2-15-11 storm discussion

Here we go again with the snow! Two different systems in the mix this weekend will bring copious amounts of powder to the slopes.  The first system will really begin to impact the Colorado Rockies with impressive entirety by early Thursday.   I do not expect a massive amount of snow to fall Thursday, 4-6" drifting to 10" in areas.  The storm will take a break Friday night through Saturday, and then early Sunday the S.Pacific moisture moves in.  I think the 2nd storm will pack more of a punch, and spill out into the Denver area by Monday morning.  This storm has a lot of help from the jet stream, and placement of the precipitation max will depend on upper level dynamics.  Both the GFS and the NAM agree on the placement of the moisture, but disagree about initiation.  As timing and placement are interrelated I have to make the call and say it's going to be a good weekend in the hills and a miserable commute on Monday.  We're used to that around here right? How much snow will we see? In the hills well, 4-6" like I said for the 1st storm, and more for the 2nd, possibly 10" in the hills, max in the southern Co mtns.  Accumulation totals in Denver will depend on a couple of things, but this mid-lat cyclone will end up affecting a large portion of the central plains into Tuesday.  If I had to call an amount for Denver now, I'd say 4", increasing to the west.  What is the likelihood that this storm will miss Denver entirely? Slim, I think we will get some significant snow as the precip footprint of this storm is extensive. 

Snowpack-cast:
The fist storm will be drier and colder, producing a lighter, fluffy snow, while the second storm will be warmer and wetter, producing a heavier snowpack.  Where the storm tracks overlap we may see an 'up-side-down' snowpack, light on the bottom, heavy on top and after only a short time of wind loading and internal vapor transport, the snow will develop a heavy solid top layer and a weak non-cohesive under-layer.  this 'stratigraphy' of snow increases avy danger, so if you go out to central Co mtns Monday, you may want to stick to the low-angle slopes!


Check back for a more detailed Sunday/Monday forecast. I might be able to update on Saturday...

Week of 2/3/11

As the Arctic air mass moves off to the ESE another system sets up, tracing the terrain of the Rocky Mountains from Colorado to Montana.  This pattern will send a series of shortwave, upper level disturbances cruising down the eastern flank of the continental divide.  Another Arctic airmass is poised to strike the intermountain and central to northern plains areas Monday through Wednesday but with less severity than the last, bringing temps down into the single digits at night but high 20's during the day which will seem relatively warm. 

The pattern will bring snow into the northern and central plains Monday through Wednesday next week with accumulations from 4-7 inches. 

For tonight in Denver and the metro: This storm is moving faster than anticipated with flakes already falling in the metro area.  Expect accumulation around 3 inches in elevated areas along the Front Range and Palmer Ddivide, but I expect only an inch in the metro areas and along the valley highway.  This will be enough to slow the evening commute, though the actual weather will not be that bad, freeze conditions do exist, and therefore slippery conditions also exist.  The pre-frozen ground and lack of liquid available to the roads will limit the icing factor in most areas not completely shaded during the day.  Denver and the adjacent plains, along with Eastern Colorado can expect another blast of snow Sunday into Monday, with blustery, snowing conditions remaining through Thursday. 

Now for the exciting stuff, the hillz:

Slight, UL disturbances like the ones I see in the forecast are notorious for causing widespread and differential snow patterns in the mountains.  Snow is already falling in the vicinity of Park Range, precipitation will spread throughout the evening, bringing lighter snow and flurries for most of the day Friday, skies will partially clear Saturday between UL disturbances, and then Sunday the heavy snow will begin, and linger through Wednesday 2/9. 

Because the 700mb winds will be predominantly from the NNW, wind loading of slopes will occur on the SSE aspect, increasing avalanche danger especially in Park and Gore Ranges as well as the Elkhead Mountains where the snowfall will be most intense.  The search for turbulent enhancement will begin on the SSE side of Lenawee face at about 12000ft on Friday morning.  Investigation will continue and move westward Saturday and finally north into Park County on Sunday.  I will be looking for turbulent cells between mountain peaks, above and vertically diagonal to valleys.  The investigation will be driven predominantly by first hand observations, if possible with this dry storm I would like to photograph a turbulent overturn being seeded by the parent storm.  I have seen it with my bare eyes before, but I need documentation to support my thesis.  The idea is that relatively warm valley air is forced into a thermal circulation which first causes sublimation and saltation from trees and forested areas, the 'sublimed' or 'saltated' moisture (further referred to as environmentally available moisture) is then transported by wind up the face of the next peak.  As the loaded 'warm' air is forced up the face, it cools, converges with the moisture in the parent storm and forms a secondary precipitation cell that feeds the available environmental moisture into the parent storm.

