By Doug Schneider

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The main weather concern for the Six Hours of Silverstone continues to be Sunday, when a low pressure system is expected to track across Scotland. One change that the models have shown in the past day has been to move this low into Scotland faster. The timing of rain chances on Sunday has shifted a little earlier, and is expected to start in the late morning hours, around 10 or 11 am, and end in the mid to late afternoon, around 4 or 5 pm. It still looks like any rain that falls will be very light and have little impact on the race. Teams may have to decide whether a switch to rain tires will be needed, but I don't expect that there will be any interruptions, as only a couple hundredths of an inch (a couple millimeters) is expected. 

By: Stephen McCoy - @stephenmmcc

Grab an umbrella and a raincoat if you're planning on attending Saturday's or Sunday's sessions of the Michelin GT Challenge at VIRginia International Raceway. Precipitation will be likely on Saturday with a chance for scattered thunderstorms in the afternoon and evening; Sunday also has a chance for scattered thunderstorms in the afternoon and evening.

In the upper levels, a shortwave trough mentioned in the previous forecasts will move eastward towards the Atlantic Ocean with its center positioned over the Midwest. As a result, winds in the upper levels will be from the southwest, bringing  moisture to the region from the southeastern states. In addition, winds in the lower levels will be from the west and southwest, advecting moisture to the lower levels as well. At the surface, a cold front extending from a low pressure system centered over New England will track southward towards central Virginia through Sunday. Due to the large amounts of moisture in the atmosphere, precipitation over the region looks likely on Saturday. Depending on how far southward the cold front extends, a chance of thunderstorms is possible as the front would act as the necessary lifting mechanism to create scattered storms. With the approach of the front, winds at the surface will be from the southwest, with some breezy conditions as the winds at both the surface and low levels are from the same general direction.

The best chance for precipitation will be in the afternoon and will likely affect the end of WeatherTech qualifying and/or the Biscuitville Grand Prix. Rain chances will continue into the evening, but will slowly decrease overnight. With cloud cover and precipitation, temperatures will likely only reach into the mid 80's, with heat index values in the upper 80's to low 90's.

On Sunday, as mentioned in the previous forecast, upper level winds are expected to return to zonal flow as the shortwave trough moves into the Atlantic Ocean. At the surface, the cold front is expected to stall before it reaches southern Virginia, creating a stationary front to the north of the track. As winds on either side of the front converge, the cooler air could undercut the warm air, creating a chance for scattered thunderstorms Sunday afternoon. As the track will be to the south of the front, winds from the west to southwest will advect warm air to the region, raising temperatures towards the upper 80's. Drier air in the upper and mid levels of the atmosphere will lead to partly cloudy conditions through the day.

By: Stephen McCoy - @stephenmmcc

More heat and humidity in the forecast for the Michelin GT Challenge at VIRginia International Raceway. Precipitation chances have shifted from Friday and Saturday to Saturday and Sunday, though there is a chance of thunderstorms on Friday after track activities have finished for the day.

On Friday, an upper level shortwave trough will be centered over the Midwest. Winds at this level will be from the southeast, which would normally bring moisture into the region. However, a pocket of dry air originating over the Gulf coast on Wednesday will be transported to southern Virginia by these winds. In the mid to low levels, winds will bring moisture from the southwest, though with dry air aloft cloud cover will be concentrated mainly at the lower levels. At the surface, winds will also be from the southwest as a low pressure system will be to the north over the Great Lakes and a high pressure system will be to the southeast over the Atlantic. With the vertical stack of winds from the southwest, surface winds will be moderate with some stronger gusts possible. The surface pressure systems are expected to funnel moisture from the Gulf of Mexico through the southeast, raising dew point temperatures throughout the region. As a result, heat index values for the track will likely reach the mid 90's.

As the surface low pressure system moves eastward from the Great Lakes on Friday, a surface trough is expected to extend southwest from the center of the system. A squall line will likely set up along this surface trough towards mid-day and progress eastward with the low pressure system. There is a chance that the squall line could produce precipitation at the track, however it should reach the track after activities have concluded for the day, though it may disrupt some dinner plans.

