(Updated) Showers, Thunderstorms (Some Possibly Severe), & Wind Sunday

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Morning update (11am Saturday)

As of 10:45am, the cold front has slid through the Sacramento area, leaving the central and northern Sacramento valley sunny to partly cloudy, and is allowing temperatures to climb into the 60s from the earlier 50s. Clearing should sweep into the Sacramento area through around noon to 1pm, though continued showers are possible. In the upper levels, 500mb (roughly 18,000 feet AGL) temperatures behind the front are around -16 to -18c, which will support some instability overtop the warming surface temperatures not only in the valley, but along the coastal mountains and parts of the coast. Low-level lapse rates are expected to climb into the 7 to 8c range through mid-afternoon as surface temperatures warm where it’s been at least partially sunny, and in those same areas CAPE values should increase to around 200 – 400 j/kg on average, mostly from the Sacramento area northward through Redding.

Visible satellite and surface wind gust barbs via METAR sites, from 11:05am Saturday morning.

Visible satellite and surface wind gust barbs via METAR sites, from 11:05am Saturday morning.

Mid-level jet in excess of 60 knots remains overhead through most of the day today, with low-level winds also remaining gusty (surface gusts up to 40 mph in some parts of the lower elevations through the afternoon) given the 988mb cyclone off the OR/WA coast tightening pressure gradients will vent and support thunderstorms that develop this afternoon across northern California. In addition, these winds veer with height, with surface winds in the valley out of the south to locally southeast, and the 500 – 300mb winds out of the WSW, creating favorable directional shear for rotating thunderstorms/updrafts. 0 – 1km SRH (storm relative helicity) values reflect this favorable shear with values across the valley and parts of the coast in the 100 to 200 M2/S2 range, which is quite decent for California standards.

0 - 1km SRH, 10m wind arrows, and METAR wind barbs from 11:05am.

0 – 1km SRH, 10m wind arrows, and METAR wind barbs from 11:05am.

As post-frontal/cold-core cumulus and convection stream onshore through the afternoon, bigger surges of these bands/clusters riding over the coastal range should spark off valley development along the west side, and these showers/thunderstorms should strengthen at least a bit as they cross over the valley in the favorable/terrain-enhanced shear available to them, along with the available instability provided the sun stays out for a while.

My thoughts on the threats of strong thunderstorms remains about the same as it did last night, as you can find below, however high resolution models aren’t indicating too much in the way of rotating updrafts, which may be due to the lack of atmosphere-wide instability in the form lapse rates, as mid-level lapse rates are quite poor, below 7c today, which could limit the amount of strong thunderstorms today.

An interesting day is in store Saturday across northern California as a relatively rapidly deepening mid-latitude cyclone off the CA/OR coast slides northeastward into the Pacific Northwest, sending a cold front and potent post-frontal cold pool, which will promote an environment quite supportive of thunderstorms. The low is currently about 350 miles off the northwest coast of California, and has dropped off to 991mb. Continued deepening is expected through the overnight hours into early Saturday, and is likely to bottom out around 986 – 988mb according to the NAM.

Inferred satellite imagery from 8pm Friday evening, along with MSLP via mesoscale analysis, indicating a sub-992mb low offshore.

Inferred satellite imagery from 8pm Friday evening, along with MSLP via mesoscale analysis, indicating a sub-992mb low offshore.

Regarding the front, models are having a tough time with how well the precipitation band will hold together. Right now, I’d say the front won’t be backed by a solid precipitation band, rather a broken line of showers, perhaps multiple lines/clusters of showers, as while the low has tapped into a decent moisture plume, it appears to be offset from the front, in front of it rather than along and just ahead of it. Timing of frontal passage looks to be between 6 – 9am in the north and 8 – 11am in the Sacramento area, sliding from northwest to southeast.

The depth/pressure gradient created by the low will support a low-level jet (at 850mb) of 40 – 50kts across the lower elevations of northern California, mainly from Sacramento north. The strength of surface winds really depends on how well precipitation bands form, and models are a bit conflicted as to how well it holds together. If the cold front holds together well enough to bring through a band of at least moderate precipitation, the downward momentum created by the falling precipitation could be enough to drag down some 25 – 35mph surface winds in the valley. The low-level jet remains in excess of 30 – 35kts north of Sacramento through the mid to late-afternoon, so post-frontal showers or thunderstorms could not only try to drag some of these winds down lower, but also tap these decent winds a few thousand feet above the surface to allow for some sustaination of convection.

