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SEVERE THUNDERSTORM WATCH UNTIL 10 PM

NORTHERN ILLINOIS SKYWARN ACTIVATED ON 147.195

 ILLINOIS COUNTIES INCLUDED ARE

BOONE                BUREAU              CARROLL
COOK                 DE KALB             DUPAGE
FULTON               GRUNDY              HANCOCK
HENDERSON            HENRY               JO DAVIESS
KANE                 KENDALL             KNOX
LAKE                 LA SALLE            LEE
LIVINGSTON           MARSHALL            MCDONOUGH
MCHENRY              MERCER              OGLE
PEORIA               PUTNAM              ROCK ISLAND
STARK                STEPHENSON          TAZEWELL
WARREN               WHITESIDE           WILL
WINNEBAGO            WOODFORD

SHORT TERM…
207 PM CDT

Through tonight…

Mainly a mesoscale discussion…Concerns center on thunderstorms
later this afternoon and evening. A severe thunderstorm watch has
been issued for a good portion of northern Illinois.

Forcing and instability is still working its way east ahead of a
cold front across Iowa. While isolated development is possible
ahead of the front, the majority of the forcing is tied to the
front itself. The corridor downstream of more active thunderstorms
across Iowa is favorable for continued development or
intensification as we have been able to heat up a bit after
morning cloudiness. There is an 18z sounding out of the Quad
Cities, KDVN, has depicted some mixing out of the dewpoints which
is fairly representative of what has been occurring across
northeast Illinois.

As far as hazards, the storms being on the front have largely
taken on a linear shape already this afternoon, with stronger
portions of the line depicting a decent wind signature on a few of
the storms. Therefore, damaging winds remains the primary concern.
Upstream in Iowa, effective shear is 45-55 kt which is more than
sufficient for rotating storms, which has been observed by radar
and spotters in Iowa. There is a closed surface low across Iowa
that will spread northeast into Wisconsin. This feature is likely
contributing to higher helicity values and will be the corridor
where if any tornadoes were to form, this would be area. This
corridor will shift into north central Illinois and the I-39
corridor later this afternoon, and thus will be something to watch
for. As the low continues northeast, this threat becomes lower
farther east, but we do expect shear to increase as storms
approach our area given the approach of the positively tilted
trough. A severe thunderstorm watch was issued as damaging winds
do remain the main hazard, but the watch mentions possible
tornadoes as well. Mid level lapse rates suggest hail is not as
big of a concern, but certainly possible with any supercells.

Most of the thunderstorms will be confined to a several hour
period around the cold front. Expect gusty southwest winds to
around 20 kt ahead of the front. A few isolated/widely scattered
showers or storms are possible by mid afternoon, but the main
window for severe storms will be late afternoon around the
Rockford area, and 7-9 pm around the Chicago area. The storms will
be arriving closer to Chicago after peak heating, and thus
confidence on how strong/severe storms will be closer to Lake
Michigan is lower. Certainly the forcing and shear will still be
sufficient. But thunderstorms should survive to the lake
regardless.

Behind the front winds will head to NW and gust into the middle
20s overnight, with lows falling into the 40s outside of Chicago

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Concerning…Severe potential

SUMMARY…Gradually increasing storm strength and organization may
result in a high enough severe threat to merit a watch. Damaging
wind gusts are the primary threat, although a tornado or two cannot
be ruled out.

DISCUSSION…Cold front continues to steadily progress eastward
across IA with a developing convective line along its leading edge.
This convective line has shown generally steady-state intensity over
the past hour or so but the downstream thermodynamic environment
appears a bit more favorable for intensification as warmer
temperatures promote stronger instability, particularly across
eastern IA. Farther east (i.e. northern IL and southern WI),
temperatures and dewpoints are a bit lower and the thermodynamic
environment does not currently appear particularly favorable. Even
so, continued heating plus mid-level cooling associated with the
approaching shortwave trough should result in enough instability for
storm persistence/maintenance.

Linear character of the ongoing storms suggests the primary severe
threat will be damaging wind gusts. A tornado or two is also
possible with the strongest portion of the line. Warm mid-level
temperatures and the resulting lack of steeper lapse rates should
keep hail sizes predominately below severe thresholds. Recent trends
suggest severe coverage may high enough to merit a watch.

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