Updated: Showery, Cool, Unsettled Week Ahead

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Monday evening update, posted at 5:25pm

Upper-level low today tracked a bit further south than what was initially progged, bringing more precipitation to the I-80 corridor than expected previously. Precipitation continues across the west slope and east side of the valley this early evening/late afternoon, with snow levels down to about 4000ft along I-80 and Highway-50, with snow levels locally down to 3000ft from near Butte county northward. Snow/low-elevation showers will continue tonight, but will decrease in coverage by Tuesday morning.

Next system slanted for Tuesday into Wednesday is expected to be a bit stronger than this first system, with a surface low pressure of around 996 – 998mb progged to impact the far northwestern coast/southwestern Oregon late Tuesday evening/early Wednesday. This will send a cold front through northern California, with precipitation beginning along the coast by late morning/early afternoon, spreading inland into interior northern California by late afternoon into the evening. This system will have similar precipitable water values (.6 to .8 making it inland), but upper-level jet dynamics will be lacking a bit… but the position/strength of the surface low will aid in low-level moisture transport and lift, making up for the lack of stronger upper-level jet support. In addition, this surface low will also bring a surge of gusty winds with and ahead of the front, with 850mb winds rising to upwards of 40 knots, with surface sustained winds currently forecast in the 10 – 15 knot range. Thus, cannot rule out some 30 – 35mph gusts through the day and evening Tuesday, especially with any heavier precipitation along the cold front.

4km NAM's forecast 850mb winds, valid at 6pm Tuesday. Note: winds at this level will not make it to the surface, but winds somewhat weaker than these forecast values could.

4km NAM’s forecast 850mb winds, valid at 6pm Tuesday. Note: winds at this level will not make it to the surface, but winds somewhat weaker than these forecast values could.

Somewhat unstable air will accompany this system as it works its way inland Tuesday evening, with CAPE forecast to increase to into the one to three hundred j/kg range mainly from Sacramento westward through the delta/northern San Joaquin valley, with better instability closer to the coast. Thus, when combined with decent lift and frontal convergence, cannot rule out some isolated thunderstorms later Tuesday afternoon into the evening… mainly near/along the coast, but can’t rule out some evening storms making their way into the valley if conditions permit. Low-level shear looks decent along the coast, thus if any decent convection develops that’s strong enough to work with some of the available shear, can’t rule out some rotating storms or even some waterspouts.

Showers could linger into Wednesday morning, but should begin decreasing in coverage through the day as the low departs eastward and lift ceases. A deformation zone slides down the coast through the day, which aids in kicking the low out. This could bring some showers along the coast, but little accumulation is expected.

Another system is expected to slide through Friday/early Saturday, but model differences still exist and will hold off on going into detail with this.

Below: updated QPF forecast through Thursday. Includes rainfall that has fallen during the day Monday, and is not just for the Tuesday/Wednesday system.

Updated total precipitation forecast, including the precipitation that's already fallen during the day Monday.

Updated total precipitation forecast, including the precipitation that’s already fallen during the day Monday.

That’s all for now. I’ll post an update if needed Tuesday some time if the thunderstorm threat looks to increase.

On a side note, the potential for a cold blast next week decreased a bit today across most models… but still cannot be totally ruled out.


 

Sunday afternoon update:

A relatively complex pattern setting early this week will bring a fairly unsettled Monday, Tuesday, and Wednesday, as a train of weak to moderate strength storms parades into California over the course of the next few days. The first system is offshore this Sunday afternoon, and is comprised of the following: 1). a cold-core upper-low/trough off the California coast, 2). a split jet including a fairly potent subtropical jet blasting up from the southwest, and 3). a cold front separating the cold troughy air from the moist/warmer subtropical air being feed in along the subtropical jet. The cold front will advance toward the coast (of northern California) through Monday morning, with precipitation beginning to spread through most of interior northern California by mid to late-morning for most locations, and early afternoon in locations further east/in the mountains. Snow levels look like they’ll start out at around 5000ft, but progressively lower through the day to around 3500 – 4500ft by Monday evening, with moderate snow at times across the mountains. Weak jet dynamics won’t be all too favorable for heavy precipitation, but a south to southwesterly mid-level flow will support decent moisture transport into the the higher terrain enhancing precipitation, and a small surface low riding the coast near San Francisco will boost lift enhancement across much of the region… and when combined with precipitable water values in the .5 to .7″ range, some moderate to spotty heavy precipitation appears to be a good bet.

