The National Weather Service in San Francisco has indicated that a significant pattern change is on the horizon, starting late Sunday night and continuing into the middle of the week. According to NWS San Francisco, we're looking at cooler temperatures and a chance for light rain from Monday through Wednesday.
As of early Sunday morning, the forecast discussion from the National Weather Service points to below-normal temperatures throughout the week. Though details on rain amounts and the chance of thunderstorms are still up in the air, they do note a moderate confidence of up to "1/10th expected in the North Bay and only some novelty drops further south." Persistently low temperatures and southwestern winds will accompany the moisture, making for a chilly, wet start to our typical Californian September.
The short-term forecast states that a patchy marine layer will fill in under a full moon early this morning, influencing cloud coverage throughout the day. Though increased sunshine may peek through, 850 mb temperatures remain below normal, ensuring that the surface temperatures will be about 5 degrees cooler. Tonight, as the trough approaches, expect earlier and more widespread marine layer clouds as any dry air is filtered out.
And the outlook for the next couple of days presents even more intrigue. After the front above sweeps through, the atmosphere will cool up at 500 mb to around -15C, a stark change uncommon for this time of year. This cooling could trigger some instability Tuesday through Thursday, "with the best chance Wednesday afternoon," as per the Area Forecast Discussion. While cumulus clouds and an outside chance for showers or thunderstorms loom, by Thursday, rain chances diminish, as does the instability.
Aviation and marine impacts are also notable with the change in weather. Light winds are expected to prevail, with stratus clouds impacting visibility in the mornings and evenings. West winds around 10 to 20 knots will blow today near SFO before weakening tonight and Monday morning. Mariners, in particular, should prepare for gentle to moderate northwesterly breezes today through Monday, with stronger gusts possible by Tuesday and Wednesday. Light rain and a possible thunderstorm could make for some tricky conditions early this week.
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