Active Several Days Ahead; Thunderstorms The Main Threat

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8:56am Monday: New update to be posted by 1pm.

A fairly deep cutoff low off the central California coast pulling in a modest monsoonal moisture plume from the southeast will begin to ignite widespread thunderstorms beginning Sunday afternoon across the sierra, possibly spreading westward into the western slopes/foothills by evening and potentially into the southern Sacramento/northern San Joaquin valley, if storms can survive the trip down the hill.

300mb winds and water vapor imagery indicating our pretty deep cutoff low offshore, and moisture beginning to rise.

300mb winds and water vapor imagery indicating our pretty deep cutoff low offshore, and moisture beginning to rise.

Sunday night into the early morning hours of (12am to around 4am) Monday, moisture will have just increased, with precipitable water values increasing to near 1″ overnight. MLCAPEs generally under 200j/kg are expected to limit the coverage of any nocturnal convection, though. Overall, the Sunday – early Monday time frame convective threat is low, but is there.

Sunday/early Monday's convective threat.

Sunday/early Monday’s convective threat.

Monday into Tuesday morning, moisture becomes more notable, with precipitable water values climbing upwards of 1.2 – 1.4″. Instability becomes a bit stronger, at least over the mountains, with MLCAPEs even after sunset remaining up near 700 – 1000j/kg… with valley values around 200 – 700j/kg. These values are more conductive for convection on a more widespread scale. Models indicate a potential MCC (mesoscale convective complex) developing along a band overnight from the sierra, across the valley, over the coastal mountains. Best chance of this occurring looks to be from Highway 50 northward. If no MCC or MCS develops, at least scattered thunderstorms are likely.

Thunderstorm threat forecast for Monday into Tuesday morning.

Thunderstorm threat forecast for Monday into Tuesday morning.

Tuesday into Wednesday morning, expecting nearly the same setup as Monday night.

Wednesday into Thursday, the cutoff low is expected to begin pushing inland. Models are a bit conflicted with the coverage of thunderstorms as the low comes inland, but it appears, at least at the moment, that the best threat of storms will occur north of Yuba City, but in accordance with other models, a lower chance remains down into the northern SJ valley in an isolated case… more likely in the sierra.

Wednesday into Thursday's threat, as the cutoff low begins to push northeast and inland into NE CA and southern OR.

Wednesday into Thursday’s threat, as the cutoff low begins to push northeast and inland into NE CA and southern OR.

Though forecast soundings across the valley indicate a strong inversion around 900mb through the event, above that inversion lapse rates are fairly steep, which should easily allow elevated convective development.

40 – 70kt winds between 500 and 300mb will allow for some better organization compared to our normal monsoonal setups with generally have little to no upper level wind. With there being some upper level winds caused by the offshore cutoff low, there will be some shear overhead to work with. Effective shear values over the several day event are forecast to remain around 20 – 30kts. Because storms won’t really be surface based, the shear may help organize convective clusters and/or MCCs and MCSs.

Forecast 300mb wind and heights valid for early Tuesday morning.

Forecast 300mb wind and heights valid for early Tuesday morning.

Some storms over the the several day event will be capable of strong, gusty winds due to the 40 – 70kt wind max accompanying the cutoff low (as the winds could be driven down by thunderstorm downbursts, microbursts, and collapsing thunderstorm outflow winds. Strong winds would also be likely with any MCCs or MCSs that develop (if any). 500mb temperatures are forecast around -10c, which combined with decent instability will be good for hail development in stronger storms as well. Some large/severe hail occurring is also likely.

Over the 5 day event, rainfall amounts will be extremely variable. Some locations could pick none, while some (mainly in the mountains/sierra) could see between 1 – 3″ of rain. Training convection could cause localized flooding.

Elevated thunderstorms, as in this case, will be capable of massive amounts of lightning discharge. Though storms will have very wetting rainfall under the precipitation cores, cannot rule out isolated strikes outside precipitation cores… and given that brush is extremely dry, fire starts are easily caused. Gusty winds near thunderstorms could also fan flames. This chance isn’t very high though given it seems pretty likely even regions outside precipitation cores could be impacted by the precip core shortly after a dry lightning strike occurs as the storm moves downstream.

Expecting things to dry out over the weekend as the cutoff low exits and a trough digs offshore, amplifying the ridge in the southwest.

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