AM Update: Front+Vortmax to Bring Showers & Thunderstorms Monday

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Monday morning update (11am)

As of 10:45am, the cold front associated with a deep trough and upper-low in the Gulf of Alaska was sweeping through northern California. Scattered showers currently exist in the cold front’s precipitation/cloud band, with clearing (partly cloudy skies) noted behind the front along the north(west) coast. Post-frontal clearing should fill into the central and northern valley late this morning or early this afternoon as the cold front slowly works its way through the Sacramento/Highway 50 & I-80 corridor during the afternoon/evening south of Sacramento. Sunny conditions ahead of the front in the Sacramento area eastward could support some convective enhancement along the cold front’s belt of showers, perhaps inducing or supporting some heavier showers or thunderstorms along the front.

Visible satellite, radar mosaic, and METARs displaying precipitation-type from 11am, Monday.

Visible satellite, radar mosaic, and METARs displaying precipitation-type from 11am, Monday.

Behind the front, the clearing should be enough to boost temperatures in the central and northern valley into the upper 60s to low 70s – which beneath a -20 to -22c 500mb cold pocket associated with a vortmax off the northern California coast will support some steeper lapse rates, which then combined with dewpoints in the valley in the low to mid-50s would allow for SBCAPE in excess of 200 – 500 j/kg to develop – generally from Yuba City northward it seems, though some CAPE under 500 j/kg is likely southward through the Sacramento area.

12z 4km NAM's forecast (ML/mixed-layer) CAPE valid at 5pm, indicating a pretty unstable environment in the central and northern valley, with 500 - 1000 j/kg CAPE values depicted.

12z 4km NAM’s forecast (ML/mixed-layer) CAPE valid at 5pm, indicating a pretty unstable environment in the central and northern valley, with 500 – 1000 j/kg CAPE values depicted.

The vortmax off the coast is fueling, or at least supporting, some fairly robust cold-core convection offshore, with some sheared cloud tops noted. Wrapped around this vortmax is a 90 – 100 knot 300mb jet, which will remain overhead through most of the day and evening tonight even as the vortmax slides overhead in the evening. The upper-level winds themselves will aid in the sustainment of convection, and more importantly provide shear. Winds are out of the south to southeast in the central and northern valley today winds propel through the delta and curve north to fill the Sacramento valley and curve south to enter the San Joaquin valley. The south/southeast surface winds below the strong southwesterly winds from 500 – 300mb create moderate to strongly effective directional shear, with 0 – 3km SRH in the 50 – 150 M2/S2 range scattered across pockets northern California, and could be locally stronger given terrain enhanced surface wind backing in the central/northern Sacramento valley. Effective bulk shear increases this afternoon to 20 – 40kts from Sacramento to Redding, which itself is supportive of some organized/strong thunderstorms, and given the veering upper-level winds may support some supercellular thunderstorms if enough instability/buoyancy is present, and lift from the vortmax is sufficient.

Mesoscale analysis-derived 500mb vorticity (fill), 500mb heights (lines/contours), and visible satellite imagery in the background.

Mesoscale analysis-derived 500mb vorticity (fill), 500mb heights (lines/contours), and visible satellite imagery in the background.

12z 4km NAM's forecast 0 - 3km SRH valid at 5pm. Note the values in excess of 100 M2/S2 north of Sacramento in the unstable, post-frontal environment.

12z 4km NAM’s forecast 0 – 3km SRH valid at 5pm. Note the values in excess of 100 M2/S2 north of Sacramento in the unstable, post-frontal environment.

Thunderstorms in the central and northern Sacramento valley today, with all of that said, could be capable of strong winds, very heavy rain, hail (potentially large), and possibly some funnel clouds or a brief tornado (or two), if conditions permit.

We’ve seen a few weak storms over the last week that haven’t dropped any major rain or snowfall amounts across interior northern California, due to the fact that the trough/upper low driving these fronts into our area is well up north in the Gulf of Alaska, which causes these fronts to get strung out and narrowed down considerably the further south they dive.

Visible satellite imagery from around 1pm Sunday, with a easily visible cold front off the west coast (the same one that will sweep through northern California Monday). Perhaps more interesting is the vort-max feature behind the front, which is shrouded in cold-core convection/cumulus clusters.

