Unsettled 2nd Half of the Week Inbound Across Northern California

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norcalweather.net has been dormant throughout the summer and early this fall due to a relatively inactive period. This summer seemed to feature less in the way of low elevation thunderstorm events than ‘usual’, albeit every warm season is different and our warm season is simultaneously considered our dry season. That said, dry season is now in the rear-view mirror and ahead is a curvy road of wet and dry spells as you’d expect (or not depending how into the weather you are). We had our first winter-like storm of the season last week, and the reason I didn’t make the time to post about it is because it wasn’t too eventful of a system. It was significant in the way that is dumped quite a bit of precipitation in the higher elevations and dampened fire-ravened regions in the coastal hills & wine country, but overall it wasn’t a significant system.

This week into the first few days of next week, we have two systems ahead of us. Both of them aren’t all that significant; however when their precipitation totals are combined into a less than 6 day period, it’ll certainly be more notable. As of today (Monday), the first system looks to be the strongest of the two — and to put that into context, it should be a tad bit stronger compared to the storm last week. Currently, the first system is located in the Gulf of Alaska as a fairly strong upper-level low with a deep, matured surface low associated with it. A 110-knot plus jet streak is rounding the low from the back side and will help the low and its associated trough dig deeper (southward) into the Pacific, and arrive off the western U.S. coast by late Tuesday night and Wednesday morning.

Graphic overview of the pattern anticipated Wednesday – Friday.

By Wednesday morning’s arrival of the upper low at the west’s front door, the surface cyclone will have past the point of maturity and begin to weaken. As the weakening low pushes closer to the Oregon/Washington coast late Wednesday, the system’s cold front will begin to sweep across northern California from northwest to southeast. Prior to the arrival of the front, moisture sourced from the central Pacific south of the Hawaiian islands will stretch out across the Pacific and begin streaming into northern California. This moisture, combined with a tad bit of lift and energy from the jet streak rounding the trough will help squeeze out some bands/clusters of showers throughout the area north of San Francisco by as early as Wednesday morning continuing through the day.

The front will begin sweeping through late/overnight Wednesday and depart southeast Thursday morning. The front itself will bring an increase in shower activity and intensity for a few hours as it passes overhead. A cool, moist onshore flow supported by a 80-knot mid-level jet ahead of a vort-max at the bottom end of the upper-low (off the north coast of California) behind the front will make for an unsettled Thursday with scattered showers throughout the day. Right now, instability looks rather limited (CAPE values <200 j/kg), but will monitor for any pockets of clearing in the cold pool as the timeframe nears and higher resolution models can get a better grasp of the system to see if any thunderstorms are possible during the afternoon Thursday. If there were to be any thunderstorms, shear would be excellent. If thunderstorms appear possible I’ll post an update specifically for that chance.

18z NAM’s forecast MLCAPE values, valid at 4pm Thursday, depicting some weak instability over parts of the Sacramento valley. If these values increase a bit, there could be sufficient thermodynamic support for some stronger showers or thunderstorms.

As the main upper-level low moves into the Pacific Northwest overnight Thursday into early Friday morning, an increase in shower coverage is possible again in the region generally north of Stockton (as with most of the precipitation from this event). Snow levels finally start to drop off later Thursday into Friday with the core of the trough from their higher point (>7000 feet) Wednesday/first half of Thursday, but still remain up around 6000 feet.

Some ridging builds for the majority of the weekend, giving us a dry Veterans day. As you read previously, there is a second anticipated system expected at some point in the next week — but it’s hard to nail down the timing right now. Models are teetering around with precipitation beginning to roll inland sometime late Sunday or sometime Monday. I’m leaning toward a Monday morning start to this precipitation right now, meaning we could end up with a (for the most part) dry Veterans Day weekend. This secondary system’s strength is also fairly hard to nail down right now as well given model discrepancies, but right now it appears it’ll perhaps be similar in strength to the mid-week storm. Models at time fancy the idea of it being a notch stronger, but at times fall back to a notch below, so right now assuming a similar strength system feels like the right way to roll forward.

Total precipitation through Monday, including the (potentially variable) amounts from the second system on Monday looks like the following. Around an inch for the eastern side of the Sacramento valley with about a half inch elsewhere in the valley, 1 – 3″ in the foothills and coastal mountains, 2 – 5″ in the orographically favored slopes of upper Butte/Plumas counties as well as the south to southwest facing slopes of Shasta county. These amounts could change in total with half of the weight being on the strength of that second system. Half of the totals are expected from the first system midweek.

Forecast liquid precipitation totals through Monday. These totals include a mean of model outputs for the second storm – and will likely change a bit. Roughly half of these totals are expected from the first storm.

Overall, an unsettled Wednesday – Friday is expected this week, with at least most of Veterans day weekend looking dry. Early next week more active weather is expected — but to what extend is still a bit unknown. I’ll have another update out sometime this week regarding storm #2, as well as an update strictly on any thunderstorm risk no later than Thursday morning if thunderstorms look possible.

Look forward to a winter full of interesting storms and patterns to write about, and welcome back to norcalweather.net to those who’ve patiently waited.

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