Severe Thunderstorm Watch including northwest Illinois being considered
The national Storm Prediction Center has issued a mesoscale analysis
discussion indicating consideration of a severe thunderstorm watch
across Iowa into northwest Illinois this afternoon (area depicted on
headlined map above).
Mesoscale Discussion 1380
NWS Storm Prediction Center Norman OK
0132 PM CDT Fri Jul 21 2017
Areas affected…much of Iowa…northwest Illinois
Concerning…Severe potential…Watch likely
Valid 211832Z – 212100Z
Probability of Watch Issuance…80 percent
SUMMARY…Severe thunderstorms are expected to develop in the
19Z-21Z time frame with a risk for damaging winds, large hail, and a
tornado or two. Short term trends will be monitored and a watch is
likely prior to 21Z.
DISCUSSION…A composite frontal boundary extended east-west from
northeast Nebraska across central Iowa and northern Illinois as of
1815Z. Elevated thunderstorms continued in the vicinity of the
Iowa/Wisconsin border, and thunderstorms have recently developed
over northeast Nebraska/northwest Iowa. A very moist air mass
(lower-mid 70s surface dew points) remains in place, and with
continued heating moderate-strong surface-based instability will
exist near the boundary this afternoon. The discussion area remains
on the periphery of the stronger mid-level flow, with deep-layer
shear averaging 35-45 kts.
Surface-based thunderstorms are expected to develop in the 19Z-21Z
time frame as convective inhibition continues to diminish due to the
combined effects of heating and ascent associated with an
eastward-moving MCV over southern Minnesota/northern Iowa. An
initial supercellular mode is expected with large hail and damaging
winds the primary threats. Favorable low-level hodograph curvature
near the boundary will support a risk for a tornado or two. With
time, one or two convective clusters should gradually evolve with a
more concentrated risk for damaging winds into the evening hours,
supported by a strengthening southwesterly low-level jet.
Given this expected scenario, one or two watches will likely be
needed for the discussion area, with an initial watch prior to 21Z.