Strong Storm Chances on the Rise Tonight Through Thursday
By Hunter Anderson
July 11, 2018 - 1:50 PM CST | 24 0
Like what I did with the "Natural" Fireworks post, let's quickly review the verification of the outlook posted for Independence Day, 2018 before we get into the meat and potatoes of the thunderstorm outlook for today and into tomorrow.
After a brief review, there were a couple clusters of severe reports (Omaha, NE area and Marinette Co, WI area) outside of the main STRONG/ISO SEVERE outlook area. In general the STRONG/ISO SEVERE outlook captured the majority of severe reports after daybreak including an EF-1 tornado that impacted Bemidji, MN around 6:20AM on July 4th. The tornado report icon is hidden behind a few wind damage reports (likely result from aforementioned tornado) in the same area. It was also a plus seeing the SEVERE outlook capture quite a few severe reports as well as a tornado warning in southern Minnesota. For what it's worth, the SEVERE outlooks are 2/2 on possessing at least 1 tornado warning (July 1, 2018 SEVERE outlook captured 7/9 tornado warnings) in its outline.
Alright, now for the meat and potatoes, with a side of forecasting...
The main feature(s) to watch today will be the position of the mid level shortwave trough as it treks across the southern Canadian prairies and far northern plains/upper Mississippi River valley. Associated with this trough is a surface low that'll be occluding in eastern Saskatchewan throughout the day. A cold front will move east-southeast today and will be placed somewhere along a Baudette, MN--Ortonville, MN--Valentine, NE line. A warm front will move northeast across northern/eastern Minnesota into western Wisconsin this afternoon and evening. The strongest mid-level flow will remain north of the international border but upper level dynamics are substantial enough across northern Minnesota.
In the established warm sector, dew points will rise into the mid 70s, strong diurnal heating, and steep mid-level lapse rates will all contribute to 4000+ j/kg of SBCAPE. Shear, especially in the lowest 3km is most impressive over northern and eastern Minnesota along the warm frontal boundary. Weaker mid level flow could hinder supercell longevity, but supercell modes are still expected initially with very large hail (>2" diameter) and tornadoes the primary hazards. Given southeast surface winds around 10-15mph with southwest 850mb winds around 35-40mph, this will create an enhanced sheared environment denoted by 0-1 SRH values exceeding 200 m^2s^-2 and 0-3km SRH exceeding 400 m^2s^-2. Convective initiation is expected to occur around 23Z-01Z 07/12 dependent on how long the cap holds prior to main forcing for ascent arriving. Initial activity is expected to spread and grow upscale overtime across northeast Minnesota into northwest Wisconsin and adjacent eastern Minnesota areas. Damaging winds and brief tornadoes are possible in these areas as well as some flash flooding given near/above-record precipitable water amounts and a stalling boundary.
Main corridor of strongest mid level flow will remain confined to the MN-Canadian border where the trough is suppressing the stubborn mid level ridge over the central CONUS. Wednesday's cold front that will be sliding through the northern plains/upper Mississippi River valley will sag south and be draped across northern Wisconsin southwestward through southern Minnesota and into northern Nebraska by Thursday evening. Main impulse arrives Thursday evening and should be strong enough to spark convection across the region.
Some questions remain regarding residual convective debris evolution from Wednesday night's activity, however highest confidence exists across the upper Mississippi River valley of old outflow becoming reinforced by strong diurnal heating which should slowly push the boundary further north/west with time. Any boundary intersections with the cold front poses an enhanced threat of robust storm development especially if cells develop in the environment characterized by >3500 j/kg MLCAPE and 30-35kts of effective bulk shear. Deep layer shear vectors seem more parallel than anything to the forcing so a quick transition from scattered storm mode to linear is likely as a sturdy cold pool becomes established. Activity should spread east into much of Wisconsin before midnight Friday morning. Main threats will be very large hail (>1.75" diameter) initially before damaging winds become main concern over time. Low level shear isn't as impressive as what will be realized across northern Minnesota tonight so not expecting much on the tornado front which will be more reliant on favorable storm/mesoscale boundary interactions.