Early August Severe Weather Opportunities
By Hunter Anderson
August 5, 2018 - 6:30 AM CST | 15 0
As we say goodbye to a rather quiet July we're welcomed with a supportive upper level flow pattern (that historically brings severe activity across/around the Badger state) in this early August. I discuss the rather complicated forecast below.
An upper trough will shift eastward across the northern CONUS Rockies Sunday. Confluent flow will exist on the southern/western periphery of this feature with diffluent flow on the eastern perimeter with associated divergence over the front range of Wyoming, Montana, and into adjacent northern/central high plains areas. Further east, multiple embedded vorticity maxima associated with a corridor of stronger mid level westerlies across the northern plains and upper Mississippi River Valley will propagate eastward throughout the period. Further downstream, enhanced mid-level westerlies/southwesterlies will reside in eastern Ontario due to an upper low nearing Hudson Bay. At the surface a cold front will move southeastward across the Dakotas, Minnesota, and extend possibly as far east as northern Wisconsin and Upper Michigan. A convectively-induced surface low may emerge over eastern South Dakota/Nebraska later on in the period and trek eastward near the Minnesota-Iowa border overnight.
A rather complicated forecast scenario is unfolding due to "multiple waves with multiple personalities" moving eastward within unseasonably strong mid-upper level flow. 80-120kt 250mb jet will be placed across the eastern Dakotas, Minnesota, northern half of Wisconsin, and Upper Michigan Sunday afternoon atop a 50-60kt 500mb jet. Differential cyclonic vorticity advection, possibly associated with an MCV will move across Wisconsin during the late morning/early afternoon. The environment ahead of this feature will be comprised of surface temps in the mid-upper 80s and surface dew points above 70F. Mid level lapse rates exceeding 7.5-8C/km will create a rather unstable environment from southeastern Minnesota to eastern Wisconsin as MLCAPE values will likely surpass 3000 j/kg. Effective shear through this corridor, especially in eastern Wisconsin will support bowing segments and embedded supercell structures. If a lake breeze develops and interacts with any established updraft/downdraft system this could enhance supercell potential as a southeasterly wind component would enlarge hodographs especially in the lowest 1-3km. Large hail up to 2" in diameter, damaging winds in excess of 70mph, and a tornado or two are all possible especially in the hatched area (eastern Wisconsin-southern Upper Michigan) where parameter space seems most appreciable. Most recent CAMS show convection either initializing over Minnesota early Sunday morning and propagating east with time, or initializing over central Wisconsin around late morning--either way this wave of activity would be associated with the mid level DCVA/MVC.
Elsewhere severe convection seems possible across the Wyoming front range into the Nebraska panhandle and northern Colorado where strong shear will overlap an unstable environment as upslope/easterly surface flow prevails. Additional strong/severe convection seems possible in eastern Nebraska into southern Minnesota/northern Iowa as an aforementioned surface low develops upstream of a moderately sheared and unstable environment. I would like to reiterate that there still is reasonable uncertainty with today's convective evolution across the Midwest and western Great Lakes but after assessing latest 00Z models what I mentioned above seems like most favored outcome at this time.
A surface low will move quickly across the Badger state overnight Sunday into Monday morning and will eventually eject into lower Michigan by Monday afternoon thanks to moderate 40-50kt cyclonic mid level flow. A cold front will be draped southwestward from this low across western lower Michigan, northern Illinois, northern Missouri and central Kansas by 07/00Z. Ahead of the front the environment characterized by MLCAPE exceeding 2500 j/kg and moderate low level helicity (200-250 m2/s2 in the 0-3km) will pose risk for strong/rotating updrafts. Large hail up to 2" in diameter and a tornado or two will be possible initially, especially in the hatched areas before the main threat transitions to damaging winds as cells merge and form into clusters/brief bowing segments. Convective segments should ideally become less organized as they propagate further south from the stronger shear and overnight stabilization occurs.
If you want a good idea on my whereabouts over the next 36-48 hours, there's a good chance I'll be located in my "hatched" area(s.) Let's see if I can salvage this miserable storm chase season with (at the very least) some decent structure documentation.