Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS63 KLOT 271912

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
212 PM CDT Tue Jun 27 2017

203 PM CDT

Through Wednesday...

Quiet weather in place the rest of today under the influence of
high  pressure, but precip chances will increase through the day
Wednesday ahead of an area of low pressure working east across the
Upper Midwest.

This afternoon, high pressure is centered over Indiana with
ridging  extending into northern Illinois providing light flow
and fair conditions. A lake breeze is charging westward across the
Chicago metro pushing temps back down a couple degrees into the
upper 60s to lower 70s. Lake breeze should wash out later in the
evening with southerly flow overspreading the CWA behind the
departing ridge and ahead of low pressure over the plains.

Attention this evening and tonight will turn off to the west where
an MCS is expected to develop around the mid and upper Missouri
River Valley on the nose of a strong low level jet. Convection is
expected to spread east across Iowa and Minnesota overnight and
eventually reach Wisconsin and potentially northern Illinois
early tomorrow. Convection is expected to outrun any instability
and the better forcing associated with the LLJ is expected to
remain to our west and northwest with mid/upper level height
rises/ridging building overhead locally. Some of the models do try
to bring in a decaying area of precipitation across northern
Illinois while a number more are dry. Given the limiting factors
mentioned above, have lowered PoPs but do maintain some slight
chance/low chance PoPs mainly across the northern two tiers of
counties in Illinois to account for the uncertainty. If something
were able to persist into the CWA, there is not much of a severe
threat given the lack of instability.

Broad area of low pressure is expected to advance east across the
Dakotas and far upper Midwest through the day Wednesday with
strengthening southerly flow expected locally. Tightening pressure
gradient and deep mixing should be able to tap into strong mid
level winds which will result in gusty conditions from mid/late
morning through the afternoon. Expect a few wind gusts to top out
in excess of 30 mph with more frequent gusts of 25 to 30 mph. Dew
points are expected to gradually increase throughout the day,
eventually pushing above 60F, especially across the western CWA
where modest MLCAPE will eventually develop mid to late in the
afternoon. Hard to pin down specifics on convection tomorrow
before 00Z with no strong focus for ascent in the area, but with
broad upper difluence behind the departing upper ridge axis and
the potential for remnant outflow boundaries cannot completely
rule out isolated storms tomorrow afternoon. If any storms are
able to develop, there would be some threat for severe
thunderstorms given a strongly sheared environment with
cyclonically curved hodographs in place. Better chances arrive
tomorrow evening as discussed in the extended portion of the
forecast discussion.



330 AM CDT

Wednesday through Monday...

A pattern change to zonal/westerly mid-upper level flow will occur
for the second half of the work week. This will favor multiple
convectively modified short-wave disturbances traversing the
region Wednesday-Friday and a return to more seasonable
temperatures, moist dewpoints and high column moisture content.
Therefore, the mid to late week period will be periodically active
with multiple opportunities for precipitation/thunder potential.
The synoptic set-up will also be one to monitor for strong/severe
thunderstorm potential, as guidance is indicating relatively
strong wind fields supportive of favorable bulk shear. As alluded
to above, models are also indicating the potential for anomalously
high precipitable water values around 2" at times late Wednesday
through Friday, so a risk for heavy downpours and at least
localized flooding may also evolve.

On Wednesday, an uncertain factor will be the likelihood of an
ongoing MCS tracking from the Plains and at least toward the area.
The latest guidance is keying on a morning to mid day arrival of
the lead MCS modified short-wave. A warm front will be in the
process of lifting north in response to pressure falls from strong
low pressure moving from the Dakotas into Minnesota. Given this,
the MCS will likely be moving in prior to the arrival of better
moisture with the front and outrunning the instability axis, so
should be on a weakening trend. Have bumped up PoPs from 12z-18z
to account for the MCS likely moving in from the west during the
morning. Would expect there to be at least embedded thunder with
it, but severe weather unlikely given limited instability.

Confidence in specific trends into Wednesday afternoon and evening
is low given that much will depend on any convective overturning
of lapse rates, the position of outflow boundaries from the
morning MCS (and possible redevelopment off these), and lingering
cloud cover effects on destabilization. Another short-wave will be
approaching from the west by the evening. Should there be enough
recovery/destabilization, the forecast wind fields are quite
concerning for a wind and isolated tornado threat in northern
Illinois. Models are indicating 50-60 kt of 0-6 km bulk shear and
0-1km/0-3km SRH greater than 200/500 m2/s2 due to a 50+ kt
southwesterly 850 mb low level jet. Discrete/semi-discrete
supercells would also contain a large hail threat despite mid-
level lapse rates not forecast to be very steep. For additional
details, see the SPC Day 2 Outlook. Overall, did not made big
changes to the going forecast for Wednesday afternoon and evening,
with general idea of increasing PoPs/shower and thunderstorm
coverage during the afternoon and evening.

Another item of note for Wednesday is the strong south-southwest
wind potential. If cloud cover issues are not too much of a
limiting factor on mixing depth and tapping into higher momentum
air aloft, gusts could reach or even exceed 35 mph during the
afternoon per forecast soundings. The cold front trailing from
the surface low will progress toward the area Wednesday night into
Thursday. The frontal boundary and/or outflow from overnight
convection then appears probable to stall out near or over the
area on Thursday.

Needless to say, confidence is also low on the position of the
likely stationary frontal/outflow boundary and in general for the
convective evolution Thursday afternoon into Thursday night. There
are large differences in the operational guidance with respect to
timing of additional shortwaves during the afternoon and evening,
which will dictate shower and thunderstorm being isolated/widely
scattered or more widespread. Sufficient flow aloft parallel to
stalled boundary could again yield a risk for strong to severe
thunderstorms, and orientation adds some concern for training of
convection (flooding risk) depending on how things play out.

