Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260036 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
736 PM CDT Sat May 25 2019

735 PM CDT

No major changes made to the forecast message on this quiet start
to a Saturday evening on our Memorial Day weekend. The timing of
thunderstorm arrival into the CWA remains mainly overnight for
much of the area, with the exception being north central Illinois
where late evening (9 pm - 12 am) isolated thunderstorms may

Numerous thunderstorms, some supercellular and severe, have
developed near the Iowa/Missouri border area early this evening,
with isolated showers and storms across eastern Iowa. Much of this
activity is being assisted in developing by a strong mid to upper
level speed maximum as seen on water vapor imagery just to the
north. This exists in an analyzed high moisture plume, with 850 mb
dew points of 15C and precipitable waters near 1.5 inches. Large
MUCAPE values of 2000-2500 J/kg and deep layer shear (45+ kt) more
than adequate for organization, support the storms sustaining
themselves and additional growth to their east northeast within
the jet streak. Further north along the surface cold front,
located near Dubuque to Cedar Rapids Iowa, isolated thunderstorm
development is possible this evening although any deeper updrafts
have been struggling thus far. So our main upstream area of storms
is expected to be southeast Iowa.

As the storms continue to increase and their east to east-
northeast movement, they will move into north central/northeast
Illinois and northwest Indiana and peak in coverage overnight.
Confidence in coverage of thunderstorms diminishes north of I-80,
but there will likely be rain with embedded thunder there. The
most sustained activity, with at times west-to-east orientation
and multiple waves, will be favored south of I-80. The low-level
jet replenishing the storms is moderate in intensity, around 25
kt, and not predicted to increase much. This somewhat "in check"
moisture replenishment and limited storm motion offset is the
limiting flash flood potential into our CWA. However, this is
still something we will have to watch as through Sunday morning
one to two inch totals are certainly possible. As for the severe
threat, the high MUCAPE values do intersect the -10C to -30C range
in the forecast soundings, so some severe hail threat will
persist with any stronger updrafts and cannot rule out some
stronger winds in any congealed storms, again both of these more
favored along/south of I-80.



151 PM CDT

Through Sunday...

Destabilization has been slow today in wake of morning
precipitation and continued cloud cover, but dewpoints have risen
into the low to mid 60s. There is some modest instability though
mid level lapse rates are fairly weak, and we are in a bit of a
lull in forcing. There is some hi- res guidance painting a few
storms along the I-80 corridor as instability rebuilds in an axis
of weak lower level converegence, but confidence on this is
pretty low and vertical growth has been limited, thus it appears
most of the area will be dry this afternoon.

The better signal for precipitation/storms will be after dark as
a more organized wave across the central/southern Plains makes
headway across the ridge and interacts with the slowly southward
shifting cold front. Initially the northward extension of any
activity will be limited closer to the better instability. These
will encounter a somewhat unstable environment aloft also. PWATs
will increase to near 1.8", such that heavy rainfall will be a
concern, and while storms will be moving, the upper flow will be
parallel to the front and would support periods of sustained
moderate to heavy rain along and south of I-80. Instability/shear
parameter space is also supportive of stronger storms and SPC has
the area in a marginal risk for severe storms. Hail and locally
damaging winds would be the main severe concerns, with the greater
threat (though still marginal) south of the Chicago area.

The cold front will bring a shift to northeast winds and a cooler
airmass for Sunday. Some showers will linger into the morning and
possibly some embedded thunderstorms, then expect some clearing
in the afternoon. Highs near the lakeshore will be considerably
cooler, with a modest cooling inland due to the northeast winds
and somewhat drier airmass.



223 PM CDT

Sunday Night through Saturday...

The start of the period, Sunday night, is appearing to be on the
drier side. Any activity remaining from the day should quickly
exit, with any upstream development to the west likely not
arriving until Monday morning. Monday morning will be the start of
another active period this next week across the region. Large
upper level low expected to move through the western CONUS on
Monday, with fairly active mid levels likely in place over the
central CONUS. Surface low/trough expected to develop across the
plains and in this pattern, will see broad and strong southerly
flow setup across much of the region with a warm front expected to
lift north through the area. Mid level energy along with WAA and
strong LLJ will likely help showers and thunderstorms spread over
the area early Monday morning but with dry conditions then
expected by midday. Will need to monitor where this trough and
boundary setup Monday afternoon, as its possible these features
remain overhead later in the day Monday. If that were to occur,
then additional thunderstorms will be likely and these will have
the potential to be strong/severe.

Once again, where the trough/boundary setup will be key for
precip chances Monday night into Tuesday. It`s possible that this
boundary lifts north with dry conditions then likely, however, if
it stays in place then periodic showers/storms will be possible.
Still some lower confidence on this period/setup. By Tuesday
afternoon, will need to turn attention to the west where
additional thunderstorm development is expected with approaching
upper level low and surface low. Setup for severe storms appearing
to be possible Tuesday afternoon to our west. Latest guidance is
showing this might stay to the west, with a slower eastward
progression with the upper level low. Given that guidance is
generally too quick with upper level lows, this latest guidance
seems reasonable. Even if the development were to develop to our
west, its possible that this upstream development shifts east into
northern IL by Tuesday night. Will need to monitor this period
for additional strong/severe storms. With a slower progression of
this system, thinking additional thunderstorms will be possible
across the CWA on Wednesday and have adjusted the forecast to
reflect this thinking.



For the 00Z TAFs...

Quiescent weather conditions will continue for a few more hours at
the area terminals, but rapidly deteriorating flying conditions
due to showers and storms are expected later this evening and
overnight. Notable impacts to southbound departures will be
possible. The main aviation weather concerns this period stem

/1/ Timing of VCTS and any on-station TSRA potential late this
/2/ Cessation of higher TS chances late tonight with a transition
to more showery precipitation
/3/ Timing a north wind shift with an incoming cold front
/4/ Any low cig/fog potential behind the front, mainly Sunday
evening and overnight

Showers and thunderstorms are beginning to increase in coverage to
the west of the C90 TRACON with an area of stronger/severe
convection straddling the Missouri/Iowa state line. Another area
of bubbling cumulus and developing showers can be seen along a
cold front from near KDBQ to KVTI to KDSM in Iowa. Anticipate a
continued increase in convective coverage through the evening
hours as this activity slowly presses eastwards. Latest model
guidance suggests that the main instability/moisture axis will
attempt to set up near and south of I-80 this evening and
overnight, and this is likely where the highest coverage of
thunderstorms will occur as the aforementioned cold front presses
southward. For the Chicago area terminals, it appears as if things
will mainly stay in the SHRA VCTS variety overnight with more
limited instability. That said, there will be a window with
somewhat higher chances for on-station TSRA at the front end of
developing precipitation tonight, likely during the 7-9z timeframe
and a TEMPO for -TSRA has been added to the 00z TAFs after
coordination with the ZAU CWSU. Additionally added a brief TSRA
potential at RFD with convection beginning to intensify along the
cold front just to the west.

The convective evolution overnight is a bit uncertain, but it does
appear as if a developing cold pool should help push most of the
heavier convection incrementally south of I-80. Multi-model
consensus supports a north/northeast wind shift into the Chicago
area terminals towards 9z or so. Precipitation chances should
come to an end during the late morning/early afternoon hours.
Towards the end of the ORD/MDW extended TAFs there is a non-zero
potential for some low cig/BR development, although confidence in
this is too low to include an explicit mention at this juncture.






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