Large Monsoonal Surge w/Upper Low Sunday – Tuesday

Large Monsoonal Surge w/Upper Low Sunday – Tuesday

August 2, 2014 Convection, Cutoff Low, Dry Lightning, Fire Weather, Monsoon, Thunderstorms 

A strengthening upper-level low off the northern Baja/southern California coast will continue its way northward through the day Sunday, bringing with it a big surge in mid/upper-level monsoonal moisture from the southwestern U.S. This upper low/moisture surge has brought a large amount of showers and thunderstorms to southern California, Arizona, and Nevada over the last couple days, and as the upper low moves northward, northern California will get in on the action over the next 72 hours or so.

Water vapor, radar, lightning, METAR weather conditions, and 500mb heights from 5:05pm Saturday. Upper-low is well-defined off the Baja/southern California coast.

Models have been fluctuating a bit over the last several runs regarding westward extend of the upper low and associated convection – with 18z NAM trending quite a bit further eastward compared to 12z – but 18z models can be strange. Even so, if the 18z were to be closer to correct than the 12z run (which would have resulted in a significant amount of convection across much of northern California), there’d still be a decent chance of showers and thunderstorms from Stockton northward in the lower elevations of northern California as a vortmax on the eastern fringe of the upper low slides over northern California, likely somewhere north of SFO.

18z 12km NAM’s forecast 500mb relative vorticity, with the positive max over northern California being the center of a vort-max. Forecast time is Monday evening.

Moisture in general, in the form of precipitable water, isn’t too significant with the initial/entry wave Sunday night, with values around .8 – 1″, but by Monday values surge to 1.2 to 1.5″ across northern California, and this is when our chance will be the best for storms to survive and develop into the lower elevations of northern California, and when instability will be at its best… however some model runs were indicating storms pushing northwest into northern California as early as Sunday evening, continuing northward slowly through the night into Monday. The best combination of moisture, instability, and (weak) upper-support is currently modeled to converge in the northern and central Sacramento valley, northern foothills, sierra, coastal mountains, and perhaps northern mountains Monday, Monday night, and even into Tuesday – though the chance of convection would continue through Monday night from around Stockton north.

Actually, Sunday through Tuesday would be more accurate.

The key factor to this setup is a vort-max rooted around 500mb on the western side of the upper-low, as stated earlier, and is what will support the development and sustainment of convection, and this is the feature models are having trouble with. If a westward trend begins again, northern California would be in for a widespread convective event, but a trend eastward as seen in 18z U.S. models (GFS, NAM) would degrade our setup and likely just lead to another wave of mid/upper-level clouds from convective remnants from the sierra and Nevada instead of live/actively-developing convection surviving into the lower elevations.

Higher resolution view via the 18z 4km NAM of 500mb relative vorticity – valid at 8pm Monday. This is about 24 hours later timing than the 12z runs of both NAMs.

Dry layer in the lower-mid levels of the atmosphere will slowly slacken up and moisten between Sunday night and Monday, but despite this, lightning striking outside precipitation cores cannot be ruled out, and if this event sets up like some of the more bullish models are advertising, storms could produce rather large amounts of lightning, and it only takes one outside of a precipitation core to potentially spark a fire. Some storms could collapse/weaken and produce outflow winds that are quite erratic as well, which can fan fire starts.

Aside from fire threats from convection late Sunday through Tuesday, increasing precipitable water values in combination with moistening soundings would support heavy rain with most thunderstorms, and steep lapse rates could support some hail with stronger thunderstorms. LCL heights will be in excess of 1000 – 2000 meters across much of California during the event, so all convection that develops will be quite elevated.

Upper-low and monsoonal surge will slowly continue northward into Wednesday and Thursday before most of the leftover moisture is out of our area, minus some sierra crest and eastward storms.