Stuck in the Middle with You

As I looked at the computer model forecasts for the next several days, the Stealers Wheel song Stuck in the Middle with You came to mind.  Although we're going to get some scraps of moisture into Utah late tonight and tomorrow, for the most part we are stuck between two streams of moisture, one penetrating inland north of the High Sierra and one penetrating inland south of the High Sierra, as illustrated by the GEFS ensemble mean integrated water vapor transport forecast for 0000 UTC Thursday (1800 MST Wednesday).

Source: NWS

Thus, we get a little bit of moisture and precipitation in here late tonight and early tomorrow (valley rain, mountain snow), but beyond that, we're stuck in the middle of the two juiciest airstreams.  Check out the GFS forecast for 0000 UTC Friday (1800 MST Thursday).  Precip to the north and precip to the south, but we're left high and dry.  Clowns to the left of me, jokers to the right, here I am stuck in the middle with you!

Just to further emphasize how we are stuck in the middle, here's the GFS total precipitation forecast through 0600 UTC 5 December.  Note how we are in an area of lower precipitation between an arm of high precipitation extending across Idaho and an arm of higher precipitation extending across Arizona.

Finally, here's an ensemble of our 22 experimental downscaled forecasts for Alta–Collins from the North American Ensemble Forecast System (NAEFS) for the next week.  There are a few members of the Canadian modeling system that are going for big totals for the event late tonight and tomorrow, but this is highly unlikely and we've seen outlier forecasts produced by some of their ensemble members before.  Most are generating 5" or less of snow through tomorrow night.  Some as little as 2".

Based on this, I'd go for probably go for 2–4" at upper elevations in the Cottonwoods, but even that might be optimistic.  I'd call it dust on crust, but it's going to be more like crud on crust since it's going to be a warm storm.  Beyond that, we have to hope for some scraps to sneak in here as there are no direct hits projected for the next week.

Life after Death

Although there has been quite a bit of variation between the models concerning the details, especially timing, things appear to be coming together for what should be an exciting period this weekend.

As discussed in the previous post, the precipitation accompanying the Pacific cold front that made landfall along the Pacific coast overnight is expected to die off as the system moves inland.  This is, however, a case of life after death as the cold front will be rejuvenated once the pre-frontal southerly flow begins to move past the Sierra Nevada, enabling juicy air over the eastern Pacific and Gulf of California to push into the Intermountain West.

The GFS loop below, which shows the 700-mb (10,000 ft) temperatures, surface winds, and precipitable water (a measure of the total water vapor content of the atmosphere, color fill).  Note as the front pushes across California and Nevada that it eventually draws moist air up the lower Colorado River Valley and into Utah.  

1200 UTC 24 Sep 2014 GFS Forecast of 700-mb (10,000 ft) temperature, surface winds, and precipitable water (color fill) from 1200 UTC (0600 MDT) 24 Sep – 0000 UTC 28 Sep (1800 MDT 27 Sep). 

Some of the moisture increase reflects what meteorologists call moisture convergence along the front, but tapping into the juicy stuff to the south plays an important role.

The GFS is currently really excited for late Friday night and Saturday, with heavy precipitation along the front over Utah (apologies for the crappy color scale; Friday night and Saturday are near the end of the loop.

1200 UTC 24 Sep 2014 GFS Forecast of 700-mb (10,000 ft) temperature, surface winds, and 6-h accumulated precipitation (color fill) from 1200 UTC (0600 MDT) 24 Sep – 0000 UTC 28 Sep (1800 MDT 27 Sep). 

Here's a better image showing the 6-hour accumulated precipitation for the period ending 6 PM Saturday.  The GFS is putting out up to just over an inch of rain in portions of northern Utah.

Who gets what will ultimately depend on factors that are not reliably predictable this far in advance, but it does look like late Friday Night and Saturday could be pretty wet with some thunderstorms thrown in just for fun.

After that, things look showery and cooler for northern Utah, but as things stand now, I don't think we'll see much more than some light accumulations in the highest elevations.

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.

