By Doug Schneider

Over the past few days, the main models that we use to make long range forecasts had major differences in the outlook for rain at Lime Rock. I'm happy to say that they have come into better agreement today, and the model that was wet is now slower to bring in rain, so I can make my initial forecast dry with some level of confidence. 

The general pattern later this week will be a high pressure area over New England, drifting east off the coast through Friday. It will provide a really nice day for racing on Friday, with lots of sunshine and temperatures in the mid 80s. Winds will be from the south at 5 to 10 mph, but the humidity won't be uncomfortably high with dewpoints in the upper 50s to around 60.

The high pressure center moves farther east on Saturday, while a low pressure system tracks into the Great Lakes region. This pattern will spread more moisture into the atmosphere from the south, but I don't expect it will be enough to produce rain on Saturday - just a little more cloud cover. The chance of rain should hold off until Saturday night and Sunday. 

With my last forecast for IndyCar in Toronto, the models started off too fast to bring in rain, and tended to get slower and slower with each run. I think this trend will continue for this forecast as well, and hopefully it won't go the other way. This is one weekend to be thankful that there's no racing at Lime Rock on Sunday.

By: Stephen McCoy

It will be a hot one on Saturday for the final day of racing at Portland International Raceway as low amounts of cloud cover and northerly surface winds do little to relieve the heat.

In the low levels, winds are expected from the east, allowing warmer, drier air to enter the region; this is in contrast to the general flow for the northwest as low level winds from the west normally bring cooler air off the Pacific. In the upper levels, a high pressure ridge is expected to move through the region over the next few days and as a result, light winds will be present, disrupting the general flow in the upper levels, and trapping the warmer air closer to the surface.

The high temperature forecast for Sunday has only gone up in the past few days. The latest model runs suggest much of the same, now indicating a high in the upper 90's. The National Weather Service office in Portland has issued a heat advisory for the area from 10am PDT until Monday at 8pm PDT as even with low relative humidity, heat index values will still reach the low to mid 90's. Surface winds are expected from the north and northwest, as they have been all weekend, with wind speeds increasing as the day progresses; the maximum wind speeds will be reached around sunset.

By: Stephen McCoy

Not much has changed for Saturday's forecast, remaining mostly sunny with some cloud cover, primarily in the upper levels. Winds will stay light for the majority of the daylight hours, however some winds closer to 10-15 mph could be possible into the evening. Conditions for Sunday are expected to be similar with some cloud cover mainly concentrated in the upper levels. However, temperatures will be warmer than Saturday as light winds will set up from the west in the mid to upper levels of the atmosphere. With dew point temperatures in the low to mid 50's, heat index values are expected to reach the low 90's.

By Doug Schneider

The model trend this week has been to slow down the approach of rain into the Toronto area this weekend. That trend has continued since the last update, and the chance of rain has been pushed into Saturday with today's update. It does appear more likely that there will be some weather impacts on the racing.

A few scattered light showers are observed on radar across southern Ontario today, moving toward the east. Light showers may move in and out of Toronto through tonight, but the main area of showers and thunderstorms associated with a cold front is expected to arrive late Saturday morning. One model, the NAM, shows quite a bit of rain falling from late morning through the early afternoon, up to one inch. I think that's probably overdone, with around a quarter inch being closer to what most models are showing. That would still be enough to cause some short delays, particularly with a chance of lightning as well. The races most likely to be impacted are the MRTI races, which run from 10:50 am to 1:40 pm, and IndyCar qualifying from 1:55 pm to 3:10 pm. 

The front should be south and east of Toronto by Saturday evening, with high pressure building in for Sunday. Race day continues to look mostly sunny and warm, with high temperatures in the upper 80s (lower 30s C). 

By: Stephen McCoy

The forecast for this weekend's races at Portland International Raceway remains mostly consistent with the previous forecast with the main exception of Sunday.

For Friday, the main concentration of clouds over the region will be focused in the upper levels of the atmosphere from moisture moving through from the south. With a temperature inversion and a dry atmosphere expected in the mid and lower levels, expect a high temperature in the upper 80's with heat index values in the low to mid 80's. Later into the evening, winds could pick up as the low level winds and surface winds are both expected from the northwest, possibly creating some moderate gusts; downslope flow from mountains to the west could contribute to stronger winds.

For Saturday, expect similar conditions to those on Friday apart from large amounts of cloud cover as drier air moves into the upper atmosphere. The moisture moving into the area mentioned in the previous forecast appears to hold off, resulting in more intermittent cloud cover, mainly concentrated in the afternoon.

For Sunday, temperatures are expected to reach into the 90's, which is consistent with the previous forecast. However, much like the previous forecast, the models are in disagreement for the high temperature, with the GFS continuing to bring temperatures closer to 100, while the ECMWF and German Global models are in the low 90's. The latter two models are slowly trending warmer, though and with the addition of the NAM and other high resolution models, the next forecast should provide a better consensus for the high temperature; heat index values will remain in the low 90's regardless. Sunny conditions are expected during the day as dry air moves in throughout the atmosphere.