Thursday, June 28, 2007

4th of July Preview

The weather in the days leading up the 4th of July could be rather unsettled. Warm and more humid air is expected to begin flowing into the region by Sunday, with high temperatures in the upper 80s to lower 90s forecast for Monday thru Wednesday. Also, this developing pattern will favor the development of mesoscale convective complexes (a big complex of thunderstorms) somewhere in the northern Plains, and these complexes could then move east and southeast toward Wisconsin. Usually, with this type of pattern, rain and thunderstorms are more likely locally during the overnight and early morning hours, with the daytime hours usually remaining dry, but very warm. These complexes can also be severe sometimes. It will be nearly impossible to pinpoint the timing of these expected storms until about a day before the event, unfortunately.

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Tuesday, June 12, 2007

The Arrival of Summer

It would seem that summer has arrived, and will be sticking around for awhile. A ridge of high pressure has parked itself over the region, and this is resulting in some very warm temperatures and no rain. Highs are expected to reach the middle to upper 80s this week, with a few 90s possible. The humidity will probably increase through the week, but it shouldn't be too oppressive. The best moisture and precip chances are well to our west, and it looks like it will be late this weekend before we pick up any rain chances. This seemingly permanent summer-like weather will be a nice change of pace from our most recent weather pattern consisting of a few days of warm temperatures, then a few cooler days, with some rain thrown in frequently.


Thursday, June 07, 2007

High Risk

5:08 p.m.: The severe weather threat appears to be over, and the tornado watch will likely be cancelled in the next hour or so. Severe thunderstorms moved through southeast Minnesota and western Wisconsin, resulting in some wind damage, very large hail, and possibly producing a tornado near Bangor. The storms have moved into northeastern Wisconsin, where several tornadoes have been reported. A tornado was reported just south of downtown Wisconsin Rapids, along with softball-sized hail. A tornado watch is in effect for eastern Wisconsin as well, but storms have not developed there yet. Overall the severe weather was not quite as widespread as expected over western Wisconsin, perhaps due to this morning's cloud cover. Another factor may have been the fact that the boundary that the storms formed along moved into the region a little quicker than expected. Still, this was a significant outbreak in areas mainly north of the Kickapoo Valley, but not as bad as feared.

12:40 p.m.: A Tornado Watch is in effect until 10:00 p.m. This is a PDS watch (Particularly Dangerous Situation), which means the threat for severe weather is very high. Destructive tornadoes, hail up to 2.5 inches in diameter, and wind gusts up to 80 miles per hour are possible. We remain under a high risk for severe weather, and the threat for tornadoes is slightly higher this afternoon than it was this morning. Thunderstorms are rapidly developing across northern Iowa and southern Minnesota, and several warnings have already been issued there. These storms will move north and east toward Wisconsin during the afternoon. The temperature has warmed into the lower 80s and dewpoints are well into the 60s, so the atmosphere is quite unstable. This is a very volatile situation and can change rapidly, so monitor the situation closely as the afternoon progresses and be prepared to seek shelter if these storms approach.

10:08 a.m.: Instead of eroding, the cloudcover has actually increased this morning. This could work to suppress the severe thunderstorm risk this afternoon. However, wind shear remains very high, and dewpoint temperatures have climbed into the middle 60s. Skies are also more clear across parts of Iowa and Minnesota where convective initiation is expected in the next few hours, so the threat for severe weather is still very real.

8:37 a.m.: There is a rare high risk of severe thunderstorms today across all of Wisconsin and adjacent parts of Minnesota and Iowa. All of the parameters for a significant severe weather outbreak still appear to be coming together. It is becoming increasingly warm and humid this morning, with some scattered clouds. Most of Iowa and southern Minnesota is cloud-free at this time, and this clear area is expected to move into Wisconsin as the morning progresses. This will allow the atmosphere to become more unstable. Wind shear values are already high, so thunderstorms are expected to rapidly develop by early afternoon and then move east. Tornadoes, damaging winds and hail are all possible. There is also a threat for some strong tornadoes and widespread damaging winds, with gusts up to hurricane force possible. There is still some chance that this forecast may be a bust, but it looks more and more like it will be a stormy afternoon across the area.

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Wednesday, June 06, 2007

Dangerous Thursday

It appears that all the ingredients are coming together to possibly create a widespread, significant severe weather outbreak across the region on Thursday. An unseasonably strong storm system is developing across the northern plains today, and this is expected to drive a strong cold front through the Upper Midwest tomorrow. Wind shear values are terrifyingly high, which leads to a high probability of severe thunderstorms. The entire region is currently under a moderate risk for severe storms. The most likely scenario would see thunderstorms rapidly developing across eastern Minnesota and Iowa early tomorrow afternoon. Tornadoes, some of which could be strong, will be possible with early supercells that develop. These discrete storms are then expected to form into a rapidly-moving squall line which is then forecast to move across Wisconsin late Thursday afternoon and evening. Widespread damaging winds are possible with this line, an event known as a 'derecho'.

There are two factors that could limit thunderstorm development. The first is the convection that is expected to affect the Dakotas and Minnesota tonight. A widespread severe weather event is anticipated there, but it is forecast to remain well to our north. However, if any of this precipitation makes it to southern Wisconsin, or debris clouds linger across the region tomorrow morning, the severe threat will be reduced. At this time, this is not expected to happen. The second inhibiting factor is the wind shear. While strong wind shear favors severe thunderstorms, in this case it is possible that the shear will actually be too strong, and the storms will be ripped apart before they can fully develop. Basically, this will probably be one of those "all or nothing" scenarios. If storms get going, they will likely be some really bad ones. If one of these two factors mentioned above comes into play, we may not get much of anything. Despite this, severe weather looks likely tomorrow afternoon, and if everything comes together right, it could be a bad day.


Saturday, June 02, 2007

Ongoing Severe Threat part 2

No severe weather developed in the valley yesterday, but a brief tornado in Lafayette County was rated EF0 and a much stronger tornado in Muscatine County, Iowa was rated EF3. There is still a severe weather threat this afternoon, as partly sunny skies will allow for some destabilization of the atmosphere. The most likely scenario is for thunderstorms to develop over central Iowa and Minnesota during the afternoon hours and then track eastward into Wisconsin. This morning's rain and cloudcover has stabilized the atmosphere for now, so the severe weather threat is not as great as it could be, but it still bears watching.


Friday, June 01, 2007

Ongoing Severe Threat

A tornado watch is currently in effect for about another two hours as thunderstorms continue to move out of Iowa and Illinois into Wisconsin. Tornadoes have apparently caused some rather significant damage north of the Quad Cities, and other tornadoes were reported in northwestern Illinois and Lafayette County in Wisconsin. All is quiet right now but another line of storms is rotating to the north and northwest into southwest Wisconsin. There could be a severe weather threat with this line as it moves though but at this time most of the storms are rather weak. This unsettled weather pattern will remain in place through next week, with nearly daily chances for thunderstorms.