If the parent storm will efficiently entrain environmental moisture through thermodynamic and hydrostatic processes, it is possible to begin understanding the differential snowfall patterns in the Rockies.  How will I determine where the moisture comes from and how much is being transported, condensed and re-precipitated? Well, the sensor on my pack tracks temp, wind, humidity, altitude, density altitude and pressure.  I will track these variables up and down one valley during the course of one day.  I will track the ascent and decent and use the altimeter log to delineate each 'datapath' in the raw data.  A datapath will be classified as any path that completes an elevation circuit up one side and down the other side of a mountain.  I hope to establish 4 ascending paths per day for three consecutive days during similar synoptic and meso scale conditions. 

This week's outlook for Colorado

Conditions in the high country of Colorado Should remain clear through Wednesday morning.  A strong upper level system will move into Colorado creating snow in the mountains by midday Wednesday.  Intermittent heavy snowfall is possible in the Rockies Wednesday afternoon into Friday afternoon, with the most intense part of the storm in south central to southwestern Colorado.  Snowfall rates will increase Wednesday night, the most accumulation can be expected Thursday.  Present accumulation forecasts predict a two foot average with the higher accumulation on the south to west oriented slopes*. 

Travel through the Colorado high country during this storm should be slow but possible.  Avalanche danger should be a consideration on newly solidified snowpacks on the southwestern side of the slopes*, and mediation may cause intermittent road closures. 

As for Denver, again, the snow amount depends on the storm track, but with the persistent cold air advection forecast with this storm, even a small amount of uplift should do the trick.  Still, I place precipitation totals below seasonal or upslope storm averages.  If I want to call it this far out I'd say 4" in Denver, 8" in the foothills by midday Friday, less if the storm tracks north.
"

Spectacular Lenticulars and a fabulous forecast

If you happened to catch the sunset this evening, the culprit behind it is the tight pressure gradient paralleling the Rockies, force vector towards the plains as displayed in the Mesoscale analysis data from the Storm Prediction Center.  These high winds across the top of the mountain warm by compression while descending the lee side of the Rockies.  This is the reason we have had so many unseasonably warm days here in Denver while the Mountains have seen more than twice the amount of snow as this time last year.  The smooth looking lenticular clouds are a result of air that moves swiftly in an out of conditions that allow for condensation.  They disappear and reappear in response to the wind only a few hundred meters above the divide. I would imagine a large degree of clear air turbulence within a couple hundred meters below these clouds in response to speed shear induced by the mountains.

This is a typical set-up in a La Nina year.  The heat cycle in the equatorial Pacific allows for moist zonal upper level flow over the Rockies.  Consider the 700hPa level to be just above the tops of the mountains, and though there is usually no physical differentiation between 700hPa and higher levels, visualize everything above it moving a whole lot faster, acting like a 'cap' this is certainly not always the case, but for our purposes, consider this cap as one side of an atmospheric mechanism that quite efficiently wrings the moisture out of the atmosphere with the help of the friction and elevation increase provided first by the Sierra Nevada Mountains and then by the Rocky Mountains.  With almost no meridional component to the UL flow, the lee-side trough initiation is not sufficient to produce the vorticity necessary for an up-slope storm. The surface low pressure system moves quickly south, reducing the time window for snow in the city.  If the north-south component of 500hPa flow changes and becomes slightly one way or the other (+ or - v component) the vorticity induced by the reduction in land elevation will be sufficient to produce a storm along the Front Range. 

I think this is great news and corresponds nicely to my seasonal forecast, besides that it is the 2nd week of Dec and we have not seen that big storm in the city.  Fine, keep it in the hills where we love to ride it! I remember last year I wanted to truck some of it from the city up to the hills just so there would be something there. 

So, about this forecast: I think this storm will be in line with the season and stay mostly in the hills.  The GFS says we will see snow in the Southern Plains of Colorado, while the NAM and I agree that it will stay  mostly in the CENTRAL ROCKIES!! Stated, I'm going out on a limb throwing a four day forecast out on the internet:

8 inches near Montezuma by Thursday morning, total of 11 inches by Friday noonish, less accumulation west of Vail, more accumulation south towards Fairplay. The storm moves out of the mountains late Friday, and avalanche conditions will continue to exist above tree line where the north and northwest winds are solidifying the snow pack; as powder falls onto smooth hard snow, it is more likely to trigger a slough slide while the wind crusts themselves can become slab slides.  Cornices may form or increase and become a threat by Thursday morning on the east-southeast side of ridges as the westerly wind moves newly fallen snow from the west side to the east side of the terrain.  Particular attention should be paid in the BC a couple of hours after sunrise (all day) Friday and Saturday on these east and southeast faces above treeline due to moisture advection within the snow pack Friday night which should be clearing and therefore very cold. Warm sun on Saturday will cause temperature changes and weaken cornices and previously weakened snow horizons.

I'm stickin' to my guns and saying there will not be appreciable snow (snow that sticks) in the Denver Metro Area.  That's a risky forecast.  Because I post this, we'll get dumped on ;)

No forecast is guaranteed.  Be prepared for ANYTHING when you explore the winter wonderland.  The mountain is your friend as long as you respect it!  Happy adventures!!

