By: Stephen McCoy

Minor changes to temperature in this update with warmer weather expected for Saturday. Sunday's conditions will begin mostly clear, though as the day progresses cloud cover will slowly move in due to southwesterly winds in the upper levels ahead of an upper level trough. Partly cloudy skies are anticipated for the Grand Prix, though heavier cloud cover will move in for the remaining on-track sessions. Northwesterly winds overnight and into the morning on Sunday will cause a cooler high for the day relative to the rest of the weekend. Fog remains a possibility for all three days, though Friday remains the most likely for foggy conditions during the morning.

By: Stephen McCoy

It'll be a nice weekend to be in California for the INDYCAR season finale, though a bit cooler than this previous weekend's IMSA race at WeatherTech Raceway Laguna Seca. A cold front moving through the region on Monday and a second cold front moving through on Wednesday will cool temperatures to the mid 70's F for the weekend as surface winds bring air in from the northwest off the Pacific. Conditions will be clear for much of the event due to dry air, also from the northwest, entering into the region through the mid-to-upper levels.

Fog is possible each day, but largely depends on the direction of the surface winds. If they come off the mountains to the east and southeast, nothing to worry about. But northwesterly winds during the morning will bring moisture off Monterey Bay and could result in fog during the morning. Friday currently looks more likely than the rest of the weekend as light to no wind will be present before increasing into the afternoon.

By Doug Schneider

Nice weather is expected in Portland throughout the IndyCar Grand Prix weekend. 

Hot temperatures can be expected in Portland today and Wednesday, then a cold front will move across the area on Thursday, bringing a chance of showers and thunderstorms. 

Cooler temperatures will follow the front on Friday, and continue through the weekend. There will be a low pressure system moving across the Pacific and approaching Portland late Saturday and Sunday, but with limited moisture for this system to work with, only an increase in clouds is expected on Sunday. Each day is expected to have high temperatures in the lower to mid 80s, with a southwest wind at 5 to 10 mph. It should be a great weekend to be at the track.

By Doug Schneider

The good news with today's forecast update is that the models are starting to agree on the timing and position of a cold front south of St. Louis, which gives me more confidence in this forecast. 

A cold front is expected to cross through St. Louis on Thursday night. It will be close enough to the track and with enough uncertainty about its speed that I will keep a small chance of rain in the forecast for Friday morning. But since on-track activity does not start until the afternoon, no impact is expected. Behind the front, drier air will be building in from the north through the day, and with mostly cloudy skies, high temperatures will be quite comfortable in the lower 80s. 

As the front is expected to stall across southern Missouri on Saturday and high pressure over the Great Lakes region will extend across Illinois, Sunday will stay dry. Previous model runs had shown a weak disturbance approaching Saturday, but that no longer appears to be the case. With more sunshine expected, highs on Saturday will reach the mid 80s, but humidity will be comfortably low. Temperatures during the race will be in the 75 to 80 range. 

I will be traveling to VIR tomorrow for the IMSA race this weekend, and Friday may be a busy day of providing weather support to the track, so I may not get to update this forecast until Friday night. 

By Doug Schneider

A programming note - Scott was originally scheduled to make this forecast, but due to a busy week at his real job, I'll be pulling double duty with IMSA at VIR and IndyCar at Gateway. Normally we would also have the forecast for the IGTC race at Suzuka, but Stephen is traveling out of the country this weekend, so that race forecast has been dropped.

A cold front is expected to move north to south across the St. Louis area on Thursday night. The front ix expected to be positioned south of the area by the time on-track activities start on Friday afternoon. However, it won't be very far away, and an upper level disturbance is expected to draw moisture northward over the front on Friday afternoon. As a result, scattered showers and possibly a thunderstorm will be possible in the St. Louis area. It appears that the afternoon hours will have the best chance of rain, which could impact the qualifying sessions for IndyCar, Indy Lights, and Pro 2000. With scattered coverage, they could be hit-and-miss, but I'd put the chance of rain at the track at 40% right now. 

The front to the south is not expected to move much through Sunday, but a little drier and more stable air may be able to build into the St. Louis area on Saturday as high pressure tracks east across the Great Lakes. With the front still being nearby, I don't feel confident enough to keep Saturday entirely dry, but the chance of rain is low, just 20%. If there is an isolated shower Saturday, it would probably be in the afternoon during the Pro 2000 and Indy Lights races. I expect that the IndyCar race in the evening will remain dry. 

One piece of good news in this forecast is that temperatures will be very pleasant through the event. Afternoon high temperatures will be in the lower to middle 80s, and temperatures during Saturday night's race will be in the 75 to 80 range. 

​I expect that there will be some changes to this forecast as we get closer to the weekend and the details come into better focus, so stay tuned.