Good Wednesday morning!

The Aspen leaves are turning, most notably over the northern 1/3rd of Colorado.

The webcam at Copper Mountain, looking north to the Gore Range, shows a beautiful stand of Golden Aspens just up the hill from I-70.

Looking at earth from 22,000 miles above in space, the GEOS-16 weather satellite shows a perfect morning in Colorado with not a cloud over the mountains.

While the skies are clear and we’ll be free from precipitation for the rest of the week, all is not quiet.

The National Weather Service map notes that a Red Flag Warning will be in effect over the central-western US, including the western half (ish) of Colorado.

A Red Flag Warning means that there is fire danger due to the combination of dry weather and gusty winds.

Forecasts for western Colorado show afternoon wind gusts in the 30-45 mph range through the week. These winds will be caused by a storm well to our northwest, which will bring snow to western Canada.

You’ll notice on the National Weather Service map that there are also lots of alerts over the eastern Carolinas. This is due to the approaching Hurricane Florence.

When you look at the graphic above, which represents the official forecast from the National Hurricane Center and incorporates many models, please notice how the ‘cone of uncertainty’ expands right around the time that the hurricane could make landfall.

Rather than focusing on the exact center (dot) of where the hurricane might be, relax your eyes and look at the total envelope of the cone.

After looking at this graphic and many of the individual models, it’s clear that Hurricane Florence is going to slow down right as it’s about to make landfall. This slow down is not good for two reasons.

The first reason that a slow down is not good is that the storm’s path and intensity become harder to predict. The storm will wobble around rather than move quickly on a direct path.

The second reason that a slow down is not good is that the storm might stay close enough to the coast to bring the highest winds ashore, but not move inland far enough to weaken quickly. The worst-case scenario (shown in some of the latest models) is that the storm slows down and drift south, right along the coasts of southern North Carolina and most of South Carolina. This would be bad for a large section of coastline.

Regardless of exactly where the storm wobbles, what we know from past slow-moving systems is that there will be a LOT of rain, likely more than 20 inches. So in addition to the wind and storm surge damage along the coast, inland areas will likely see incredible flooding.

As I said in my previous post, let’s hope for the best but it’s wise to prepare for the worst.

Also, it bears repeating that when storms slow down to a crawl, forecast accuracy plummets. So be ready for many adjustments to the forecast that you see above, and these adjustments could lead to a forecast that shifts the hurricane south and keeps it close to the coast for 2-4 days.

Back here in Colorado, the 10-day precipitation forecast is lackluster.

And the snow will generally stay over western Canada. You can check webcams in Alberta and British Columbia here on OpenSnow to keep an eye on this early-season storm.

The only potential for active weather in Colorado could be around:

* Sunday, September 16th. The American GFS model shows showers. Other models show nothing. Right now my bet is for nothing.

* Wednesday, September 19th. Most models show a surge of cooler air and moisture over the eastern plains, maybe touching off some showers over the eastern mountains.

* Saturday, September 22nd. A fast-moving storm could bring a few showers and cool air to the northern and eastern parts of Colorado.

* First week of October. The longest-range European model shows increasing chances for storminess, so this could be the time of our first cool system of the season.

Thanks for reading … next update on Friday, September 14!

JOEL GRATZ

OpenSummit

We have an iPhone app that provides detailed weather forecasts for your hiking, biking, and climbing adventures. OpenSummit now includes forecasts for 1,000 of the highest and/or notable summits and hiking areas across the United States. Download OpenSummit (iPhone only)

Upcoming Presentations

* September 20 in Boulder. Part of Ignite Boulder. Details & tickets here.

* September 27 in Aspen. More details soon.

* October 18 in Colorado Springs. More details soon.

* Early November in Summit County. More details soon.

* Early November in Boulder. More details soon.

 

Geography Key

Northern Mountains
Steamboat, Granby, Beaver Creek, Vail, Ski Cooper, Copper, Breckenridge, Keystone, Loveland, Abasin, Winter Park, Berthoud Pass, Eldora, Rocky Mountain National Park, Cameron Pass

Along the Divide
Loveland, Arapahoe Basin, Winter Park, Berthoud Pass

East of the Divide
Eldora, Echo, Rocky Mountain National Park, Cameron Pass

Central Mountains
Aspen, Sunlight, Monarch, Crested Butte, Irwin, Powderhorn

Southern Mountains
Telluride, Silverton – north side of the southern mountains | Purgatory, Wolf Creek – south side of the southern mountains