Outlook for the Twin Cities

(Update: 7:15 pm). Where are the storms?Great question. The atmosphere over southern Minnesota is primed for a potentially wild display of severe storms. But the atmosphere is also “capped”. A layer of warmer air has “put a lid” on the airmass, at least temporarily preventing warm, rising thermals from mutating into thunderstorms. The thought was that an upper level storm, a wrinkle of cold air aloft approaching from Montana, would be strong enough to break the cap, but now SPC meteorologists in Oklahoma aren’t so sure and neither am I. Conditions for severe weather are off the scale, but if we can’t get the process started, then it will be a much quieter evening than just about anyone anticipated. Stay tuned…

Tornado Watch for east central and southeastern MN and much of western WI until 10 pm Saturday night
. Many ingredients are in place for a significant outbreak of tornadoes, even a couple of longer-lasting, long-track, violent tornadoes capable of significant damage. Go about your normal activities, but keep an eye on the sky, and stay connected with local media for possible warnings, issued on a county by county basis. Most towns and suburbs will merely see a generic thunderstorm or two, any severe weather will impact less than 1/2 of 1% of the watch area. But much like a sneeze or hiccup, it’s impossible to tell precisely where Mother Nature may launch deadly winds, much more than 20-25 minutes in advance.

That said, I think the MN River Valley, from Redwood Falls into the southern suburbs of the Twin Cities, is most ripe for a family of tornado touch downs.



SPC Update. Risk for much of southern/central MN has been upgraded from slight to moderate

Risk of Tornadoes (composite index from SPC in Norman, OK below). This is an experimental forecast tool, but one of the more reliable indices from SPC is highlighting a corridor from Windom and Redwood Falls into the southwestern suburbs of the Twin Cities as “most ripe” for tornadoes into the late afternoon/evening hours. A tornado watch remains posted for much of central and southwestern MN through 7 pm and the risk of large, damaging hail, straight-line wind damage, even 2-4 tornadoes, is potentially higher than it’s been all summer. Stay alert, and stay tuned to local media, including Internet for updates – be ready to move to a shelter, even if a warning hasn’t been posted or the sirens aren’t sounding. No substitute for common sense. I have a hunch someone south/west of the Twin Cities is going to get blasted by a pretty strong (EF2-3) tornado by 6 pm. Just a strong gut feel.


VGP, or Vorticity Generation Parameter. Usually anything greater than .2 m/s2 gets our attention. Note the .3 field expanding across southern Minnesota. Another ominous sign.

Doppler Radar at 3:45 pm, showing a “supercell” thunderstorm gaining strength near Marshall and Tracy, dropping estimated 2″ diameter hail, tracking toward Redwood Falls. This cell, or a nearby cell, is capable of spinning up a tornado in the coming 1-3 hours. Stay alert, especially if you live near the Minnesota River. There is enough low-level wind shear (helicity), instability and moisture (dew points in the mid 70s) for something more than just a tiny, brief, garden-variety twister.

Saturday Update

* Lots of weather: 2-5″ of rain since Thursday night, some flooding reports, downed trees, powerlines, lightning strikes, 1-2″ hail reports.

* Isolated severe storms may flare up AGAIN later today – slight risk.

* Heat Advisory: could feel like 100 over far southern/southeastern MN later today.

* Sun returns for a time this afternoon, but the atmosphere remains ripe for rough storms.

* Behind cool front: Sunday should be sunnier, drier, less humid – a few degrees cooler, but probably a better outdoor day across most of Minnesota.

Storm Rainfall estimated by Doppler Radar. The “key” is on the extreme left of the graphic. Everything within the yellow-shaded area picked up 1.5″+ rain, within the light red area: over 3″. The dark red regions signify 5″ or more of rain in the last 24 hours. As you can see many communities soaked up over 3-4″ of rain overnight, over a MONTH’S WORTH of rain in 24 hours, give or take. This was on top of the .50 to 1.5″ that fell Friday morning, by the way. So the bottom line: anywhere from 2-6″ of rain has drenched central Minnesota, including the Twin Cities metro area. Not enough to break the drought altogether, but certainly put a big (runny) dent in the middle of it!



