Stormy Saturday, Colder Storm Monday & Tuesday

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The third in a series of systems that has impacted northern California over the last several days will continue to slide across northern California through the day Saturday, as it’s already slid through half the state as of Friday evening.

March 28th Map

The upper low driving this front into northern California is located off the Pacific Northwest coast (again), which is driving a strong 110 – 130kt upper jet into northern & central California. The jet itself is a cross over of the subtropical and main Pacific jet, which transports in some modest moisture along/ahead of the low’s associated cold front. The front will slowly slip through the region through early afternoon (Saturday) in the southern Sacramento valley, and clear the northern & central valley by late morning. Precipitation with the front will likely be moderate most of the time, but could be locally heavy given pockets of stronger lift along the east side of the valley and especially in the foothills and mountains where uplsoping and orographics enhance precipitation.Snow levels with the front overnight (Friday) into Saturday will stick around 5500 – 6500ft, but by mid to late-afternoon, colder air begins working it’s way southeast overhead, and supports snow levels around 4500 – 5000ft.

0z 4km NAM's forecast 300mb winds and 500mb heights valid Saturday afternoon.

0z 4km NAM’s forecast 300mb winds and 500mb heights valid Saturday afternoon.

Behind the front, the cold(er) air aloft will promote steeper mid-level lapse rates in a 150 – 200 mile wide plume associated with a vort-max associated with the upper low offshore. The timing of this vort-max will be key to convective chances, as it is what will be the key initiator convection wise behind the front. It does appear to be quite likely that things dry out and clear up behind the front in the central/northern valley by late morning and early afternoon, allowing surface temperatures to rise into the upper 50s to low 60s in the valley north of Sacramento. These temperatures will promote decent amounts of low-level CAPE, with values in the 100 – 300 j/kg range becoming more widespread from the northern San Joaquin valley through the northern Sacramento valley, with values in the Sacramento valley potentially exceeding 500 j/kg, with values upwards of 800 j/kg being modeled by some high resolution models including the 4km NAM.

0z 4km NAM's forecast MLCAPE and 10m wind directions arrows.

0z 4km NAM’s forecast MLCAPE and 10m wind directions arrows.

Directional shear will be decent once again Saturday given the upper low off the PNW coast and a vort-max off the northern CA coast, with surface winds out of the south to southeast and mid/upper-level winds from the southwest to west. As the vort-max comes inland, bands of better speed shear in association with the decent veering promote o – 3km storm relative helicity values in the 50 – 150 M2/S2 range, which is a bit less then the Wednesday event, but is sufficient for rotating thunderstorms given appropriate instability/buoyancy, and lifting mechanism. Forecast hodographs vary a bit by model, but in general do indicate some small clockwise hook features, also supporting the idea (or, well, probable fact) that directional shear will be decent in the lower levels at least.

0z 4km NAM's forecast 10m winds and 0 - 3km storm relative helicity.

0z 4km NAM’s forecast 10m winds and 0 – 3km storm relative helicity.

With these decent parameters forecast, the only question lies in the timing of the vort-max to aid in the initiation of thunderstorms. Right now it appears the timing may be a bit too late to kick things off, with the NAM bringing it inland by mid-evening, and by then surface temperatures will have plummeted and buoyancy would be weak. If the v-max comes in about 3 hours sooner, in the later afternoon at least, thunderstorm development would be likely, and given decent shear and instability in place, strong to severe storms would also be quite possible, including potential for large hail, strong winds, and another shot at funnels & brief tornadoes. Again, that’s if timing of the v-max is quicker than currently indicated.

0z 4km NAM's forecast 500mb relative vorticity and heights, valid at 5pm Saturday. Note the base of the vort-max is located offshore, at this time... it'd need to move inland near this time to support convective initiation behind the front.

0z 4km NAM’s forecast 500mb relative vorticity and heights, valid at 5pm Saturday. Note the base of the vort-max is located offshore, at this time… it’d need to move inland near this time to support convective initiation behind the front.

Precipitation amounts through Saturday night are expected to be around .25 – .75″ in the valley, with local amounts around an inch (mainly along the east side), with .75 – 2″ in the foothills, and upwards of 3 – 4″ of liquid in the mountains. This translates to 1 – 2.5 feet of snow above 6500 – 7000ft, with local amounts around 3ft above 8000ft.

0z 4km NAM's forecast snow through Saturday night.

0z 4km NAM’s forecast snow through Saturday night.

Cold air aloft remains overhead Sunday, but jet dynamics and lift wane, so even though there will be some low-level instability, cumulus and possibly some showers are about all that is expected.

Another, deeper low drops further south through the day Sunday and positions itself off the California coast by Monday, sending inland another cold front. This system will be quite a bit colder than previous systems, with snow levels dropping off to 2000 – 2500ft in the northern foothills/mountains, and around 3000ft in the rest of the motherload foothills. Timing of frontal precipitation is expected to be by morning along the coast, and afternoon across the rest of the region. Front currently looks to push east by late afternoon along the coast and early to mid-evening inland. Surface low itself begins to rotate into northern California early Tuesday, with another round of precipitation. The surface low could bring with it some thunderstorms during the day Tuesday, but hard to tell if any clearing will occur and if there is a severe threat. Precipitation amounts could be similar to that of the Friday/Saturday system, however there’d be more snow in the mountains given higher snow ratios due to the colder air.

0z 4km NAM's forecast 500mb heights and 300mb winds, valid Monday evening.

0z 4km NAM’s forecast 500mb heights and 300mb winds, valid Monday evening.

Another system could arrive Thursday into Friday, but beyond that it’s hard to tell what could evolve out of the pattern.

I’ll have an update Saturday or Sunday evening with more information on the Saturday/Sunday system.

Showing 2 comments
  • Kip

    Great weather info for California. I really appreciate your posts, and check them regularly. Thank you.

    • wxtracker15

      My pleasure, and thanks for viewing!

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