Very Strong Low to Slam California Tonight Through Saturday

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While we’re in a break through this evening, don’t be fooled: one heck of a strong storm is offshore, and it’s going to pack a punch when it nears shore and slows down early Friday morning. This secondary storm is much stronger than the previous; to compare, the last system dropped to around 998mb. The next system offshore is already down to around 986mb, and dropping in pressure by around 1mb an hour… possibly more at times.

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As you can tell from the satellite image above, this system is already looking very impressive, and by late tonight it should be a fully wrapped up, mature cyclone. In the early morning hours of Friday, the surface low is currently forecast to dip down to around 968mb by the 18z NAM. Other models indicate a depth of around 970 – 974mb. Either way, it’ll still be a very powerful storm – one of the deepest lows offshore in a few years, and the biggest precipitation event for southern California in at least a year.

18z 4km NAM's forecast MSLP, 10m winds, and 10m wind vectors arrows. At this given time, (around 6am), the low's depth is forecast at 968mb.

18z 4km NAM’s forecast MSLP, 10m winds, and 10m wind vectors arrows. At this given time, (around 6am), the low’s depth is forecast at 968mb.

Tonight, as the low deepens, the cold front begins to surge east, sending it’s associated precipitation band inland with it. Currently appears precipitation will begin to push eastward between 11pm and 1am for most of interior northern California. Cold frontal precipitation continues into Friday morning until the front clears to the east by the mid to late morning. As this front comes through, a belt of strong belt of low/mid-level winds streams over the region, including 50 – 60kt winds at 850mb and 925mb (closer to the surface) winds of 45 – 55kts from around Colusa southward. Given moderate to heavy precipitation along/ahead of the front, currently believe 40 – 50mph gusts are possible in the southern Sacramento valley south through the San Joaquin valley. 40 – 50mph winds are also likely in coastal areas.

18z 4km NAM's forecast 850mb winds/vector arrows, and MSLP for around 4am.

18z 4km NAM’s forecast 850mb winds/vector arrows, and MSLP for around 4am.

Between the cold front and the occluded front associated with the deep low lies a little vort max along a band of cold-core convection. This is expected to rotate into northern California some time Friday, bringing the next round of precipitation. Between the cold front and the small vort-max Friday morning and perhaps early afternoon there could be some clearing. Any clearing behind the cold front given proximity to the very strong low offshore and -20 to -25c cold pool aloft. If we see any sunshine Friday morning, surface warming beneath the cold pool would lead to some rapid destabilization across the lower elevations.

Models are somewhat conflicted on whether or not any clearing will occur. The 4km SPC WRF indicates sufficient clearing for the development of most unstable CAPE values in excess of 200 – 500 j/kg. The 12z NAM 4km indicated close to the same, but the 18z run indicates cloud cover and showers continuing through most of the day, limiting instability. The SREF is up there with the SPC WRF being quite bullish with clearing… and supports 400 – 800 j/kg MUCAPE values from Yuba City to Modesto.

If clearing occurs and sufficient instability is able to arise, thunderstorms would be very likely given strong large-scale accent to get things going on top of any robust buoyancy. Given the deep low offshore directional shear is decent, with surface winds out of the southeast and 700 – 500mb winds out of the southwest by afternoon. 0 – 1km and 0 – 3km AGL SR helicities of 75 – 200 M2/S2 support some clockwise hodograph curvature, which is indicates potential for some rotating storms. Temperature/dewpoint spreads of only a few degrees also support deep convective development, and smaller T/D spreads have also been tied to better tornado potential in combination of course with other thermodynamic and kinodynamic indicies.

4km NAM's forecast 0 - 3km AGL helicity for mid-afternoon Friday. Yellow colors and deeper are 100 M2/S2 values plus.

4km NAM’s forecast 0 – 3km AGL helicity for mid-afternoon Friday. Yellow colors and deeper are 100 M2/S2 values plus.

There may be some issues with some veer-back-veer in some portions of northern California, so in some regions rotational development could be hindered. Even if notably rotation were to be hindered, 20 – 30kts of effective bulk shear would support hail development… and given the decent cold pool aloft, hail (potentially large) would be possible.

In short, if sky’s can at least partially clear for a few hours Friday morning/early afternoon ahead of a vort-max between fronts, thunderstorms would be quite likely from around Chico or Yuba City southward through at least the southern or central San Joaquin valley. Thunderstorms Friday afternoon/early evening, if any were to develop and become strong, could become capable of producing hail (possibly large), very heavy rain as most convection does, gusty winds, and a tornado threat depending on location. Generally the further south you were the better veering and less veer-back-veer shear vertical wind patterns occur.

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Along the coast, convection is likely as soon as Friday morning, continuing through the day even into Friday night until the low finally pushes inland. Coastal storms also pose a hail and waterspout/weak tornado threat given proximity to the deep surface low and ample available low/mid-level instability and steep lapse rates.

The low itself fills/weakens through the day Friday, and is expected to push into the central California coast Friday evening into the overnight. The low’s occluded front rotates in with the low, bringing precipitation from around Chico southward through from Friday evening through Saturday morning. Precipitation with this feature could be heavy at times given it’s directly associated with the low, even though it fills rapidly as it nears the coast. Low impacts the central coast Saturday afternoon after the osculated front falls apart inland through the morning. As the low impacts, it’s last bit of lift and instability ahead of the low may keep showers going through the rest of the afternoon across northern California, with heavier rain along the south-central coast where  the strong south/southwest flow southeast of the low provides the best lift. Can’t rule out some additional thunderstorms from the southern Sacramento valley southward throughout the San Joaquin valley Saturday afternoon into the evening as the low pushes through northern California.

Wintery precipitation wise, snow levels in the sierra are expected remain around 5500 – 6500ft for the most part. Generally they’d dip under the 6000ft mark more often during periods of intense precipitation or convection. Friday night as the low comes inland and the occluded front wrapped directly around the low pushes inland, snow levels are forecast to drop a bit more… perhaps down to 5000ft or near the Blue Canyon level by then, though accumulation down to 5000ft may be tricky.

While this system has a significant sub-tropical moisture tap, with precipitable water values in excess of 1.3 – 1.4″ off the southern California coast, the best moisture will be shunted south of northern California, rather blasting southern California with the best moisture, thus they’ll score the best precipitation of this system… especially in the mountains. Even so, strong lift combined with the moisture we have available (.6 – .9″ precipitable water values through Saturday) will still produce some decent QPFs across the region. .75 – 2″ of rain is possible across the valley depending upon convective coverage… with 1.5 – 3″ in the foothills/mountains/sierra. Locally 3 – 5″ is possible in orographically favored regions/south/southwest facing slopes. Along the central coast amounts of 2 – 5″ are possible… with several inches in the southern California mountains, even a couple inches in the lowlands.

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Periods of very heavy rainfall could lead to some flooding problems across California over the next 48 hours or so. In addition to flooding potential, strong winds combined with heavy, soaking rain will also likely lead to many power outages… especially in regions that have already seen some rain in the last 24 hours.

Things clear out Saturday night into Sunday, though another weak storm is possible Monday or Tuesday. A stronger storm is possible by mid-week, but we’ll look into that after we’ve dealt with this current beast of a storm.

I may put out a short-term update Friday morning depending on how convective chances are looking. If the day is looking bleak, I’ll probably refrain, and if the day is looking rather active thunderstorm wise, expect a short update on the top of this post probably before noon.

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