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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 241140

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CDT Mon Jun 24 2019

330 AM CDT

Through Tuesday...

Today through Early this Evening:
Main concern is the risk for strong to severe thunderstorms
capable of large hail and damaging winds during the mid to late 
afternoon and possibly into the early evening.

Early this morning, the area resides in the moist warm sector of
low pressure over northern Wisconsin, with temperatures and dew
points in the mid 60s to lower 70s. The primary feature of
interest that will drive the threat for thunderstorms this
afternoon is a stout mid-upper trough over the central and
northern Plains that will pivot eastward through this afternoon. 
This upper trough had unusually cold h5 temps in the minus mid to
upper teens Celsius analyzed on the 00z RAOBS, with tropopause
height possibly just below 500 mb.

Large scale ascent will increase as the short-wave trough
approaches this morning along with a anomalous for late June 
90-100 kt 300 mb jet streak nosing eastward. Anticipating that
there will be an uptick in showers this morning due to the
increasing large scale ascent interacting with moist air mass in
place. Through mid day, thunderstorm coverage should be low due 
to plentiful cloud cover and forecast soundings indicating minimal

Turning to this afternoon, there may be a small gap between the 
morning/mid day showers and the next round organizing. A dry slot
associated with the trough will wrap into the region, which could
allow at least partial sunshine to break out, along with southwest
winds gusting up to 30-35 mph. The core of the trough/upper low 
will be to our north, but cold pool of it will bring h5 temps down
to the minus low-mid teens Celsius over the area. This will bring
freezing levels down close to 10kft AGL. Upper 70s temps and mid 
60s Td with the unseasonably cool temp profile aloft would enable 
moderate instability up to 1500-2000 j/kg to be realized if there 
are sufficient breaks in the clouds. Conceptually, on the leading 
edge of the strongest ascent with the upper trough (with strong 
mid- level PVA), a broken arc of low- topped thunderstorms should 
blossom in the uncapped environment just to the west of the CWA in
the vicinity of the Mississippi River.

While there are uncertainties in this setup regarding how much
instability can be realized and coverage of thunderstorms during
the prime period of concern, there are things going for it in
support of the Slight Risk issued by SPC for a large portion of
the CWA. With seasonably fast flow aloft, effective southwesterly 
deep layer shear of around 35 kt (locally 40 kt) will support
storm organization. Given corresponding cold temps aloft/low
freezing levels, most intense updrafts, including isolated
supercells, could produce severe criteria hail, despite
equilibrium levels of less than 30kft. In addition, steep low
level lapse rates could support damaging downburst winds. With
limited directional shear, unclear how prevalent supercellular
structures are or if there's more mixed mode multi-cell,
supercell, and broken line segments. These storms will be moving 
at a fast clip off to the northeast at 40-45 mph, so duration of 
strong/severe thunderstorm risk in any given location should be 
fairly brief on the order of 2-3 hours. Approximate timing of 
severe risk from west to east is 2pm-7pm or so. The latest NAM 3km
Nest simulated reflectivity has a reasonable depiction of the 
expectations for today.

Mid Evening Tonight through Tuesday: 

Cold front passage will quickly follow the exit of anticipated
broken line of convection, which should shut most of the precip,
though can't rule out a stray shower while the trough is still
overhead. A cooler and temporarily drier air mass will move into
the area, along with clearing skies, with early Tuesday AM lows
close to seasonable upper 50s-lower 60s. 

Tuesday will start out pleasant and dry under close to full sun. 
It appears that return flow warm advection should ensue fairly 
quickly by mid day, though it's uncertain how quickly surface dew 
points recover and how high they get during peak heating. 
Southwest winds gusting up to 20-25 mph and 925 mb temps warming 
to the lower 20s support highs for most locations reaching the mid
80s, with isolated upper 80s possible. There is some concern for 
thunderstorms in the mid to late afternoon, as models are 
indicating fairly strong instability in 2000-3000+ j/kg range 
(though with aforementioned dew point uncertainty in how high dew 
points get which could limit these values) and little/no capping.
Big question is exactly what will trigger convection, with
possibility of a subtle wave in brisk west-northwest flow aloft.

Considering that there is a large variance in the models and
possibility of insufficient forcing to get convection to develop,
added only slight chance PoPs for now for Tuesday afternoon. 
There's a conditional risk for strong to severe thunderstorms with
more than sufficient northwesterly deep layer shear of 40+ kt and
moderate to strong instability. See the Long Term discussion for 
additional details on Tuesday evening/night. 




Tuesday night through Monday...

252 AM...Forecast concerns this period are convective chances 
and high temperatures.

A weak trough/cold front will move into the area Tuesday night
with a chance of showers/thunderstorms. Convective trends are
still quite uncertain Tuesday and how activity may trend into
Tuesday night. Current guidance trends would support activity
developing well to our southwest and then moving southeast. Thus
best potential may end up in the southwestern cwa. Any precip 
that does materialize in the cwa will likely be weakening/exiting 
early Wednesday morning.

Broad weak high pressure will settle across the area Wednesday
which is looking mainly dry but there could be some isolated
afternoon showers or thunderstorms in the far southeast cwa.
Westerly winds will be diminishing in the afternoon and with the
high overhead...a lake breeze is possible. Otherwise with plenty
of sunshine high temps should be well in the 80s.

The upper ridge will slowly build northeast across the region
Thursday into Friday with high temperatures warming in the upper 
80s and perhaps into the lower 90s as forecast by mex guidance. 
These temps look on track provided the area remains dry with sun. 
There does appear to be some potential for convection to develop
across MN and move southeast both Wednesday night and again
Thursday night before the upper ridge more firmly builds into the
western Great Lakes region this weekend. Have maintained low
chance pops for this uncertainty. Overall flow at the surface
looks weak during the end of the week and there should be some
lake breeze potential...perhaps remaining close to the lake if
they form. cms


For the 12Z TAFs...

640 AM...Forecast concerns include...

Ifr/lifr cigs this morning. 
Scattered showers/isolated thunderstorms this morning. 
Scattered thunderstorms mid afternoon. 
Strong/gusty winds mid/late afternoon into the evening.

Ifr and some patchy lifr cigs are widespread across northeast IL
this morning and this low cloud cover is expected to lift through
mvfr this morning and become low vfr by early afternoon.

Some showers have begun to develop across northwest IL and these
are expected to increase in coverage over the next few hours and
then become prevailing for a few hours in the late morning. The
thunder threat remains low but opted to maintain vicinity thunder
with this forecast.

Forecast for this afternoon looks on track. A cold front will move
across the area during the mid/late afternoon. Scattered
thunderstorms are expected to develop ahead of this front and
should be moving rather quickly as winds aloft increase. Duration
at any one location probably only an hour or less. There could be
a few lingering showers into early this evening.

South/southwest winds will increase during the morning with gusts
into the 20kt range by early afternoon. Winds will shift more
west/southwest behind the front with gusts increasing into the
25-30kt range...which will slowly diminish during the evening. cms


LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 PM Monday to 1 AM Tuesday.