Rockford Scanner™: Storms Later This Evening, Tornado Watch To Our SW




Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
711 PM CDT Wed May 22 2019

.MESOSCALE DISCUSSION…
711 PM CDT

A few showers and storms developed a bit earlier across portions
of northeastern Missouri, but have struggled to maintain intensity
as they progressed into northern Illinois, likely as they outran a
narrow plume of increased low-mid level moisture. With limited
instability present across our region, think the severe threat in
the immediate term (over the next hour or two) appears fairly
low although a stronger storms will be possible across our
northwest.

However, strong flow in the 850-700 mb layer will continue to
transport a higher theta-e airmass into the region over the next
several hours along with an associated uptick in elevated
instability. Based on the most recent guidance, it appears as if
the most favorable axis of moisture and instability will develop
essentially across the southeastern half of the CWA (perhaps from
a La Salle to Midway line). Farther to the north and west of this
line, increased westerly flow aloft should drive a drier airmass
eastward, reducing the amount of instability available to support
more robust thunderstorm development.

The kinematic environment will be favorable for severe storms,
with deep layer shear values pushing 50-55 kts at times. Recent
point soundings reveal a lingering layer of warm air in the mid-
levels centered around 500 mb, which will limit the overall amount
of instability this evening, but steep lapse rates do exist in
narrow layers under 500 mb which, when combined with the
aforementioned robust deep layer shear, will support rotating
thunderstorms with a potential for large hail (quarter to golf
ball sized based on SHIP parameters). In addition, recent CAM
guidance suggests a propensity for storms to quickly grow upscale
into a surging MCS later tonight, which could deliver an
increasing threat for strong to locally damaging wind gusts
potentially for locales along and east of I-55 later tonight.
Current thinking is the main window for severe weather across our
region will be during the 9 PM to 3 AM timeframe or so.

While plenty of clockwise curvature is noted in regional
hodographs this evening, we believe the tornado threat will remain
tempered somewhat by a lack of truly surface-based instability as
the low-levels begin to decouple with the loss of daytime
heating. That said, if a surging line of storms can get going, any
segments that orient in a more NW to SE manner would have access
to ample low-level shear for a spin-up QLCS tornado, once again
mainly for locales east and south of I-55.

Finally, while a threat for heavy rainfall exists, the push of
westerly momentum aloft should keep things moving along tonight–
especially if a MCS materializes as many of the CAMs indicate. As
a result, while locally heavy rainfall will be possible but this
threat should remain fairly localized and there are no plans to
hoist a Flash Flood Watch at this point.

 


Warmer air has finally moved into the area, and it will stick around into the holiday weekend. There will be periodic chances of thunderstorms from tonight into the weekend. The storms tonight and on Friday could be strong to severe, with damaging winds and hail a possibility. Another concern is the locally heavy rain tonight, as the ground is already saturated.

4:18 PM Wednesday May 22nd:
A Tornado Watch has been issued off to the west and southwest of our county warning area counties. We do expect a severe threat to evolve this evening for portions of the area. Continue to stay tuned for updates

Latest trends an increasing threat of severe thunderstorms tonight across northern and central Illinois and possibly into northwest Indiana.

A cold front will encounter the warm and increasingly humid air mass and result in rapid thunderstorm development this evening and continuing into portions of the overnight hours.

The primary risk will be hail, possibly up to golf ball sized, locally damaging winds up to 70 mph, and a small threat for tornadoes, especially southwest of a Dixon to Pontiac line





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