Six Questions to Answer When Forecasting

Exciting weather is happening and on the way, so it seems fitting to organize this post around the six questions to answer when forecasting. 

1. What has happened?

This "winter" has thus far been like 3.5 consecutive Novembers rather than a typical November, December, January, and February sequence.  The average temperature at the Salt Lake City International Airport for 1 December - 17 February was 37.8ºF.  That's just a shade lower than the November mean of about 40ºF.  As everyone knows, we're well below average for snowfall and snowpack, especially in the lowlands.  The photo below was taken this morning looking up City Creek Canyon and indeed it looks more like a scene you might see after a November snowstorm than one would expect in mid February. 

 
This context is important as I suspect most people are entirely unprepared for what is coming.  It will probably seem like the first storm of the year.

2. Why has it happened?

This is a good question and one that I can't answer satisfactorily.  The easy answer is that the warmth and snow drought reflects persistent high pressure and a storm track that has remained predominantly north of northern Utah.  Why that has been the case remains a subject of debate.

3. What is happening?

Wow, what a windy night.  Strong south winds at all elevations.  In the past six hours (ending at 9:20 AM MST), several sites in the northern Wasatch and Bear River Range have gusted over 70 mph and ridgelines in the central Wasatch have seen gusts as high as 71 mph.  I can find many sites in the mid elevations reporting gusts over 50 mph.  Sherwood Hills (5658 ft) near Sardine Canyon guested to 64 mph.   If there was much powder left over from yesterday's feast, I suspect it's been blown to Jackson Hole.  Peak gust at the Salt Lake Airport so far is 43 mph.

With these strong winds, we are seeing some dust.  Concentrations were especially high in the western Salt Lake Valley, which I suspect is due to emissions from the area west of Utah Lake as we have seen in recent events. 

4. Why is it happening?

The answer here is an approaching frontal trough that at 1500 UTC (0800 MST) was sagging southward into northern Utah.  This has created a strong pressure gradient to its south, with strong gusts at all elevations. 

5. What will happen?
6. Why will it happen?

Loaded questions!  I'll answer them together as it is easier.  As I write this, the surface front just passed Hill Air Force Base.  The HRRR shows it progressing slowly southward today, with frontal passage in the northern Salt Lake Valley around 2000 UTC (1300 MST).  Thus, expect to see a wind shift early this afternoon, if not sooner in the valley. 

Periods of snow are likely in the northern Wastach today and will develop in the central Wasatch later this afternoon.  It's a bit of an oddball situation as the latest NAM shows the frontal band over far northern Utah through 2100 UTC, but then rather than bringing it through continuously, redevelops it to the south tonight. 

Tomorrow brings the post-frontal crapshoot beneath the upper-level trough where much depends on flow direction, moisture, and instability.  The NAM forecast below isn't too bullish on snow, but the 6 Z GFS is more enthusiastic and keeps us in wrap-around moisture (not shown). 

It's worth a look at the 12-km NAM-derived forecast for Alta.  Measureable precipitation begins around 5 PM and is strongest from about 6 PM to 11 PM with the frontal forcing.  Periods of snow continue through 6 PM tomorrow in the unstable post-frontal period.  Total water equivalent is 0.6" with 10" of snow by 9 AM tomorrow and 12" by 7 PM tomorrow. 

Take a peak also at the temperatures for Mt. Baldy.  Keep in mind these are for 11,000 feet, a bit above the lift-served terrain, but this provides some idea of how cold the airmass will be.  It goes sub zero by noon tomorrow and down to -8ºF by 9 AM Tuesday.  I don't think we've seen air this cold yet this year.

To summarize, this looks to be an all elevation storm and you should be prepared for winter conditions.  We haven't had a stress test like this in some time. Thankfully, it is the President's Day Holiday, which will hopefully help with tomorrow's commute.  For Alta-Collins, I lean toward 7-14" by 9 AM tomorrow.  I have a bit more heartburn for totals after that given the variability I'm seeing in the models.  Some snow is likely, but the range of possibilities is large.  Hug a ski patroller or avalanche forecaster when you see them.  They will have a tough job the next few days. 

Finally, this is a high-impact, rapidly evolving situation.  Keep an eye on official forecasts at http://www.weather.gov/slc/.