Monday, March 31, 2014

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All Aboard, Last Stop, The Sierra

Well we do have one more storm in this wave train. It will impact the Sierra today and through tomorrow night with the heaviest snow tonight. This will be a cold system with a limited moisture connection, so the bust out some bubbly to celebrate champagne powder in April.

The proper way to open champagne (there are many other videos of popping/spraying champagne, but wasn't quite sure they were appropriate...)

Quick and dirty forecast: 12-18" of snow for areas near the crest and further south (Mammoth) with areas east of the crest looking at 6-12" (Mt. Rose, Heavenly, Northstar) (Figure 1). There is an outside chance some areas along the crest at the highest elevations could push 2'. 

Figure 1: HPC 72-hour precipitation forecast. 0.5-1.5" expected by Wednesday night with most of it coming tonight through tomorrow night.

There will be very good lifting (dynamics) with this storm due in part to the very cold air aloft (cold air over warm air, air rises and therefore more snow) (Figure 2). The moisture connection is not nearly as good as this weekend's storm, but convective elements within the storm should help increase snowfall.

Figure 2: 9-10000' ft. winds, heights, temperatures. Breezy but not too windy conditions tonight and tomorrow with very cold temperatures. This means a better snow-to-water ratio and powder conditions.

The first part of the storm will move through this evening with the larger trough and cold air pool moving over the area tomorrow (Figure 3). The heaviest snow will be tonight with lingering snow showers through tomorrow. The southern Sierra will get another round tomorrow night as the trough moves to the south and east.

Figure 3: Mid-level winds, vorticity, and heights. Top image valid tonight and bottom image valid Wednesday morning. Lots of cold air and instability moving across the Sierra in the next 72 hours.

After hearing the great reports yesterday, the stellar conditions look to continue through midweek. So go out and enjoy the next few days! And ride the late (midnight) season train!

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Friday, March 28, 2014

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Riding the Spring Wave Train

Well the waves are stacked up in the Pacific and due to some changes in the hemispheric long-wave pattern, they are dropping south and impacting the Sierra. With four different waves forecasted to impact the Sierra in the next 8 days, I think it's time to jump on board the Love Train!

The first wave will impact the Sierra tonight and into early Sunday morning. The crest will see the most snow, with less snow the further east you head (as usual). So we are forecasting 12-18" along the crest and on the west shore, with less snow further east for areas like Northstar, Mt. Rose, and Heavenly (6-12"). We also wouldn't be surprised if some of the highest elevations near the crest see 24" of snow by Sunday morning (Figure 1). 

Figure 1: 24-hour precipitation forecast totals valid tonight through Saturday night.

Figure 2: Mid-level vorticity (measure of spin and instability in atmosphere), winds, and heights. X's and color-shaded areas represent shortwaves/storms. Stacked up across northern Canada and all through the Pacific.

Overall the pattern has shifted in the past week. Ridging and a blocking pattern has developed over the eastern United States, which has helped create more meriodinal (N-S, S-N) flow over the western United States. This has allowed that 'backdoor' cold air to slide south out of NW Canada as we have seen in our previous winter storms this year. Additionally, a ridge has developed further west in the northern Pacific than it normally has this winter, and is not quite as strong as it has been. This has allowed an active Pacific to move troughs up and over the ridge and back down south to the California coast with some help of the cold Canadian air (Figure 2).

Figure 3: Precipitable water valid tomorrow morning

The first wave will move through tomorrow, but good moisture advection with southwest flow ahead of the trough will start producing precipitation this evening and overnight (Figure 3). There is a better moisture connection with this storm than the previous one and the moisture connection will be better than the two waves early next week.

Figure 4: 5-day forecasted precipitation totals. 2-5" expected in the Sierra by Wednesday morning.

Overall, we are looking at 1-3" of precipitation from tonight to Sunday morning (Figure 1). However, some of the higher precipitation totals will be at lower elevations. The snow level will begin around 7000' and drop tomorrow to around 5000' by tomorrow afternoon.

We will have a break in the snow late Sunday morning through Monday afternoon before the snow starts up again. There will be two shortwaves the impact the Sierra Monday night and again Tuesday night. As of now, we are forecasting 4-8" for the first wave (Monday PM - Tuesday AM) and 3-6" for the second wave (Tuesday PM - Wednesday AM). So by Wednesday morning we should have another 7-14" of snow.

