UPDATE: Upper-Low to Bring Showers & Thunderstorms Tonight/Sunday

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6:30pm early-evening update

Showers and thunderstorms have spread from the western slope/sierra crest westward into the southern motherload foothills and northeastern San Joaquin valley. The activity is developing northwest further, and believe the Sacramento area could get in on the action between 7 and 10pm, with additional development possible through the night into Sunday morning.

Radar, lightning data, and surface dew points via METAR sites at 6:45pm this (Saturday) evening. The red outlined polygon is a Storm Prediction Center Mesoscale Discussion indicating potential for a few strong storms through the evening.

Radar, lightning data, and surface dew points via METAR sites at 6:45pm this (Saturday) evening. The red outlined polygon is a Storm Prediction Center Mesoscale Discussion indicating potential for a few strong storms through the evening.

Instability wise, 300 – 700 j/kg MUCAPE exists throughout much of northern California, while the valley has considerable CIN/capping. Nevertheless, given most of this instability is quite elevated, convection is generally developing well above the capping inversion, thus there’s limited negative effects on the development so far. Precipitable water values are between .8 and 1″ across a good chunk of the lower elevations with dewpoints in the mid to upper 50s… which indicates a fair amount of moisture present at the surface, and given the .8 – 1″ precipitable water values, in general there’s a fair amount of moisture to play with, but nothing extreme.

Effective shear generally between 5 to 15 kts indicates potential for slight organization of storms rooted as deep as they can go, with at least modest upper-level southeasterly winds in the 300 to 500mb range also wrapping around the low, which could aid in the sustainment of convection slightly as well through the night.

HRRR currently not handling the situation very well and is under-doing convective coverage. Most WRFs are also off with placement and timing, thus it’s more of a wait-and-see type deal than one that’ll be perfectly forecast, though, the forecast in general of scattered showers and thunderstorms looks golden.

Gusty winds and lightning could be a hazard fire weather wise and will be something to watch cautiously through the night. On the positive side, most storms should be wet, and the coverage we may see tonight could drop anywhere from a few hundredths to a few or several tenths of an inch of rain across the foothills, mountains, and possibly parts of the valley. Limited severe threat expected due to a lack of stronger nocturnal instability (though lift makes up for it in the presence of an upper-low), but isolated strong storms aren’t at all out of the question, some capable of hail/strong wind.

Saturday morning discussion:

A closed upper-level low that originated from the weak system that traversed through the area earlier in the week that is currently sitting off the central/southern California coast will begin to lift to the east-northeast through the night tonight across central and northern California, then off into Nevada by Monday. This will put northern California in a somewhat favorable upper-level flow for mountain convection this afternoon, continuing into the night and morning by Sunday. Fair amount of moisture resides down in southern California and Arizona, and it appears some of this will be drawn northeast into northern California through the day/night tonight. It appears this moisture, combined with bands of weak lift rotating around the low, westward down the west slope of the sierra and instability supported by a -10c 500mb cold pocket aloft associated with the closed low looks to be well-supportive of convective development across the sierra and western slope, especially tonight when moisture and lift increases a bit.

12km NAM's forecast 500mb heights and relative vorticity, valid late this evening.

12km NAM’s forecast 500mb heights and relative vorticity, valid late this evening.

Most high resolution models support the idea of showers and thunderstorms traversing from the western slope of the sierra into the foothills and valley, generally from Stockton to Chico. In the foothills, chances exist anywhere from Mariposa county northward to Butte county. The RAP indicates quite a bit of westward extend, with showers/storms trying into the delta. Total totals are forecast in the mid 40s to low 50s in the valley/foothills, which itself indicates good mid-level support for thunderstorms. MUCAPE values are forecast in the 100 – 500 j/kg range from the Nevada border west to the east side of the valley, which combined with the better lift the further east you go would suggest the best chance of valley storms would be east of Sacramento.

September 20th

Aside from models, water vapor imagery indicates moistening on the north side of the low over northern California and most of Nevada… indicating the mid/upper-levels of the atmosphere are beginning to react to the moisture being drawn northeast from the southwestern U.S., and the models seem to have a relatively good handle on this, increasing my confidence in the potential scenario tonight… however, cutoff lows and subtropical moisture are not at all the models cup of tea, and are quite often over and under doing some setups/events.

Water vapor satellite imagery combined with lightning data and 500mb heights analysis. Upper-level low can be noted off the central coast, with moistening over northern & central California, as well as most of Nevada, where thunderstorms are forming this afternoon as of 12:15pm.

Water vapor satellite imagery combined with lightning data and 500mb heights analysis. Upper-level low can be noted off the central coast, with moistening over northern & central California, as well as most of Nevada, where thunderstorms are forming this afternoon as of 12:15pm.

If thunderstorms do develop tonight into Sunday morning, they’ll be elevated, and typically elevated thunderstorms produce a fair amount of lightning. Increasing moisture and cooler air aloft will also support hail and heavy rain with more organized storms… however, there is always potential for lightning to occur outside of precipitation cores. Also, if storms (do) develop and collapse upon weakening, gusty winds wouldn’t be a good thing for fires… however, models indicate potentially some wetting rain tonight from convection along the sierra into parts of the west slope of the sierra – hopefully overtop the King Fire.

Since there’s still a good amount of inconsistencies and discrepancies with events like this, I’ll post another update late today or this evening regarding this event, and how chances are looking.

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