Quick Update for a Quick Wednesday Storm

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I wish I could have wrote this sooner, but time and work got the best of me. In any case, it appears the last storm we’ll see for a while will swing through northern California Wednesday afternoon into the evening, with a quick burst of rain and mountain snow as a narrow cold front slides through. This front will be driven through the area due to a trough rapidly diving into the Pacific Northwest & Great Basin. It isn’t a full-blown inside-slider, but it is taking a much more inland track than many of the storms we’ve seen this season. Usually these more interior-based systems are cooler than storms that have more offshore land to venture over, and this trough will indeed be a bit cooler than the last few, but nothing extraordinarily cold.

NAM's forecast 500mb wind and heights, valid at 4pm Wednesday. The trough axis can be noted clearly dipping into northern California with the dip in the heights.

NAM’s forecast 500mb wind and heights, valid at 4pm Wednesday. The trough axis can be noted clearly dipping into northern California with the dip in the heights.

The front looks like it’ll bring precipitation to areas north of Chico between 8 – 11am, and between 11 and 1pm from Chico to Yuba City. In the Sacramento/I-80 & Highway 50 corridor, 1pm to 4pm looks to be the timeframe for precipitation to begin and start winding down. The frontal band looks like it’ll be pretty narrow – perhaps just 50 – 75 miles wide at best. Parts of the central valley may get rain shadowed as the westerly flow of the moisture plume associated with the front gets blocked partially by the coastal mountains. Lift up the west slope and banking against the foothills should allow the east side of the valley to get in on the action, but parts of the western valley adjacent to the coastal mountains may little to locally no precipitation. Slightly better moisture is able to make it into the Sacramento/I-80 corridor through the delta, and may allow accumulating precipitation to fall through most of the I-80 and Highway 50 corridor.

Precipitation will be mostly light to moderate in the valley, with moderate to locally heavy precipitation in parts of the foothills and especially further up the hill where the upslope flow wrings more liquid out. Snow levels look like they’ll begin around 5000ft, but lower quickly to around 3000 – 4000ft along the back edge of the precipitation band/cold front. Snow will be heavy at times over the passes, and it appears a quick 6 – 12″ is possible above 5000ft over the course of 4 – 8 hours!

4km NAM's forecast 700mb vertical velocities, indicating rapid vertical motion (warm colors) up the west slope, indicating good lift/upslope flow.

4km NAM’s forecast 700mb vertical velocities, indicating rapid vertical motion (warm colors) up the west slope, indicating good lift/upslope flow.

In the valley, if clearing occurs in the morning ahead of the front in the central and southern parts, enough heating may occur to allow for some convective development along, ahead, and behind the front. Currently looks like 100 – 400 j/kg of CAPE forecast along the front, which combined with any lower-level buoyancy (a sun-warmed surface), may support the development of deeper convection or thunderstorms… mainly from Chico southward, and much more likely along the east side of the valley up into the foothills & mountains, where moisture & lift is maximized. Strong mid-level jet traveling along the base of the trough will aid any thunderstorms that develop. Shear is decent… but it doesn’t currently appear like a favorable setup for discrete/isolated convection to develop, rather more linear. 40 – 50kt 850mb winds jetting overhead Wednesday could get tapped into by any heavier precipitation and especially any thunderstorms, and could drag down some gusts upwards of 30 – 40mph if thunderstorm development were to occur, but at the moment no real severe threat appears likely.

The east side of the valley may pick up up to around a quarter to three tenths of an inch, with a tenth or so further west, and even less along the west side of the central and northern valley. A quarter to half inch looks like a good bet in the foothills, with around the same to perhaps three-quarters of an inch of liquid precipitation equivalent in the highest elevations and where upsloping is maxed out.

Riding rapidly takes over behind the system, creating a tight pressure gradient as a deep surface low develops in the Rockies. Breezy to gusty winds will become prevalent across a good chunk of northern California from Wednesday night through Thursday… lingering into Friday, mainly in the mountains. Other than that, the pattern looks like it may stay quite through the end of the year… possibly into January. Definitely not what we need if we’re trying to bust out of our historic drought.

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