An incredible severe weather event may
be on tap for Monday in Minnesota. Although the computer models are unreliable
at best this time of year, it does appear that there may be some truth to this
A frontal boundary will extend through the length of Minnesota
by mid-afternoon. Ongoing convection in the northern part of the state may
inhibit significant severe storms by mid-afternoon, but farther south (lets say
near New Ulm, MN) I anticipate a supercell to develop around 4 p.m. in an
unstable airmass along an outflow boundary. At this point in time, the lifted
index is expected to be near -5, MLCAPE near 1500. Surface dew point in the low
70s and 850mb dew point near 60 should be more than enough instability and
moisture for fuel.
A significant shortwave coming through the area by mid-afternoon
should be sufficient to lift the boundary layer. That will be the match to start
the fire. Convective Inhibition or negative buoyancy is projected to erode in
said area due to strong mechanical forcing. This will be along an area of
increasing convergence on the surface boundary at the nose of the moisture axis.
A bit of a warm front should be laying just to the south,
somewhere along the MN/IA border. This warm front may be prevented from
traveling too far to the north due to cloud cover caused from overnight
convection. This will strengthen the warm front and become a focal point for
tornadic supercells as the boundary creates backed winds at the surface and
increases convergence in the area of the triple point.
A low level jet (850mb) from northwest Iowa into southeast
Minnesota will intensify mechanical forcing. These winds from the SW working
together with the backed winds at the surface should help to generate incredibly
high levels of 0-1km Helicity, or low level corkscrew turning in the
atmosphere... key for tornadic development.
The 850mb and 700mb temperatures along with CINH values suggest
that capping inversion will not be a problem. Incredible amounts of 500mb dry
intrusion are anticipated by late afternoon. As this dry intrusion moves into
the area, central and southern Minnesota should have explosive development of
severe storms. Associated with this dry intrusion will be increasing mid level
winds which will increase the amounts of speed and directional shear, supportive
of supercells and tornadoes. Given the extent of area effected and the strong
500mb winds expected, a segmented bowing line of severe storms are likely to
develop later in the period. The area most likely affected by the bowing line of
severe storms would be East-Central MN into Western Wisconsin and far Southeast
Minnesota. Farther south and west of that area is where the greater tornadic
potential would be as cells would have better opportunity to become isolated
supercells, especially early in their development stage.
As the storms that develop near 4 p.m. in
Southwest/South-Central Minnesota get going, they will travel east-southeast
along the warm front to near Mankato and Albert Lea. The atmosphere in this area
expected to be more than favorable for long-lasting significant tornadoes.
The Energy-Helicity Index (EHI) is forecasted to be over a 4,
500mb to Surface wind differential of about 40 kts, 0-1km Helicity of around
200, VGP of near 0.50, LCL of about 875mb, LFC of about 800mb and CINH less than
abs(-50). All of these parameters indicate significant tornado potential, with
potentially tornadoes producing F2 to F5 damage.
My target for the day is Mankato, MN.
Now... with all of that said, the threat of inhibited storms
from overnight convection is real. However, I do believe that most of the
blow-off will remain to the north and east of the target area. Primarily north
of I-94. The remaining outflow boundaries may help to align the warm front
parallel to the mean storm vector for supporting long-lived supercells along
The other questionmark is whether or not model data will let me
down again. If it does, then I'm helpless. However, latest model data seems to
indicate the above scenario... in my opinion. The SPC at the hour I'm writing
this puts their target much farther south into Iowa. We'll see what