Potent Spring Storm to Bring Heavy Rain, Snow, & Thunderstorms to Northern California

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Off the northern California & Oregon coast lies a quickly developing/strengthening surface low beneath a cold upper-level low that’s been sitting off the Pacific Northwest coast since the weekend, and what drove showers into northern California on Sunday. It currently appears there’s sufficient moisture/temperature contrasts ahead of the low to allow it to strengthen, but to what extent isn’t too clear at this point due to models having some slight differences… but a median of model solutions generates a low bottoming out around 995mb. In any case, the low will swing its cold front through northern California late tonight into early Tuesday morning. This front will be much more robust than the previous systems’, and has a fair amount more vertical motion and moisture to work with (precipitable water plume of near a inch, compared to under .7″ with the last wave), which combined with a fair amount of cold air aloft and steepening lapse rates supporting a tad bit of instability with the front will support some moderate to heavy precipitation along what is expected to be a fairly narrow cold frontal precipitation band – perhaps 50 to 75 miles wide.

Infrared satellite imagery, 500mb heights (brown contours), and MSLP (blue contours) from 2pm Monday depicting the offshore developing system.

Infrared satellite imagery, 500mb heights (brown contours), and MSLP (blue contours) from 2pm Monday depicting the offshore developing system.

Wintery Precipitation

Snow levels will be fairly low compared to the other storms we saw through the season, with snow levels in the coastal and northern mountains currently looking to hover around 2500 – 3000ft. Along the west slope,  snow levels could set up around 3000ft initially, but may locally drop down to near 2500ft toward the back edge of the front with a bit snow perhaps mixing in some foothill communities such as upper Magalia, Grass Valley, Colfax, and Camino. Snowfall amounts in excess of a foot look quite possible over 4000ft Tuesday, which will likely cause some notable travel issues, as well as the fact that snowfall rates could be in excess of an inch an hour.

Potential for thunderstorms

It appears the cold front will exist the region by mid-morning Tuesday, or late morning if the system slows down a bit. In either case, clearing is quite possible behind the front as more open-cell cumulus bands/clusters associated with the core of the low begin rotating inland. If enough breaks open up and sunshine becomes scattered to widespread enough for temperatures to rise into the upper 50s or even low 60s in the valley, fairly strong destabilization could occur (for California’s standards) given the anomalously cold upper-low overhead (-30 to -32c at 500mb). Morning rainfall should boost dewpoints at least slightly into the mid to upper-40s, which combined with the steep to very steep mid-level lapse rates and boosted surface temperatures if the post-frontal environment isn’t completely shrouded in cloud cover could boast surface-based CAPE anywhere from 400 to 800 j/kg on a widespread scale according to high-resolution models, with some values upwards of 1,000 j/kg being modeled. Shear is a bit tricky Tuesday given the core of the upper-low/trough slides overhead through the day, putting us under the dead-zone for mid/upper-level winds. This doesn’t mean there isn’t any wind, though, as it appears 500 – 300mb winds will still be in the 30 to 50kt range across northern California, with much stronger winds in central California where the jet blasts overhead. Given the low will be off the northern California coast Tuesday afternoon, there could be locally enhanced mid-level winds, though, that aren’t well modeled. In the low levels, surface winds will be present and potentially breezy given the southerly flow ahead of the surface low offshore. These gusty southerly low-level winds combined with the modest westerly to northwesterly 500 – 300mb winds equate to decent directional shear and support nice clockwise curved and looped hodographs, indicative of a decently sheared environment supportive of rotating storms.

Once again I’d like to point out that the potential of rotating storms is highly conditional, and is going to require at least some sunshine to get enough buoyancy for storms to become robust enough to tap into the shear available. Most models do support the idea of at least some scattered clearing patches across the valley, though, so there is real potential for strong thunderstorms in the valley from as far south as perhaps the Modesto area, northward throughout the Sacramento valley to Redding. Just about anywhere in this stretch of the valley appears to be potentially primed for rotating storms, and if clearing is widespread Tuesday morning into the early afternoon, things could be a bit hectic.

Storm-Potential_1

Severe threat

Even if clearing doesn’t occur, as the core of the low moves inland convection will likely develop. It currently appears that the low itself or some sub-vorticities will venture inland anywhere from Point Arena northward, but they’ll be tracking southeastward, which would probably take them over the valley and aid in the development of afternoon convection. If things are cloudy all morning the convection would likely be weak with sparse lightning, but if clearing were to be prominent across the valley in the morning ahead of these moving inland, they could really help out with the development of strong to severe thunderstorms in the Sacramento valley depending on track, and could make for interesting local wind enhancement near/in storms.

Cutting to what thunderstorms could be capable of Tuesday, it’s conditional answer. I’ll answer it in two contrasted setups.

1). Little to no cloud breaks: storms capable of small hail, heavy rain, and gusty winds, as well as locally lower snow levels.

2). Widespread clearing ahead of afternoon development: storms capable of larger hail, strong winds, funnel clouds or tornadoes, heavy rain, and locally lower snow levels.

If the mid/late morning hours are clear in a good chunk of the valley, I’ll probably end up heading out in hopes to chase some decent storms and see what they have to offer. I’ll post an update to the top of this post Tuesday morning looking things over to see how the afternoon looks thunderstorm wise, and if it at all looks favorable for anything decent.

Rainfall wise through the day the valley will likely pick up anywhere from a quarter to three quarters of an inch of rain, with around an inch or so in the foothills and higher terrain. Locally (much) higher totals are possible anywhere thunderstorms venture overhead. Dry weather takes over from Wednesday on as ridging returns, with 70s perhaps returning by late in the week.

 

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