Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241142

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 AM CDT Fri May 24 2019

345 AM CDT

Through Saturday...

Challenging forecast in the short term with convective processes
likely to dictate the eventual evolution of things.

Early this morning, a large, loosely organized MCS and associated
MCV are moving northeastward across Iowa. Strengthening warm air
advection out ahead of this MCV is beginning to result in isolated
to widely scattered storms developing over northern IL to the
north of the warm front that`s draped across central IL. This warm
front and associated pool of warm, moist, unstable air is progged
to begin lifting north early this morning. The northward advance
of this boundary may ultimately be retarded for a time by
convective cold pool and associated outflow boundary re-enforcing
the boundary later this morning. As the MCV over IA tracks
northeastward, it should become increasingly separated from the
better pool of instability and moisture, so while the MCV tracks
into WI the greater concentration of convection this morning will
probably be over northern IL, especially early this morning when
aided by the enhanced WAA associated with the low level jet (LLJ).

All indications are that there should be a lull in the activity
over our CWA later this morning through the mid-afternoon in the
wake of the first MCV and following the weakening and veering of
the LLJ. There are some sizable differences among the various
models with respect to the placement of the boundary this
afternoon, likely due in large part to models struggling and
attempting to resolve the impacts of potential convective cold
pool. Guidance is advertising some modest pressure falls to our
north this afternoon while we see low/mid level height rises, so
warm front may mix northward some this afternoon. High bust
potential with temps this afternoon, with easterly flow north of
the boundary likely to result in some lake enhanced cooling over
northeast IL through much of the afternoon.

By late this afternoon, if atmosphere is able to recover and
destabilize, there is a threat of storms developing near the warm
front. Generally speaking, most models suggest that our area
should see some capping inversion aloft, which conceptually would
make sense given the rising heights today. If the cap is weaker
than progged and the boundary layer were to recover sufficiently
from this morning, then given the kinematic fields there would
certainly be a threat for isolated severe thunderstorms.

While forcing looks to be somewhat nebulous, guidance is in pretty
good agreement in lighting up convection late this afternoon from
the southern plains north into eastern IA and northwest IL. This
convection is likely to spread eastward this evening into the
remainder of northern IL with strengthening low level jet likely
to provide a continuous feed of unseasonably moist and unstable
air into the region. While there is some lingering uncertainty
still regarding the coverage of storms and placement of storms
tonight, there is enough of a model consensus pointing to the
northern half or so of our CWA to go ahead with a flash flood
watch given the ingredients progged to be in place. The
ingredients include precipitable water values over 200% of normal
and in the 99th percentile for May, 40kt+ low level jet, high
freezing levels/very efficient warm rain processes, and Corfidi
Vectors dropping to near zero raising concerns for potential back-
building convection and some threat for training storms. Soils
remain very moist and unable to hold much more water and rivers,
creeks, and streams are also running above normal. Potential
exists for storms to easily produce 1-2" per hour rainfall amounts
and if any back- building/training were to occur the potential is
there for totals easily over 3" which given the already fairly
low flash flood guidance would pose a flash flood threat.

In addition, to the flash flood threat, strong shear profiles and
progged ample instability also raise the risk for severe
thunderstorms. Damaging winds and large hail appear to be the
primary threat, which could persist into even the early parts of
the overnight hours. There is also a more conditional tornado
threat as well, particularly near the warm front where more backed
low level flow will result in larger low level hodographs. The
magnitude of the severe threat this evening is a bit uncertain due
to the lingering questions regarding forcing, if it becomes
apparent that forcing will be sufficient for organized convection
then a more substantial severe threat would likely exist given the
likely strong shear profiles and ample instability.

Convective activity will likely linger into Saturday morning with
a tendency for storms to settle southward into the better
instability as air mass over northern IL gets worked over by
potential repeated rounds of storms and low level jet supporting
that renewed development weakens and veers a bit. Convection
should wane in the afternoon, especially northern CWA.

- Izzi


317 AM CDT

Saturday night through Thursday...

The main forecast concerns through the period will continue to
focus on the chances and timing of showers and thunderstorms
through mid week as we remain in a very active weather pattern.

A cold frontal boundary is expected to sag southward over the area
at the beginning of the period Saturday night, before stalling
out somewhere across central and southern IL on Sunday. This front
is likely to be the focus for additional shower and thunderstorm
chances Saturday night into Sunday, so its exact placement will be
key in identify storm chances. At this time it appears the best
chances of storms will mainly south of I-80 Saturday night, with
the focus then gradually sagging southward over central portions
of IL and IN during the day Sunday. Some strong to severe storms,
with heavy rainfall will continue to be possible with this
activity, again with the best potential looking to be south of
I-80 during this period.

It does appear the area will have another short break of dry
weather later Sunday into early Monday as high pressure builds
over the western Great Lakes region. Again this will be a short
period, as yet another storm system is forecast to spawn over the
central Rockies Sunday night as an upper level impulse ejects out
of a southwestern CONUS upper low. This area of low pressure
should then track northeastward to the western Great Lakes by
Tuesday evening. This would likely result in the cold front to our
south shifting back northward over the area as a warm front
Monday, with showers and thunderstorms probable over at least
northern sections of the area late Monday into Monday evening.

Yet another period of showers and thunderstorms, with possibly a
much better severe and heavy rain threat is then looking to be on
Tuesday and Tuesday night. This is the period when the main
southwestern CONUS upper system is forecast to shift northeastward
towards the western Great Lakes. The thermodyamics and kinematics
will certainly be favorable for severe thunderstorms with this
activity into Tuesday night, and very heavy rainfall will again be
of concern.

Following this period of active weather into early Wednesday, it
does appear that we could get another break in the active weather
for a couple of days.



For the 12Z TAFs...

The main forecast concerns continue to focus on the timing of
thunderstorms today and tonight.

Showers and embedded thunderstorms are currently moving across
terminals. It appears that this activity will shift out of the
area in the 13-14z timeframe. However, another healthy complex of
storms is currently ongoing farther west across IA and northern
MO. Forecast guidance suggests that this activity will weaken as
it shift eastward this morning, but this may not happen as quickly
as guidance suggests. Given the continued uptick in warm air
advection/isentropic ascent and moisture transport off the
surface, it appears that sufficient elevated instability may
allow this complex of storms to make a run at terminals around
midday today. This is a change from the previous forecast, and
this convective activity could impact the timing and placement of
our next batch of thunderstorms tonight.

There continues to be concerns for more thunderstorms over the
area very late this afternoon and into tonight. However,
observation trends will need to be monitored following the
morning thunderstorm activity. For now, no major changes have been
made to the timing for storms tonight.



IL...Flash Flood Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     PM Friday to 10 AM Saturday.

IN...Flash Flood Watch...INZ001...7 PM Friday to 10 AM Saturday.




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