Core Dump on Our Pending Storm

The Doppler on Wheels (DOW) has been in town now for two weeks, and we've yet to have a significant storm during that period.  At Alta, the best we've done is 4 inches on November 4th, which isn't much of a storm.  We've kept ourselves busy with various educational and outreach activities, but are in desperate need of a storm.

Fortunately, it looks like Mother Nature will give us something tomorrow afternoon through Friday.  It's been an interesting storm to follow in the forecast models for a number of reasons.

1. It's taking forever to get here.

The GFS forecast initialized at 0000 UTC 12 November (5 PM MST Saturday) showed the upper-level trough making landfall onto the Pacific Northwest coast at 1800 UTC 16 November (11 AM MST Thursday), with precipitation across the Wasatch Mountains, Uintas, and even western Colorado.  Under this scenario, we'd want to be out storm chasing early tomorrow (Thursday).

During the past three days, however, the GFS has really slowed the progression of the trough.  The forecast from 0000 UTC 15 November (5 PM MST Tuesday) has the trough much farther west and well off the coast at 1800 UTC 16 November (11 AM MST Thursday).  Precipitation is just sneaking into the northern Wasatch and Bear River Range area, and there's no precipitation over Colorado.  Instead, we'll be able to sleep in tomorrow!

2. The Sierra Nevada really take a bite out of storms

Through flow blocking and water vapor depletion in mountain-induced rain and snowfall, the Sierra Nevada have a dramatic impact for the worse on moisture transport into the Great Basin.  This can be seen in the GFS forecast pannels immediately above.  Note in particular the how the column-integrated relative humidity in the lower left panel decreases abruptly across the southern "High" Sierra, with moisture only able to sneak in across the lower northern Sierra north of Lake Tahoe.  This effect is also apparent in theNAM forecast for the same time and, in this case, it is a contributor to the delay of precipitation spreading into northern Utah.  Without the High Sierra, moisture would penetrate more easily into the Great Basin and the Wasatch would light up even earlier.  Pity.

The time-height section from the NAM shows a classic "cloud-storm" environment tomorrow over the Salt Lake Valley.  Cloud storm is a phrase we jokingly call events with high clouds and virga, but little precipitation reaching the valley floor.  There's copious moisture at mid levels, but dry environment down low.  Deep moisture doesn't penetrate into the Salt Lake Valley until Friday night.  More evidence of further delays in the storm really getting going over the Salt Lake Valley.

Add all this up — the delay in the arrival of the trough, the drying influence of the High Sierra, and the dry low levels over northern Utah — and you have a recipe for restless natives anxious for the arrival of a storm that has been promised for Thursday.

3. Many storm chasing options

We of course have a mobile radar, so we can put it wherever we want and don't have to necessarily wait in the Salt Lake Valley for weather.  That being said, it takes time to move the DOW around and configure a reasonable scanning strategy to do real science.   We have a number of possible targets through Friday afternoon, including the spillover of precipitation across the northern Wasatch Mountains and into the Ogden Valley, multi-ridge interactions between the Stansbury and Oquirrh Mountains or Oquirrh and Wasatch Mountains, a cold frontal passage presently forecast for Friday morning, and post-frontal convection in northwesterly flow in the wake of the cold front.  It's going to be a busy time!  We'll have an interesting planning session this afternoon and then will need to keep a close eye on things in the field to maximize our opportunities.

4. Mountain snow possibilities

The situation this week has been pretty grim for skiers.  Not only has it been dry, but it has also been warm.  I don't follow the snowmaking activities at the resorts, but I suspect they were limited at best.  That situation will continue today and even tomorrow looks to be pretty warm.  In addition, the early phases of the storm when it does arrive look quite warm.  For example, the NAM forecast for 1200 UTC (5 AM MST) Friday morning has 700-mb temperatures of around -2ºC.  That equates to a snow level around 7000 feet or so.

Thus, while the upper elevations of Snowbasin are likely to get a pasting, the base may see rain for at least a portion of the storm.  PCMR may also see rain at the base during the early stages.

However, the snow level will be lowering during the period, especially on Friday.  Friday has some potential to be productive in the Cottonwoods due to cold, unstable, northwesterly flow.  Overall, the NAM-12km is generating about an inch of water and 9 inches of snow at Alta Collins through late Friday.  The numbers, however, vary widely across models and ensemble members.  At this point, I'd lean toward 6-12 inches at upper elevations in Little Cottonwood, with the potential for more if the post-frontal environment is highly productive.  Water totals in the northern Wasatch should be higher.

Snowmakers had better be ready to release the torrents Friday, Friday night, and Saturday morning.  After that, ridging returns and more marginal snowmaking conditions return for a couple of days.

Addendum at 10:15 AM 15 November

The SREF plume diagram below was unavailable when I wrote this post, but I've added it here and it shows remarkable spread for the event at Alta Collins.  Talk about forecaster heartburn!  Hope for the high members to verify.