Organizing Precipitation Band

It's been quite impressive how precipitation has organized in a line extending from central Nevada to northern Utah over the past two hours.  This has occurred south of the band that was over extreme northern Utah this morning, as anticipated by the NAM.

The full extent of the band is clearer in this satellite image, which includes a lightning strike overlay (thanks Chris A!).

Yes, we've been Thunderstruck in portions of northern Utah, but not yet in the Salt Lake Valley.

I see a lot in this case that reminds me of strong cold front events and other situations with strong confluence and convergence that extend across the Great Basin from the High Sierra.  Work is needed to better understand these events and the role of flow-mountain interactions in producing them.

Thunderstruck?

It's going to be a very interesting day in northern Utah.  The morning sounding shows steep lapse rates extending through the middle troposphere.  CAPE is limited, but with some surface heating, may get sufficiently high for us to see good rumblers in northern Utah.

Source: NCAR/RAL

At present, precipitation and a sharp wind shift and temperature contrast are draped across extreme northern Utah.

The NAM, however, generates a new band to the south over Salt Lake, that becomes dominant and quite intense.  At issue is why this occurs, although it forms along the Great Basin Confluence Zone that frequently forms during large-scale southwesterly flow.  

We're still not sure why this kinematic feature forms, but it appears to play a role in frontal development (West and Steenburgh 2010) and I guess we'll see today if perhaps it can contribute to convective initiation.  

Will we be Thunderstruck?  Perhaps only Angus Young knows.  

The Mammoth Mountain Microclimate

It has been a fantastic snow season in Utah this year, but also in the Sierra, which are going to get the goods yet again today.

The snow depth at the Sesame Snow Study Plot at Mammoth Mountain presently sits at about 180" and has increased over 40 inches in the past 3-4 days.  This site is located at about 9000 feet, not far from the main lodge at Mammoth.  Snow depths on the upper mountain are certainly higher.

As far as Sierra ski areas go, Mammoth is a very interesting meteorologically for a number of reasons.  First, it has altitude on its side, with a summit elevation of over 11,000 feet.  Second, although it is thought of as an eastern Sierra resort, it actually sits on the Sierra Crest in an area where the height of the High Sierra is relatively low and there is a dearth of high topography upstream, especially to the southwest where the San Joaquin River lies.

Mammoth Mountain is located very near Mammoth Lakes in this image.

What this means is that airmasses traversing the Sierra Nevada undergo less loss of water vapor (to precipitation) as they approach Mammoth Mountain, enabling Mammoth to be an exceptionally snowy location.

In addition, the isotopic composition of the snow that falls at Mammoth is somewhat unique.  A small fraction of water molecules that fall from the sky during a snowstorm (or rain storm) are built with a hydrogen isotope known as deuterium that includes a neutron (regular hydrogen does not include a neutron).  Because of this extra neutron, these deuterium-based water vapor molecules are a bit heavier than your standard water vapor molecule, and condense more easily into water or ice.  Thus, airmasses traversing the Sierra typically become depleted of deuterium because it preferentially rains out on the windward slopes.

Mammoth is unique because there is less upstream precipitation and depletion of deuterium.  As a result, there is an unusually large amount of deuterium in the snow around Mammoth.  This was discovered 40 years ago by Friedman and Smith (1970) in a classic paper published in Science.  There is a maximum in the ratio of deuterium to regular hydrogen (D/H) in Sierra snow observed just south of Mono Lake in the Mammoth Mountain area.  This maximum can be seen in the plot below, which includes contours of the departure (in per mil) of the ratio of observed deuterium to regular hydrogen (D/H) compared to that of standard ocean water.  A -100 indicates that the sample has 10% less deuterium than standard ocean water, and more negative numbers indicate less deuterium.

Ratio of deuterium to hydrogen (D/H) in Sierra snow expressed as a per mil
 departure from that found in standard ocean water (Friedman and Smith 1970).

Deuterium is a stable, naturally occurring isotope (i.e., it is not radioactive), so you Mammoth skiers have nothing to fear.  Further, deuterium has no measurable impact on how "heavy" the snow is, so you can't blame Sierra cement on it.  Stable isotopes like deuterium can, however, be used by clever geochemists, meteorologists, and hydrologists to understand the movement of water through the hydrologic system and even long-term climate change.

