Unsettled weather with minimal precipitation to start the week

April 8, 2019 – Another weather system will spread precipitation southward across the region later today and tonight. Thunderstorms with heavy rain will be possible over Shasta County this afternoon and evening potentially leading to flooding issues. Showers decrease by Tuesday evening with dry weather expected Wednesday. Another system may bring some showers to the mountains Thursday.

Discussion

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Partly to mostly cloudy skies cover the region early this morning with showers currently limited to northern Shasta County and areas to the north and west. Southerly winds have slackened a bit compared to 24 hours ago, but current temperatures remain very mild ranging from the mid 30s and 40s over the mountains to the upper 50s to mid 60s across the Central Valley.

Upstream trough is on the move and will displace the ridge eastward that`s presently over NorCal. Satellite imagery continues to indicate a wide swath of deep moisture (TPW of 1-1.3 inches) moving up from the southwest into NorCal. Showers and thunderstorms are expected to increase across the northern portion of the forecast area by this afternoon as large scale upward vertical motion and low-level convergence increase ahead of the approaching trough.

HREF and HRRR ensemble continue to point toward the potential for strong convection, and its associated hazards, centered on Shasta County later this afternoon into this evening as several lines of thunderstorms move through. Main threat appears to be heavy rain with the potential for flash flooding given the deep available moisture and strong south/southeasterly low-level inflow/ convergence (flash flood watch has been issued for 3 PM until 9 PM). 00Z HREF indicating high potential (> 50%) for hourly rainfall rates exceeding 1 inch an hour, and their ensemble even shows non- zero chances for 6 hour amounts to exceed 3 inches into this evening. Given the forecast instability and shear, hail and even a brief tornado would be possible with any persistent discrete cells.

Showers will extend southeastward across the region tonight as the cold front moves through. Snow levels forecast to lower to around 5000 feet early Tuesday with a few inches of snowfall accumulation possible across the higher passes before showers taper off late in the day.

Dry weather along with breezy north winds expected Wednesday, then a “slider” type system may bring some showers to mainly the mountains on Thursday.

Extended Discussion (Friday through Monday)

No significant weather impacts are expected for the extended period, with periods of mainly light, mountain precipitation expected.

Lingering mountain showers forecast for Friday, mainly in the morning, as a relatively cool and dry trough exits. Most of the area should remain dry and mild. GFS and European ensembles suggest Saturday during the day will remain dry across the forecast area with shortwave ridging. Temperatures should trend up, with highs several degrees above normal, similar to what we saw yesterday. Saturday night into Sunday light precipitation is expected to spread in from north to south, mainly over the mountains. Some of this could linger into Monday.