Purpose:
This extended forecast (10-14 days out) is supposed to help you see what we could expect to see on Friday (October 11th, 2019) to Tuesday (October 15th, 2019). I will include a public discussion which will be quick and short. The other discussion is technical. The area we will look at will be the Metro area which includes:
· Southern Illinois
· Southern and Southwest Indiana
· Western and Northern Kentucky
Always keep in mind to be flexible with the weather and it may change. I hope this assists with planning, and be sure to check out the Metro Weather Watch (MWW) Facebook Page.
Public:
Friday (Oct. 11) weather will be quite active for the metro. Temperatures should reach into the upper 50s, however; a breezy morning will cause temperatures cooler than expected. Thunderstorms will be present early in the morning and rain showers will appear throughout the day. The severity of the thunderstorm will be mainly from the wind. Precip should be around an inch or higher. Saturday (Oct. 12) temperatures will reach the upper 40s with calmer winds and no clouds. Sunday (Oct. 13) temperatures will reach the lower 50s with a clear day. Clouds will only appear in the overnight hours. Monday (Oct. 14) temperatures will reach the upper 60s with calm winds and a mostly clear day. More clouds will appear in the overnight hours. It will not be until Tuesday (Oct. 15) for another chance of showers. Temperatures will reach the lower 60s with calm winds. Precipitation should be around 1/4 inch. Overall, there look to be more sunny days than rainy.
Technical:
On Friday, Oct 11th, a low-pressure system is noticed. The first evidence that supports this is the 250mb level. It shows a longwave over the Midwest with a jet core in Kansas extending into Missouri at 6UTC. By 18UTC, the trough continues to move where the metro area will be in the exit region of 70 – 110 kts. This means divergence is occurring at the lower levels. Supporting evidence from the 500mb vorticity values are present across the metro area ranging between 16-24microb/s supports it. Furthermore, humidity at 700mb is above 90% throughout the day meaning moisture is available. This means rain showers will be present throughout the day and will accumulate to at least an inch. However, the chance of thunderstorms will only be in the morning only because vorticity values decrease as the day progresses. Surface winds will be between 25-30kts and SW at 6UTC. 18UTC surface winds will decrease over time and switch to NNW.
Saturday, Oct 12th in the aftermath of the low-pressure system. Supporting evidence from pressure levels of 250mb, 500mb, and 925mb proves this. At 250mb, the jet core weakens over the metro as the day progresses and little to no vorticity values are seen at 500mb. Next, humidity is below 70% at the 700mb level. Finally, the 925mb level shows a high-pressure system entering the area. The temperature gradient (red contours) expands after the low-pressure system passes between 06UTC and 18UTC.
Sunday, Oct 13th shows some movement across all pressure levels. At 250mb, the jet streak continues to weaken as it moves off to the east coast. Zonal flow can be seen across the entire CONUS. On the west coast, another jet streak begins to form over Nebraska. This will become important because it will have impacts over the next couple of days. Small values of 500mb vorticity can only be seen in the afternoon to overnight. The high pressure continues to persist throughout the day after viewing 925mb. Depending on how quickly the new jet develops, temperatures could be in the lower 50s or 60s.
Monday, Oct 14th shows lots of movement in the atmosphere. At 250mb, the jet streak over Nebraska will move up into the trough over Canada because it will couple with another. This will change from being in zonal to meridional flow. In other words, the trough over Canada will stretch down into to Midwest overnight. This chain reaction will induce frontogenesis over Kansas and Nebraska throughout Monday. It will also cause a high-pressure system over the metro area to shrink as temperature (red contours) gradient tighten.
Tuesday, Oct 15th will be the spark of another low system bringing rain across the area. How do we know? From 250mb, the jet streak extends the trough into Missouri and Illinois. The jet core is over Illinois and Indiana. At 500mb, vorticity values are between 16-28microbars/s north of the Metro area and move south by the evening. Humidity at 700mb is above 90%. At 925mb, winds are from the NE at 15kts. Rain showers should accumulate up to 0.25 inches. A chance of thunderstorms is possible, but the severity is small.
In conclusion, 2 storms are likely to occur in this forecast period. The GFS ensemble was used for this forecast along with GFS Mean Spread. I also took a look at the 12Z GFS model from the College of Dupage to see how many changes have occurred. The results were astonishing because the 12Z GFS model shows a very different output than the 6Z model. The 12Z GFS shows the metro area is not in range for any rain for Friday. Furthermore, it has a more aggressive outlook for the 2nd storm system putting it on Sunday night into Monday morning. Thus it shows that we know there are 2 storm systems that will occur. But the timing of when it happens is up for further questioning because a lot will depend on the jet streaks.
Meteorologist Sean Danaher
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