Sunday, September 5, 2010

Thundery - Potentially Severe Labor Day?

What Month Is This Again? This is September. Our weather is supposed to settle down in September, now that the sun is as high in the sky as it was in early April (but the upper atmosphere is still relatively warm, suffering from a lingering summer-hangover). That SHOULD lead to a relatively stable atmosphere, and a rapid downturn in severe storms. In a perfect world, yes. Wind shear, low-level moisture and instability may be just sufficient for a few rough storms later today - tune in later today for updates.

Slight Risk. SPC has east central southeastern and south central MN under a slight risk of severe storms later today - unusual for early September. Conditions are marginal, but we can't rule out a few (isolated) severe storms, especially near the Iowa border - after 3 or 4 pm today.

So-So Sunday. There were some intervals of sunshine out there - but the mid and high level clouds were a bit thicker and more abundant than predicted, keeping us 1-3 degrees cooler than expected. Highs ranged from a chilly 58 at Grand Marais to 62 in Duluth, 70 in St. Cloud and 73 in the Twin Cities.

Paul's Conservation MN Outlook for the Twin Cities and all of Minnesota:

Labor Day: Some sun early, then increasing/thickening clouds. A few strong/severe storms are possible later today. Winds: SE 15-25. High: 75

Monday night: Showers and T-storms, locally heavy rain possible. Low: 55

Tuesday: Blustery with more showers, temperatures more typical of early October. High: 62 (winds gust as high as 30-40 mph).

Wednesday: Plenty of sun, less wind - a much nicer day. High: near 70

Thursday: More clouds, showers likely. High: 69

Friday: Windy and warmer - more humid with showers and a few heavy T-storms. High: 78

Saturday: Plenty of sun, drying out (probably the nicer day of the weekend). High: 75

Sunday: Sunny start, clouds increase with showers possible late. High: 74

Happy Labor Day! So here we are in the relatively sedate, normally-uneventful month of September - we should be waving goodbye to the school bus, tossing footballs, enjoying smoky bonfires, tracking foggy mornings, savoring comfortable sunshine and a fresh Canadian breeze (most of the time). It's been a crazy summer (145 tornadoes and counting...) so why should the last holiday of summer be any different? Severe storms, on September 6? Unusual, but hardly unprecedented. Severe storms, even isolated tornadoes, have been reported in Minnesota as late as mid October.

September Soaker? The NAM/WRF model is printing out some very heavy rainfall amounts for parts of central and northern MN, and much of the Dakotas, some 1-3" amounts later today through Tuesday morning. The model is predicting .36" for the Twin Cities, but .69" for St. Cloud, well over an inch from Alexandria and Detroit Lakes to Bemidji and the Brainerd Lakes area.

An approaching storm will set the stage for strong wind shear today, winds changing speed and direction as you rise up through the atmosphere. Low-level moisture is marginal, so is instability (the rate at which a warm thermal of air can accelerate skyward). Even so, shear may be sufficient for a few rotating, "supercell" thunderstorms, especially south and east of St. Cloud, where a watch may be needed later today. Good grief - enough already.

Trying to salvage some of your Labor Day? Tired of staring at your Mother In Law - hoping to escape outside for a few hours? You best bet will come during the morning and midday hours, when the sun may be peeking through, but clouds will tend to increase and thicken as the day goes on, the chance of showers and T-storms spiking as we approach mid and late afternoon. Again, I'm not terribly impressed with the odds of a major outbreak - the best chance of a few rough storms later today will come closer to the Iowa border, where the moisture field is a little richer, and temperatures will warm more. Models print out about a third of an inch of rain tonight and Tuesday.

Tomorrow will feel like early October (again) - temperatures hurtling through the 60s into the 50s as northwest winds gust over 30 mph at times. An utterly, thoroughly lousy, irredeemable day, weatherwise, a reminder that BIG changes are looming on the horizon. We'll see more 70s and 80s - sure of it, but the pace and intensity of Canadian air will be on fast-forward in the coming weeks, each cool front a little cooler, lasting a few hours longer, the pace of warming afterward increasingly sluggish and restrained.

The next round of showers and embedded T-storms arrives Thursday and Friday. With any luck skies will clear in time for a pretty nice Saturday, enough sunshine for 70s with comfortable humidity levels. Sunday will probably start out sunny and pleasant, but clouds increase as the day goes on, the next round of showers possible as early as Sunday afternoon, more likely Sunday night into Monday.

Stay alert later today - at the rate we're going, the kind of (Congo-like) summer we're experiencing with jungle-like humidity levels and a potentially record-setting number of severe storms - I guess anything is possible. Better to err on the side of safety and caution.

The Day After. Earl knocked out power to portions of Martha's Vineyard, but the hurricane weakened (dramatically) before grazing coastal Massachusetts with winds as high as 60-70 mph, just below hurricane strength. As much as 6" of rain fell on Martha's Vineyard, Earl going on to score a direct hit on Nova Scotia as a strong tropical storm. More on Earl's aftermath here.

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