Rockford Scanner™: Update On The Weather, From The NWS



A few damaging thunderstorm gusts are possible today over parts of
Illinois and vicinity.

Only appreciable changes made to the previous outlook include
1) removal of low-severe probabilities over southern WI due to the
lack of forecast destabilization for a hail risk. The surface
boundary will likely remain south—likely nullifying potential for
a strong gust.
2) reduce the spatial extent of the 2-percent tornado area to be
confined to the warm frontal zone. The 18z ILX raob showed very
poor low-level moisture (6.4 g/kg lowest 100mb mean mixing ratio).
Some sheltering/reduced mixing of moisture may occur in the
immediate vicinity of the warm front. However, confidence is
increasing that any tornado potential is likely very low to nil
given the relatively dry boundary layer.

..Smith.. 04/11/2019

.PREV DISCUSSION… /ISSUED 1124 AM CDT Thu Apr 11 2019/

…Mid MS Valley…
Water vapor imagery shows a powerful upper low over NE/KS, with a
strong mid-level jet extending from TX into IA/WI. At the surface,
the main low is over southeast NE with a cold front stretching
southward from the low across western MO. This front will
eventually be the focus for thunderstorm activity across parts of
MO/IL. Strong southerly low-level winds are transporting 50s
dewpoints northward ahead of the front, and some daytime heating is
apparent in visible satellite imagery. Forecast soundings show
strong deep-layer flow over area, supporting fast-moving convection
with a risk of gusty/damaging winds. Thermodynamic profiles are
rather weak with a deep warm layer above the boundary layer. This
will limit the amount of CAPE. Present indications are that
fast-moving bowing line segments may pose some risk for damaging
wind gusts across the SLGT risk area – mainly late this afternoon
and evening.

Another area of some concern is along the retreating warm front
across northern IL and extreme eastern IA. Mid-level lapse rates
are steeper in this region, perhaps leading to more robust updrafts
this afternoon and a chance of hail in this strongest cells. Most
of this activity will likely be elevated atop a cold/stable boundary
layer. Any storm that can remain in the warm sector could pose a
limited tornado threat as well.

125 PM CDT

No major changes to forecast thinking for timing of thunderstorms
later today into this evening and Wind Advisory headline for
areas mainly south of I-80. Severe threat remains rather
uncertain, however, with timing of any severe storms still
appearing similar to previous forecast thinking. Recent AMDAR
soundings from MDW and RFD show a pretty stout capping inversion
to surface based convection based at 800 mb, with temperatures at
+12 to +14 Celsius. There are steep lapse rates and modest
elevated instability above this capping inversion. With steep
mid-level lapse rates continuing to be advected northward and
strong deep layer south-southwest flow, the capping will be slow
to erode. Areas in the far west/northwest CWA closer to better
forcing could get some showers and possibly a thunderstorm after
20 or 21z, as some lightning is noted in area of showers near the
MS River slowly working eastward. Elevated convection here is able
to tap into the instability above the capping inversion.

Some red flags with respect to the potential severe threat
include: expansive mid and high clouds limiting insolation and
dew points generally running below guidance thus far serving to
limit SB/MLCAPE as warm front lifts northward; and the deep layer
shear vector parallel to the cold front orientation. Limited
instability could prevent for intense updrafts and the unfavorable
deep shear vector orientation could preclude noteworthy forward
propagation of line segments. Both of these could preclude a more
widespread risk for damaging winds, so message will be potential
still existing of isolated to widely scattered severe instances.
Severe threat should wane after midnight as available unstable
wanes. Overall parallel orientation of shear to boundary and
convection ahead of it could tend to yield a transition to more of
a heavy downpour/isolated minor flooding risk (for more details
see WPC Day 1 Marginal Excessive Rainfall Outlook).

isolated tornado risk, the warm front continues to lift north at a
steady pace and the evolving triple point of secondary low
pressure lifting northward near MS River could be area to watch
over parts of central/north central IL late afternoon/early



323 AM CDT

Through Friday…

A classic transition season major storm will have impacts across
the Midwest through the next 36 hours. The main forecast messages
for northern Illinois and northwest Indiana are 1.) prevailing
gusty winds throughout much of today through Friday peaking near
45 mph at times, 2.) widespread thunderstorms this evening with a
scattered severe threat mainly of gusts to 60 mph and the
possibility of a few brief tornadoes, and 3.) sharp warm up
occurring this afternoon into early evening with morning wind
chills in the 20s turning into later day temperatures of 70+ up
to at least I-80. A few seasons bottled up into 18 hours!