0z 4km NAM's forecast 850mb windspeeds valid at 1pm Saturday. Note: these speeds won't reach the surface, bu winds about 30 - 40% weaker could.

0z 4km NAM’s forecast 850mb windspeeds valid at 1pm Saturday. Note: these speeds won’t reach the surface, bu winds about 30 – 40% weaker could.

Behind the front, -15 to -18c 500mb cold pool slides aloft, supporting some instability when coupled with valley surface temperatures climbing into the mid to upper-60s according to the 4km NAM, however temperatures will be highly dependent on the amount of clearing that occurs. If a few to several hours of clearing occur behind the front during the morning and early afternoon ahead of what is in the upper-levels a negatively tilted trough axis with a piece of robust energy swinging around the surface low, moderate instability should be present with SBCAPEs in the 100 to 500 j/kg range from Sacramento northward, which will lead to what should be large-scale re-development of showers and thunderstorms during the afternoon and evening. Some high resolution models depict CAPE values approaching the 600 – 800 j/kg range between Yuba City and Redding, but is highly dependent on the timing of frontal passage and arrival of the trough axis and that belt of energy.

October 24th Graphic 1

In the upper levels, the trough/surface cyclone will be supported by a fairly powerful upper-level jet, with 100 – 120kt WSW 300mb winds blasting northern California during the afternoon. The winds remain pretty through the low levels (as explained above), with notable backing of valley surface winds – out of the south in the Sacramento valley, creating close to a 90-degree angle in wind vectors from the surface to 500mb, with winds out of the west to west-southwest at that level through 300mb. Hodographs aren’t significantly impressive due to the lack of a stronger westerly component in mid/upper-level winds, or more of a southeast wind at the surface in the valley, and exhibit more along the way of a center to the top right, instead of a much more favorable curve indicative of primed directional shear. Nevertheless, some looping can be noted in hodographs, and in some terrain-enhanced regions of the valley modified hodographs would exhibit more curvature. Forecast 0 – 3km SRH from Sacramento to Redding is rather impressive with 100 – 300 M2/S values in general noted, though in the lower levels from 0 – 1km, SRH is forecast to remain around 100 – 150 M2/S2, which is still relatively decent.

0z 4km NAM's forecast 0 - 3km SRH valid at 4pm Saturday afternoon.

0z 4km NAM’s forecast 0 – 3km SRH valid at 4pm Saturday afternoon.

With all of that back information provided, it appears that if sufficient instability arises Saturday afternoon/evening across the lower elevations of northern California from about Sacramento northward, enough shear is present to support the development of strong to severe thunderstorms, possibly some supercellular, capable of large hail, strong winds, torrential rain, and possibly weak tornadoes. If surface winds in the Sacramento valley end up having more southeast than south vectors, the tornado threat would increase from a higher-end chance to more of a low-end moderate chance.

Rainfall amounts from this system as a whole will be quite tricky unless a band of widespread thunderstorms develops and delivers more uniform precipitation, but that doesn’t look too likely at this point. It appears that the Sacramento valley should see anywhere from several hundredths to two to three tenths of an inch of rain from the front and a few post-frontal showers, though the north end of the valley is almost certain to get multiple showers or thunderstorms, and amounts up there are more likely to hit a uniform half inch or so, more directly under thunderstorm precipitation cores (which goes for the rest of the region). In the foothills and western slope from San Andreas northward, .2 – .75″ of rain is likely, with up to around 1 – 2″ in south/southwest facing slopes of Plumas, Butte, and Shasta counties, and other localized orographically enhanced regions. Around .75 – 1.5″ is also likely along the northwest coast, with several hundredths to three tenths in the bay area and delta. In the higher sierra, between 6500 and 7500 feet, rain will likely turn to snow Saturday night, with a few inches possible by Sunday morning.October 24th Graphic 2

A dryer pattern looks to take over by early next week, but models become much more conflicted by late week regarding the potential of another storm impacting California.

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