It should be some fun riding over the next week with the wave train coming through, albeit a late wave train. So enjoy it while its here and stop wishing it didn't come 2 months ago! We will get some nice winter riding in and this will extend our spring skiing further into April. Ride the train!!!

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Tuesday, March 25, 2014

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More snow at the end of the Tunnel: Round 1

Ullr looks to reward us over the next seven days as he sends a series of storms barreling toward the Sierra. The first of these storms is currently driving a cold front through the Central Valley as is visible on radar.
Regional Radar Reflectivity from 1100 PDT
Moisture looks to be limited behind the initial surge of cold air as can be seen by the thin band of showers on the radar.  However, still expect some high elevation snow this afternoon with the initial frontal passage before things dry out a bit overnight.  Cold air will continue to filter into the Sierra overnight, dropping snow levels below the Lake by dawn.

The real action begins tomorrow around daybreak as increasing moisture and upper level support combine to produce widespread Sierra snowfall. Models have shifted the best dynamics a little farther to our south than in previous runs favoring the Mammoth Area. This means the lift over the Tahoe area will be weaker than previously anticipated.  Weaker lifting reduces the amount of moisture that gets wrung out of the column.  Ultimately, reducing snowfall amounts for the Tahoe Basin.
The forecasted Jet Stream intensity and location for 800 PDT tomorrow morning.  Note the left exit region of the Jet is directly over the Mammoth Area
 However, the best moisture remains over the Lake and points to the north, still keeping the higher snowfall totals in our neck of the woods.  High resolution models are still producing 1.5 to 2.5 inches of liquid equivalent over the crest with lesser amounts to the south.
24hr accumulated liquid precipitation (in inches) from tonight at 1700PDT through tomorrow at 1700PDT.

 Models are also prolonging the event with moisture drying out more slowly on Thursday.  
Even with weaker dynamics in place, still think original snowfall forecast is on track with: 18-24" along the crest, 12-15" mid-mountain and 4-8" at Lake level.
Upper limits are a bit optimistic but cannot rule out some pockets of heavier snow due to decent instability and the possibility for isolated snow squalls. 

The next round of snow looks to impact the Tahoe Basin Saturday night with another system possible on Monday night.
Model depicted 500 mb vorticity or spin, with maxes denoted by crosses over the Pacific at 1100PDT Wednesday.
 Check out the next two storms of the series, with the first underneath the base of the Aleutians and the other, moving south of  Kamchatka.
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Monday, March 24, 2014

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Midweek Madness

One more day of spring before winter's icy grip returns to the Sierra Tuesday afternoon.  Confidence continues to increase in a moderate snow event for the Sierra focused on the Tahoe Basin, a rare and welcome occurrence this season. Unseasonably cold air associated with this system will provide more snow and less rain with snow levels crashing to Lake level before the bulk of the precip arrives.

Current thinking is still on track with 18-24" along the crest, 12-15" mid-mountain and 4-8" at Lake level.  
The Hydrometeorological Prediction Center (HPC) is giving it a 20% chance for 18 inches of snow along the crest through Wednesday at 1700 PDT. 
 An impressive cold front will move over the Sierra Tuesday afternoon/evening with some snow beginning for the highest elevations at that time.  Depending on the model, snow will either continue through the overnight or take some time to become widespread.  Either way, the best dynamics look to move into the area Wednesday morning as the left exit region of a 125+ kt cyclonically curved jet moves overhead.
The forecasted Jet Stream location and intensity Wednesday morning at 800 PDT
 The left exit region is notorious for producing copious amounts of precipitation due to enhanced lift through a deep layer of the atmosphere.  Simultaneously, the cyclonic, or counter clockwise, curvature of the jet provides favorable vorticity, or spin, further intensifying the lift.  For those of you physicist out there, just think about the "Right Hand Rule." For everyone else, more spin equals more lift and that means the snow piles up deeper and faster.
The forecasted mid-level vorticity Wednesday morning at 800 PDT

With the dynamics in place, the real question is: "What about the moisture?"

That is where the bulk of the uncertainty remains among the models.  Luckily for us, the models continue to trend towards the wetter GFS model. The folks at HPC are beginning to buy into that trend and have ramped up their precipitation forecast for the Sierra.
HPC precipitation forecast for through Friday at 1700 PDT with 2.8 to 1.5 inches along the Sierra Crest.
The bulk of the storm will come to an end late Wednesday night /early Thursday morning but not before it leaves a fresh blanket of surprisingly light Marchmellow fluff.

