Wet System Late Friday/Saturday, Thunderstorms Again Possible

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What a week it’s been! Two days of supercells in Glenn and Butte counties, with Wednesday having supercells from the northern San Joaquin valley to Glenn/Butte counties. With all the action we’ve seen, we still aren’t done just yet.

For more information on the Glenn & Butte county tornadic supercell, check out this here review I posted: Glenn County Tornadic Supercell Review

Yet another large upper low and associated surface low will push eastward and become quasi-stationary off the Pacific Northwest coast Friday. It has already tapped into a subtropical moisture plume from the Hawaiian Islands, and though not a significant moisture tap, it includes precipitable water values in the 1 – 1.2″ range being transported into OR/CA via a combination of the main Pacific jet and a subtropical jet. This strong/dynamic jet streak energy, moisture, and cold air aloft associated with the upper low will dig the trough further south and enhance a vorticity max off the California coast.

March 27th Map

As the vort max pushes toward the California coast Friday night into Saturday, a cold front will slide through bringing a band of moderate to locally heavy rain given the above mentioned moisture and jet dynamics in addition to strong lift. The frontal precipitation band should arrive along the north coast by Friday morning, into the coastal mountains, northern mountains, and northern valley by mid to late-afternoon, and begin filling into the central valley by early to mid-evening. Front slows as it slides further south, and finally reaches the I-80 corridor by the early morning hours of Saturday. The band may stall out from around SFO up the I-80/Highway-50 corridor through at least the late morning hours Saturday, possibly into the afternoon. It should finally arrive in the San Joaquin valley by mid-morning.

Convection could once again be something to keep a close eye on with this system. Given a more noticeable system closer to our coast (in this case the vort max), and a deeper trough, colder air aloft with punch further south compared to storms earlier in the week. -25c 500mb temperatures arrive overhead by the afternoon and evening, which will promote some steepening lapse rates along a leading plume of the cooling upper-level air. Given a 100 – 130kt jet carving through California Saturday, speed shear will be strong, but directional shear is a bit more iffy. The GFS indicates great low-level southeasterly winds and decent southwest to WSW winds at 500mb, while the NAM indicates westerly surface winds and westerly 500mb winds from Yuba City to Stockton… with somewhat in the way of better veering north to Redding. Overall, for now, I’d expect great speed shear and at least modest directional shear.

300mb winds and 500mb heights plotted through the 0z 12km NAM, valid for around 2pm Saturday.

300mb winds and 500mb heights plotted through the 0z 12km NAM, valid for around 2pm Saturday.

Clearing is again iffy with this setup. If the front exists by early to mid-afternoon in at least half of the Sacramento valley and allows for some partly cloudy skies to develop and boost surface temperatures into the upper 50s to low 60s, buoyancy and CAPE would be sufficient for thunderstorm development again. The NAM indicates MLCAPEs in the 100 – 400 j/kg range Saturday afternoon in the northern San Joaquin valley north to around Chico, and that’s without widespread clearing. If clearing were to occur, CAPE/LIs would be higher than that, and (low-level) lapse rates would also skyrocket.

Forecast mixed-layer CAPE via the 0z 12km NAM, valid for 2pm Saturday.

Forecast mixed-layer CAPE via the 0z 12km NAM, valid for 2pm Saturday.

There is certainly potential for strong thunderstorms Saturday if clearing occurs, but confidence in rotating storms in my opinion needs another day to elapse for models to get a better handle on timing of all the elements.

Aside from convection, snow levels initially with the front Friday night should start out around 6000 – 6500ft, but drop off to around 5000ft later Saturday.

Precipitation totals will range from 1/2 – 1″ of rain in the valley in general, but along the east side of the valley upwards of 1.5″ is possible given decent upsloping/orographics in the sierra banking up against the foothills into the east side. Higher up, the foothills should see anywhere from .75 – 1.5″ in the south, to 1 – 3″ in the central and northern foothills. The western slope of the sierra is expected to pick up 2.5 – 4″ of liquid, locally a bit more in orographically favored areas. .5 – 1.5″ along the coast, with perhaps locally up to or near 2″, and lastly, 1 – 3″ in the coastal and northern mountains. These liquid QPFs would translate to 1 – 2ft of snow above 6000 – 6500ft late Friday – early Sunday.

Forecast QPF via the 0z GFS. 0z NAM/4km NAM came in dryer, thus for now I'll disregard it's QPF.

Forecast QPF via the 0z GFS. 0z NAM/4km NAM came in dryer, thus for now I’ll disregard it’s QPF.

Sunday for the most part should be generally drier, however leftover cold air aloft could support some scattered convection, but nothing widespread/significant is expected.

A larger and stronger low drops out of the Gulf of Alaska over the weekend and winds up off either the Pacific Northwest or OR/CA coast by Monday, and brings another round of rain, snow, and potentially another thunderstorm threat. This system could dump precipitation amounts similar to the Friday/Saturday system, except it’d be a bit cooler, with lower snow levels and more snowfall in the mountains. More information on this over the weekend.

Forecast 500mb relative vorticity and heights valid Monday afternoon, via the 0z GFS.

Forecast 500mb relative vorticity and heights valid Monday afternoon, via the 0z GFS.

I’ll have a update out Friday night regarding Saturday’s system, mainly the convective threat, and an update Saturday night for the Monday/Tuesday system, and probably again Sunday night to get a closer look at the possible convective threat Monday. It’s certainly been a busy week, perhaps some early and belated birthday presents given mine is Friday? Probably not, but nonetheless very enjoyable! Stay tuned!

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