Visible satellite imagery from around 1pm Sunday, with a easily visible cold front off the west coast (the same one that will sweep through northern California Monday). Perhaps more interesting is the vort-max feature behind the front, which is shrouded in cold-core convection/cumulus clusters.

Tonight/Monday, a deep low (which actually has some of the remains of what once was Super Typhoon Vongfong) will slam into the British Columbia coast, sending its front southeast though the night, arriving at our coast by Monday morning, sliding inland through the early/mid-afternoon. Band of moisture associated with the front equates to at or under an inch of precipitable water, which isn’t too questionable this time of year, but better than anything more meager of course. Vertical lift isn’t very significant with the front itself, however convective elements could come into play through the morning and afternoon to make up for the more meager frontal lift.

4km NAM's forecast precipitable water, forecast valid around 11am Monday.

4km NAM’s forecast precipitable water, forecast valid around 11am Monday.

4km NAM indicates temperatures rising into the low 70s in the valley by late morning and early afternoon, which would be ahead of the front in the Sacramento area. In the upper levels, at say 500mb, temperatures will cool to around -16 to -18c through the day as the post-frontal airmass nears and slides overhead through the day. These (cold) temperatures overtop the relatively warm surface temperatures forecast in the valley (which could be overshot a bit) would support some fairly steep lapse rates, which combined with available moisture (dewpoints in the valley are progged in the low to mid 50s Monday afternoon) also supports feasible CAPE values in the 200 – 500 j/kg range from the highway 50/I-80 corridor northward, with some highres models indicating post-frontal regime CAPE in excess of 500 j/kg in the central and northern valley.

4km NAM's forecast surface temperatures at 1pm Monday.

4km NAM’s forecast surface temperatures at 1pm Monday.

95 – 105kt upper-level jet lagging behind the front a bit will rage overhead during the afternoon and evening, primetime for convection. The jet wraps around a vort-lobe/vortmax that slides overhead northern California late in the afternoon into the evening, which itself could aid in the development of convection, especially given the upper-level wind support available. Given the decent upper-level jet and vort-lobe/max offshore warping the jet around it and creating some interesting shear. At the surface, winds in the valley jet through the delta and become south to southeasterly in the Sacramento valley. At 500mb (and above), winds are southwesterly to west-southwesterly – creating quite nice directional shear with the veering of upper-level winds. 0 – 3km storm relative helicity reflects this good directional shear, with values in the 50 to 200 range across much of northern California, with the bulk of the best values along the front, which isn’t as primed for convection as the post-frontal environment.

300mb (upper-level) winds wrapping around an offshore vort-max, modeled through the HRW-N, valid at 5pm Monday (primetime convective time, rhyme not intended, but it happened).

300mb (upper-level) winds wrapping around an offshore vort-max, modeled through the HRW-N, valid at 5pm Monday (primetime convective time, rhyme not intended, but it happened).

To sum things up, it appears one round of showers and potentially isolated thunderstorms will evolve out of the cold front itself as it slips through the region through the morning and afternoon, with the best chances of thunderstorms front the front being north of Highway 50. Behind the front, if enough clearing occurs to bring surface temperatures into the upper 60s to low 70s, more showers and a better chance of thunderstorms exists as a vort-max off the coast increases lift a bit and intensifies the upper-level jet. Best instability is currently placed between Yuba City and Redding, which combined with decent shear could support some more organized thunderstorms. Any thunderstorms that can become organized behind the front Monday afternoon into the early evening have enough thermodynamic and shear support to promote thunderstorms capable of hail, strong winds, and very heavy rain. If several hours of clearing occurs and instability/buoyancy becomes more significant, some mesocylonic action in thunderstorms could arise.

October 19th

Currently looks a few hundredths to a tenth of an inch of rain is possible in the Sacramento valley, with a few tenths to half an inch in the foothills and western slope of the sierra. In the northern mountains, about the same amounts as the foothills and west slope go, though there’s a better chance for amounts closer to an inch further north. These amounts don’t take into account all of the convection that could develop, thus some locations could see drastically varying amounts.

I’ll probably post a quick update Monday morning or early afternoon as the day brews up and a better idea of the extend of convective strength potential becomes clear.

Another weak clipper storm may affect northern California by Wednesday – but currently not seeing any widespread significant precipitation or convective threat with this system, with another weak system or two over the weekend. No significant storms in sight, unfortunately.

Comments
  • Mary B.

    Here’s hoping for rain in Sacramento!

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