temperatures will continue to be below seasonal normals through the weekend and into early next week, with Valley temperatures in the 80s and periods of breezy onshore winds, especially in the Delta.
Extended Discussion (Tuesday through Friday)
Ensemble guidance and cluster analysis indicate NorCal will be caught in between patterns Tuesday-Thursday, with the low to our south shearing apart, leaving a small piece of energy off the SoCal coast and another moving east into Plains and then a board ridge to our west in the Pacific. As a result of this pattern, thunderstorm chances will be limited to mainly the mountains with decreasing chances throughout the week as drier air filters in from the west. Dry conditions across the Valley with 1-2 degrees of warming each day.