Strong Storm Impacting Northern California Tonight into Thursday

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This is a post solely on the incoming storm affecting the region through late Thursday. More on the potential storm early next week in the coming days.

The strongest storm of the season and in many months is currently beginning to spread prefrontal precipitation across much of northern California this Wednesday morning, however this is just a teaser of what’s to come overnight into Thursday morning. Offshore, a long fetch of subtropical moisture sourced from a cutoff low west of Hawaii is being fed northwestward right on into northern California via a subtropical jet. This subtropical jet is being merged with a strong Pacific jet streak driven by a potent & cold upper-level low off the British Columbia coast, driving inland cold-pool showers and thunderstorms into Washington today.

This merger will fuel the development of a potent cold frontal rain band upon the arrival cold front, which is expected to slide southward this evening into the northern Sacramento valley and reach the Bay Area, Sacramento region, and west slope of the sierra later in the evening. Precipitable water values with the moisture plume being entrained are in excess of 1.4″, with a 1.5″ maxima just offshore. This high concentration of total atmospheric moisture combined with a favorable and rather strong southerly low-level jet beneath a 90 – 100 knot mid-level jet (overlapping the subtropical jet) tonight with the front will help intensify precipitation rates rapidly as it slides further south closer to the moisture plume.

The front will start off up in the northern coast & northern valley appearingly rather weak tonight, however once it reaches the better moisture a bit further south in addition to the two jet streams merging, the front will rapidly intensify. This intensification is currently anticipated as it reaches around Yuba City, and is also expected to slow up once it reaches a point somewhere between Yuba City & Stockon for a few hours. Where this occurs, heightened precipitation amounts are of course expected as well as increased risk of flooding.

High resolution models have recently begun suggesting the cold front will also potentially include embedded thunderstorms upon the arrival of the subtropical moisture and merger of the Pacific jet from the upper-low in the Pacific Northwest. Potential thunderstorms or simply embedded convective bits could increase rainfall rates even further and bring about more localized flooding, in addition to driving downward to the surface strong, gusty winds given the aforementioned southerly low-level jet (in excess of 45 knots a few thousand feet above the surface). Stronger winds are expected in the higher elevations in the mountains, with and without thunderstorms, with gusts perhaps in excess of 50 mph ahead of the cold front.

High resolution (3km) NAM forecast MLCAPE, valid at 8pm PST. Note the band of values in excess of 100 j/kg pushing as far inland as the lower foothills as the cold frnt begins to arrive in the southern Sacramento valley & Bay Area. Some of the areas with higher values suggest enough potential instability for embedded thunderstorms this evening, in combination with a variety of other available environmental factors.

Flooding potential

The front will finally begin to exit the region sometime Thursday morning, but not  after slowly barraging north-central California for upwards of 12 hours in some locations with moderate to heavy rain. Over this period of time where the front slows down, rainfall rates in excess of a quarter to half an inch an hour at times could cause serious flooding concerns along burn scars from the past couple of summers — especially this summer in from the fires in October across portions of the northern California foothills/mountains and especially the coastal hills north of the Bay Area (commonly dubbed the ‘wine country fires’). If you are. or know anyone located near burn scars from this summer, make sure you’re/they’re aware of the possibility of debris flows and mudslides. Vegetation holds in the soil, and when its burned away there’s nothing to hold it down and keep it from moving with the water runoff itself.

If any cloud breaks occur behind the cold front after it passes Thursday morning, the cold pool now bringing inland some showers/storms into WA/OR could bring about the same thing our way. We’ll need breaks in the cloud deck to allow for some solid surface heating in order to gain enough instability to help generate some convection inland, otherwise if it remains mostly cloudy and showery, thunderstorms would be limited to the immediate coast.

Rainfall amounts in this ~24 hour period will range from .5 – 1″ in much of the western side of the valley, to upwards of 2 – 3″ along the east side of the valley against the foothills. In the Bay Area (lower elevations), 1 – 3″ are expected, with 2 – 5″ falling over the coastal hills north of the Bay and mountains northeast of Santa Cruz. Inland, 3 – 6″ of rain are expected across the majority of the sierra foothills, with 6 – 12″ of liquid precipitation above around 3000 to 4000 feet from Plumas/Butte counties to Tuolumne county. Once again, these rainfall amounts (primarily the regions expecting >2″) in/near hilly terrain where water will feed into streams could cause some flooding, especially if thunderstorms develop and locally increase these amounts forecasted.

Snow levels will hover around 7000 – 8000 feet with the cold front and the subtropical moisture plume it interacts with, however given the immense amount of liquid precipitation expected in the higher elevations – the translation of 8 – 10″ of liquid above the snow level equates to around 2 – 4 feet of snow at the highest elevations.

There’s potential for another storm of unknown strength at this time late Sunday into at least Monday next week. I’ll have more on that in a few days.

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