Into the Moderate Risk

The Storm Prediction Center has moved our area into a "moderate risk" zone. This is their 3rd of 4 severe weather categories...and the 4th and final one is rarely issued anywhere, and I cannot say that I have seen it used up here. I would say we see a moderate risk every other year or so. Sometimes we get in a pattern where we get a couple in a short time span, but then there are occasions when we don't get any in a whole year. So, in conclusion of all that...when the SPC puts out the moderate for our area, its time for me to really look at things.

So what is going on that has prompted this? A strong area of low pressure with strong winds aloft will be moving eastward towards us overnight tonight. This system has produced plenty of severe weather already. Currently, there is a line of thunderstorms stretching from Michigan down into Indiana, then stretching back into Illinois and Iowa as the low moves across Wisconsin. These storms will not effect us. The part of the line moving across Michigan will turn southeast and die out. However, in its wake, new storms will develop later this afternoon and form into another cluster. This cluster will move eastward into Western New York later this evening and into the first part of the night and then towards our area after midnight. As it crosses Michigan, southern Ontario and into western New York, it will be producing widespread damaging winds...and it is for that reason that the moderate risk has been posted.

The question for us, on the far eastern edge of the moderate risk, is whether or not the line's severe potential continues overnight. We are in the 45% range for damaging winds. That percentage means the SPC thinks there is a 45% chance of damaging winds within 25 miles of a point. Statistically, from the center of the Grotonweather forecast area, a 25-mile radius covers the entire forecast area and some...so we can assume just a slightly lower probability of damaging winds in our area...very roughly 40%. That may not seem like a very high chance, but in reality, it is. I, however, am not so convinced, and would put the threat at maybe 20-30%. Here is why:

By the time the storms come over here, it will be well after dark. Typically, this would be very unfavorable for any severe weather with the loss of the daytime heating. However, our instability will actually increase overnight and, if the models are correct, peak sometime near or after sunrise tomorrow morning. From what I have been seeing on the models, the line should run our ahead of the main instability and thus weaken itself. Additionally...it will also likely outrun the upper level forcing and stronger winds aloft, which also come through tomorrow morning. If the timing of these matched up better, I would be more convinced a once-every couple years event was indeed unfolding.

Does this mean severe weather is not possible tonight? Absolutely not. These complexes of thunderstorms often develop their own mechanisms for keeping themselves going. If this were not the case, I wouldn't be overly concerned at all. Our instability will still only be marginal to moderate and, as I said earlier, the forcing and upper level winds aren't quite as good as a real classic event would have them. But since these things have a life of their own, there very well could be damage across our area overnight tonight. I do not feel at this time that the damage will be widespread, and certainly not on the scale of our area's most infamous overnight line of storms: The Labor Day 1998 event that destroyed the New York State fairgrounds.

Bottom line is this: Sometime after midnight, a line of thunderstorms should roll through our area. Frequent lightning, heavy rain, maybe a little small hail and gusty winds are likely. Some wind damage is not out of the question, but shouldn't be widespread. Rain and thunderstorms may continue to impact the region tomorrow morning though. Some models have shown very heavy rain in here tomorrow morning, so flash flooding could become a threat.

If anything looks like it has changed, I will post another update this afternoon. Otherwise, an update will be issued around 9m tonight to see how things have evolved through the day.

Round Two Begins!

The clusters of thunderstorms that moved through earlier today have shifted well off to the south and east. While these storms stabilized the atmosphere for a while, in the long run, they only made things worse. Over an inch of rain fell in Groton earlier. Now, with the sun back out, that inch of rain is returning to the atmosphere. Dewpoints have shot up into the mid to upper 70's, making it feel over 90 despite temperatures being in the low to mid 80's. All this moisture is fueling another round of thunderstorms which is just now starting to explode.

Right now, most of these storms are in the southern counties of New York, but there is a large cell over Lake Ontario as well. Further rapid development of storms is certainly possible over the next few hours. There is currently (4:30) a small cell near Trumansburg that will have to be watched for rapid development in the next hour or so. A severe thunderstorm watch remains in effect as well.

Looking into this evening and overnight, there is a very strong line of severe thunderstorms over Michigan right now. These storms have a severe thunderstorm watch for Michigan, and a torando watch issued by the Canadian Weather Service for Southern Ontario. It is hard to say if this line will hold together long enough to effect us, but it is something I will be watching through the evening.

On this radar image (from weathertap.com) you can see the numerous cells firing up in New York, along with the line of storms in Michigan marching eastward.

Severe Update: 4:30PM

Here is the latest on the severe situation...there is good news, and there is bad news.

The good news is, storms aren't forming ahead of the main line of thunderstorms that stretches from Ontario down into the Deep South. This line is a it broken in Ohio, and it is there that there are a few super cell thunderstorms, which have produced funnel clouds, 4.25" hail, lots of wind damage and, probably a tornado or two. However, it looks like our chances for tornadic storms are slightly less than earlier.

Now the bad news. According to Henry Margusity, the severe weather expert at Accuweather, the reason storms are not firing ahead of the main line is because the main line is so strong. Basically, the line is using up the energy in the atmosphere to fuel its self. According to the software I am using to track these storms, the should begin to enter the area sometime after 8. With the intensity of this line, I am not sure it will weaken as it loses its day time heating. It may, so that is something to watch as the evening progresses.

Overall, our chances for storms to fire ahead of this line are decreased, but not gone. An isolated tornado or two is still possible in the general area. Later tonight, expect a line of storms to come through with strong, damaging winds, especially for areas west of us.