Thunderstorms Likely Through Wednesday

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Thunderstorms have developed over the southern/central sierra over the weekend as monsoonal moisture and lift/upper level support begins to affect northern California… however more widespread storms are expected to develop Monday through Wednesday as a weak offshore cutoff low rotates further northward and along with it, deeper monsoonal moisture.

Map depicting the placement of the cutoff upper low and the associated thunderstorm chances.

Map depicting the placement of the cutoff upper low and the associated thunderstorm chances.

On Monday, a much stronger, more noticeable wave of moisture, instability (which will still be fairly weak), and lift will rotate north into northern California from central/southern California as the weak cutoff low offshore spirals around smaller waves of energy. Ahead of the wave, thunderstorms are expected to begin developing over the mountains ahead of the wave in the afternoon, and possibly become more widespread by evening… possibly into the lower elevations through the overnight hours and into Tuesday morning.

It’s a bit hard determine the threat of the thunderstorms in the valley with these summer setups, however it does happen occasionally (especially when there is a decent source of lift, such as the cutoff low). This specific event is lacking instability, so I wouldn’t get any hopes up, rather just keep on eye on things as the setup is semi-conductive of valley activity. The higher in elevation you go, the better chance of storms you’ll have due to the lower amount of instability needed to spark off/support storms compared to the valley.

Band of upper level energy associated with the offshore cutoff low rotating north into northern California Monday night.

Band of upper level energy associated with the offshore cutoff low rotating north into northern California Monday night.

The same pattern is expected to repeat Tuesday/Tuesday night into Wednesday as well… however the threat of convection may slide further north compared to Monday/Tuesday.

Instability both days is expected to remain marginal… with surface CAPE values generally under 500j/kg (weak) and LIs above zero. Lapse rates aren’t the best, but are expected to remain around 7c, which isn’t strong, but also not weak. Precipitable water values between 1.3 and 1.6″ will support very heavy rainfall with thunderstorms that penetrate the initially dry sub-cloud layer. Some hail and strong/gusty/erratic winds are also possible from thunderstorms that decay due to outflow and possibly a microburst or two given inverted-v soundings.

Inverted-v in the low levels indicating a dry sub-cloud layer. You can also note a inversion up to 850mb, with decent lapse rates above that. This means convection will be rooted around or above 850mb in the more favorable thermodynamic zone.

Inverted-v in the low levels indicating a dry sub-cloud layer. You can also note a inversion up to 850mb, with decent lapse rates above that. This means convection will be rooted around or above 850mb in the more favorable thermodynamic zone.

As a trough pushes into the Pacific Northwest/BC, the flow aloft may favor mountain thunderstorms to continue through the weekend… mainly along the coastal mountains and in the northern mountains. Models aren’t in very good agreement regarding coverage and timing, with the ECMWF cutting off convective chances on Saturday while the GFS continues the chance through the Sunday. Not seeing much of a valley threat at the moment within this timeframe, though.

Post in the comments if you receive any interesting weather over the coming days, as well as on the Facebook page (where you can also find short term updates regarding local weather).

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