Current Conditions at Sanborn Field
Thursday: Mostly sunny. High: 75
Thursday Night: Increasing cloudiness. Low: 52
Friday: Becoming Mostly Cloudy, Late. High: 83
Friday Night: Cloudy. Low: 50
Saturday: Becoming Sunny. High: 73
Discussion:
After a clear and crisp morning, temperatures for Thursday will rebound nicely into the middle 70s with a mostly sunny sky. By Friday temperatures will warm to the lower 80s as high pressure to our southeast sends us a southerly flow. However, ahead of an approaching area of low-pressure clouds will build into the region, late Friday afternoon into Friday night. After, the passage of the trough early Saturday, the sky will become sunny and temperatures will be cooler, with highs in the lower 70s.
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Forecasters: Duff, McDonald, Aldrich
Date Issued: 09/23/21 9:45 AM CST
Technical discussion:
The 06z GFS20 was used for today's forecast because it has been handling the temperatures and moisture more accurately than the NAM. GFS soundings were used to aid in forecasting sky conditions and the chances for precipitation over the forecast period.
Looking at the 300-mb heights, the earlier area of low-pressure continues to move off to the east, allowing for calm and quiet conditions for the Midwest. In its wake, another area of low-pressure and an associated trough begins to dig deeper into the Mississippi River Valley over the forecast period, allowing upper-level cloudiness to increase by 21z on Friday. This area of low-pressure does contain quite a bit of upper-level moisture, but it's rather progressive in nature, allowing for a cloud-free day by Saturday. Until then, however, expect a mostly sunny sky through the day on Thursday.
500-mb plots of heights and vorticity show the arrival of the new trough arriving into the region by 21z, Friday. Considerable amounts of vorticity were observed across the upper Midwest by 00z Saturday, however, that forcing looks to stay north of our forecast area, indicating that cloud cover should prevail for the region into Friday afternoon, Friday evening, and during the overnight. By Saturday morning, this entire system pushes off to the east and as high-pressure builds in, sky conditions should become increasingly sunny on Saturday.
From evaluating relative humidity, heights, and vertical velocity on the 700-mb map, we see upper-level moisture increasing into the region as soon as 18z, Friday. Further forecast periods of the model plots indicate that this moisture remains in the upper-levels and doesn't really impact us at the surface. The end result for us would be increasing cloud cover with very limited chances for precipitation (sprinkles) as we approach 06z on Saturday. Winds will also change to more of a northwesterly component after the passage of this frontal system by 12z Saturday.
850-mb heights, winds, and temperatures show minimal moisture at the surface from 18z Thursday, through the period, although, upper-level moisture continues to increase due to the placement and energy of the upper-level trough and low-pressure system. Winds, although light from the south, will allow surface temperatures to warm into the 80s on Friday, but cool back into the more comfortable low 70s by Saturday.
The next forecasting shift should pay attention to the opportunity for low-level moisture (showers) as the upper-level trough and low-pressure system move through the region late Friday into Saturday.
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