12-14-QuickMap

Precipitation from the first system begins to decrease from west to east Monday night into Tuesday, but showers will remain possible during this period. The next system begins to roll in right on the heels of the first, with precipitation redeveloping/spreading inland by Tuesday afternoon into the night. This system will feature a surface low off the Oregon coast, and will appears that it’ll sit off the Oregon coast through Tuesday night into Wednesday morning. This will send precipitation southeast around the low, with the coast getting the heaviest precipitation, with progressively lighter/less precipitation the further inland you go. Looks like the trough as a whole splits, sending another low directly into southern California Wednesday morning, delivering better precipitation into the LA Basin. This system will be a bit more unstable, so convection could help aid precipitation rates inland to make up slightly for the somewhat unfavorable setup. Snow levels with this system look similar to Monday’s, though amounts will be a fair amount less.

Showers may continue into Thursday as the cold air aloft combined with some leftover lift/moisture from the low-progressing low, but a deformation zone offshore pushes the low out by Thursday evening, and the deformation zone rapidly drops along the coast and falls apart/collapses, with little to no precipitation expected along the coast Thursday night. Another system looks possible at some point Friday or Saturday, but there’s still some model discrepencies with timing and strength and will hold off until a later time to go into detail on this system.

Precipitation amounts

Rainfall amounts from Monday through Thursday look to range from around a inch to two inches across northern California, with locally higher amounts in the Shasta county mountains due to some weak orographic aid. These liquid QPFs in the mountains will support snowfall amounts in the 1 – 2ft range above about 4000 – 4500ft, which isn’t to shabby for these weak systems, and this does not include the potential system on Friday or Saturday.

Updated total precipitation forecast, including the precipitation that’s already fallen during the day Monday.

Convective potential

Given the colder air these systems have to work with, some convection potential is expected every day from Monday through Thursday. Tuesday – Thursday looks a bit more favorable for convection compared to the Monday system given increasingly cooler air aloft to work with. Each afternoon Tuesday through Thursday potential will be maximized, but is conditional, and the chance would be highest if cloud cover were to break and some sun could warm the surface and boost buoyancy.

The long range outlook, say for the week of Christmas, is interesting. There is generally decent agreement that the east-Pacific ridge will set up in the Gulf of Alaska, which is a very favorable and notable position to drive (very) cold air from Canada into the western U.S., and is one of the main features I look for when looking for arctic blasts. The ECMWF and GFS both indicate this setup, but differ greatly with what occurs from it. Both drive cold air down into the west between the 23rd and 24th, in time for Christmas day… but the ECMWF is for the most part dry outside of a weak cold front, while the GFS has a fair amount of precipitation with a fairly strong “inside-slider” type low that dives into Nevada, which would set up a good pattern for significant mountain snowfall… possibly some foothill snow as well. There’s still a massive amount of time when it comes to forecasting until this time-frame comes up, so I wouldn’t put a lot of hope into anything just yet… but given the overall agreement that things will at least get cold, it’s something to monitor.

Long range teleconnections support this idea of a cold/troughy western U.S. pattern, with the PNA generally forecast to go deeply negative, and the AO forecast to go strongly positive. It’s a bit debatable when it comes to a positive AO and a troughy western U.S., but usually when the AO is negative the eastern U.S. scores more in the way of cold/troughyness, and the west usually ridgy. The NAO is forecast to go positive during the week too, which supports the idea of ridging along the east coast in this case, and generally when the east is ridgy the west has the opposite pattern (troughy). Aside from this likely confusing-to-most meteorologist gibber-jabber, it appears a consensus of long-range forecast & climactic anomalies support the idea of a much colder pattern for the west coast during the week of 22nd, or as stated before, Christmas week. More to come on this later this week.

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