 Lingering moisture and instability will combine to produce scattered snow showers through the afternoon with maybe an inch or two of additional accumulation for a few lucky spots.

Another forecast will be issued on Tuesday before the system is slated to arrive.  Hopefully models continue to trend wetter with this system and we get in on a little March Madness of our own.

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Friday, March 21, 2014

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Winter in Spring?

Well the models have been consistent for past five days or so bringing a trough and some much needed and wanted snow to the Sierra. It will start impacting the Sierra Tuesday evening and last through Wednesday night. The heaviest snow will be Wednesday morning and afternoon. However, we know with the trough being 5 days out, that the timing could change.

Our preliminary snowfall forecast: 12-18" mid-mountain and above and 6-12" below. The snow level will drop down below lake level on Wednesday. This is a pretty cold system so expect some nice powder turns Wednesday and Thursday (Figure 1).

Figure 1: 7-day precipitation forecast from HPC. 0.25-1.5" of precipitation is expected. 

We are at the southern edge of a trough once again, but we may just luck out and get on the cold/north side of the jet. If this happens, we get less wind, better snow-to-water ratios, and better spillover.

Figure 2: Mid-level winds, temperatures, and heights. Valid next Wednesday morning. Cold air advection on the west/back side of the trough will help strengthen the trough and give us more snow.

For a better chance of this, we really need the mid and upper levels to get some cold air advection (movement of air) on the backside of this trough (Figures 2-3). This will deepen the trough and push the jet further to the south.

Figure 3: Upper level winds and heights. Valid next Wednesday morning. We hope we stay to the north of the jet, which means better snow and less wind.

Additionally, if we can get better vorticity (measure of spin in the atmosphere) advection on the backside of the trough, it will help propagate the wave further to the south (Figure 4). A general rule of thumb is thermal advection deepens or weakens a trough, while vorticity advection influences the propagation of the trough/wave.

Figure 4: Mid-level vorticity, winds, and heights. Hoping the vorticity will increase enough to the south to push the trough further south and give us better conditions.

However, the trajectory of the trough and the absence of prolonged strong south-southwest flow ahead of the trough is limiting our moisture connection (Figure 5). Temperatures will be cold and instability associated with the trough will do their best to squeeze out all the moisture possible though.

Figure 5: 9-10000' winds, relative humidity, and heights. Limited south-southwest flow is cutting off our best moisture source.

We will have another post up by Sunday night updating our forecast. So put up the mountain bike this week, and break those skis and boards back out!
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Sunday, March 9, 2014

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Update for Sunday-Monday

As suspected, the eastward progression of this event has slowed. In addition, the main dynamics for this storm stay a bit further north than anticipated. Isolated light rain (snow) showers will arrive overnight for below (above) 8,000 ft. Snow amounts will be reduced as well with 1-3" possible below 7,500 ft; 3-6" above 8,000 ft. Winds will crank up in advance this afternoon and overnight from WSW. The best timing for a short burst of Pow, above 8,000 ft, will be just before and during cold fropa which drives through Monday late morning. Monday afternoon will be breezy and cold above 8,000 ft, wind shifting from WSW to N, with isolated snow showers likely. If you dress warm, Tuesday has potential at high elevation; blue early and isolated snow showers later, but the N wind will bite.
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Friday, March 7, 2014

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High Snow Levels Continue for Sunday-Monday

This weekend takes a split weather wise with a bluebird Saturday, at least early on, and a grey day Sunday as light rain (snow) showers commence below (above) 8,000 ft in advance of more significant moisture Sunday night into Monday. The subtropical feed associated with this event will keep snow levels above 8,000 ft until late Sunday. Snow levels will drop to Lake level on Monday as cold fropa passes through. At this time, snow amounts look to be limited due to the wetter/warmer nature of this storm with totals on the order 1-3" below 7,000 ft; 3-6" between 7,000-8000 ft; and 6-12" above 8,000 ft. 

Over time, forecast models continue to show a slower progression for this storm. The momentum slowdown appears hinged to the developing nature of the upper-level flow where the upper-level ridge building in overnight through Saturday positively amplifies across the Great Basin in conjunction with a downstream shortwave that digs back across western Texas / eastern New Mexico (see Figure 1). The net effect of this ridge-trough sequence facilitates the behavior of the upstream trough that we await for Sunday. The orientation (the positive tilt) of the latter then reveals a subtropical fetch along the central California coast Sunday into Monday (see Figure 2).

