A Tornado Watch has been issued until midnight for Winnebago, Lee, Ogle, LaSalle, Grundy and Livingston counties. Have multiple ways to receive warnings if any are issued for your area and a plan to access safe shelter if needed
URGENT – IMMEDIATE BROADCAST REQUESTED
Tornado Watch Number 236
NWS Storm Prediction Center Norman OK
625 PM CDT Fri May 24 2019
The NWS Storm Prediction Center has issued a
* Tornado Watch for portions of
Northern and central Illinois
Far northeast Missouri
* Effective this Friday night from 625 PM until Midnight CDT.
* Primary threats include…
A few tornadoes possible
Scattered large hail and isolated very large hail events to 2
inches in diameter possible
Scattered damaging wind gusts to 65 mph possible
SUMMARY…A few supercells should develop along a residual boundary
from eastern Iowa into west-central Illinois. This activity should
spread east-northeast in northern Illinois through late evening.
The tornado watch area is approximately along and 85 statute miles
north and south of a line from 45 miles north northwest of
Burlington IA to 40 miles north northeast of Bloomington IL. For a
complete depiction of the watch see the associated watch outline
update (WOUS64 KWNS WOU6).
REMEMBER…A Tornado Watch means conditions are favorable for
tornadoes and severe thunderstorms in and close to the watch
area. Persons in these areas should be on the lookout for
threatening weather conditions and listen for later statements
and possible warnings.
553 PM CDT
The weather forecast over the next 12+ hours remains fairly
complicated, and uncertainties abound mainly due to the weak
nature of large-scale forcing for ascent. While a threat for
strong to severe thunderstorms certainly exists this evening and
into the nighttime hours, how this threat materializes remains
tied to mainly subtle mesoscale processes and subtle waves of
synoptic ascent which are both difficult to diagnose and pinpoint
with much certainty.
Surface analysis late this afternoon depicts a northwest to
southeast arcing warm front–reinforced by this morning’s
convection which laid out an outflow boundary–likely just
entering portions of La Salle, Livingston, and Ford Counties.
Dewpoints immediately to the south of this boundary jump into the
70s with breezy south to south-southwesterly winds, and this
seems to be demarcated pretty well by an area of bubbling Cumulus
and HCRs which are steadily building northward. Recent RAP
soundings and SPC mesoanalysis reveal that this is a relatively
high-quality warm sector, with mean mixing ratios pushing 16 g/kg
and decent moisture through a fairly deep layer (up to 700 mb). An
earlier tornadic supercell, which was riding along the northern
extent of this warm frontal boundary, quickly dissipated a few
hours ago as it approached Bloomington, likely as it encountered a
pocket of warmer air aloft with 700 mb temperatures analyzed at
+9 to +10C. This is indicative of lingering capping, which has
thus far suppressed any additional convective initiation attempts
in the warm sector. Recent GOES-16 visible satellite loops reveal
some towering Cu development taking place across West Central
Illinois, however, where capping is lower due to cooler
Farther to our west, an additional area of bubbling cumulus has
been noted across central and eastern Iowa, although recent radar
trends suggest incipient updrafts are struggling to develop.
Large scale forcing for ascent is not strong here, but modest
mid-level height falls (20-40 m/12 hours) are noted in recent
analyses nosing into far northwestern Iowa at the leading edge of
a very subtle shortwave. It’s possible some lingering mesoscale
subsidence is still in place across eastern Iowa in the wake of
this morning’s MCS, tempering additional robust convective
development at this juncture, but conditions do appear favorable
for robust updraft development over the next few hours.
With this all laid out, it does appear there may be two favored
corridors for potential convective development over the next few
hours: /1/ near and south of the incoming warm front and /2/
across eastern Iowa. All modes of severe weather would be possible
across our area, including the threat for a few tornadoes. Deep
layer shear in excess of 50 kts will support rotating storms and
supercell structures and sizable CAPE in the hail growth zone
will foster large hail development, potentially to the size of
golf balls or even larger with some analogs supporting 2″+ hail
not out of the question.
Recent runs of the HRRR have been a bit concerning, developing
robust storms near the incoming warm front and into the I-80
corridor. Locally backed surface flow would support a tornadic
potential with these storms as they interact with the front and
gain access to considerable streamwise vorticity. Think it looks
too aggressive based on latest satellite trends, but we will need
to keep our eyes peeled for development near and south of I-80
during the 8 to 11 PM time frame. The other area of convection–to
our west in eastern Iowa–may attempt to develop into our western
counties later this evening, likely after 10 PM or so. A tornado
threat will continue here, although CAM guidance indicates some
propensity for storms to congeal into clusters which may
eventually deliver more of a damaging wind and hail threat as this
second area of activity pushes eastward into the overnight hours.
Finally, a flash flood potential does exist this evening and
overnight, although the spatial breadth and magnitude of this
threat still is uncertain due to the mesoscale processes involved.
Current thinking is that the Flash Flood Watch captures the
favored corridor well, and no immediate changes are planned.