Showers, Thunderstorms Across Northern California Monday [Updated]

Home / Cold / Showers, Thunderstorms Across Northern California Monday [Updated]

Thunderstorm-Focused Update

Sunday evening, 11pm

The current trough’s cold front has slid through northern California, and will traverse through the San Joaquin valley through the the night. Now, northern California lies within the post-frontal environment — a cold, showery environment at that. Along the north coast of the state, showers and thunderstorms are impacting already as the core of the vort-max winds up in cold core convective cloud cover. The vortmax has invigorated the core of convection offshore, increasing lightning activity and strengthening individual storms. These storms will roll ashore from the Bay Area, north to the Oregon boarder, through the night tonight and especially by Monday morning as the trough axis pushes inland, putting northern California directly under this vast, unstable cold core. Models suggest perhaps a band or two of showers and storms rolling inland through the day Monday, however the strength of these bands is dependent on timing. If the bands hold off until late morning to breach the valley floor, there could be enough breaks in the clouds to support better instability for these storms to work with. If the storms breach the valley earlier in the morning, before temperatures have a chance to warm up a few degrees to strengthen destabilization, chances are we’ll be in for a somewhat more simple day of widespread showers and perhaps a few isolated thunderstorms, but nothing major, aside from potentially heavy rainfall from strong showers along the band.

Water vapor imagery depicting the cluster of convection off the north coast, with some lightning strikes noted near shore via lightning detection networks.

Water vapor imagery depicting the cluster of convection off the north coast, with some lightning strikes noted near shore via lightning detection networks.

If scenario one plays out, and sunshine is able to break out in parts of the valley ahead of the vortmax and it’s associated showers and storms, there is potential for some stronger, more intense thunderstorms due to lift being on the better side, with surface winds out of the south, and west to southwest mid-level winds, creating favorable directional shear for rotation in updrafts of well-organized thunderstorms, thunderstorms that will require decent instability to get going — thus at least a partially sunny morning to allow surface temperatures to at least approach the mid to upper-50s. If that occurs, strong thunderstorms capable of larger hail and strong winds are a possibility, but cannot go without mentioning that shear profiles such as what are forecast for Monday, are what I’d look for to mention a chance of a funnel cloud or brief tornado, and again, that’s only if the conditional threat is met with some broken cloud cover.

current event map

Best chance of thunderstorms through Monday evening before showers dwindle as the trough begins to exit our region lie along the coast, inland into the delta, valley and adjacent foothills — especially the southern Sacramento valley, south through the central San Joaquin valley. I placed the strong possibility over this region, as this is where the vort-max is expected to traverse inland, maximizing lift and shear locally with it.


 

Previous update, posted Saturday night;

A cold, mid-winter like system off the Pacific Northwest coast will drive into northern California over the next couple of days as the associated trough moves inland. This will send a cold, showery inner-trough environment inland, with the cold front clearing through the region here on Sunday. The cold front itself will slide through the north end of the Sacramento valley during the late morning, and by early afternoon, enter the Sacramento region. The northern San Joaquin valley, and points south and east will pick up frontal precipitation in the late afternoon into the evening. This front isn’t the most robust nor have very much of a moisture plume to work with (precipitable water values <.80″), however decent jet dynamics in the left exit of the trough’s jet combined with some instability right on the heels of the front should aid in the development of a band of showers, possibly even heavy, given the partially convective nature of it. Large field of cold air cumulus behind the front associated with the vortex max/upper-low will continue showers behind the front through the overnight hours.

precip

This post-frontal environment is what will make this forecast a bit tricky. Models vary regarding the strength and track of this vortmax in the base of the trough within the field of cold air cumulus, a fairly unstable thermodynamic environment. It appears a majority of models suggest a fairly robust vortmax embedded in the convective cloudbase offshore moving inland late Monday morning into the afternoon. There are some models that depict a much less active Monday with the trough digging a bit further south, seemingly placing the piece of energy out-of-touch with the convective cloudcover, thus no way to invigorate further development and deepening of convection. Other models keep the trough a bit further north, and this small positional difference seems to be enough to support the deeper and more widespread convection during the day Monday.

NAM's forecast 500mb wind and heights, valid at 1pm Monday. The vort-max can be noted off the San Francisco coast, where a small belt of mid-level winds wraps ahead of it.

NAM’s forecast 500mb wind and heights, valid at 1pm Monday. The vort-max can be noted off the San Francisco coast, where a small belt of mid-level winds wraps ahead of it.

