First Fall Storm of Season Expected Sunday

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Believe it or not, this corner on the internet dubbed norcalweather.net still exists! It’s just been hibernating through the dry, overly predictable pattern we’ve been in for months. It’s perhaps been one of the most lackluster monsoon seasons this summer across northern California compared to many of the previous years’ monsoon seasons keeping this blog dormant. Now with monsoon season behind us, we’re here knocking on October’s door and the first fall-like storm of the year is approaching. Over the past several days, a steady cooling pattern has gripped the west as a Gulf of Alaskan trough nudges the persistent eastern Pacific ridge westward, switching a previously notable offshore wind event over the weekend into early in the week into a noticeable onshore flow. Temperatures has tumbled from the upper 90s to low 100s over the weekend into Monday to the mid to upper 80s as of Thursday, and by Friday into the weekend, 60s & 70s across the lower elevations, and it isn’t just temperature falls we’re talking. We’ll have a real shot at some widespread precipitation including mountain snow come the end of the week.

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This trough will have sent an essentially dead first cold front through northern California by early Friday, placing us in northern California under the very eastern edge of the trough. The original longwave/ex-upper-low will slide into Oregon & Washington late Friday into Saturday, giving the Pacific Northwest the first taste of precipitation from this trough in the western U.S., while temperatures continue to cool down south in northern California. Temperatures will fall from the mid and upper 80s Thursday into the upper 70s to low 80s (in the lower inland elevations). Some showers could begin brushing the northwest part of the state as early as Saturday morning as the trough begins to “reload” with a new, stronger upper-level low as a new piece of energy drops out of southern Alaska/northwest Canada into the base of the trough off the west coast. The rest of the state will remain dry Saturday, but temperatures fall into the low to mid-70s in the lower elevations, well below normal for early October.

Stronger secondary low dropping into the trough Saturday afternoon off the Pacific Northwest coast after the initial wave’s inland push.

As the trough reloads with a stronger low, it switches from a neutrally-oriented trough towards more of a negative tilt — something that’ll come into play Sunday when the low rotates around the core/base of the trough and pivots toward northern California & Oregon. As the low begins to near the coast Sunday, it sends a cold front that keeps its act together far more than the initial dead front the trough first sent Thursday/Friday, bringing some showers into the coast by early to mid-morning. By late morning to early afternoon, the front will be sweeping through the valley and sierra, bringing showers to these inland regions as well. This system isn’t exceptionally moist and doesn’t have taps to any active tropical activity in the Pacific, but will feature a belt of precipitable water values around .7 – 1″, which isn’t horrible when combined with a fairly dynamic divergent upper-level jet & flow as the polar jet pulls northward the sub-tropical jet and they work with each-other simultaneously.

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Showers will continue into Sunday evening/overnight as the upper-low continues to venture southeast, clipping through part of the valley and sierra, locally enhancing lift & precipitation. In the sierra given presence of a -25 to -30c 500mb cold pool aloft, translating down to -6 to -8c temperatures at 700mb during peak period of precipitation, snow levels will drop off to around 6000ft in the I-80/Hwy-5o corridor times Sunday through early Monday, with snow levels around 5000 – 5500ft in the northern mountains. Snowfall amounts will be sketchy given the precipitation with this system will be showery & lacking well-forecasted solid precipitation bands, anywhere from a quick dusting to several inches are possible around pass level through early Monday.

Convection

With that cold pool still in-mind, factoring in valley surface temperatures in the mid to upper-60s Sunday afternoon, lapse rates will be steep without a doubt. When moisture is thrown into that equation, some legitimate instability should coincide with the afternoon and early evening hours — especially if some patches of sun sneak in through breaks in cloud cover. With lift certainly present given proximity to the low off the northwest coast & Oregon’s coast, igniting convection wouldn’t be a problem if the instability is there to help with convective intensity. As for shear, directional shear in the valley isn’t too shabby. Surface winds appear they’ll be backed out of the south to south-southeast in the Sacramento valley, undercutting veering mid & upper-level winds out of the southwest to west-southwest, though the speeds of these winds won’t be too strong as the core of the jet slides southeast. That said, with all the available ingredients and the still relatively high angle of the sun we don’t typically have during wintertime thunderstorm events suggests thunderstorms are a good bet Sunday afternoon & evening across northern California. At this point still a few days out it’s hard to say if there’s a subtle severe threat, but with how consistent models are with the overall setup — a few strong storms cannot be ruled out. If this is the case, expect a quick update to this post Saturday or Sunday morning.

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Another system is expected to brush northern California late Monday into Tuesday keeping ridging from strengthening much and preventing temperatures from the 80 degree mark in the lower elevations until Wednesday, though nights will be fairly cold. Models diverge after that with the ECMWF suggesting another system by next Friday, while the GFS keeps us dry until early the following week. In both solutions, however, we may not have to wait very long for more interesting weather to be knocking on the door as it appears the persistent and large-scale eastern Pacific ridge may be defeated. Stay tuned.

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