Chase target:
Lake Mills, IA (25 miles northwest of Mason City).
Timing:
Storms will move into the target area at 5 PM CDT.
Comments:
An area of mostly non-severe storms in central through western MN will develop south towards the target area late in the afternoon. Expect one or more supercells to develop ahead of the earlier precipitation, and these should ride east-southeast along the surface front. A full spectrum of severe weather can be expected with these storms, including tornadoes and hail to baseball size. By mid-evening, storms will evolve into a forward-propagating MCS with severe winds to 70 mph as it tracks east along the IA/MN border.
Discussion:
This is somewhat of a complicated FCST, as it depends on subtle timing of SHWV features, the evolution of early precipitation, and the timing of very strong mid-level WAA. Early in the day, a large area of precipitation should break out in response to the first of several waves which will track trough MN along the SRN fringe of the stronger ULVL flow. In the wake of this feature, very strong UVV will result from a punch of mid-level WAA. Towards evening, a 50kt LLJ will provide a storm inflow of 20C dewpoints as well as providing an environment with very large, curving hodographs, with SFC-3km SRH’s in excess of 500m2/s2 just to the N of a SFC boundary.
00Z analysis indicates low-amplitude ULVL ridge with a 100kt H25 streak over MT. H85 moisture return is evident by a plume of 10C dewpoints streaking through NE. At the SFC, low-pressure is organizing over the WRN Dakotas with a developing WF trailing to the SE of this feature. The 00Z WRF initialized slightly low on SFC dewpoints in IA and SRN MN, but was good on temperature. The GFS initialized best with SFC moisture but was 5 degrees cool on H85 temps in SRN and SERN MN whereas the WRF handled this level best. The WRF initialized well with regard to H7 temperatures in ERN NE and SD while the GFS was 2-3C low in some areas. Both models initialized well on the aforementioned H25 streak, as well as a 70kt H5 reflection of this feature in ERN MT. The WRF has been tending north of the GFS with regard to storm track, while the latest WRF run has shifted slightly more to the south with the H5 SHWV track. The GFS progression of the SHWV has slowed down from the last run, and is now more in line with the WRF. The latest GFS run has pushed the 00Z position of the SFC front, which is tomorrow’s playground, slightly further to the S to a location just to the N of the IA/MN border. Conversely, the new WRF has gone further N and is now closer to the GFS. Given 00Z model performance, will go with a blend of the two.
Despite very warm mid-level temperatures (at 00Z tomorrow, a sharp H7 thermal gradient will exist from N to S across the IA/MN border, with temperatures ranging from just over 10C to 16C in IA), it seems reasonable to believe that outflow from an established area of convection in MN will cause new storm initiation in and near the target area. This is especially true where outflow interacts with the synoptic front. SFC dewpoints to 80F beneath good mid-level (H7-H5 layer) lapse rates of around 8.5C/km should result in SBCAPE’s to 4000J/kg. This degree of instability in combination with the aforementioned very large hodographs and deep layer shear to 70kts beneath a 60kt H5 streak should create an environment ripe for extreme severe WX. Finally, it looks as though there will be a narrow area just to the N of the SFC front where cloud base heights will be in the 800-1000m AGL range which should increase the chance of tornadogenesis within rotating updrafts.
- bill
