Upslope Storm to Bring 4-8″ of Snowfall to Denver Metro Area

Update: Models have diverged slightly in maximum snowfall totals with
some predicting a bit less and one predicting a bit more. I'm staying 
in the center at the original 4-8", which I think is realistic. The
Short Range Ensemble Forecast (SREF) model has upped its totals, but
it tends to get a bit crazy with its accumulations as we approach
major events. So, we'll stick with the others that are generally 
agreeing on the position of the system and the precipitation that we
can expect. With that being said, focal points for this storm such as
the foothills, Palmer Divide, and higher mountain terrain could see
amounts a bit higher in the 6-12" range with localized amounts even 
a bit higher.

Greetings! After what seems like years, we finally have an exciting winter storm that will be coming through the Denver and Front Range regions!

ermahgerdWith very low seasonal snowfall accumulations leading into this weekend, we need to embrace our wonderfully overdue moisture-brining storm! However, this storm has a bit more in store for us than just snowfall. We’ll see a decent drop in temperature with this system and that could contribute to freezing drizzle before the snow starts. So, this could definitely end up as a a fairly impactful and, at times, dangerous storm.

We’ll break down the forecast like this:

  • National Weather Service Winter Storm Watch
  • Storm Timing and Development
  • Expected Accumulations and Threats

National Weather Service Winter Storm Watch

...WINTER STORM WARNING IN EFFECT FROM MIDNIGHT TONIGHT TO 5PM MST
SUNDAY AFTERNOON THAT INCLUDES FORT COLLINS, BOULDER, DENVER, AND 
GREELEY. HEAVY SNOW POSSIBLE. PLAN ON DIFFICULT TRAVEL CONDITIONS 
REDUCTIONS IN VISIBILITY ARE POSSIBLE DUE TO BLOWING SNOW. TRAVEL 
WILL BE IMPACTED SUNDAY MORNING AND INTO THE AFTERNOON HOURS. 
PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Winter Storm Warning means significant amounts of snow, sleet
and ice are expected. This will make travel very hazardous or
impossible. The latest road conditions for the state you are
calling from can be obtained by calling 5 1 1.

Storm Timing and Development

Screen Shot 2018-01-20 at 10.30.11 PM

GOES Infrared Satellite Imagery at 5:22 am MST

Currently, clouds related to the current system can be seen in the image above draped over northeastern Colorado into Utah and Nevada and down into Arizona. Cloud cover will continue to increase and will move eastward during the day today and we should start seeing heavy coverage around the area in mid- to late-afternoon.

Screen Shot 2018-01-20 at 10.17.41 PM

Forecasted Mean Sea Level Pressure and Temperature at the surface at 5pm Mountain Time on Saturday

The low pressure system that is currently sitting in eastern Colorado will continue to strengthen and will center itself over southeastern Colorado near the Oklahoma panhandle late Saturday night into Sunday morning before moving eastward Sunday afternoon. As it progresses, we will notice temperatures that will cool off beginning at 2pm when winds should shift from easterly/northeasterly to predominantly northerly. Temperatures will fall from the upper 40s on Saturday until daybreak on Sunday when a slight increase during the day will occur. Lows over night are expected to be in the low 20s and highs for Sunday will be in the mid- to upper 20s.

Precipitation is expected to start in the region around midnight on Saturday/early Sunday morning. Currently, the precipitation is expected to start off as freezing drizzle or freezing rain because of the relatively warm temperatures that will exist until late Saturday night. The freezing drizzle should turn to snow quickly and snow will remain dominant until the storm system moves out on Sunday afternoon.

You’ll notice that the low pressure system in the image above (black lines) looks particularly circular and has a good position for creating snowfall for Denver. But, there are a couple ingredients missing for that blizzard that everyone wants. First, this is a relatively dry system with minimal moisture input which will limit the amount of snow that we get. Another reason that we won’t see feet of snow is that the storm system will move out of the area too quickly. We really need a blocking pattern (holds weather pattern still-ish for a while) to really give us those storms that last days and give us heavy dumps of snow. We’re just not going to get that out of this one, unfortunately.

Expected Accumulations and Threats

The Global Forecast System (GFS), North American Mesoscale(NAM), Short Range Ensemble Forecast (SREF) models all agree on the Denver metro area getting between 4-8″ of accumulation for this system. As this is an upslope event, meaning that the winds will be coming from the east/northeast and will travel “up-slope” when they hit the mountains, I expect the largest accumulations to be along the western foothills and south along the Palmer Divide in areas like Castle Rock and Highlands Ranch. It will be important to monitor the latest model output as the High Resolution Rapid Refresh (HRRR) model comes into range and begins to create snowfall predictions as well.

The biggest concerns with this storm will be the freezing drizzle falling early and occasionally heavy snow. If freezing drizzle builds up on the ground and a layer of snow is combined with it, roads will be very icy and dangerous on Sunday morning. However, current thoughts are that the impact from freezing drizzle will be minimum due to the short period that it will fall. Driving conditions will be hazardous when the icy and snow-packed roads are combined with reduced visibility from heavy and blowing snow. Avoiding travel is a good idea if it is non-essential.

I will update this post as new model runs come out and new information becomes available.

Thanks for reading!

-Andrew

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