Update: Potent Cutoff Low to Bring Very Active Tuesday & Wednesday to California

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Monday evening update, as of 9pm

Closed low offshore has slowly been pushing eastward, while at the same time pulling in and significantly enhancing a powerful subtropical jet blasting southern & central California. The low is currently estimated to be at a depth of about 988 – 989mb, and is on a a slow weakening trend. The cyclonic pull of the low will drag a good chunk of moisture out of the subtropical jet overnight tonight into Tuesday as the front begins to push ashore. At the moment, there are two clear areas of thick/precipitation-supportive cloud cover; one directly associated with the low’s cold front off the northern California coast, and the other associated with the expansive subtropical jet, with a clear gap between the two. Over the last few hours, however, this gap has been closing off as moisture from the subtropical jet gets pulled into the low’s cyclonic flow. This moisture will enhance coverage and intensity of the frontal precipitation band/shield as precipitation fills in across much of the state through the morning and afternoon Tuesday. Pre-frontal precipitation will occur during the morning hours, which should be lighter than the band along/just ahead of the cold front, which slides through northern California through the afternoon/early evening.

Infrared satellite imagery from about 9:30pm Monday.

Infrared satellite imagery from about 9:30pm Monday.

Lightning has been noted with the front and moisture plume itself given the immense amount of moisture and lift available with this setup, with weak to moderate instability to set this convection off. NAM indicates some weak to modest CAPE across a good chunk of northern/central California during the day Tuesday with and behind the front, so cannot rule out some isolated thunderstorms during the day. Tuesday evening instability increases a bit more behind the front, so showers and possibly thunderstorms will remain possible through the night ahead of the low. By Wednesday, the low nears the coast, sending in another band of precipitation – this one potentially more convective in nature, which keeps things wet for what looks like most of the day, and again, thunderstorms are possible, with precipitation continuing into the night Wednesday.

Things ease up a bit Thursday as the low weakens considerably and rotates offshore, but showers will remain possible, just not nearly as widespread as we will have seen Tuesday and Wednesday.

Thunderstorm Potential

Best chance of thunderstorms at the moment for the Sacramento valley and adjacent foothills appears to occur Tuesday evening as a potential band of convection rolls through northern California behind the front, where instability is maximized. While shear is decent, and instability is modest, the timing and lack of sunshine to provide stronger buoyancy will likely limit the severe threat. Plenty of moisture available and a 30 to 45kt low-level jet at 850mb will support organized thunderstorms if development occurs, though, with heavy rain, hail, and strong winds possible from convection Tuesday night. Wednesday lots of cloud cover associated with the low’s precipitation band may limit strong buoyancy (once again), limiting updraft strength. That isn’t to say thunderstorms can’t/won’t occur, though. Short updates on convective chances will be posted if needed Tuesday, Tuesday evening, and Wednesday.

December-1st

Otherwise, previous information given in Sunday evenings update still appears adequate.

 


 

Sunday evening update:

An offshore closed low slowly gaining strength well off the California coast, which has kept northern California unsettled this weekend, will continue to strengthen into early this week before beginning a surge northeast and inland, with several sorts of impacts possible across California through the middle of the week (at least). Currently, the low is centered around 36 N / -138 W, or about 700 miles off the northern California coast. The low isn’t just mid/upper-level based, it’s also surface based, with a minimum surface pressure around 996 – 998mb. Well to the south of the low, by nearly 1,000 miles, lies a quickly maturing subtropical jet with deep convection blossoming. This subtropical jet is currently aimed at central Baja California, but is being drawn northward due to the closed low’s wide cyclonic flow pulling the band north. The combination of a weak polar jet and a moderate to strong subtropical jet will result in immense amounts of subtropical convection blowing up during the day Monday, which will feed northward into the low as the low progresses eastward towards the California coast through the day.

