Winter-Like Storm to Bring Severe Threat to Sacramento Valley Thursday

Home / Active / Winter-Like Storm to Bring Severe Threat to Sacramento Valley Thursday

Howdy everyone, I missed posting about the last thunderstorm threat due to a lack of sufficient time to throw together a post (these take a lot of time to put together) — but I couldn’t let two events pass norcalweather.net’s view. Currently, a deep trough filled with a handful of surface lows and vort-maxes off the western U.S.’s coast is preparing to send inland a narrow, but relatively potent cold front. Ahead of and with this cold front, a relatively narrow but focused band of moderate precipitation will sweep through the region — bringing anywhere from a tenth to a half inch of rain in the valley, and 1 – 2″ of liquid precipitation for the higher elevations — although some of that precipitation will fall as snow above around 5500 – 6000ft overnight, dumping a quick 4 – 8″ above the snow level. Gusty winds ahead of and along the front are also expected, as the low-level jet amps up to around 40 – 50 knots during the evening hours Wednesday.

Radar and visible satellite imagery from ~3:30pm Wednesday, with the vort-max “comma” noted in the middle of the frame, expected to help generate thunderstorm activity Thursday.

As the cold front pushes ashore and through interior northern California Wednesday night, the troughs orientation will change from a neutral orientation to a negative tilt by Thursday as a strong mid-level jet streak (90 – 110 knots) rounds the base of the trough. This orientation will come in handy when we get into the specifics on the threat of thunderstorms later in this post. Behind the cold front Wednesday night, a dry slot will rotate overhead between the back edge of the front and a small surface vorticity currently visible on satellite offshore wrapped in convective cloud cover and showers. This dry slot will stay overhead into the mid t0 late-morning hours Thursday, likely allowing some surface heating to occur in parts of the valley with some high resolution models suggesting surface temperatures making it into the upper 50s to low-60s across much of the Sacramento valley. These warm surface temperatures along with the good low-level moisture leftover from the overnight precipitation beneath the overhead trough’s -28 to -30c mid-level cold pool will steepen mid-level lapse rates quite a bit — potentially to 8c (very steep). These steep lapse rates combined with CAPE (convective available potential energy, a measure of atmospheric instability) values in excess of 400 – 600 j/kg from Sacramento northward should create an environment primed for thunderstorm development by California’s standards.

High resolution NAM’s forecast MSLP (blue contours), surface wind azimuths (directional arrows), 500mb heights (brown contours), and CAPE (fill). The surface low/vort-max sits west of Chico in this image, and is where instability is maximized. Instability can also be noted to be highest along the east side of the valley elsewhere.

Timing of the surface vorticity will be key to thunderstorm development Thursday. Currently, models suggest this vorticity moving over the coastal mountains and into the central & northern Sacramento valley by early to mid-afternoon, remaining overhead into the late afternoon before sliding east into the foothills after around 6pm. This timing during peak afternoon heating will likely help provide the lift to support robust thunderstorm development in the Sacramento valley as it moves overhead. In addition to providing the extra lift to get thunderstorms going, it’ll also strengthen the low-level flow ahead of the vorticity out of the south. Models currently place surface winds in the 15 – 25 knot range across much of the valley Thursday afternoon out of the south to southeast locally. These south to southeast winds, backed (easterly flow enhancement) more by that precious surface vorticity moving into the valley, will create excellent directional shear given the mid & upper-level flow associated with the large-scale upper-low’s jet will be out of the west to southwest. The surface winds out of the south overall beneath the westerly upper-level flow creates close to a 90-degree angle in wind directions with height — perfect to support rotating thunderstorms given turning wind with height. The backed low-level winds and decent westerly flow in the mid & upper-levels is a key trait of being in the SW corner of a negatively-tilted trough — right where interior northern California will sit Thursday.

Forecast soundings suggest the best shear & instability combination will exist from the middle of the valley east from Sacramento north, although once you get north of the Sutter Buttes much of the valley lies in the favorable conditions for strong thunderstorm development and sustaination. A couple of issues with the setup include 1). the core of the jet will be focused on central California Thursday afternoon — keeping stronger mid-level winds out of Sacramento valley thunderstorms’ reach; this will primarily mean storms move slower and loose out on a bit of speed shear. 2). secondly, if if the vort-max speeds up, thunderstorms could develop too early and miss out on peak afternoon heating & instability, limiting the severe threat. Overall, if current model guidance is correct or even off by a hour or so of timing: this event warrants the always anticipated “chance of a tornado or a few funnel clouds” wording given the available shear, instability, and lift. This events setup lines up like plenty of typical northern California supercell producing systems do do, and they all have their individual setup pros and cons.

NAM’s forecast sounding for the Glenn/Butte county border valid at 1pm Thursday. With the moderate amount of instability available and the curved/hooked hodograph along with backed surface winds out of the southeast, small rotating supercellular storm structures would be possible if enough instability arises.

Generally, thunderstorms will be possible from the northern San Joaquin valley northward — with the severe threat beginning from around the Sacramento area northeastward from around Highway 70 & 99 eastward, then envelops the entire valley north of the Sutter Buttes. Best threat of funnel clouds and perhaps a tornado looks to be from around Live Oak to Anderson (between Redding & Red Bluff), with strong winds and large hail possible from Roseville northward.

Storm chasers: if you live in the Sacramento valley — especially in the central valley and have availability Thursday, be ready to jet after lunch time. Southern Sacramento valley & San Joaquin valley chasers: don’t completely rule out something interesting between Stockton and Modesto. The northerly surface flow in the San Joaquin valley will make it tough for rotating storms to drop anything, but hail is a good bet.

Leave a Reply

%d bloggers like this: