Heat advisory, severe storms later

Get ready for a hot, sweaty, potentially volatile day of weather, possibly the hottest day of the year so far for much of Minnesota. The (dreaded) Heat Index will be in the 100-105 range, with the greatest risk of heat-related ailments in the Twin Cities, where warmth will get a boost from the urban heat island. This has nothing to do with global warming & climate change, and everything to do with metropolitan areas, more asphalt, concrete, businesses, dark surfaces that absorb heat during the day, and slowly re-radiate that warmth at night, keeping temperatures as much as 5-10 degrees warmer during the nighttime hours, and 5 degrees hotter during the heat of the afternoon.

With dew points near 70 Tuesday afternoon, and “pooling” of moisture ahead of an oncoming cool front, the Heat Index will flirt with 100 by early afternoon. South and west of the Minnesota River the combination of heat and humidity could make it feel more like 105, even 110, up in the danger zone. For people working outside the risk of heat exhaustion (clammy skin, nausea) will be significant. Watch for symptoms of heat stroke (dry skin, rapid pulse, disorientation, slurred speech) which can be fatal if not caught and treated early. Take the heat seriously later today – there’s a good chance it will take your breath away – literally – after 2 or 3 pm.

Weather Headlines

* Heat Advisory in effect for much of the Midwest, including the immediate Twin Cities region, through Tuesday night. Factoring temperature and relative humidity it may FEEL like 100-105 by mid afternoon.

* Slight severe risk later today: isolated large hail, damaging straight-line winds, even an isolated tornado or two possible – best chance around the dinner hour. Watches and warnings likely.

* Minnesota Lakes (Faribault county) reports 4.67″ of rain from Sunday night storms!

* Models print out .25 to .50″ of rain tonight as cooler front arrives.

* Drying out Wednesday, only a minimal, isolated shower/thunder risk by late afternoon.

* Dry Thursday, warm sunshine – quiet.

* More showers/storms possible late Friday and Friday night.

* Potential for a fine, lake-worthy weekend, with ample sun and daytime highs well up into the 80s, even up north at the cabin.

SPC Outlook for Tuesday, showing a slight risk of isolated T-storms over central and southern Minnesota. I do expect some watches and warnings to be issued after 2 pm or so, the best chance of severe weather coming right after the maximum heating of the day, after the daytime “high”, which usually comes around 4 pm or so. Any reports of large hail, damaging winds or isolated tornadoes will be spotty and extremely rare – most of us will just see a generic (heavy) thunderstorm. There’s no way to predict, even 12-24 hours in advance, exactly which part of a county will be impacted by violent weather. All we can do is say that conditions should be ripe overhead, conditions condusive for isolated severe weather, which should affect far less than 1/10th of 1% of the state.
WRF/NAM Forecast for 7 pm today showing heavy/severe storms marching east across Minnesota. Much of the day will be sunny, but clouds increase during the afternoon hours, the heaviest rains coming from late afternoon into the nighttime hours.
Observed rainfall over the last week, showing significant, 1.5-3″ rains soaking much of west central and central Minnesota, along with south central counties around Albert Lea and Austin. But very little rain fell along the Minnesota River Valley and along the North Shore – the rain later today will be welcome indeed across much of the state.
I am cautiously optimistic for the upcoming weekend. This is the 132 GFS Outlook, valid 7 pm Saturday evening, showing rain/storms moving into Milwaukee and the Chicago area, and dry weather for much of Wisconsin, Minnesota and the Dakotas, with a west to northwest wind flow at the surface. Right now it appears dry, sunny, seasonably warm weather will linger most of the weekend. What can possibly go wrong?
The Heat Index (courtesy of the National Weather Service). Find the predicted temperature and relative humidity and you can calculate the Heat Index, which later today should be approaching 100 over central Minnesota, maybe 100-105 over southern Minnesota and the immediate Twin Cities metro area.

All those counties in pink (including Hennepin and Ramsey Counties) are under a Heat Advisory through Tuesday night, calling for an elevated risk of heat-related ailments. Map courtesy of Weather Underground.

(graphic courtesy of the National Weather Service office in Chanhassen, MN).

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