The amount of cloud cover on Thursday from convective debris and
new convection will play a significant role in high temperatures.
Progged 925 mb temperatures in the lower 20s Celsius at peak
heating supports highs in the upper 80s based off local
climatology in late June. In the grids went with somewhat of a
middle ground between the warming potential and most of the
deterministic guidance, which is cooler, with a forecast of
solidly mid 80s highs. With forecast dew points in the upper 60s
to lower 70s, current forecast yields max heat index values in
upper 80s to lower 90s. If warmth is maximized, these values could
easily reach the mid 90s (and vice versa if temps are cooler).
Maximizing warmth on Thursday will also yield greater instability,
as pertaining to the thunderstorm and strong/severe risk discussed

Additional upstream MCS activity appears likely on Thursday night
over the Plains and then shifting over the local region. A
surface low is expected to take shape from a short-wave emanating
from this activity, with the low tracking over the area on Friday,
though there is a good deal of uncertainty on exact track and
timing of this. Global guidance is indicating a significant QPF
signal moving over the area late Thursday night into Friday
morning with this wave, so given time of day and likelihood of
plenty of cloud cover it`s possible that the biggest threat Friday
could be heavy rainfall and flooding. If any pockets of heating
and destabilization can take place Friday afternoon in more
subsident regime behind lead wave, would need to monitor for a
strong/severe risk in any convective redevelopment given
supportive wind fields/bulk shear.

A cold front passage should end thunderstorm threat Friday evening
and likely set the stage for a somewhat drier/quieter Saturday
and Sunday. A fast moving trough/upper low will be pivoting
across the Great Lakes region on Saturday possibly additional
northwest flow energy on Sunday. Instability will be much lower
both days, but with the presence of forcing from aforementioned
waves, threat for diurnally driven isolated to widely scattered
showers and thunderstorms is there. Much of the time will likely
be dry, so not at all appearing to be a washout, with temps near
seasonal. A stronger system could then move in on Monday with
additional shower/thunder potential.

The main message for the Wednesday-Friday period continues to be
this: confidence in the day to day sensible weather details is
low as is common this time of year with mesoscale convective
uncertainties that will need to be sorted out. Should things come
together, a period or two of more widespread severe and/or
flooding potential may evolve. Please stay tuned for later



For the 18Z TAFs...

Main forecast concerns will be westward extent of a lake breeze
this afternoon, convective chances overnight and Wednesday, and
very breezy conditions developing by mid morning Wednesday.

For the rest of this afternoon and evening, high pressure centered
over Illinois will continue to gradually slide east across the
region providing relatively light flow (around 10 kt or less) and
VFR conditions at the terminals. A lake breeze has already
developed this afternoon and visible on TORD but has made little
inland progress thus far. Given the modest opposing westerly flow
in place, it will be one of those days that the boundary will get
close to both ORD and MDW and likely stall somewhere in the area.
Confidence in exactly how far west the boundary will progress and
if it is able to pass over the terminals turning winds easterly
remains low thus have not made any significant changes to going
forecast with respect to winds this afternoon. Still maintain a
wind shift at MDW which has the best chance to lake breeze passage
but keep out of the ORD TAF in the meantime. Some of the hi-res
models do bring it through ORD late this afternoon/early evening.

Expect SSW flow to overspread the terminals again mid to late this
evening once the lake breeze washes out and continue through the
overnight hours. Low pressure advancing east across the plains
will result in a sharp increase in winds tomorrow morning, with
gusts at times pushing to near 30kt late morning through the

Meanwhile, thunderstorms are expected to develop to our west this
evening and overnight eventually spreading east into far northern
Illinois and Wisconsin Wednesday morning. Precipitation is
expected to outrun the instability and the better forcing and
support for sustained nocturnal convection is generally expected
to stay to our west and northwest with low but non-zero chances
for a decaying area of precip to overspread the terminals tomorrow
morning. Have introduced a prob30 for showers at RFD but will
call for dry conditions for the Chicago area terminals where there
is only a slight chance for precipitation. Instability doesn`t
arrive in the area until beyond the current TAF period and it does
currently appear that Wednesday evening and overnight could be
active with areas of thunderstorms for the northern IL and
northwest IN terminals.



410 AM CDT

The main marine concern is strong southerly winds on Wednesday
through Wednesday night. As high pressure crests the lake and then
departs east today, moderate west-northwest winds will back to
southerly this afternoon. On Wednesday, an unseasonably deep
surface low will move across the Upper Midwest by mid day. The low
will track to near or just north of the northern tip of the lake
by Thursday morning. This is a favorable path for strong southerly
winds, with the potential limiting factor on even higher speeds
being the stable conditions over the open waters. Am expecting
speeds/frequent gusts to 30 kt, with the potential for occasional
gale force gusts in the afternoon and evening, especially on the
north half. Cannot rule out prevailing gales, but confidence is
low, so have not issued any gale headlines with this forecast. A
Small Craft Advisory will be needed for the entire nearshore.

West-southwest winds will gradually diminish on Thursday as a
trailing cold front settles toward or over the lake, with a more
pronounced wind shift possible on the north half. The front and
wind shift will then shift further south down the lake Thursday
night, followed by another surface low tracking over the central
or southern portion of the lake on Friday. Southwest wind
speeds hazardous for small craft are possible for at least a
portion of as a warm front lifts north. A cold front will move
across the lake Friday evening and possibly clear the lake by
Saturday morning.

A few periods of fairly widespread thunderstorms are probable in
the Wednesday-Friday timeframe, with some risk for strong to
severe thunderstorms, mainly over the central and southern
portions of the lake.






WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.