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Models have been having somewhat of a tough time it seems handling the trajectory of the moisture given off by this deep convection well off the Baja/southern California coast, with some runs taking it further north than others, and some further south. The amount of moisture that gets entrained has also fluxuated a bit, and if more were to be entrained than modeled, it could cause the low to “wobble”, strengthen a little more, or dump more precipitation compared to model output. This is something to watch as we near Tuesday, when the bulk of this moisture will move inland.

As the low nears the coast further by Tuesday, it’s expected to peak in intensity (depth) in the upper 980s to low 990s (in millibars), and will begin spreading cold frontal precipitation across California through the day, with mid to late-morning timing expected here in northern California. Models remain conflicted as to where exactly the low and bulk of the deep subtropical moisture goes, but in general it appears northern California is in for quite the wet day Tuesday as the potent cold front combined with precipitable water values in excess of 1.2 to 1.3″ and strong upper-level divergence support a wide swath of moderate to heavy precipitation. Behind the front, a continued very moist and favorable flow ahead of the low continues, keeping things relatively wet through the night Tuesday.

12z GFS's forecast MSLP and 500mb winds, valid at 4pm Monday, roughly when the low is expected to reach maximum depth/strength.

12z GFS’s forecast MSLP and 500mb winds, valid at 4pm Monday, roughly when the low is expected to reach maximum depth/strength.

By Wednesday, the low will approach the coast, potentially slide inland slightly, and recurve back offshore. As the low slides very near the coast Wednesday, another front sweeps through with another round of moderate to heavy precipitation. Precipitable waters remain near an inch with the this wave, which combined with even stronger lift could result in locally even more intense precipitation. Colder air aloft directly associated with the core of the upper-level low could also support some convection within the frontal band and especially behind it. Moist and at least slightly unstable flow continues behind this front as the low slowly rotates away from the coast and northward. Models become conflicted at this point by Thursday, with the ECMWF indicating the low will have weakened significantly and moved into OR/WA, while the GFS suggests the low gets pulled into a deeper low in the Gulf of Alaska, which would keep us wet/unsettled through Friday, with the ECMWF indicating ridging taking over by Friday.

Breaking Down Impacts

Precipitation amounts through Thursday

Currently looks like a good bet that much of northern California will receive an inch plus, with 1 – 3″ across the Sacramento and northern half of the San Joaquin valley, with 2 – 3″ across much of the motherload/foothills and west slope, 1 – 2.5″ along much of the coast, 2 – 5″ in the western Shasta county mountains, and upwards of 3 – 6″ along orographically enhanced slopes from Placer county northward through Butte county.

Nov 30th 2

Snowfall and snow levels

Given the very warm origin of the moisture tap, the system as a whole is very warm, with 850mb temperatures remaining in the 5 – 8c range through the bulk of the event. This will keep snow levels around 8000ft much of the time, but could try and trickle down a bit at times during heavier precipitation or convective bands. 1 – 2ft of snow will be possible above 8000ft through Thursday.

Winds

Tuesday, during the day, winds won’t be too significant due to the low being pretty far offshore still, with sustained winds in the valley likely only a few miles an hour to perhaps 10, with isolated gusts to perhaps 20 or 25mph. The low nears the coast Tuesday night into Wednesday morning, with the pressure gradient becoming much more noticeable, which combined with 850mb winds in the 35 to 45kt range and moderate to heavy precipitation, some gusts in excess of 30 to 35mph appear likely in the valley, with gusts in the 45 – 55mph range along the sierra crest/ridges.

Convection/thunderstorms

Some pockets of convection cannot be ruled out with Tuesday’s cold front, however, a lack of more substantial instability will likely prevent significant thunderstorm activity. As the low approaches the coast Wednesday, better lift and cooler air aloft associated with the core of the upper-low should bring inland better instability with the second front, as well as behind it. It doesn’t look very likely that any widespread breaks in the clouds will occur though, limiting more ambient buoyancy, thus, any convection should remain weak to moderate strength, but cannot rule out any lightning, especially along the coast where precipitation bands rotating around the low will likely have the most convection blowing up.

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