Showing posts with label Mountain Waves. Show all posts
Showing posts with label Mountain Waves. Show all posts

Monday, December 17, 2012

A Menacing Frontal Passage

Some nasty looking clouds are accompanying the cold front as it moves across the University of Utah.  Radar shows precip is starting to move into the Salt Lake Valley, but the coverage is far spottier than I would like to see.  Nevertheless, the precip is about to move in and I just heard thunder. Whoot Whoot!

Salt Lake will get some snow out of this eventually, but the GFS has shifted the heaviest accumulations to the south just a bit compared to this morning's run, suggesting the big winners might be to the south.

We'll see how things come together tonight.

A Maalox Moment

Today is the kind of day when a meteorologist could use a bottle of Maalox.  The forecast through tomorrow is very difficult, with divergent views being produced by our two main forecast models: The GFS and the NAM.

To illustrate this, I've put together a paneled chart that shows the guidance being produced by the two models side by side.  The GFS is on the left, the NAM on the right.  The panels are the 6-h accumulated precipitation (snow-water equivalent) ending at the time indicated in the legend at color filled intervals of .01, .05, .10, .25, and .5 inches.  The forecasts start with this afternoon (1800–0000 UTC/1100–1700 MST) and end tomorrow afternoon.  Click to enlarge further.

These panels illustrate a common dilemma facing meteorologists in the Intermountain West.  The GFS (left-hand-side) is a lower resolution model.  It doesn't handle topographic effects very well.  For example, note the lack of structure in the precipitation forecast compared to the NAM on the right.  However, the GFS runs later, sometimes ingests data that the NAM hasn't, and frequently does a better job handling the movement of large scale features like fronts.  For this forecast, that could be very very important.

Notice how the GFS develops an elongated west-east oriented frontal precipitation band that sags very slowly through northern Utah and Nevada.  If such a forecast verifies, there would be major accumulations not just in the mountains, but also the lowlands of Salt Lake and Utah Counties, as well as portions of western Utah and central Nevada.

In contrast, the forecast produced by the NAM is very different.  Beyond the greater detail owing to its higher resolution, the frontal precipitation band is weaker, especially over western Utah and central Nevada, and moves more quickly into southern Utah.  Does one lean toward this given the better handling of topographic effects?

One thing is for sure.  Snow is coming.  Major accumulations are likely in the mountains, but let's hope the GFS verifies so that we get a pasting in the valleys too.

Tuesday, November 20, 2012

This Forecast Is a Turkey

The prospects for a deep-powder day for the holiday weekend are not zero, but are fairly low.  The latest computer model forecasts keep the action largely to our north.  The total accumulated precipitation forecast by the GFS between 1200 UTC this morning and 2100 UTC Sunday afternoon shows heavy precipitation over the northwest, and dry conditions over the southwest.  The Wasatch sit just to the south of the goods.

Two weak systems graze northern Utah during this period, one Wednesday night, the other late Saturday and Sunday.  We'll have to hope these storms are more productive than currently forecast if we're to get a good freshening up during the holiday weekend.  Yes, it's a turkey of a forecast, but there is something to be thankful for.  The snowpack SWE at Snowbird is 140% of average.  At upper-elevations in the central Wasatch, we will have an above average snowpack for this time of year.  

Source: NOAA/NWS

Wednesday, November 7, 2012

Change You Can Believe In

No, not Obama.  This is a science blog.  The change is a meteorological one, with the deep upper-level trough swinging into the western US later this week.  For Utah, it will bring the coldest air of the season thus far.    

Mountain and valley snow?  You betcha.  How much?  On that, the polls are still open.

Tuesday, May 8, 2012

Nevada Mountain Waves

The flow over mountains constantly induces waves in the atmosphere.  Sometimes clouds indicate their presence, other times, the air is clear and they are undetectable to the naked eye.

Recall from the previous post that a close low formed over Utah yesterday and was forecast to move southward to the Gulf of California by Wednesday.  The circulation center is presently parked over the Lower Colorado River Valley, with easterly–northeasterly upper-level flow over southern Nevada where several dark bands cut across the flow in the water vapor imagery.  

These dark bands result from mountain waves induced by flow over the ranges of southern Nevada.  The water vapor channel is very sensitive to the amount and temperature of water vapor in the upper troposphere.  The dark bands are generated in areas where the mountains cause the air to sink, warm, and dry.  The band near the head of the arrow appears to be related to flow over Mt. Charleston and other high topographical features east of the Amargosa and Pahrump Valleys along the CA-NV border.  

These mountain waves are only briefly detectable in conventional infrared satellite images because the air is too dry and clouds are only generated for a short time at the beginning of the loop in areas where the mountain waves are causing rising motion.  For most of this period, the atmosphere is "severe clear."

Mountain waves are sometimes associated with aircraft turbulence, which can be particularly problematic when there is wave breaking in the atmosphere, which is similar to wave breaking on the beach.  

Source: Whiteman (2000)

Thus, water vapor imagery can be useful for helping anticipate areas of clear-air turbulence.  

Saturday, October 15, 2011

Mountain Induced Cirrus

We have a nice example this morning of upper-level cirrus clouds being generated near or downstream of the Sierra Nevada and Cascade Mountains of northern California.

In particular, note how the coverage of cirrus clouds increases near Lake Tahoe and how you can see a back edge to these clouds near or just downstream of the Sierra–Cascade crest.

Here's an even better image (thanks Dale Durran) of mountain induced cirrus downstream of the High Sierra from another event.

Most of us think about clouds forming on the windward side, but some mountain waves tilt upstream with height and, while the low-level flow might be sinking downstream of the mountains, at upper-levels it is rising.  For example, in the model simulation below, the flow moves from left to right and roughly parallels the black contours.  Note how contours slope downward in the lee of the barrier at low levels, consistent with sinking motion.  The mountain wave, however, slopes upstream with height, so that the strongest sinking motion sits over the barrier when you get to around 5000 m or higher.  Further, downstream of this subsidence, the air rises, much like one sees when water flows over a rock in a river.

Source: Jim Doyle, NRL
 It is in this area of downstream rising motion that mountain induced cirrus clouds form.  It is also in this area that gliders can reach extreme altitudes.