A classic fall storm—just like the ones you used to know—promises to put a big pause on our Septemberest November Ever. This picture-perfect low pressure system will bring blizzard conditions to Colorado, severe thunderstorms with tornadoes to the Midwest, and ripping winds from the Plains to the Great Lakes.
A trough in the jet stream digging across the Rocky Mountains—along with strong winds at the base of that trough—will join forces to create diverging winds in the upper atmosphere in an aptly-named process known as divergence. These diverging winds create a void, and air from the surface rushes skyward to fill that emptiness.
When that happens, it leaves less air at the surface, so a low pressure forms, and this low will be a sizeable one.
The low won’t be all that deep in the grand scheme of things, but what it lacks in central pressure it will make up for in sheer size. A low of this caliber will bring lots of different types of dangerous weather to lots of places; it’s one of those dynamic storm systems that we’re supposed to see during the fall.
Ah, normalcy! How abnormal these days.
This low will start its life in eastern Colorado this evening, which is a pretty typical spot for one of these things to get going. As it gets its act together, it will trigger steady snow across much of the area. The greatest totals—a foot or more—will occur over the mountains, but a formidable blanket of white doom is possible.
Most spots in the Denver area should see a couple of inches of snow—possibly five or six if you get a nice thumping from a heavier band—which is completely manageable in this part of the world, but it’s something you haven’t had to deal with since last winter. As long as you keep your wits about you, you’ll be fine.
The problem with the snow is that the winds will pick up as this low rapidly gets its act together. As such, blizzard warnings (in red above) are in effect for northeastern Colorado, northwestern Kansas, and extreme southwestern Nebraska. The combination of snow and strong winds are predicted to send visibilities below a quarter of a mile for several hours at a time during the height of the storm, which is dangerous to both travelers and pedestrians caught outdoors.
Playing the odds, it’s unlikely that any of you reading this actually live in the blizzard warnings, but it’s more likely that a few of you or someone you know will have to travel through the area over the next couple of days. Be mindful of watches and warnings, and take it easy if visibility drops below your ability to drive safely.
As this low strengthens and swirls toward the Midwest, it will suck up unstable air from the Gulf of Mexico, allowing a relatively soupy airmass to greet the approaching cold front. The combination of instability, lift from the front, and very strong wind shear in the atmosphere are a recipe for a severe weather outbreak.
The Storm Prediction Center has issued an enhanced risk for severe weather—a three on a zero-to-five scale—for a chunk of the Midwest centered around Quincy. Cities like Des Moines, Columbia, Springfield (Ill.), and Peoria are all in or right on the edge of this area of greatest risk. The threat for severe weather extends almost the entire length of the cold front, stretching down into eastern Texas. The most likely time for thunderstorms from the late morning hours through the evening hours, from west to east.
Damaging winds—gusts in thunderstorms that exceed 60 MPH—are going to be the primary threat for anyone under the risk for severe weather on Wednesday, but large hail and even some tornadoes can’t be ruled out. The tornado risk is especially worrying given the fact that people don’t necessarily pay attention to that sort of thing outside of the spring months.
Make sure you have multiple ways to receive watches and warnings tomorrow, including cell phone, internet, television, radio, and NOAA weather radio. Don’t rely on tornado sirens to alert you to approaching hazards—these are outdoor systems are unreliable and they’re not designed to be heard indoors.
If you’re not an avid weather geek, it can seem a little foreign to talk about severe weather as we approach the middle of November, but this is a pretty common time of the year for bad thunderstorms. The atmospheric setup during November resembles what you would see in early spring, with an active jet stream spawning lows that send fronts crashing into warm, unstable air lingering over the southern United States.
Between 1950 and 2014, we’ve seen 2,312 documented tornadoes in the United States during the month of November, and a handful of those were on the stronger side. One of the most memorable November tornadoes in recent history was the EF-4 that hit Washington, Illinois, back in 2013.
Stiff Gales a Blowin’
If you’re enjoying what’s left of the fall foliage, take it in while you still have a chance. If you don’t get snow or thunder from this storm system, you’re still going to see some pretty strong winds, and it’s going to strip away what’s left of the leaves still clinging to their trees. High wind watches and warnings are up for counties from Colorado to Minnesota, and they’ll likely be expanded east across the Midwest and Great Lakes as the storm organizes and draws closer.
The above map shows surface winds, in knots, on Thursday morning according to this morning’s run of the GFS model. This storm’s wind field will be very large, producing sustained winds of 30-50 MPH across a decent swath of land from Colorado to New England during its lifespan. I wouldn’t want to be on the eastern shores of the lakes until the winds die down—not only for the discomfort, but for the rough waves and minor coastal flooding that will result.
Make sure you don’t have any weak trees or limbs hanging over your house, and secure any loose objects that could blow down the road or through your sliding glass door.
Once this storm jets off into the Canadian wild, a ridge will build right back in and allow temperatures to park themselves well above average as we’ve seen for the past few weeks, and they’ll stay there for a little while longer.
Our Septemberest November Ever continues, right after this.