A picture perfect mid-latitude cyclone structure is seen on
satellite imagery early this morning, with the primary upper low
across the western Plains and vast diffluence aloft and deep moist
advection ahead. At the surface, a 986 mb low is just west of
Kansas City with a warm front extending eastward into the southern
half of Illinois. Water vapor imagery shows a couple small
perturbations in the southwest flow, one over northeast Missouri,
that may trigger spotty showers into north central Illinois this
morning. For the most part though, a slower evolution of this
system will keep most of the CWA dry through at least mid-
afternoon, with little focus aloft and capping atop the warm front
as it moves into the area. The tight pressure gradient over the
area north of the warm front this morning will continue to support
occasional easterly gusts of 40 to 45 mph, especially north of
I-80. These winds and synoptic placement will keep temperatures
slow to climb, again especially north of I-80 where the marine
layer will be advected well inland. Minor immediate lakeshore
flooding along Lake Michigan’s northeast Illinois shore,
especially north of Chicago, will continue to be an issue through
midday with 10+ ft waves.

The surface cyclone will begin to start a very gradual occlusion
early this afternoon. However, a potent 135 kt upper jet and
associated dry slot punching into the Iowa/Illinois border area at
midday will support concentrated ascent and a continued low-level
baroclinic response, with the warm front advancing northward
through the CWA. There may even be a second surface low around 997
mb to develop near the Quad Cities. How far north the warm front
progresses in the CWA is a key player today, and see it not having
much of an issue getting to at least near I-88 due to the slower
evolution of this system. Warm fronts tend to get hung up with
the lake this time of year so that adds greater uncertainty for
far northeast Illinois. Temperatures and dew points will take a
spike with the warm frontal passage and remain elevated until the
storms this evening, and southerly wind gusts should at least be
close to Advisory criteria. South of I-80 will have the longest
time in these higher gusts while Chicago it will be more short-

Guidance is in really sound agreement with a general north to
south axis of convection developing ahead of the dry slot and its
mid-level cooling and along a low-level trough axis. This looks to
develop late afternoon and mature early evening as it moves
eastward across the CWA. While the most robust convection may be
in a fine line, there is likely to be scattered convection apart
from that especially across the far north and into Wisconsin, but
possibly ahead of the main line too. Some of this other convection
may have little lightning due to small CAPE to -10C and not as
deep of updrafts as the main line. Mixed layer instability should
at least reach 500-1000 J/kg up to the warm front, with no
shortage in kinematics given around 70 kt of deep layer shear. The
shear orientation with respect to the boundary is such that it
favors individual cellular structure in the first several hours,
but all remaining fairly close to that boundary given the rapid
north-northeast individual storm prorogation. This motion of
cells will mean some crossing the warm front, and this is where
a brief tornado potential may exist. This could be maximized a tad
in north central Illinois if a triple point were to take shape
just west of there. Short residence time of cells on the warm
front points toward any rotational structure probably being short-
lived and not ideal for a significant tornado threat. What should
become a slowly progressing linear or quasi-linear MCS by late
evening will continue to possess gusty winds, again possibly
scattered severe, as it moves into northwest Indiana.

The nature of this system owes to higher than normal confidence
for timing, with general evening for the scattered severe chance,
and for the Chicago area that falls between 8 pm – 12 am. The
confidence in hazard potential noted above also is with fairly
good confidence, it’s just a bit tricky to say if things will mesh
enough for scattered coverage across the entire Slight Risk area
and the severe window may be more concentrated. But with dynamics
like this, a convective wind threat and localized tornado threat
can occur fairly easily and linger later into the evening/early

For Friday, the continued slow occlusion will be occurring with
the system over Minnesota. Cold advection wrapping around the low
will drive gusty southwest winds, especially given sun and mixing
expected through at least early afternoon. Gusts of 40-45 mph
remain likely. High temperatures toward Rockford where more
stratocumulus is expected in the afternoon, and possibly even a
stray shower, should stay below 50.



206 PM CDT

Friday Night through Thursday…

Main concern in the extended continues to be with a rain/snow mix
on Sunday into Sunday evening, with some potential for a period
of all snow in some locations across north central IL.

Late Saturday night through Sunday night looks to have another
strong system impact the area. This will find us more in the cool
sector portion of the storm, or at least that continues to be the
trend in solutions. This gives concerns for the possibility of
numerous hours of wet snow or a wet snow/rain mix in places with
even potential for light slushy accumulation.

After high pressure brings us a pleasant weather Saturday, this
next closed upper low will move northeastward from the Southern
Plains. The current forecast track continues to favor the
deformation area over the CWA, which along with a stiff northeast
wind should support enough precipitation to help offset any
daytime warming on Sunday. Still too far in time to say if it is
enough to keep a wet snow mix going on Sunday, but enough of a
signal to continue this mention in the forecast through the day
and into Sunday evening. Highs look to remain below 40 for much of
the CWA with this regime, with the exception of areas south of
the Kankakee river valley where temps will rise well into the 40s.
Again strong wind gusts and potentially lakeside flooding
concerns could be raised with this system.