Subtropical Visit?

After a fantastic couple of days of skiing, a bit of a pattern change is in store for this week.  In particular, over the next couple of days, cyclogenesis will occur over the eastern Pacific, leading to a surge of subtropical moisture into California and the Sierra Nevada late Tuesday and Wednesday.  As this occurs, we will see the large-scale flow over Utah shift to southwesterly with a strong connection into the subtropics.  Note the surge of high precipitable water air into California late in the loop below.

0600 UTC 28 Feb initialized GFS sea level pressure
(black contours), preciptable water (color fill) and 250-mb
(jet level) wind vectors (red).

The Sierra in in store for a round of storminess for sure, although snow levels may rise to as high as 6000 feet near Lake Tahoe during the storm.

Due to the large-scale southwesterly flow, this will be a good case to examine the transformation (drying and warming) of the airmass across the Sierra Nevada and the evolution of the "leftovers" over the Intermountain West.

Change is coming

After a couple of days of sublime snow conditions and ideal weather, major changes are now underway that will bring much warmer and wetter weather to the Wasatch.

A narrow filament of moisture, known as an atmospheric river, is being pulled from the subtropics and into California over the next couple of days, with the leftovers after passage over the Sierra Nevada and other western ranges penetrating into Utah.   The evolution of this atmospheric river is well captured by the latest SSMI/AMSRE-derived total precipitable water imagery from the Space Science and Engineering Center at the University of Wisconsin-Madison.

In this loop, subtropical moisture is pulled northward just west of Hawaii where there is strong southerly and southeasterly flow between the subtropical high centered east of Hawaii and a subtropical low near the dateline (Long = -180).  This moisture curves anticyclonically (clockwise) around the subtropical high and then spreads eastward toward California as the westerly flow is enhanced by the digging cyclone over the North Pacific.

A lot of large-scale weather features are coming together to bring you this weekend's weather.  For example, without the extraction of moisture from the tropics west of Hawaii, the atmospheric river would likely be much less potent.  It does pay to examine the entire Pacific Basin when forecasting for Utah.

Update 1:35 PM Dec 17

The subtropical low west of Hawaii is an example of a Kona Low.  See Simpson (1952), Morrison and Businger (2001), and Otkin and Martin (2004).

Subtropics to pay a visit

A remarkable week is on tap as a series of storms will pummel the western United States and bring the warmth and wet to Utah.  Winter should persist until Friday, when the first surge of warm air and moisture moves into Utah late in the day.

We will have it easy compared to California, which will be seeing heavy rain associated with two atmospheric rivers that are expected to impinge upon the Sierra Nevada the next few days.  An atmospheric river is a narrow plume of moisture that is typically connected to the subtropics or tropics and extends into the mid latitudes.  The GFS precipitable water and sea level pressure forecast for 1800 UTC 18 Dec (Saturday) produces a marvelous atmospheric river (black arrow) that extends from a region of subtropical moisture just north of Hawaii into California.

72-h GFS forecast of precipitable water (color fill) and
sea level pressure valid 1800 UTC 18 Dec.

The atmospheric rivers are especially dramatic if you examine the GFS forecast loop.  You can see how they are extracted from the reservoir of moisture over the tropics and subtropics.

What I find remarkable about these rivers is how much moisture they lose over the Sierra Nevada.  Note how the PW decreases markedly across that barrier, which reflects water vapor loss to precipitation.  If we could just remove the Sierra, Utah would be a much wetter place!

Sierra Nevada and airmass transformation

During southwesterly flow, a pronounced gradient in precipitable water (i.e., the PW wall) often develops over the High Sierra crest with a cloud and precipitation shadow extending downstream over the Intermountain West.  During the day, sensible heating within the cloud and precipitation shadow can enhance frontogenesis directly through differential heating (assuming post-frontal cloud cover) and indirectly by driving a thermally forced circulation that reinforces the cross-front ageostrophic circulation.  Koch et al. (1995) describe this process nicely.