Predicted Temperature at 7 pm this evening. 90 is still a pretty good bet from the Twin Cities south to the Iowa border. Highs reach the 80s over central Minnesota, but check out the readings near Bemidji and Grand Rapids: holding in the 60s? That’s right, a 30-40 degree temperature spread is possible later today from north to south, especially when you consider that dew points in the 70s will make it FEEL like 100 degrees over far southern Minnesota.

Dew Point Temperatures predicted for 7 pm this evening. Mid 70s? Yikes! We are talking breath-TAKING humidity levels later today. When there’s this much water in the air your body can’t cool itself naturally by evaporating sweat. The evaporation process results in cooling (that’s why you feel chilled when you step out of the shower). But when the air is thick with water vapor the evaporation process breaks down and you can overheat much faster.

SPC Outlook for later today – 30% risk of severe weather within 25 miles of any point within the red/hashmark region – including the Twin Cities. More watches/warnings are possible later on, especially south/east of the Twin Cities. I expect a lower risk of severe weather (and less heat/humidity) for St. Cloud and the Brainerd Lakes area, where the cool front will come through earlier in the day.

Summarizing…

Heat Advisories are posted across southern Minnesota later today – take the heat seriously.

Near 90 later today with hazy sunshine on the increase.

Another .25-50″ of rain possible by late afternoon/evening as a cool frontal passage sparks more storms.

Slight risk of isolated severe storms later today, can’t rule out an isolated tornado.

Sunnier, drier, a couple degrees cooler Sunday – overall a BETTER DAY for the lake or pool

Weather outlook

Saturday: Partly sunny, hot and very humid. Chance of a few nighttime T-storms (some severe). High: 92 (heat index nearing 100).

Tonight: Showers and storms possible. Low 72.

Sunday: Mix of clouds and sun, best chance of a T-shower early morning and again late afternoon. High: 89

Monday: Sunnier, slightly cooler, less humid. High: 84

Tuesday: Mostly sunny and pleasantly warm. High: 86

Wednesday: Sunny, bordering on hot. High: near 88

Thursday: Less sun, chance of a T-shower. High: 87

Ahh, the mighty BUFKIT – one of my favorite forecasting tools, which looks like a bunch of garbage, but to be honest, it can tell a lot… The meteorological equivalent of looking into a crystal ball. Hour by Hour this computer model can look into the future, tell us what may be. Keep in mind that this isn’t etched in stone, but it gives us (meteorologists) a good idea of what to expect or what to watch for given atmospheric conditions at that time. It’s a meteoroligists job to decifer the information and make an educated guess on whether or not the information, given by ridiculous mathematical calculations, are true or not. With that said, I’ve highlighted a few main points from BUFKIT at a given time period for Saturday (5pm), which is highlighted by the blue circle in the lower right. Take a look at the blue cirle in the upper left. This is the models interperetation of the amount of instability in the atmosphere, CAPE or Convective Available Potential Energy, at that moment. All the red lines reaching horizontally below that, indicate other instability parameters that are sufficient for T-Storm development. The only other two blue circles I’ve highlighted, mark the temperature in degrees Celcius, which is the number/number format seen in the bottom part of the BUFKIT. Just above that is the last blue circle, which indicates rain falling at that moemet. Again, this is not to be taken litterally at 5pm, but it may be a good indication of what may happen if everything comes together just right.

This image shows the risk of severe weather for our Saturday:

The image below shows the best potential for storms by percentages, a 30% chance through central Minnesota is quite impressive. Hail, high winds and an isolated tornado can’t be ruled out today.

Keep an eye on the sky and an ear on your favorite website, tv station and or radio for any severe watches or warnings that may be issued later today. Have a good weekend.

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