SPC AC 201728
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1228 PM CDT Fri Sep 20 2019
Valid 211200Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
KANSAS INTO IOWA AND OVER NORTHWEST MINNESOTA...
Severe storms are possible across parts of the Plains on Saturday,
most concentrated from Kansas into Iowa. Severe storms are also
expected across northwest Minnesota. Other more isolated activity is
possible over parts of the southern High Plains.
Progressive shortwave trough embedded within a broad upper trough
will move northeastward across the Dakotas and into southern
Manitoba on Saturday. Its parent upper trough will also gradually
move eastward as a second shortwave trough rotates through its base
across the Four Corners region. Overall evolution of these features
will result in strong mid/upper-level southwesterly flow spreading
across the northern/central Plains into the mid/upper MS Valley and
Upper Great Lakes.
The surface pattern early Saturday will feature a cold front
extending from a low centered over the central Manitoba/Ontario
border south-southwestward to another low over the southern CO/KS
border. Northern portion of this front is expected to progress
quickly eastward/southeastward throughout the day, likely reaching
the Upper Great Lakes by early Sunday morning. Southern portion of
front is also expected to move southeastward, but more slowly than
portions farther north. Expectation is for this portion of this
front to extend from southwest KS into northern MO by early Sunday
morning. Thunderstorms are expected along the length of the front as
it interacts with the moist and unstable air mass ahead of it.
...Central Plains into the Mid MS Valley...
Highest thunderstorm coverage is expected along the front from KS
northeastward into northwest MO/southern IA. In this region, several
mechanisms for ascent will overlap, including convergence along
front, persistent warm-air advection, and large-scale lift provided
within the entrance region of the upper jet. This ascent combined
with the moist low-level air mass and moderate vertical shear will
support some more organized updrafts and potentially a few
supercells. Given the sagging cold front and persistent warm-air
advection, storms may have trouble remaining surface based,
potentially limiting a more widespread severe threat. Even so, the
more organized storms will be capable of large hail, with some
instances of very large hail (i.e. greater than 2" in diameter)
...Far Eastern Dakotas across MN into WI...
Showers and thunderstorms will likely be ongoing at the beginning of
the period, supported by a strong low-level jet. A few of these
storms may be strong enough to produce hail. These showers and
thunderstorms will drift eastward/northeastward and weaken during
the morning as the low-level also shifts eastward.
Moisture advection ahead of the approaching front is expected to
result in mid to upper 60s across the region by mid-afternoon. Lapse
rates are also expected to steepen as cooler mid/upper-level
temperatures gradually move into the area while low-level
temperatures remain relatively warm from antecedent convection.
These thermodynamic conditions coupled with strong mid-level flow
aloft result in an environment conditionally supportive of strong to
severe thunderstorms. Main deterrents for storm initiation are the
warm low-level temperatures (i.e. between 850-700 mb) and the lack
of stronger convergence along the front. As such, much of the region
will likely struggle to produce deep convection. Most probable
location for thunderstorms is across northern MN where the frontal
ascent is well correlated with ascent provided by the shortwave
trough. Here, a few supercells capable of large hail and strong wind
gusts are possible.
...MAXIMUM RISK BY HAZARD...
Tornado: 5% - Slight
Wind: 15% - Slight
Hail: 15% SIG - Slight
CLICK TO GET WUUS02 PTSDY2 PRODUCT
NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 0600Z