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Slide: 1 On Sunday afternoon at 2:44 PM, fall officially
begins. It seems fitting then that Sunday will feature a range of weather phenomena
beginning with conditions typical of the warm season with a quick transition to more winter-like
conditions in the higher elevations as a strong Pacific storm system moves through the area.
The storm will pass over the region Sunday into Sunday night with conditions improving
Monday in its wake.
Slide: 2 During the late morning and into early evening,
showers and thunderstorms will be the rule across eastern Utah and western Colorado.
There is a possibility that some of those storms may be severe with 1 inch hail and
wind gusts to 60 mph possible. In addition, heavy rain may bring about flash flooding
and/or mud and debris slides. Ahead of a cold front associated with this storm, breezy southwest
winds and mild temperatures will prevail across the area. The front sweeps rapidly eastward
across eastern Utah and western Colorado Sunday afternoon causing snow levels to drop from
above 12,000 feet to around 9,500 - 10,000 feet on Monday morning. This could bring from
2 to 4 inches of snow to areas above 10,000 feet overnight which may result in winter
driving conditions over the high Colorado passes.
Slide: 3 At noon Sunday, the cold front associated
with a strong Pacific storm (depicted by the large ‘L’) over eastern Nevada will have
moved to the Utah/Colorado border. Ahead of the front, deep monsoonal moisture will surge
northward over the region which is depicted by the red, yellow and green colors in the
image. This moisture combined with the strong Pacific disturbance will fuel numerous showers
and thunderstorms across the entire forecast area. A portion of the area may see severe
thunderstorms with the main threat being hail to 1 inch diameter and wind gusts of 60 mph
or more. ‘Training’ storms could result in excessive rainfall and localized flash
flooding.
Slide: 4 This image, generated by the Storm Prediction
Center, or SPC, shows a probabilistic forecast of severe weather within 25 miles of a given
point for Sunday. Local analysis performed here in Grand Junction favors the Four Corners
region in particular from late Sunday morning through the afternoon. Again, the main severe
thunderstorm threats are for large hail and wind gusts in excess of 60 mph, the latter
appearing more likely given the relatively strong southwest flow ahead of the Pacific
storm’s center.
Slide: 5 This image shows the storm center depicted
by the large ‘L’ at midnight, Sunday night. The cold front ahead of this system has moved
swiftly onto the plains and cold air will be filling over the region in its wake. This
will result in a lowering of the snow level with accumulations spreading to near 10,000
feet. Though precipitation will remain widespread in the mountains, the intensity of this activity
will be decreasing. However, instability and moisture lifting over mountain peaks and ridges
will continue to bring snowfall to the higher elevations through much of the night.
Slide: 6 The amount of snow is more difficult to pin
down during the transition season, but models suggested upwards of 4 inches are possible
above 12,000 feet with roughly 2 to 4 inches possible in the 10,000 to 12,000 foot range.
The image shown was taken from the 12-hour snowfall accumulation of the NAM40 projection,
valid at 6 AM, Monday morning.
Slide: 7 To summarize, scattered to numerous showers
and thunderstorms are expected Sunday, with the strongest storms across the Four Corners
area. Breezy winds will make travelling on east-west oriented roads a nuisance for high
profile vehicles with local areas of blowing dust possible. Once the front pushes through
Sunday afternoon, snow levels lower with accumulating snow possible in the high mountains.
Slide: 8 In conclusion, if you are planning travel
over the mountains Sunday night, be prepared for winter driving conditions. Hikers who
might be planning a late season assault on the areas many ‘14ers’ may want to hold
off for now. Enjoy the fall season, and thanks for listening.