Friday Night: Showers and thunderstorms, some may be severe. Low: 35
Saturday: Exiting clouds. High: 47
Saturday Night: Clear. Low: 32
Sunday: Sunny. High: 53
Sunday Night: Clear. Low: 35
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Date Issued: 12/10/21 05:00 PM CST
Several models were consulted in regards to the guidance of this forecast; both NAM and GFS height contours had a decent handling of the positioning of the deep trough moving through the northern plains. With that said, the GFS picked up on a cold front in New Mexico where the NAM did not necessarily ignore it, but paid less attention to its intensity. After observed surface temperatures were compared with the model outputs, it was then decided to use GFS as the primary guide. RAP was also used as a high-resolution model to help pinpoint the timing and intensity of incoming storms. Soundings were viewed in both GFS and RAP formats.
An amplified trough has been working its way over to the Midwest CONUS. A decent amount of divergence aloft indicates surface-level convergence, which helps encourage vertical lift. This divergence peaks at around 03Z, which is correlated to a burst in 500 mb circulation around the same time. This disturbance already provides evidence of some early evening thunderstorms. Further evidence could be spotted by examining the vertical lift associated with the upper-level divergence - omega values were well below -10 according to the RAP soundings, indicating a strong vertical lift in the mid-altitudes.
Provided with the aforementioned information, the possibility of severe weather Friday night was given closer examination, particularly in regards to modest CAPE values in the GFS of just over 300 j/kg. This was compared to CAPE values in the RAP soundings, which were measured exceeding 1,000 j/kg, which is a sufficient amount for the development of thunderstorms. This, mixed with a veering wind profile throughout the day on Friday and unseasonably warm temperatures, provides a possibility of severe thunderstorms Friday night.
There are some factors that could inhibit the development of this severe weather event. For starters, there is a capping inversion present in the early evening hours on Friday and noticeable CINH values exceeding 150 j/kg at the surface. Although this is a fair amount, it may not be enough the entirely prevent convection. However, persistent cloudiness on Friday will keep temperatures cool enough to keep surface-based convection from occurring. Therefore, the possibility of severe weather is decreased, yet it can not be completely ruled out given the sufficient CAPE values and above-average temperatures.
By Saturday morning, the cold front will have passed and the remainder of the weekend will remain calm. Once the trough makes its exit, upper-level flow will become more zonal, lending way to a more persistence-based outlook. The sky will quickly clear out and is expected to remain so for the rest of the forecast period. Temperatures will decrease back into seasonal averages, particularly on Saturday. CAA will take over behind the cold front, as indicated by a northwesterly flow at the surface. Temperatures may begin to increase once more on Sunday, both as a result of an abundance of sunshine and a progressively southerly flow which will take hold throughout the day.