Unsettled Days Ahead; Thunderstorms Possible

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Sunday morning update, as of 10am

Currently at/as of 9:45 this morning, the first band of precipitation was working its way into the central valley and into the delta, with more showery bands evident just off the coast. These bands will push inland through the rest of the morning into the afternoon, later from Sacramento southward. Snow levels in the mountains are running around 2500 – 3000ft, though, precipitation is still sparse this morning due to dry a low-level atmosphere/surface. Precipitation will ramp up in the sierra this afternoon as semi-convective bands push inland up upslope, during the process bringing dew points and surface temperatures closer as precipitation begins to fall. In some cases, where surface dew points are very low, and temperatures are a fair bit warmer, evaporative cooling could take place, locally lowering snow levels in the process.

Precipitation and cloud cover may be too widespread for decent convective development. Some models still suggest potential for some isolated to scattered storms this afternoon into the early evening, but overall it appears the strong/severe storm threat is a bit lower – but not impossible, and I’ll continue to keep an eye on trends into the afternoon in case things change.


 

April initially came in dry, but this is about to change as a pattern change unfolds across the western U.S., subjecting California to some much-needed rainfall over the coming several days.  Currently, off the west coast, a cold upper low is diving out of the Gulf of Alaska, currently located off the British Columbia coastline as a core of cold core cumulus signifying fairly cold and unstable air aloft with the low. In addition to this low lies a small shortwave to its south, off the California coast. These two features will close in closer to each-other tonight, with the upper low strengthening the offshore wave a bit as it pushes inland into our region tonight, increasing showers over the northern sierra and mountains. The main band/cold front will slide through Sunday morning, bringing periods of showers to a good chunk of northern California. The frontal band shouldn’t be too robust, though, given the lack of a decent moisture source, with precipitable water values under an inch (pretty measly, but better than nothing). After the first band slides though, there could be some breaks in the clouds given the lack of a consistent stream of moisture and clouds. This could allow surface temperatures to rise into the upper 50s in general, however some pockets of the valley could warm to the 60s. These temperatures beneath fairly cold mid-level temperatures should support steep low-level lapse rates, which in addition to locally boosted low-level moisture (dewpoints) from showers earlier in the day could support CAPE values in the 200 – 600 j/kg range, depending how much moisture gets boosted and how warm temperatures get.

Dual-GIF

If CAPE values get to the higher spectrum of that range in the valley (which it appears these values would generally be confined to the Sacramento valley north of Sacramento), there is indeed supportive shear available given the upper low’s position off the OR/WA coast, and due to the tilt of the trough being negative – both of which is historically favorable for valley thunderstorm development under the right conditions. The mid-level jet is fairly modest, with 500mb winds near or under 50 knots… though, the upper-level jet (300mb) is a bit stronger at 80 – 90 knots. At the mid and upper-levels, the orientation of the wind/jet is out of the west-southwest, while at/near the surface, winds in the valley should become backed out of the south to southeast as these westerly winds curve out of the delta to stream up northward into the Sacramento valley. This creates good to very good directional shear, with in some cases near 90-degree turning of wind directions between the surface and 20 – 30,000ft AGL.

Thunder

Given good shear and potentially good instability if conditions play out in favor of warmer temperatures and boosted moisture, there is or could be potential for some strong thunderstorms in the Sacramento valley… generally north of the Yuba City area it appears, where convergence for development appears to be best along a line from Arbuckle to west of Red Bluff. If the conditions are met in which instability can become sufficient (CAPE >500 j/kg) for robust thunderstorms to develop, strong storms would be most capable of large hail, and gusty winds. If conditions really become supportive, in which case there’d likely need to be at least a few hours of sun, cannot rule out a few rotating thunderstorms or potentially even a tornado.

4km NAM's forecast 10-meter winds valid at 2pm Sunday. Where the westerly winds intersect with the southerly winds, convergence is best.

4km NAM’s forecast 10-meter winds valid at 2pm Sunday. Where the westerly winds intersect with the southerly winds, convergence is best.

Snow levels with this system look like they’ll hover around 3000 – 4000ft in general, but given the convective potential things could be a bit tricky, and cannot rule out locally higher or lower levels. QPF (basically total precipitation) through Sunday evening will also be tricky given the mostly showery nature of this system, but in general it appears the valley could receive locally little or no precip to perhaps a few tenths, with about the same in the foothills (perhaps locally a half inch of rain), and around a quarter of an inch of liquid to an inch of liquid in the mountains. The snow equivalent to .25 – 1″ of liquid would be around 4 to 12″ of snow above about 4000ft, with local accumulations below 4000ft possible but tough given the fact it’s been warm lately, and surfaces such as roads retain warmth for a while.

A bit of a break is expected Monday, though showers will likely continue over the mountains (both coastal and inland). Another wave drops down the back side of the upper-low Monday, and strengthens quite rapidly as pretty strong cyclogenesis occurs as the low pulls in some warmer, more moist air up ahead of it and into it, while the cold air behind it the upper-low has to offer creates a potent contrast, and thus allows the surface low to blow up. The NAM has a 24 hour pressure drop of 17mb, with the low beginning around 1009mb, and bottoming out at 992mb. However, models have been pretty steadily trending stronger with the surface low… for instance last nights NAM run had the low drop to 995mb instead. The GFS and ECMWF keep the low in the 1000 – 1005mb range, though, which is quite a bit different from the NAM, though. Even if the GFS/ECMWF solutions are correct, we’re still in for essentially the same impacts. Currently looks like much of northern California could pick up a half inch to two inches of QPF, with 8 – 16″ of snow in the mountains, and potentially another thunderstorm threat setting up in the valley behind the low’s cold front Tuesday afternoon. I’ll have more on this later Sunday or Monday after this first system exists.

12z NAM's forecast 850mb wind, 500mb heights, and MSLP (with tooltip info at center of low).

12z NAM’s forecast 850mb wind, 500mb heights, and MSLP (with tooltip info at center of low).

I’ll probably post a quick update Sunday morning regarding the thunderstorm potential to the top of this (same) post.

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