Friday Storm Update

Advisory and Warning Update

There were a couple of changes to the advisories issued. They increased the totals for Telluride and added Cortez and Mancos to the advisory. When I woke up and read the forecast discussion (the AFD), then read the warning and advisory wording I was pretty anxious to see the models. I have to say with the exception of the NAM 3km run this morning and the Canadian the other models are much less enthusiastic to support some of the totals in the advisories.

COZ019-270500-
/O.CON.KGJT.WS.W.0022.191227T0600Z-191228T1200Z/
SOUTHWEST SAN JUAN MOUNTAINS-
INCLUDING THE CITIES OF SILVERTON, RICO, AND HESPERUS
200 PM MST THU DEC 26 2019

…WINTER STORM WARNING REMAINS IN EFFECT FROM 11 PM THIS EVENING
TO 5 AM MST SATURDAY…

* WHAT…MODERATE TO HEAVY SNOW EXPECTED. TOTAL SNOW
ACCUMULATIONS OF 7 TO 14 INCHES WITH LOCALLY HIGHER AMOUNTS UP
TO 18 INCHES.

* WHERE…SOUTHWEST SAN JUAN MOUNTAINS.

* WHEN…FROM 11 PM THIS EVENING TO 5 AM MST SATURDAY.

* IMPACTS…TRAVEL COULD BE VERY DIFFICULT TO IMPOSSIBLE

 

/O.CON.KGJT.WW.Y.0047.191227T0600Z-191228T0600Z/
ANIMAS RIVER BASIN-SAN JUAN RIVER BASIN-
INCLUDING THE CITIES OF DURANGO, BAYFIELD, IGNACIO,
AND PAGOSA SPRINGS
200 PM MST THU DEC 26 2019

…WINTER WEATHER ADVISORY REMAINS IN EFFECT FROM 11 PM THIS
EVENING TO 11 PM MST FRIDAY…

* WHAT…SNOW EXPECTED. TOTAL SNOW ACCUMULATIONS OF 3 TO 6
INCHES WITH LOCALLY HIGHER AMOUNTS UP TO 7 TO 9 INCHES
POSSIBLE IN THE FOOTHILLS.

* WHERE…ANIMAS RIVER BASIN AND SAN JUAN RIVER BASIN.

* WHEN…FROM 11 PM THIS EVENING TO 11 PM MST FRIDAY.

* IMPACTS…TRAVEL COULD BE VERY DIFFICULT

 

FOUR CORNERS/UPPER DOLORES RIVER-
INCLUDING THE CITIES OF CORTEZ, DOVE CREEK, AND MANCOS
200 PM MST THU DEC 26 2019

…WINTER WEATHER ADVISORY IN EFFECT FROM 11 PM THIS EVENING TO
11 PM MST FRIDAY…

* WHAT…SNOW EXPECTED. TOTAL SNOW ACCUMULATIONS OF 3 TO 6
INCHES WITH LOCALLY HIGHER AMOUNTS IN THE FOOTHILLS.

* WHERE…FOUR CORNERS/UPPER DOLORES RIVER.

* WHEN…FROM 11 PM THIS EVENING TO 11 PM MST FRIDAY.

* IMPACTS…TRAVEL COULD BE VERY DIFFICULT

 

NORTHWEST SAN JUAN MOUNTAINS-
INCLUDING THE CITIES OF TELLURIDE, OURAY, AND LAKE CITY
200 PM MST THU DEC 26 2019

…WINTER WEATHER ADVISORY REMAINS IN EFFECT FROM 5 AM FRIDAY TO
5 AM MST SATURDAY…

* WHAT…SNOW EXPECTED. TOTAL SNOW ACCUMULATIONS OF 6 TO 12 INCHES
WITH LOCALLY HIGHER AMOUNTS UP TO 15 INCHES ALONG THE NORTHERN
SAN JUAN PEAKS.

* WHERE…NORTHWEST SAN JUAN MOUNTAINS.

* WHEN…FROM 5 AM FRIDAY TO 5 AM MST SATURDAY.

* IMPACTS…TRAVEL COULD BE VERY DIFFICULT

Model Updates

It is so nice to roll from one storm into another rather than speculating when the next storm will arrive! I want to revisit the differences between low resolution and high-resolution models using this mornings’ NAM 12km and NAM 3km model runs and hopefully, this will help you understand normal model error.

A model’s resolution is indicated by the size of the grid it samples. With the NAM model, there are two versions: a low-resolution model with a 12km grid and a high-resolution version sampling a 3km grid. Low-resolution models even out precipitation totals over a larger area, in this case, 12km or 7.5 miles. So when you find your location on one of these maps the totals you see in your area could occur 7.5 miles away. Think of it as a margin of error, the lower the resolution the higher the margin of error.

The high-resolution version of the same model would then have a margin of error of 3km or 1.8 miles. Of all of the models I reference, the Canadian has the lowest resolution at 15km the GFS is 12km, and the Euro is 9km. High resolution doesn’t mean high accuracy, a high-resolution run of a bad model run is just detailed inaccuracy.

Today was a great example, here is the lower resolution 12km–look at Cortez it is showing 6.8 inches of snow.

NAM122612z12k

Now check out how the 3km version of the same run handles the snowfall with the smaller grid.

Nam3k122612z.png

6.8 inches turned into 1.9 inches with the higher resolution.

These are very aggressive runs for this model and if it is correct a lot of people are going to get a decent accumulation of snow. Before jumping all in on this thing I waited for the afternoon run of the same model it has backed of some.

6 hours later here is what the same model is showing

Low-resolution afternoon NAM model

NAM12k122618z.png

It is similar to the morning run but in the more detailed high-resolution version, you get this.

NAM3k122618z.png

Here are the GFS and Euro for comparison

Euro

euro122612z.png

The Euro is trying to bring the storm in further east again.

Here is the GFS

gfs122612z

The GFS and Euro seem similar but there are huge differences with how they interpret the way this storm departs the area. If you go back and check the accumulation differences for the Front Range they are a few hundred miles apart with where heavy snow will set up.

The low-resolution Canadian model deserves a look because it is the only model that has not wavered all week long about this storm (previously referred to as the “Second” storm).

Of course, as I explained earlier it has spread those higher totals over a nearly 10-mile grid.

CMC122612z

My Take

If this storm comes in on schedule (according to 2-hour old model data), things should stay cold enough for all snow down to about 6,000 feet. In the mid and higher elevations, snow ratios should range from 12-1 to 14-1, with 15-1 and higher above 9,000 feet. If precipitation does not arrive until late morning or later things could go wrong. I like the look of the Nam 3k, even this afternoons run, but I am slightly concerned with the lack of agreement with the GFS and Euro. These high ratios could deliver high amounts of snow and if all goes according to plan 10-15 inches seem likely at Purgatory. with 20-30″+ for Wolf Creek. Telluride will depend on when the SW flow shuts off, but I generally like the advisory wording and 10″ seems likely, maybe more.

If you are driving tomorrow the biggest concerns will be from Durango to Coal Bank and the entire 160 corridor to Wolf Creek Pass Resort. NWS is already calling for snowfall rates of 2 inches per hour by late morning especially above 8,000 feet. My best advice if you are leaving Durango tomorrow try to leave before sunrise. Next update early tomorrow morning.

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