Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271043
AFDLOT

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

.SHORT TERM...
235 AM CDT

Today and Tonight...

Dry and slightly warmer weather can be expected today as high
pressure develops into the region from the west. Cloud cover will
increase late tonight as winds shift south and begin to pull
warmer and more humid air back into the area.

Upper trough which has lingered over the Great Lakes region for
the past several days will move east today, nudged by Plains upper
ridge which will build in from the west through tonight.
Subsidence, indicated by 50-60 meter 500 mb height rises and
building surface high pressure, will result in mostly sunny and
dry conditions today, along with slightly warmer temperatures.
Model 925 mb temps of +15-17C should support mid-upper 70`s for
afternoon highs this afternoon. Weak lake breeze is expected, as
boundary layer winds start out less than 10 kts today, though a
gradual increase in 950 mb winds later this afternoon should limit
the inland push across the Chicago area. NW Indiana will likely
see the lake breeze push a little farther inland as winds start
the day NW there.

Winds turn south tonight as surface ridge moves east of the area.
This should help keep temps in the 55-60 degree range for
overnight lows. High cloud cover will likely increase toward
morning especially across western parts of the cwa, in association
with overnight thunderstorm complex progged to develop across
IA/MN and western WI. This activity expected to remain west of the
cwa until after sunrise Wednesday morning.

Ratzer

&&

.LONG TERM...
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
above.

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
updates.

Castro

&&

.AVIATION...
For the 12Z TAFs...

Quiet VFR aviation weather day today...with the only concern being
how far inland the lake breeze moves this afternoon across
ORD/MDW.

Surface high pressure ridge over the mid-Mississippi Valley will
continue to drift east today...with weak gradient supporting light
westerly winds initially. Boundary layer flow is weak for the most
part, though 950 mb winds turn southwest and increase a bit later
this afternoon especially across northwest/north central IL.
Expect lake breeze will develop by early afternoon and affect GYY.
High-res guidance has become a little more bullish in moving the
boundary inland across MDW by mid-afternoon, and coming very close
to ORD. Circulation appears to be pretty weak however, thus
confidence in it pushing across ORD is low at this time. Surface
ridge moves east of the area by evening...allowing light south
winds overnight.

Mid-level disturbance approaches from the west by early Wednesday,
and should result in the development of a complex of thunderstorms
across the Plains late today/overnight. Guidance indicates
weakening remnants of overnight upstream convection will push into
northern IL after sunrise...and may reach Chicago area by mid-late
morning. Have indicated a prob30 in the later hours of the ORD
TAF, though with 06Z wrf soundings showing little instability this
far east at that time have held to just shra for now. Gusty south
winds also develop Wednesday, in response to low pressure lifting
toward the Upper Mississippi Valley.

Ratzer

&&

.MARINE...
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.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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