This Week's Storm Focus - The Tetons

This Week's Storm Focus - The Tetons

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This Week's Storm Focus - The Tetons

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Good Evening!

Thanks for tuning-in to The Barlometer Snow Forecast. Tonight’s segment focuses on the Tetons, and prospective chances for snowfall at the end of this week.

JHMR and Targhee have benefited from a wee-bit of snowfall over the past (5) days, as a series of northern disturbances clipped the region. Though, things have been generally quiet since the first week of February, following a nice multi-day cycle. Folks on both sides of the range have enjoyed a pretty decent winter thus far. JHMR (278“) / GT (242“) total for the season. I spent 8-days up there in the middle of January for an avalanche course with AAI, touring several days off the Pass, GTNP, and near Togwotee. Conditions were very good, but the snowpack was a little bit spooky…

The trend of northern disturbances riding down the backside of The Ridge looks to continue, with energy tracking further west this time around. Accumulating snowfall is forecast for the Tetons early Friday into the weekend, before things settle down on Sunday. How much are we talking? Let’s have a look.

Mesoscale Analysis – Tetons

The upper-level pattern remains ~generally the same as it has for much of this winter. Dominant eastern Pacific high pressure is firmly at the helm, though as the ridge pinches and becomes more amplified, disturbances have been allowed to stream down the backside of the ridge throughout the past few weeks. This has been increasingly-common as of late, bringing quick-hitting shortwave troughs and accompanying cold fronts to portions of the Rockies. These events have been sandwiched between 3-4 day stretches of transient high pressure, as the ridge axis creeps overhead from the west.

Below, I have the visible satellite channel toggled this afternoon. A great image of the west as the GOES satellite sees it today. It’s rare that a visible image and snow-cover can so clearly tell the story of a winter. But that is the case at this point in February, with portions of the Rockies looking good, and the Cascades and Sierra nearly devoid of snow-cover entirely.

goes west

GOES-WEST Visible Satellite – 02/17/15

I’m tracking a piece of energy dropping out of the southern Gulf of Alaska by the end of the week, associated with the large maritime cyclone pictured above. As this feature climbs the ridge, a disturbance will break off from the main system and dive back down the ridge into SW Canada. It will quickly pick up steam, carving out a trough and eventually dropping into the northern Rockies along with some cold Canadian air.

Below, I have 500mb constant-height analysis courtesy, of the 12z NAM model. The contours on this product are known as isohypses not isobars. They are lines of constant geopotential height at the 500mb level (~18,000ft), which we look at to study the general flow in the upper-levels of the atmosphere (troughs, ridges etc). Lower heights are associated with colder air aloft. This image is for 5:00pm Friday evening. I’ve also highlighted the mentioned storm feature, along with its general expected path.

500mb trough

12z NAM – 500mb Geopotential Height + Storm Feature

As the trough digs south, it will push a cold front into NW Wyoming Friday afternoon, turning winds west/northwest. Though, the timing of this event is not well agreed upon by models. The European ECMWF and latest 18z GFS push the front into the Tetons Friday morning, which is something to keep in mind.

Ceilings will lower and snow will develop along the front, spreading south into the overnight hours. The heaviest period of snowfall over the Teton range will follow the frontal passage (FROPA), slowly clearing into Saturday as dryer air works into the region from the north, shutting down lift and precipitation. With a general absence of jet stream energy and overly-deep Pacific moisture, the main driver for this system will be upslope flow (E/NE) from the Canadian high pressure as it pushes in following the front. This doesn’t really work for the Tetons, which is one of the reasons snow totals will be curbed a bit. The best accumulations will fall along and east of the Continental Divide, into portions of the Winds, Gros Ventre and Bighorns.

Another area that may do better is higher elevations in the Snake River and Wyoming ranges – to the south of our forecast area. The deepest moisture looks to hang to the west, generally following the Snake River Valley in Idaho. Though, a slight shift in the track of the shortwave could bring better snow north. Below, I have 18z NAM output – simulated composite reflectivity (radar) into Saturday. You can see the progression of the cold front punching into the state, with showers persisting on Saturday and a general clearing trend from north-to-south by Sunday. Colorado is next in line. Yippee!

nam radar tetons

18z NAM – Simulated radar through late Saturday evening – 2/21

Concerning totals, 4/5 models keep the best snowfall just to the south of the Teton range. These solutions paint generally 4-7″ for GT, JHMR and the passes. The storm track and orographics favor the Idaho side, while elevation favors the Jackson side. As a result, I think amounts will be similar. But Jackson may be able to briefly capitalize on some easterly flow on Saturday, which could drop a few more inches. The UKMET model seems to be picking up on this a little bit, with accumulations perhaps in the 6-10″ inch range. I’ve been liking this model quite a bit this winter on my Colorado forecast site, so we’ll see how it does this time around.

The outlier right now is the Canadian GEM, which holds out for lighter amounts – likely in the 2-4″ range. I’m not really sure what to go with right now, as my forecast confidence is below average. If you read my Western Weather Round-Up yesterday, you know the models have been struggling with pattern evolution for this weekend. Agreement is poor concerning how these smaller features will phase as they drop down the backside of the ridge. Slight nudges in the upper-level pattern can have a major influence on snowfall amounts and distribution.

Regardless, let’s give you some visual QPF output. That’s why you’re here. Below, the latest 18z GFS output, total accumulated snowfall through early Sunday morning. Image courtesy of Weatherbell Analytics.

gfssunday

18z GFS – Total snowfall through early Sunday. Image courtesy of Weatherbell Analytics

Following our Friday/Saturday weather-maker, things look to clear out on Sunday. Though, the Rockies remain unsettled as the prevailing upper-level pattern evolves. Disturbances will continue to ride down the ridge into next week, along with an interesting feature developing to the south –  a closed low pressure over the Great Basin by Monday. Right now, moisture associated with this feature looks to remain south of the Tetons, but we’ll need to watch how this evolves.

Thanks for tuning-in today, and I hope you enjoyed your time. Keep an eye out on Unofficial Networks for subsequent regional write-ups and large-scale synoptic discussions. Along with my dedicated Colorado-specific forecast on Barlometer.com, there’s some supporting pages with information about myself, the forecast process, credibility and my vision. Check it out if you’ve got the time!

Cheers!

Nick Barlow
The Barlometer
@Barlometer

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