Showers & Thunderstorms Tonight into Friday

Showers & Thunderstorms Tonight into Friday 

July 10, 2014 Cutoff Low, Dry, Dry Lightning, Fire Weather, Monsoon, Thunderstorms

Monsoonal moisture combined with forcing/lift from an offshore closed low will work together tonight to support nocturnal thunderstorm development across northern California, but mostly focused in the foothills, western slope of the sierra, and northern mountains. However, it does appear that at least some places in the Sacramento valley could score some action, whether it be some light showers or thunderstorms.

Currently, offshore, a pair of closed lows sits spiraling around. It appears the easternmost upper low, centered about 200 miles off the San Francisco coast, will venture eastward into northern California overnight into Friday according to most models. As the upper low pushes inland, what appears to be a weak front or boundary pushes east ahead of the low, across the valley into the foothills & mountains. Mid/upper-level monsoonal moisture that’s been feeding northward due to the flow around a ridge in the southwest will be aloft as the boundary slides overhead, which combined with the forcing and lift provided by the upper low should support the development of some convection.

Water vapor imagery from 4:30pm combined with 500mb heights.

In the low-levels, dry air surrounds the closed low, which will limit precipitation that reaches the ground initially with any thunderstorms that develop. While dew points in the valley are in the low to mid-50s in general with precipitable water values pooled up in the valley between .9 and 1.2″. However, forecast soundings tonight indicate inverted-v profiles with a dry layer around 850mb, thus, a good amount of evaporation will occur until that layer is locally saturated enough to allow precipitation to reach the ground. Due to this, thunderstorms being initially dry, a fire weather threat is present given dry lightning can easily spark new fires, plus the dry layer is also known to support strong/erratic winds with storms, from outflow and even microbursts, though relatively rare, can fan any new fire starts from lightning.

Due to this threat, the National Weather Service in Sacramento issued a Red Flag Warning.

High resolution models indicate potential for a decently organized MCC (mesoscale convective complex) that develops in the western slope of the sierra extending as far west as the western fringes of the foothills to possibly parts of the east side of the valley, sliding north and strengthening as it makes its way into the northern mountains, where damaging winds could become more prominent.

Nocturnal convection will likely die off after sunrise Friday, however another round of mountain storms will redevelop after some solar heating. Most of the storms Friday afternoon should be wet as moisture increases on a more widespread scale.