Wednesday Night: Increasing Clouds. Low: 56°F
Thursday: Thunderstorm Potential. High: 76°F
Thursday Night: Party Cloudy. Low: 42°F
Friday: Increasing Clouds. Low: 57°F
Friday Night: Cloudy. Low: 45°F
=============================================================================
General Discussion:
The warm temperatures and calmer conditions experienced Wednesday will not remain as Thursday has an increased potential for showers and thunderstorms with the passage of a cold front moving through the forecast area.
-Robinson
=============================================================================
Forecasters: Cargill, Robinson
Date Issued: 04/17/2024 4:00 PM CST
Technical Discussion:
Key Messages:
- Thunderstorms are expected Thursday as a cold front moves through the area.
- Accumulated precipitation are estimated to reach 0.75”.
Both the NAM and GFS were used in this forecast. At 250mb, the polar jet stream will move over the northern CONUS and faze with the subtropical jet stream over the Midwest beginning Thursday 09Z. This upper-level pattern will remain for the remainder of this forecast period as it moves eastward. Further north, over the Great Lakes region, a low-pressure system moves east towards the Atlantic Coast. Following behind this system is a second low-pressure system positioned in the northwest CONUS along the Canadian border as of Wednesday. This low-pressure system will propagate east for the entirety of this forecast period, but due to its northern position, zonal flow over the CONUS will begin Thursday 03Z and remain through the weekend. The GFS and NAM are in disagreement with the significance of divergence, with the GFS showing more significant lift Thursday 12-21Z.
At 500mb, the GFS again shows a stronger system developing, with pockets of vorticity occuring Thursday 18Z and Saturday 12Z. This circulation is associated with the second low-pressure system moving along the northern CONUS border.
Moving closer to the surface, at 700mb, moisture and lift are displayed on both maps for Thursday 12-15Z and Friday 09Z through the end of this forecast period. While WAA moves through the forecast area between Thursday 03-21Z, it will be followed by CAA Friday 03-18Z that will drop temperatures heading into the weekend. At 850mb, the second low-pressure system will move moisture into the forecast area between Thursday 18Z and Friday 03Z. This influx of moisture is supported by the weak LLJ.
At the surface, winds will transition from west to southeast Thursday 03Z. However, a surface cold front that moves through the forecast area Thursday 21Z will steer winds from a southerly to northerly flow. The passage of this cold front not only supports the analysis of CAA in the upper-levels of the atmosphere, but will increase the chances for showers and thunderstorms occurring between Thursday 18Z and Friday 03Z.
A modeled sounding for KCOU Thursday 18Z showcased indices that support this claim. With a SFC CAPE value of 2510 J/kg, LI of -9, and strong omega values signaling prevalent lift, thunderstorms are expected to form Thursday afternoon with precipitation totals estimated to reach 0.75”.
-Robinson