The aforementioned upper level trough will continue to bring moisture from the west and southwest along with winds in the low to mid levels from the same direction. Moisture in the atmosphere will gradually increase on Saturday, increasing chances for precipitation in the afternoon and evening and will likely affect the various on-track sessions during these times. There is some disagreement among the models, though as the latest runs of the GFS show precipitation occurring overnight Friday, then remaining dry on Saturday. The ECMWF and NAM, however, both lean towards precipitation during the day. The surface low pressure system mentioned for Friday will continue to move eastward towards the north Atlantic Ocean on Saturday. A cold front extending west from the system will remain north of the track while another surface trough will stay to the east; without either of these lifting mechanisms, thunderstorms are unlikely.

Upper level flow will return to more zonal conditions on Sunday, with winds directly from the west throughout the day. The cold front extending from the surface low pressure system will slowly track southward towards the region, bringing a slight chance for scattered showers. With the passage of the front, temperatures will cool to the mid to low 80's, however with surface winds from the northeast, dew point temperatures will reach the upper 60's to low 70's. The resulting heat index will be in the low 90's.

By Doug Schneider - @Race4caster

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The initial forecast that I made on Monday still seems to be generally on track, but I've made a few tweaks for today's update. The main change has been to add a slight chance of rain on Sunday afternoon, however I still expect that impacts on the race will be very low. 

Rain associated with a deep upper level trough and surface cold front is still expected to move across England late tonight and into Thursday. Although it will likely be wet for scrutineering, the rain will be long gone by the time on-track activity begins on Friday. Behind the departing trough and cold front, temperatures on Friday will be cooler than normal (the normal high is 75 F/24 C), with a west wind at 10 to 15 mph (8-16 km./h). 

Another low pressure system will be tracking east between Iceland and Scotland on Saturday, but it will be too far north and too weak to have any real impact on Silverstone. A weak ridge in the upper levels will build slightly over southern England, which should provide for some periods of sunshine and warmer temperatures in the mid 70s F/mid 20s C. 

Yet another fast-moving low pressure system will track between Iceland and Scotland on Sunday, but it appears that this one will be farther south and a bit stronger than the one on Saturday. It will shift winds to a more southwest direction, and bring a little more moisture into central England. This may produce some light rain at Silverstone in the afternoon. However, it appears that it will be light rain, with maybe only a couple hundredths of an inch expected (around 0.5 mm) at most. I am keeping the impact level at "Green" as I don't expect there to be enough rain for any weather-related interruptions. 

By: Stephen McCoy

Conditions look hot and humid for the Michelin GT Challenge at VIR this weekend with chances for precipitation on Friday and Saturday. Heat exhaustion can be a real problem this weekend with heat index values well into the 90's so be sure to take precautionary steps to stay cool and hydrated.

During the week, an upper level shortwave trough will be moving eastward across the country, originating in the Pacific northwest and setting up over the Midwest Thursday into Friday. Ahead of the trough, a surface low pressure system is expected to form over the Great Lakes region and track eastward with the trough. A cold front extending southward from the system will likely affect the track on Friday and/or Saturday. The disparity as to what day the front will pass through is due to the GFS and ECMWF in disagreement. The Euro indicates the frontal passage in the late afternoon or evening on Friday while the GFS shows the front passing Friday night. In both cases, precipitation is expected behind the front and with the front moving through earlier with the Euro, a chance for rain to disrupt on track sessions in the afternoon; precipitation would move through overnight according to the GFS.

With the front passing through, warm winds from the southwest will be expected at the surface, raising air temperatures to the lower 90's in addition to bringing moist air from the south to the region. The increased humidity at the surface will result in heat index values in the mid to upper 90's.

On Saturday, the center of the upper level trough will move through the Mid-Atlantic, continuing to push moisture into the region from the west and southwest. Through much of the region, there will be a slight chance for scattered showers, especially in the afternoon. Where there aren't showers, partly cloudy conditions are expected as moisture persists in the low levels. Surface winds from the west will continues to keep temperatures in the 90's, with dew points in the upper 60's causing heat index values to reach the mid 90's.

For Sunday, the trough will be to the east of the region, tracking into the Atlantic Ocean, resulting in the track being upstream of the center of the trough. Winds in the upper levels will be from the west to northwest bringing drier air to the region. Partly cloudy conditions are expected as the low to mid levels of the atmosphere will have moderate amounts of moisture and with the dry air above, conditions will likely remain the same for the majority of the day. Light winds at the surface from the north and northeast will keep temperatures around 90, though with dew point temperatures in the upper 60's, heat index values will be in the low 90's.