Showers & Thunderstorms Today & Tonight, Dryer Later this Week

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A potent low off the Oregon coast will swing a cold front through northern California later today into the evening, bringing with it bands of showers and possibly thunderstorms. This update mainly focuses on the convective aspect to this system, given it’s the potential is present, but I’ll certainly go over the general aspects to this system.

March 5th

The low itself is around 984mb, but will be tracking northwest towards the Washington coast through the night into early Thursday. The position of this low puts northern California in a typically good sector of the system for decent lift and moisture. In this case, it’s not really much different. The low is supported by twin jets: one notable subtropical jet blasting northeast from Hawaii, which is transporting some fairly deep moisture into northern California, and the main Pacific jet, which wraps around the low’s center and will blast Oregon and Washington through the night. The subtropical moisture plume being transported by the subtropical jet includes precipitable water values in excess of 1.2 – 1.3″, which will push into northern California through the evening… with 1.5″ values a few hundred miles offshore, though those won’t make it inland.

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This moisture combined with the low’s cold front, which will slide through the region this evening, along with decent jet dynamics at play will make for a decent, but rather quick (<6 – 8 hours) precipitation event across northern California through the evening. Showers should begin to become widespread after 4 – 5pm in the north valley, and by 6 – 7pm in the southern Sacramento valley and a little later in the northern San Joaquin valley.

Cooler air aloft with the front combined with decent moisture along and ahead of the front makes for some notable instability, with the cooler air aloft supporting mid-level lapse rates in the 7 – 7.5c range, and MUCAPEs of 100 – 500 j/kg along/ahead of the front. Through 8 – 9pm, valley surface temperatures should remain in the low to mid 60s, which should promote at least weak buoyancy, enough to support thunderstorms along and ahead of the cold front.

HRRR's forecast mixed-layer CAPE for around 9pm, indicating widespread 400 - 800 j/kg values. This is likely overdone by at least a couple hundred j/kg, though.

HRRR’s forecast mixed-layer CAPE for around 9pm, indicating widespread 400 – 800 j/kg values. This is likely overdone by at least a couple hundred j/kg, though.

Shear is available, including 0 – 1km helicities of 50 – 150 M2/S2 and effective shear values in the 20 – 30kt range, so any thunderstorms that develop would have the potential to produce some hail if they could grow strong enough. Given the decent low off the PNW coast, winds in the mid-levels will be pretty strong, with 850mb winds in the 20 – 45kt range (stronger further north). These winds could be easily drove down the to the surface with thunderstorms, in addition to the hail threat. Good moisture through the mid-levels will also promote potential for very heavy rainfall, which could cause some localized flooding, in the case thunderstorms do develop. This event will have little to no tornadic threat as buoyancy withers through the evening as surface temperatures drop. Some high resolution models have tried to indicate a line of thunderstorms along the front, and in this case, a significant amount of lightning could be discharged, along with strong winds, very heavy rain, and pockets of hail in embedded updrafts. This isn’t a certain scenario, but something to keep in mind.

4km NAM's forecast 0 - 1km effective bulk shear forecast for around 9pm tonight.

4km NAM’s forecast 0 – 1km effective bulk shear forecast for around 9pm tonight.

Even if thunderstorms don’t develop, lift without convective enhancement will be strong enough to support bands of moderate to heavy rainfall. It’s just that, without convection, localized pockets of extreme lift would be limited, and we wouldn’t see as many pockets of much higher QPF. Without convection, QPF would be a bit more uniform.

18z 4km NAM's forecast 700mb vertical velocities for mid-evening, indicating strong upward motion along/ahead of the cold front.

18z 4km NAM’s forecast 700mb vertical velocities for mid-evening, indicating strong upward motion along/ahead of the cold front.

Precipitation should become more scattered after frontal passage for the overnight hours, and not as heavy. Snow levels will start out rather high… AOA 8000 – 9000ft, but should lower to perhaps 6000 to locally 5000ft Thursday morning.

QPFs will be decent given the bulk of precipitation falls within a 6 hour period, with .25 – .5″ in the valley to locally upwards of an inch if thunderstorms develop, with 1 – 2″ in the foothills, and 2 – 4″ along the west slope and in orographically favored regions, and of course a bit more if that line of storms were to develop.

12z 4km NAM's forecast precipitation through Thursday.

12z 4km NAM’s forecast precipitation through Thursday.

Snowfall amounts will be limited to perhaps 3 – 7″ above 8000ft given snow levels drop after the bulk of the precipitation is over with.

Leftover cold pocket aloft will promote some instability across northern California Thursday, which may spark off some showers and possibly a few thunderstorms, but right now nothing widespread or strong is expected. If that changes, I’ll type out another convective update Thursday morning.

We dry out and warm up Friday and Saturday as ridging returns, but another system impacts Sunday into Monday. I’ll probably end up putting out a post on this system by Friday or Saturday. For now, let’s focus on tonight’s and tomorrow’s convective potential.

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