Inversion lingers, but will soon give way

Posted Tuesday, November 20th, 2018 by Pete Saunders

As promised, upper-level ridging/high pressure has allowed for continued dry, sunny and calm conditions across the intermountain west since Sunday. The sinking motion associated with this high pressure also has allowed for a relatively strong inversion to set up over the region, which has given us some pretty…gross haziness the past couple of days. An inversion, of course, is when the air temperature warms with altitude. At 1100 MST this morning in the mountains at Solitude APEX, which is over 9,000 ft in elevation, the temperature was 11 degrees Fahrenheit warmer (49 degrees Fahrenheit) than SLC International Airport (38 degrees Fahrenheit) which is at an altitude of 4,226 ft.


The map below shows a larger scale view of why this is happening. Here in Utah, we are directly under a high pressure system, with the polar jet stream arcing over us to the north. This has allowed us to stay dry and under large subsidence (sinking air). However, a trough that has been digging south from Alaska is currently off the coast of California, moving its way toward the continental U.S. Assuming you’re on this site because you love to ski and/or board…this is great news! We’re gonna ditch the inversion for a bit and welcome in a more “troughy” pattern. We’ll just have to wait a little bit longer, though.

Water vapor map valid at 0730 MST (1430 UTC) showing a large area of high pressure over the western U.S. Meanwhile, a trough and associated polar jet stream are digging south off the Pacific Northwest coast.
Water vapor map valid at 0730 MST (1430 UTC) showing a large area of high pressure over the western U.S. Meanwhile, a trough and associated polar jet stream are digging south off the Pacific Northwest coast.

Short Term Forecast (Wednesday – THANKSGIVING – Friday) 

I’m not gonna lie, Wednesday is gonna be another borefest. We’re still locked in to high pressure over the region tomorrow, and it’s going to be sunny (valley hazy) and calm. Wednesday night, though, winds will increase out of the southwest and should start mixing out the inversion. By early Thanksgiving morning, the aforementioned trough will have weakened a bit but will begin moving in from the west. Precipitation will increase and become widespread over the region by 0500 MST. Freezing levels around this time will be ~8,000 ft, lowering to ~6,600 ft by 1100 MST with 700 hPa temperatures lowering to -7 degrees Celsius. The below image shows time-height profiles of temperature, wind and moisture (relative humidity [RH]).

NAM 12km time-height section from weather.utah.edu (University of Utah Dept. of Atmospheric Sciences) showing dry conditions through Wednesday (yellow/brown RH contours), with lowering freezing levels (dark blue line) by Thursday at 12Z (0500 MST) and increased moisture (green RH contours).

What does this mean? A fairly decent amount of Thanksgiving SNOW!  Every mountain resort in the region should start out with snow early Thursday morning, increasing in intensity through noon. Orographic snow will continue through the evening and northwest flow on the back side of the trough moves in and rides up the high terrain of the Wasatch Mountains, and should taper off by midnight. The models are in a bit of disagreement with regard to total amount and location of heaviest precipitation. It looks right now, though, that the upper Cottonwood resorts will see 0.5-0.8″ of liquid water through the day on Thursday, which with an average of 10:1 to 15:1 ratios should equate to ~6-10″ of snow with areas of slightly higher amounts. For the northern Wasatch around Ben Lomond Peak, totals could be a little bit higher, around 0.8-1″ of liquid water on Thursday, or 8-12″ of snow.

On Friday, another trough moves through the region, bringing more snow into the mountains.  Moisture is a bit more limited with this trough, though, and snow ratios might be a bit lower with warm advection ahead of this trough late Thursday night. Below is the ECMWF 24 hour precipitation forecast for Friday valid at 00Z 24 November. Expect 3-5″ of snow in both the upper Cottonwoods and around Ben Lomond.

24 hour total precipitation ECMWF forecast for Friday, valid from 06Z 23 Nov to 06Z 24 Nov. Precipitation totals look to be a bit more modest for Fridays system. models.weatherbell.com
24 hour total precipitation ECMWF forecast for Friday, valid from 06Z 23 Nov to 06Z 24 Nov. Precipitation totals look to be a bit more modest for Fridays system.
models.weatherbell.com

 


Mid-Range Forecast (Friday night – Sunday):

Friday night into Saturday, through Saturday night look very interesting as a cold Pacific storm makes its way into the region, continuing this active wet weather pattern. More specific details of this event will be posted in the next couple days, but as of right now it looks like this storm will produce more snow than the previous two troughs with 700 hPa temperatures dropping to -10 to -14 degrees Celsius with a Saturday morning frontal passage. If things hold, we can expect larger mountain snow accumulations this go around with perhaps 20:1 ratios due to much colder air in place. Powder skiing Saturday/Sunday?? Just for fun, here’s a look at the ECMWF total snow accumulation forecast for Thursday’s, Friday’s and Saturday’s storm:

ECMWF total snow accumulation forecast valid from 12Z 20 November until 00Z 25 November using a 15:1 ratio. Note the 2+ feet over the upper Cottonwoods, with over 30" near Ben Lomond. These storms are still aways out, but it is always interesting to see what the models are forecasting for interesting events like these. models.weatherbell.com
ECMWF total snow accumulation forecast valid from 12Z 20 November until 00Z 25 November using a 15:1 ratio. Note the 2+ feet over the upper Cottonwoods, with over 30″ near Ben Lomond. These storms are still aways out, but it is always interesting to see what the models are forecasting for interesting events like these. models.weatherbell.com

By Saturday night/Sunday morning, things should be calming down quite a bit, and ridging looks to again make its way back into the area, perhaps lasting through the middle of next week…

Pete

Leave a Reply

Your email address will not be published. Required fields are marked *