By Doug Schneider

If you've read my previous forecast discussions, you'll know that I didn't feel very confident in the specifics of what the weather would bring at Indy this weekend. Today, I'm feeling much more confident about the forecast, and some details of timing are coming into better focus. 

A trough in the mid and upper levels of the atmosphere is approaching Indiana from the west today, while moisture continues to increase from the south in the low levels. This combination will bring showers and thunderstorms across the state tonight and into Saturday morning. The best chance for weather impacts at the Speedway will be in the morning hours. I'd put the chance of rain after 8 am at the Speedway at 30% .The chance of showers will be decreasing in the afternoon as the trough exits and drier air aloft moves in from the northwest. 

Sunday is looking better for a dry race. A ridge of high pressure in the mid and upper levels will be building in from the west, while a southwest flow at the surface will keep warm and humid conditions. With the building ridge, it will be hard for afternoon showers and thunderstorms to develop. It may still happen, but I expect that the potential for that to happen will come after the race. So while an isolated shower or storm may be possible in the late afternoon, I do not have a chance of rain mentioned or any impacts for the race. It will be rather hot, with a high temperature approaching 90, and with the humidity level being relatively high, it will feel like the mid 90s at the track. So be sure to drink plenty of water and wear sunscreen if you're going to the race.

By: Stephen McCoy

Not much has changed in the forecast for this weekend’s Pirelli World Challenge Grand Prix of Lime Rock. Winds in the upper levels of the atmosphere are expected to stay constant from the northwest, bringing dry air from Canada. At the lower levels, the general flow will be from the west and will be warmer than the surface in the early morning hours. This will set up a temperature inversion, and likely cause some low level clouds to persist. Once the surface temperature is warm enough to overcome the inversion, conditions will begin to clear. With westerly flow at both the surface and the upper levels, some windy conditions are likely with some stronger gusts.

The low pressure system mentioned in the previous discussion is expected to now move out into the Atlantic Ocean during the day on Friday. However, a second low pressure system is expected to move into the Midwest region on Saturday. A warm front is expected to extend eastward out of the system into New England. There is a slight chance of showers associated with this front, but should hold off until after track activities have ended.

For Monday, the current long-range models are in some disagreement for the likelihood of precipitation and the high temperature. The current runs of the GFS model indicate that for Monday, a moist atmosphere in the lower levels and cooler temperatures are likely to result in rain during the morning and early afternoon. However, the Euro model suggests warmer temperatures with little precipitation occurring at the end of the second GT SprintX race. Because of this disagreement, I’ve decided to indicate a slight chance for showers, though I don’t have very high confidence in the likelihood for precipitation. I’ll have a better understanding of what to expect for Monday in the next update.

By Doug Schneider

The model trend over the past couple days has moved toward a drier direction, which is good news, but there is still a lot of uncertainty about the chance of rain over the weekend, and my forecast confidence remains low. 

A big reason for my low confidence is that the weather pattern across the area is not well defined. In other words, the flow through the mid and upper levels of the atmosphere is weak, so there is not a significant trough or front that will be moving through the area to provide lift and help to focus thunderstorm development. In a weak pattern like this, thunderstorm development will depend mainly on weak surface boundaries and afternoon heating that creates instability. That is very difficult to predict this far out. It may be a situation where we will just have to monitor the radar each day and see how things develop. 

I still think Friday is going to be a dry day as there is reasonably good model agreement that there will be dry and stable conditions in place, A southwest flow will bring warm temperatures in the upper 80s in the afternoon. 

In my previous forecast I mentioned a low pressure system that was expected to move across the Plains and Great Lakes over the weekend. That system now looks weaker and farther north than before. However, there will still be increasing low level moisture in the southwest flow, and a little greater instability that may allow for afternoon showers and storms to develop on Saturday. The pattern doesn't change much for Sunday, so both days should be pretty similar to each other. I only have a 20% chance of rain each day, so there's an 80% chance of dry conditions and no interruptions to the race. Again, we'll just have to wait and see how things develop, but I can't rule out an isolated storm at the Speedway each afternoon.

By: Stephen McCoy

The Pirelli World Challenge series will be heading to New England for Memorial Day weekend for the Grand Prix of Lime Rock with racing on Friday, Saturday, and Monday. Conditions are looking nice for Friday and Saturday, but could get are a little more uncertain for Monday.

Starting with the upper atmospheric setup for Friday, the upper level jet stream is expected to set up over the Northeast US/Southeast Canada, bringing cool, dry air into the region. The low level jet stream will be bringing air into the region from the northwest as well in the morning hours before turning towards the west. The direction of the two jet streams in the morning will keep temperatures cool to start and will bring clear skies for most of the day. Little cloud cover, with the addition of westerly winds at the surface and lower levels, will allow for plenty of daytime heating and cause temperatures to reach into the low-to-mid 80’s. With both the surface and low level winds coming from the west, expect some breezy conditions with some gusts approaching 20mph.

A strong low pressure system over southeastern Canada is expected to track eastward Friday into Saturday. According to the latest runs of the GFS model, a cold front will set up north of New England and slowly move southward during the day on Saturday. With this setup, temperatures will likely remain warm overnight as winds will be from the south and southwest. Expect heavier cloud cover than Friday as moisture moves into the area in the lower levels of the atmosphere. A threat of rain is possible with the cold front; however frontal passage should occur after track activities have ended.

On Sunday, a high pressure system will develop over Canada before tracking southward over New England and into the Atlantic Ocean on Monday. Cool winds from the east off the Atlantic will keep temperatures cooler, with a low in the low-to-mid 50’s and a high around 70. Moisture off the Atlantic will likely result in some cloudy conditions for Monday.

By Doug Schneider

After some showers and thunderstorms move across Indiana today, the weather looks good through the rest of this work week as high pressure builds over the area on Tuesday. Unfortunately, that high pressure system will be losing its grip on the area by the weekend, and a low pressure system will be approaching from the west, which will bring a threat of rain for Saturday and Sunday at IMS. 

Carb Day weather looks great for the Indy Lights Freedom 100, the Pit Stop Challenge, the Carb Night Classic at Lucas Oil Raceway, and the evening concerts. Be sure to bring some sunscreen to the track, as it will be a mostly sunny and warm day with highs in the mid 80s. 

With the high pressure system moving off the east coast and a low pressure system tracking across the northern Plains, a southerly flow will start to increase moisture over Indiana on Saturday. The moisture and warm temperatures may be adequate to allow afternoon showers and thunderstorms to develop for Legends Day activities. The late afternoon and evening concerts will have the greatest chance of being impacted, but as of today it only looks like a 20% chance of a storm hitting the Speedway. 

On Race Day the low pressure system is expected to be over the Great Lakes region. Although there are some model differences in the position of the low, which will affect the timing of any showers and storms, they are in good enough agreement about the pattern that I have to include a 40% chance of showers and storms in the forecast. One model (GFS) is currently quicker to exit the rain chance on Saturday night, with drying conditions on Sunday. Hopefully this will happen and I can adjust the forecast in a drier direction over the next few days.