The next bet for snow

It's a good thing I haven't placed any bets this year...

 Looking at current surface conditions, it seems that 850mb moisture transport is attempting to bring more precip to the Colorado Rockies, however, similar to the last storm, current precipitable moisture over the region does not exceed 3/10".  700mb winds are from the west (I use 700mb winds because that is the pressure level in the atmosphere synonymous to the highest peaks in the mountain ranges),  surface wind direction is variable and pressure fluxuations are currently causing precipitation west of the divide.  Denver will see very little if any snow, it will be of the heavy wet type and will most likely fall after 5pm tonight.  Expect less than an inch of accumulation in the city. 


Forecast for the Loveland pass area:
Today:    Low: 8F,
                High: 25F,
                Wind: NW 17mph, Gust: 30
                Skycover: 90% Surface level clouds all day
                Total Snowfall: 6-8 inches. (by midnight)

Wednesday:   Low: 10F
                        High: 26F
                       Wind: W 13mph, Gust 20
                       Skycover: 25% above surface clouds all day
                       Total Snowfall: trace before 9:00am

Forecast for Denver Metro:
Today:    Low: 30
               High: 50
               Wind: W 15mph, Gust: 20
               Skycover:  50% increasing
               Total Precipitation: scattered light rain showers begin around noon, turns to snow after sunset

Wednesday:    Low: 29
                        High: 44

                        Wind: SW 5mph
                        Skycover: 50%, decreasing, possibility of a beautiful sunset
                        Total Precipitation: none

That's my forecast and I'm stickin' to it!!

Also: some helpful links that I use:
CDOT cameras
Storm Prediction Center

Review of the last storm, discussion about the upcoming storm

So the last storm was a bust as far as location is concerned.  Most (ok almost all) of the precip fell north of Denver and north of I70, here are a couple of pics from the storm: The first of the trees is a good representation of what <0.3" of precipitable moisture looks like.  This was taken last Thursday, the day I thought would be covered in powder.  It DID snow all morning, just not as much as any forecaster thought.  When the resorts said they got 6" they were LYING!!! it was closer to 3". In that way, precipitable moisture in the atmosphere is a good way to determine the amount of snowfall, it is, however a diagnostic tool, not a prognostic tool, usually.  I find looking at the 850mb moisture transport can shed more light on where the precip will actually fall.  Next, I took a close look at the kind of flakes that fell during the storm.  This second picture was taken at about 9400' ASL and shows shattered stellar crystals.  The third picture was taken at about 11,000' during the storm and clearly shows a dendrite and a platelet.  Both are indicative of a stably stratified atmosphere within the valley. Snowflake growth can be quite rapid within a stable layer.  One of the most challenging aspects of snow forecasting in the high country is boundary level transport!

The mountains and the plains

This discussion is for the week of November 8th. There will be two weather producing systems this week. The first one comes through on Tuesday, it will bring precipitation to both the mountains and the plains. Tuesday's system will drop as much as six inches in the Vail area and as much as four inches will accumulate west of the Front Range, however, most of the precipitation from Tuesday's system will fall on the western slope. The same system will likely only bring rain to the metropolitan area and the valley corridor. Some flakes may fall on the Palmer Divide and the Cheyenne Ridge with the greatest accumulation on the north side of these ridges. As for the ski areas: nothing to write home about, but something is better than nothing.

Last week I put in a request to take Thursday off work, not Tuesday. Snow will fall in the mountains Tuesday, but more will fall on Wednesday making for one of those legendary 'bluebird' days Thursday.

The low pressure system causing the precipitation is a part of a synoptic scale ridge-trough pattern, a wave. This wave is posed to amplify (become more meridional in orientation) through Thursday. Often when this happens, a system will slow down, thus dropping more moisture in one place. That is exactly what I am predicting will happen to Thursday's weather pattern. If the system amplifies significantly and slows, we are talking feet in the central mountains of Colorado! As for the metropolitan areas and valley corridor, we need only for the surface low to align with the southeastern corner of Colorado. This will produce a legendary upslope storm and pile the snow onto Denver and the Front Range cities.

Experimental forecast format: for Arapaho Basin and Keystone areas:

Tuesday: 4" by 10:00pm MST
Wednesday: 6" by noon, 12" by midnight MST.
Thursday: 18" and clear skies by noon.
Friday: ultimate bliss

Note, this system should produce enough snow to cause dangerous conditions in the back country. Avoid leeward slide zones and keep an eye on the wind.

Experimental forecast format: for metropolitan areas:

Tuesday: rain beginning 5:00am, wet snow possible but unlikely, precip will fall throughout the day
Wednesday: Partly cloudy skies, snow possible after 5:00pm, accumulation of less than an inch.
Thursday: Finally some real snow,as much as four inches in the metro area!

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