 A marvelous example of the Sierra PW wall and downstream cloud and precipitation shadow has developed.  Note the local minimum in PW that extends downstream of the High Sierra across nearly all of Nevada, the lack of cloud-cover within this minimum.

2045 UTC 19 Nov 2010 Visible Sat & RUC PW and 925 mb Wind

One can literally see the influence of the varying crest height of the Sierra Nevada in this image.  Higher PW air is able to penetrate across the relatively low Sierra crest north of Lake Tahoe and around the southern flanks of the High Sierra and through the Mohave Desert.  The PW minimum is found directly in the lee of the High Sierra, which is the most formidable part of the barrier.

At issue are the processes responsible for the PW minimum.  One possibility is airmass transformation in which water vapor is lost as orographic precipitation on the windward side of a mountain barrier, leading to a drier airmass in the lee (Smith et al. 2003).  In the present case, radar imagery suggests there is little windward precipitation over the High Sierra, except near Lake Tahoe.

2045 UTC 19 Nov 2010 Radar Composite

Perhaps radar coverage over the High Sierra is poor (anyone?), but even if it isn't, there are other mechanisms besides water vapor loss to orographic precipitation that might contribute to the Sierra PW wall including:

1. Differences in elevation upwind and downwind of the Sierra.  Because the Intermountain West is at least 1500 m higher than the central valley, a portion of the PW decrease is simply the lack of the lower-troposphere.  On the otherhand, this doesn't account for the pronounced minimum in the lee of the High Sierra.
2. Topographic blocking.  This is my personal favorite.  The Sierra frequently produce a strong barrier jet that advects large quantities of moisture poleward over their windward slopes (e.g., Marwitz 1987).   Essentially, the barrier jet acts like a robber baron.  Instead of the windward airmass being forced directly over the barrier and into the lee, blocking results in the low-level flow being diverted poleward until it encounters the lower portion of the barrier north of Lake Tahoe where it turns anticyclonically and penetrates into the Great Basin.

There is some evidence of the topographic blocking effects in the analysis above.  Note how the flow upstream of the high Sierra is SW, but the flow north of Lake Tahoe is more westerly.

Heather Reeves did a really nice job of explaining this process and its role in producing an isolated maximum in precipitation in the northern half of the Sierra Nevada (Reeves et al. 2008).  At issue is whether or not this process also contributes to the leeward PW minimum.  This would be an interesting topic for future work.  

Local and remote effects of Sierra Nevada

The next two days offer a great opportunity to examine the influence of the Sierra Nevada on the meteorology of the Intermountain West.  As of 1430Z this morning, a high-amplitude upper-level ridge and surface anticyclone were planted over the Gulf of Alaska with a positively tilted upper-level trough off the Pacific coast of the contiguous US.  As a result, large-scale southwesterly flow extends across California, the Sierra Nevada, and the Intermountain West.

IR Sat/GFS Forecast Valid ~1430 UTC 19 Nov 2010

On the regional scale, the leading edge of 700-mb baroclinity is draped across central Nevada and northern Utah and is collocated with a sea level pressure trough that extends from the Sierra Nevada across the Great Salt Lake Basin.

IR Sat/RUC Valid 1500 UTC 19 Nov 2010

The trough lies within the so-called Great Basin cyclone region identified by Jeglum et al. (2010) as having a high frequency of Intermountain cyclone occurrence and genesis downstream of the High Sierra.  Thus, although the existence of frontal troughing and baroclinity is consistent with the large scale pattern, I suspect the Sierra Nevada are enhancing the trough and influencing the position and intensity of the baroclinity over Nevada.  See also Shafer and Steenburgh (2008) and West et al. (2010).  

This pattern will persist over the next 24-36 hours as the large-scale trough amplifies and digs off the Pacific coast.  Keep an eye on things and consider the role of the Sierra Nevada.  In addition to how that mountain barrier affects mass and momentum, consider how it alters the thermodynamic and moisture characteristics of airmasses and how this in turn might affect the frontal and precipitation dynamics over the Intermountain West.