The “coldness” of this trough is something generally not seen until perhaps December to perhaps late November, and could catch some mid-elevation locations by a bit of a surprise if not keeping up to date with weather information. -25 to -30c 500mb (~18,000 ft) temperatures down at our latitude will make this one of the coldest troughs at it’s latitude in the Northern Hemisphere, an interesting note. Down in elevation a bit further at around 850mb (~5,000 ft), temperatures near-zero (Celsius, or 32 degrees Fahrenheit) will exist, and support snowfall at that elevation and below it to a point by Monday. Snow levels with the cold front Sunday should begin up around 6000ft, and drop off to near 5000ft in the evening as showers continue. If showers continue through the early morning hours of Monday, snow levels could drop to as low as 4000ft, or around Pollock Pines in the central sierra foothills, and near 3000 – 3500ft in the northern foothills of Butte/Plumas counties — with similar levels in the northern mountains ans coastal range.

The cold air associated with this trough, not only supports low(er) snow levels, but an overall unstable environment, as mentioned. If models suggesting the slightly further north-positioned trough with a vortmax embedded in cold air cumulus are correct, Monday could be a fairly active day shower and thunderstorm wise. I’m leaning toward the idea of this active Monday, as models have underestimated convection associated with cold pools before, however in this case it’s a partially mixed bag of solutions, so the idea that the majority is leaning toward a more active scenario would make sense to roll with. With that said, it’s also possible Monday could be more held-back with isolated showers and storms. In either case, thunderstorms will be possible. With that, the strength of storms will depend on cloud cover in the morning and into the afternoon. If cloud cover is broken in the morning/early afternoon, it’d allocate increased surface heating (warmer temperatures at the surface), creating more rapid rising of this warm surface air, thus stronger instability (fuel) for showers and thunderstorms thrive. If cloudcover is widespread, surface temperatures would be hindered and cool, negating potential instability… and either case is possible Monday, making the chance of robust thunderstorms a conditional risk. Shear is present, both directionally and velocity-wise, with forecast hodographs in the valley indicating clockwise curvature in the early/mid afternoon, before the back edge of the trough enters and essentially ruins shear. Early/mid-afternoon would be “go-time” for any strong thunderstorms to present themselves and take advantage of the available shear — but these strong storms of course need the instability to strengthen in the first place.

Currently forecasted thunderstorm chance for Monday, however the 50% region will likely be expanded further south with the Sunday night/Monday morning update.

Currently forecasted thunderstorm chance for Monday, however the 50% region will likely be expanded further south with the Sunday night/Monday morning update.

Best chance of strong storms lies nearest to the aforementioned vortmax — wherever it moves inland, the lift it provides will support shower/thunderstorm development, as well as feature slightly better mid-level wind wrapped in front of it. Right now, models suggest this feature moving inland near the Bay Area, which would place the central Sacramento valley south into the northern or perhaps central San Joaquin valley under the greatest chance of robust thunderstorms, especially if it holds off on spreading clouds and showers inland until late morning or beyond, which at the moment, is the current timing models present. Thunderstorms Monday will have a general chance of small hail and gusty winds, but if the conditional threat is validated with morning sunshine beneath broken/limited cloudcover, I wouldn’t place a zero threat on larger hail and perhaps a rotating thunderstorm or two. I’ll post a thunderstorm-focused update to this post later Sunday or Monday morning depending how things look, but there will be more on this aspect of the system. I’d also like to point out that on Sunday, some thunderstorms are possible from Sacramento north, mainly at the far north end of the valley, where the typical Shasta/Tehama convergence thunderstorms could develop in the afternoon and early evening.

Hodograph produced by the 06z NAM in the southern Sacramento valley, valid at 4pm Monday. From the beginning to about a third of the way through the line, you'll note a clockwise curve, or hook. This is indicative of winds turning with height, and is typically found in environments supercellular thunderstorms are able to form given the turning of wind with height.

Hodograph produced by the 06z NAM in the southern Sacramento valley, valid at 4pm Monday. From the beginning to about a third of the way through the line, you’ll note a clockwise curve, or hook. This is indicative of winds turning with height, and is typically found in environments supercellular thunderstorms are able to form given the turning of wind with height.

Things clear and dry for Tuesday and beyond. Models are hinting at a system somewhere around the 15th – 16th (basically one week from the current system), so that provides something to keep an eye on as we get done with this system. Otherwise, stay tuned for updates regarding Monday’s thunderstorm chance. Sunday’s setup is pretty well-covered.

Leave a Reply

%d bloggers like this: