Severe Weather Possible Saturday (07/23)

 Severe Thunderstorms This Weekend 

by Aidan Cera 


I have been keeping a close eye on Saturday across central and eastern Wisconsin with regards to the possibility of severe weather. The Storm Prediction Center has been highlighting this possibility for the last 2 days, below are the graphics issues by the SPC.


The SPC has issues an enhanced risk of severe weather for southeastern Minnesota, northeastern Iowa and central Wisconsin for 07/23. 


A "Sig" or significant risk for severe weather has been issues by the SPC for a decent chunk of Minnesota, northern Iowa and most of Wisconsin. 

Synopsis

A negatively tilted shortwave trough accompanied by a jet streak will advect out of the northern Great Plains and into southern Canada and into Minnesota Saturday afternoon then Wisconsin Saturday night. 


The key take away from looking at the 250 mb level of the atmosphere is that there will be ample upper level divergence over Wisconsin Saturday afternoon into the evening. This will enhance surface convergence as a form of compensation to balance the atmosphere. 


Surface flow will be out of the south/southwest across much of Wisconsin ahead of the low pressure thanks to a warm front lifting north. This flow will advect ample moisture and stifling temperatures in the upper 80s and lower 90s, with dew points in the 70s. Given these high temperatures and dew points, instability will certainly not be lacking, especially if there is ample sunshine as expected on Saturday during the day. Relative humidity is also expected to be quite high throughout the atmosphere, as shown below in the second figure




Since flow at the surface will be more southerly in nature and flow aloft will be more westerly in nature, there is expected to be substantial shear parameters accompanies by this frontal system. In fact, most of the global models indicate that shear values could top 70 knots. They are also showing that CAPE values could be close to 4,000 J/kg in some spots across Wisconsin. 
These conditions suggest that all modes of severe weather will be possibly, including large to very large hail, damaging winds and even a few tornadoes due to the strong shear. Supercell storm mode is more likely in western Wisconsin while conditions will foster more of a linear to bowing storm mode in central and eastern Wisconsin making damaging winds and heavy rainfall being the primary concern. 
I do want to put out there that the high humidity, temperatures, CAPE and shear will likely lead to the development of a powerful Mesoscale Convective System (MCS). If conditions align just right, especially with regards to downdraft behavior and where the CAPE gradient sets up, central Wisconsin could be exposed to a derecho threat.

Timing

The good news is that most of Saturday during the day should be dry especially across eastern Wisconsin. Once we head into the evening however, storm chances will increase from west to east, especially after 8-9PM CST in central Wisconsin. There will likely be ample warning of these storms before they hit as well, so there will be more time to anticipate the in climate weather before it arrives. Eastern Wisconsin appears to be in line to get storms around midnight or so, but this timing is highly variable on where convection initiates in Minnesota and how quickly it tracks eastward. Finer details will be determined tomorrow and Saturday throughout the day.


This event is still 2-3 days out so there is still plenty of time to take a closer look at the finer details of this event, so more hashing out is expected tomorrow. More information can be found at:












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