By: Stephen McCoy

The start of the TOTAL 24 Hours of Spa currently looks to have the largest chance for precipitation for the race. A few outlier models indicate that precipitation may start in the morning, however model consensus supports the idea of the main showers beginning in the early afternoon during the build-up to the green flag. CAPE values for the start of the race are projected between 500-800 J/kg, which speaking from an American viewpoint is a small amount, however from a European standpoint it could be a moderate amount and enough to kick of thunderstorms in the mid-to-late afternoon. As Sunday approaches, thunderstorm potential should decrease as the convective energy will be used up by storms on Saturday. Fog may become an issue near sunrise due to the air at the surface becoming saturated either by precipitation or by ground evaporation. Precipitation chances will begin to trail off towards Sunday afternoon, though the high resolution models indicate that there may still be some isolated showers in the area; forthcoming runs could place one of these showers directly over the track, but for this forecast conditions remain mostly dry.

By: Stephen McCoy

Conditions have far from improved for the TOTAL 24 Hours of Spa. Starting with tomorrow, temperatures are likely in the low 90's °F, low 30's °C with the latest model runs in a good amount of agreement. There could be a bit of variance in the temperature, as was seen today where a maximum temperature of 98 °F, 37 °C was recorded at Spa (my forecasted high temperature from Tuesday was 95 °F, 35 °C). The cold front mentioned in the previous forecasts is expected to pass through the region in the late afternoon to evening on Friday, bringing with it a slight chance of isolated showers. Model guidance suggests that most of the on-track sessions should be run as normal, though the Super Pole session could be impacted by showers in the area should they occur early enough; recent runs indicate showers starting around or just before 8:00 pm local time. Northwesterly surface winds behind the front will cause temperatures to drop during the morning with a low temperature on Saturday in the mid 60's °F, upper teen's °C.

I mentioned in the previous forecasts that if the front were to stall, the chances of precipitation would likely go up, which, according to the recent model runs, will turn out to be the case. A surface low pressure system to the southeast over southern Germany (extending into northern Italy) will cause winds from the southeast to push back against the cold front, stalling it to the east of the track. The cooler winds from the northwest will undercut the warmer air from the southeasterly winds, causing widespread precipitation throughout the day. Depending on timing/intensity of precipitation some of the sessions during the day may be delayed or even red-flagged. Showers will continue into the evening and overnight with chances tapering off somewhat into Sunday. However, chances for scattered showers are still possible, though timing is less concrete. Fog may be possible during the morning as air temperatures approach dew point temperatures. The surface to 500 millibar bulk shear indicates that the aforementioned low pressure system will move to the northeast during Sunday, allowing the front to continue to track eastward. This will allow winds to shift fully to the west to northwest, bringing the coolest temperatures of the weekend to the region.

By: Stephen McCoy

The forecast looked fairly straightforward for the TOTAL 24 Hours of Spa on Sunday, however today's model runs paint a bit of a different picture for this weekend's conditions.

On Thursday, surface winds are still expected from the south to southeast due to the combination of a high pressure system to the northeast and the aforementioned low pressures system to the northeast of the region. An upper level ridge centered over central Europe will suppress the air closer to the surface which will lead to warmer temperatures and mostly clear conditions through the day.

In the previous forecast, the models indicated that the cold front off the low pressure system would move through the Benelux region during the day on Friday. Since then, a trend among the models has pushed the frontal passage closer to Saturday with the it possibly stalling into a stationary front. This later passage means that Friday will have similar conditions to Thursday, though slightly cooler as the upper level ridge begins to move northwards out of the region. The models look to be in a general sense of agreement with timing of the initial passage as output suggests the high temperature for the day will occur in the early afternoon, a few hours before the high temperature usually occurs. Much like in the initial forecast, there is a chance of scattered showers with thunderstorms possible behind the front, but isolated showers ahead of the front are also not out of the question. Surface winds during the day will be continuing from the south to southeast initially before turning to the west to northwest after the front passes.

A lot of uncertainty has arisen for Sunday's conditions as the models are not in agreement for the front's position in the region. The GFS and German models indicate that the front will fully pass through the region, with temperatures dropping to the upper 50's °F, low-to-mid teen's °C  during the day as winds pile in from the west to northwest. Meanwhile the ECMWF shows conditions much warmer as the front stalls just to the west of the region, causing warmer temperatures closer to those seen on Saturday. In the former scenario, precipitation has a slight chance to linger into the morning, though without the convective environment from Saturday. Whichever the case may be, the conditions in the graphic will most likely change as the race draws closer.

By: Stephen McCoy

Through mid-week, an upper level trough will track eastward across the Atlantic Ocean with a low pressure system accompanying it at the surface. As the trough approaches the British Isles overnight Wednesday into Thursday, it is anticipated to stall, weaken and cause upper level winds to return to zonal flow into the weekend. However, the bulk shear between the surface and 500 millibars (according to the GFS) will be from the direct south, which will steer the surface low due north. A cold front extending out to the south from the center of the low is expected to move through Benelux in the late morning to mid-day on Friday. On Thursday, winds ahead of the front will be form the south to southeast, bringing high temperatures into the mid 90's °F or mid 30's °C; under an upper level ridge, cloud cover will be minimal during the day. With the timing of the frontal passage on Friday, temperatures during the day will likely remain fairly steady; maximum temperatures will be only a few degrees warmer than the morning low as the winds shift to the north to northwest behind the front.

Temperatures will continue to cool into Saturday morning with the low near 60 °F, 15 °C. In addition to the cool down, there is a chance of scattered showers with thunderstorms possible between Friday afternoon and Saturday morning. The front is expected to be an Ana Cold Front, where the majority of precipitation occurs after it has passed. What could increase precipitation chances is the fact that model runs have shown the front largely stagnating as it becomes disjointed from the low that is still continuing its way northward.

Surface winds will continue mostly from the northwest through the end of the weekend with temperatures around 70 °F, 20 °C on Saturday and in the mid 70's °F, mid 20's °C on Sunday. Mostly cloudy skies are likely on Saturday as upper level winds continue to bring moisture from the Atlantic to the region due to the aforementioned zonal flow. Once upper level winds transport the moisture out of the area, conditions on Sunday will be mostly clear to partly cloudy with the main concentration of cloud cover in the lower levels. Cooler temperatures overnight paired with clearing skies could result in some fog during the morning hours on Sunday.

By: Stephen McCoy

Only minimal changes were made to the forecast in this update for the California 8 Hours, most notably a lower chance for precipitation on Thursday. A slight chance for scattered showers is still present, but confidence is waning. Of the major computer models that we use for our forecasts, the Euro now stands alone when expressing rain during the day. However, the iterations of the model over the past day have yielded the same result, albeit indicating rain occurring more towards mid-day, which is too significant of a result to fully ignore. Should the Euro be correct, scattered showers will move through the region around noon from the north/northwest, though the model estimates precipitation rates around 1/100th of an inch per hour which would provide more of a nuisance than a major impact for the sessions on Thursday. The Euro is also indicating precipitation near the same magnitude Thursday night into Friday, but this would have no impacts to Friday's sessions.