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.

By: Stephen McCoy

Chances for precipitation have gone down for Sunday's ABC Supply 500 at Pocono Raceway, primarily due to rainfall overnight which used much of the available CAPE in the atmosphere. Recent model runs show CAPE values not reaching as high as initially anticipated during the day, again due to overnight storms using up the available energy. Still, mixed-layer values indicated by the GFS will still be near 1000 J/kg which is backed up by the higher-resolution HRRR model; the RAP and NAM 3km models show MLCAPE values exceeding 2000 J/kg. All this is to say a few isolated showers may not be out of the question, especially towards the end of the race as initially thought. Fog will be likely in the region during the morning due to the aforementioned overnight rainfall, with saturated air present under a temperature inversion below 1 km.

By: Stephen McCoy

Conditions have changed a bit for Saturday with partly cloudy conditions continuing to persist through the day, though with a now slight chance for scattered showers. Recent model output is very mixed on timing of precipitation, but should rainfall occur, totals are anticipated to be less than 1/10 of an inch. This is primarily due to the models predicting less energy in the atmosphere: CAPE values being predicted under 1000 J/kg, which is less indicative of convective storms. The cause of this is likely from surface winds out of the southwest instead of the southeast which would have brought more moisture to the region from the Atlantic Ocean. Ultimately, impacts to the track remain low as a result, however a light shower may be enough to stop or prematurely end any of the on-track sessions. Saturated air during the morning near sunrise may result in fog through the region.

On the other hand, Sunday's conditions have remained mostly the same from Wednesday's update, barring the maximum temperature. The models have come to a good consensus of convective precipitation around noon to the early afternoon. Mixed-layer CAPE values from the GFS, NAM, and NAM 3km are all above 1500 J/kg, indicating the increased likelihood of thunderstorms in the region. Temperatures are not predicted to be as high as initially thought due to precipitation cooling the area before the maximum temperature normally occurs. However, dew point temperatures in the low 70's will likely cause heat index values in the low 80's.

By: Stephen McCoy

The updated forecast for the ABC Supply 500 remains mostly consistent with the initial forecast, though with some warmer temperatures both days and a slight increase in precipitation chances on Saturday.

The atmospheric set-up for Saturday is similar to the previous forecast, with winds from the south to southeast now throughout the day, which will result in warmer air temperatures. Dew point temperatures are also anticipated to be warmer: mid-60's during the morning increasing to the upper 60's to low 70's in the afternoon. The low levels aren't anticipated to be as saturated as in the previous forecast, however with warmer temperatures, the chances for convective precipitation have increased. Mixed-layer CAPE values indicated by the GFS are now upwards of 1000 J/kg for Saturday. The NAM shows MLCAPE values well under what the GFS is predicting, though surface CAPE values are similar. Regardless, conditions will be similar to those seen today: hit-or-miss showers with a few thunderstorms possible. The models seem to indicate two standout times for precipitation: mid-morning and late afternoon. Impacts remain low as the uncertainty of where/when rainfall occurs is still high, however if showers do occur they will likely stop any on-track sessions. Much like in the previous forecast, fog is possible during the morning.

Sunday's set-up has changed since Monday, as the aforementioned surface low pressure system is now expected to weaken as it approaches Quebec. However a high pressure system sitting over the central Atlantic will cause winds from the south to southwest, resulting in warmer air temperatures for the day; dew point temperatures near 70F will continue from Saturday. Precipitation chances remain the same, though showers and/or storms will be initiated by convection as opposed to the cold front mentioned in the initial forecast. CAPE values are expected to be higher than those for Saturday and may lead to stronger storms. Impacts remain the same due to the models suggesting the highest chance for precipitation being in the early to mid afternoon. Should rainfall occur on Saturday, fog may be possible again on Sunday morning.