Figure 1 illustrating the upstream ridge-trough sequence that slows the eastward
progression and sets up the subtropical fetch depicted in Figure 2.

Figure 2 depicting the subtropical fetch (precipitable water) spreading into northern-central California.

Because the models continue to change the progression of this event, a short update will be issued early Sunday incorporating new details. In the meantime, get out early on Saturday and enjoy bluebird conditions.
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Wednesday, March 5, 2014

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Another Storm! Wait, how high are the snow levels?!

Another storm will affect the Sierra tonight, but it is of the warm variety. Ridging over the West Coast will give way to a shortwave emerging from a larger trough in the Pacific and then traversing through the Pacific Northwest and the northern half of the Sierra tonight and tomorrow. This will bring mostly rain and some snow to the Sierra as the subtropical connection is strong.

Precipitable water (in) valid tonight. The shortwave has a strong moisture connection with the Pacific. 

The precipitation will begin to fall tonight with the heaviest between 10pm and 4am. The snow level will start out around 9000' and be there during the heaviest precipitation. The snow level falls tomorrow morning to 7000-8000' and finally to lake level and below in the afternoon. However, the precipitation will be light in the afternoon with almost all of it coming before noon tomorrow.

Snowfall totals:
Above 9000': 6-10"
8000-9000': 3-6"
Below 8000': 1-3"

24-hour precipitation totals valid tomorrow evening. Expecting 0.5-1.5" for most of the Sierra high country.

Since we are on the warm/southern side of the jet, it will limit spillover. This means the highest totals will be near the crest and diminishing further east, although the higher terrain around Mt. Rose will help out totals.  It looks like Kirkwood might win the snowfall total crown again with this storm due to its location and elevation, but it will be close.

9000-10,000' heights, winds, and temperatures. Temperatures will be just below freezing at this level during the heaviest precipitation tonight. Also strong winds will begin later today and continue tomorrow.

Also another effect from being on the warm/southern side of the jet is the strong winds. Winds will be 40-55 mph sustained with gusts exceeding 70 mph tonight and into tomorrow along the ridge tops especially along and east of the crest. So expect some wind holds or closures tomorrow.

Well it is March and so these higher snow level storms are more prevalent. The problem is, we had more of these earlier in the year due in part of the drought feedback. However, we will take what we can get in terms of moisture (and snow) to help mitigate the drought. So head for the higher elevations Thursday (winds permitting) and Friday.

We do have another storm coming Sunday, but that also looks to have higher snow levels as the stubborn ridge hangs around.

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Monday, March 3, 2014

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Mini-Storm Week

Well the pattern as promised has stayed active, and it looks like those storms are and will impact the Sierra this week. The first storm is occurring today/tonight, and the second one will hit late Wednesday and into Thursday. Neither storm will drop that much snow and snow levels will be high, but a couple of small refreshers are always welcome.

NOAA HPC 72-hour forecasted precipitation. The Sierra will receive 1-2" of precipitation over the next 72 hours, although much of it will be rain

Expect 3-6" of snow above 7000' by tomorrow morning with highest totals closer to the crest. For Thursday, the snow levels will be higher closer to 8000', but will fall during the latter half of the storm. The snowfall totals for this storm are tougher to forecast, but for now we are forecasting at least 3-6" above 8000'. Confidence in the 2nd storm's forecast is moderate right now. 

24-hour forecasted precipitation via CEFA-CANSAC WRF. Forecasted precipitation for the Sierra and around Tahoe is 0.25-1"

Two shortwaves are passing just to our north this week, which means a few things: 1) snow, but not a lot; 2) wind; and 3) higher snow levels. The models have been bouncing back and forth for the last few days with these storms so getting a handle on them has been difficult.

 Precipitable Water for this evening. Excellent subtropical moisture connection that will provide ample moisture for today/tonight's storm

Some model runs have pushed the storms to our north, and some have pushed them a little further south. Either way, we knew if the storms were going to affect the Sierra and Tahoe, that we would be on the southern half of the storms. Both storms will have a good subtropical moisture connection, which will provide ample moisture, but also keep the snow levels elevated.

Mid-level heights, temperatures, and winds for today/tonight's storm. A weak shortwave will mean higher snow levels and weaker lifting which means most of the precipitation will come from orographic lifting.

So we have two mini-storms to look forward to so if you can take a couple of mornings off this week (probably tomorrow and Thursday/Friday) go out and enjoy some turns. The second storm will be windy so lift slowdowns/closures are possible Wednesday and Thursday.

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