Friday: Mostly sunny. High: 92°
Friday Night: Mostly clear. Low: 72°
Saturday: Increasing clouds, possible rain around midday. High: 82°
Saturday Night: Cloudy. Low: 69°
Sunday: Cloudy, rain likely. High: 77°
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General Discussion:
Friday is expected to be just as hot as the rest of the week, but incoming clouds will keep temperatures somewhat cooler on Saturday and Sunday. Temperatures are expected to cool even further following a cold front passage Sunday evening, which will also bring good chances for rain.
– Scheerer, Iffrig, Collier
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Forecasters: -Scheerer, Iffrig, Collier
Date Issued: 09/12/2024 9:30 AM CDT
Technical Discussion:
Key Messages:
– Increased rain chance with a non-zero chance for severe weather this weekend.
– Increased cloud cover will be keeping temperatures cooler as we head towards the first day of Fall.
The ridge that was causing hot and dry conditions for this week will push farther south throughout and being replaced by a strengthening trough. A high pressure system to the east will be bringing in moisture from the Pacific causing a chance for showers for the weekend. However, there is some disagreement between the NAM and GFS in terms of where the area of low pressure to the west will be sitting at 250mb. By Monday at 0:00z, the NAM has the low sitting over NE Colorado and the GFS has it sitting over eastern Nebraska.
Moisture is not expected to be advected into the region in any significant capacity until Saturday morning, so we’ll be expecting clear skies with patchy cloud cover on Friday. This will also leave temperatures warmer than average before the first day of Fall. There is slight disagreement between the GFS and NAM on when moisture will arrive during the forecast period. The NAM has it arriving on 9:00z Saturday and the GFS has moisture arriving to the area by 15:00z on Saturday. Ensembles put the best chance for rain on Saturday around midday, but forecast soundings show a significant dry layer near the surface that could prevent rain from reaching the surface. Regardless, cloud cover will keep temperatures in the low 80s (compared to the low 90s on Friday). The overnight low will be around 70, and the vertical profile will be very moist. Some rain is certainly possible.
Sunday seems to be the best chance for widespread precipitation, as it will continue to stay moist and multiple shortwaves/disturbances will sweep through the region, providing needed forcing. Precipitable water Sunday during the day will near 2″. A cold front draped across the CONUS from the Great Lakes region to Texas will pass through the forecast area around 21Z Sunday afternoon. This will work to cool us down later in the week, and will provide more lift and best chances for thunderstorms. Daytime heating will be limited due to cloud cover, but if enough instability can be generated, we may see some damaging winds and thunder/lightning. Low level shear is not insignificant, but overall does not seem to be high enough for more organized convection. Some deterministic points do output intriguing severe weather indices, such as a lifted index of -4 and K index as high as 39, which would indicate we should expect widespread thunderstorms and moderate potential for severe weather. This result is incredibly inconsistent, however, and changes when even a slighly different location is selected to generate a modeled sounding.
Regarding how long the moisture sticks around, the NAM uncharacteristically has moisture exiting by Sunday evening and the GFS has it staying through the end of the forecast period on 0:00z Monday.
– Scheerer, Iffrig, Collier