Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261750
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT Mon Jun 26 2017

.UPDATE...1051 AM CDT

Satellite pictures suggest two areas forcing, one across central
IL and the other associated with the upper low across northern WI.
In between, weak forcing and shallow cumulus may lead to an
isolated shower in any deeper cumulus development, but RAP
forecasts of a slight uptick in instability ahead of the energy
across central IL and extending to along and south of the I-80
corridor this afternoon would hold better chances, though till
widely scattered shower or storm activity, and then far northeast
IL as well later this afternoon when the upper low dives in and a
small pocket of better lapse rates aloft sneaks in. The forcing
wanes quickly near sunset and what minimal moisture should erode.

Expect clouds to fill in a bit more with time during the daylight
hours as well, capping our temperature rise in the coming hours.
Clearing skies and lighter winds will lead to another seasonally
chilly night.

KMD

&&

.SHORT TERM...
330 AM CDT

Today and Tonight...

Short term portion of the forecast continues to feature cooler than
normal temperatures for late June, with the potential for isolated
to scattered showers and thunderstorms later today in association
with a pair of mid-level disturbances and a weak frontal trough.

Broad upper level trough remains across much of the eastern CONUS
this morning. A fairly vigorous short wave is noted over western
Lake Superior and far northwestern WI in GOES vapor imagery, and
this feature is progged to dig southeast across the western Great
Lakes region through this evening. High res guidance is in decent
agreement in depicting a surface trough/cold front in association
with this wave, which pushes south into far northern IL and
northwest IN by late afternoon and early this evening. Weak
instability, generally less than 200 J/kg of MLCAPE is indicated in
forecast soundings, which should be enough to support isolated to
scattered showers/thunder given the available forcing, as indicated
by the 3km WRF and hinted at in some of the other CAM output.
Farther west, another mid-level speed maxima is noted across the
Northern Plains, also propagating to the southeast. An area of
precipitation was already occurring with this second wave across
parts of Iowa early this morning, and will spread southeast into
western/central IL today, primarily affecting southern parts of the
cwa during the midday/afternoon hours. In between, there may be a
corridor, roughly from Rochelle to south of Valparaiso, where precip
potential remains minimal, with better chances over far northern and
southern parts of the forecast area. Both mid-level features push
southeast of the area this evening, with subsidence developing
overnight. This, combined with loss of diurnal instability
component, should allow showers to dissipate or settle south of the
area by later this evening.

Similar to the past couple of days, breezy west-northwest winds will
develop after sunrise this morning, with steep low level lapse rates
leading to gusts around 25 kt and a stratocu deck by mid-late
morning. Low level temps (around +12C at 925 mb) should support high
temps from near 70 far north to the lower 70`s most other areas.
The late afternoon cold frontal trough will likely turn winds
north to northeast along the lake, making for a quick cool down
along the shore early this evening. High pressure builds in later
tonight with clearing skies and diminishing winds, setting the
stage for chilly overnight temps in the upper 40`s to around 50
away from the core of the Chicago urban heat island.

Ratzer

&&

.LONG TERM...
330 AM CDT

Tuesday through Sunday...

The anomalous mid/upper trough that has been bringing the cool
weather will kick east on Tuesday, with height rises favoring fair
weather conditions. After a chilly start to the day, plenty of
sunshine and the onset of low level warm advection with still
unseasonably dry 40s dew points in many areas will enable
temperatures to warm well into the 70s. Surface high pressure
around 1020 mb will be over the area through mid day and then
slide east during the afternoon, with light to moderate synoptic
south-southwest winds developing. Sufficiently weak 950 mb flow
on northwest side of the surface high and land/lake thermal
differential should enable a lake breeze to form and bring some
cooling to the Illinois shore and possibly along portions of
immediate Indiana shore.

A pattern change to zonal will quickly occur in the midst of a
quiet overnight Tuesday night. Broad lower than average heights
will be in place over the Intermountain West and Plains by
Wednesday, with subtropical ridging setting up off the Southeast
and Mid Atlantic Coast on Thursday and Friday. Ensembles are in
decent agreement in the large scale pattern, so confidence
continues to be decent in this.

The zonal/westerly mid-upper level flow pattern 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 still looks to be fairly
active with periodic precipitation/thunder potential. The synoptic
set-up will also be one to monitor for a 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 over the Plains, with respect to possible MCV
formation from this, the trajectory and timing of the wave, cloud
cover limiting destabilization, impact on mid-level lapse rates
and influence of outflow boundaries. Furthermore, a warm front
will be lifting north in response to pressure falls from strong
low pressure moving from the Dakotas into Minnesota. An earlier
arrival of the MCS modified short-wave prior to the arrival of
better moisture with the front and resulting destabilization could
limit the magnitude of thunderstorms over the area, with possible
subsidence behind this wave until later in the evening. On the
other hand, a more favorable timing could certainly yield a
notable severe/flooding risk somewhere in the region. Confidence
is low on exactly how things will play out, so have not made big
changes to the going forecast, 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 and then appears probable to stall out near or over the
area on Thursday. Needless to say, confidence is also fairly low
on the position of the likely stationary frontal boundary. Current
forecast favors areas especially along and south of I-80 for
potential shower and thunderstorm redevelopment Thursday afternoon
and evening. Another convectively modified wave may try to
impinge on the warm sector late in the day or in the evening and
the timing of this will impact whether coverage is only
isolated/scattered or more widespread.

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).

Additional upstream MCS activity appears likely on Thursday night
over the Plains. 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 uncertainty on exact
track. Global guidance is indicating a significant QPF signal
moving over the area 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 night
and possibly set the stage for a somewhat drier/quieter Saturday.
This will be determined by the timing of a fast moving trough
moving across the Great Lakes region either Saturday or Sunday.
With confidence very low this far out, have only partial covering
of slight chance PoPs over the weekend. The main message for the
Wednesday-Friday period is this: while confidence in medium- high
in large scale pattern as mentioned previously, 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. Stay
tuned for later updates.

Castro

&&

.AVIATION...
For the 18Z TAFs...

Concerns center on isolated shower or storm development this
afternoon, along with a slight wind shift later this afternoon,
which could briefly shift ORD/MDW to possibly a more northerly
direction. Lake breeze development on Tuesday is also on the
horizon.

Two focus areas for showers include the sheared out energy along
and south of I-80, which will lead to isolated to widely scattered
showers, possibly turning into a storm or two. But this is largely
south of the terminals. Next is the upper low across northern WI.
This will rotate through NE IL mid afternoon, though there is
some weak convergence along a surface trough in NE IL ahead of it.
Moisture is not great with either wave so anything would be very
short lived, but already seeing showers develop ahead of this
axis, so VCSH will be carried out of the gate with this set. CAPE
is very small and not forecasted to increase much, so held off on
thunder mention.

A weak surface trough will flip winds more northwesterly through
the afternoon. This pattern can sometimes lead to winds shifting
more northerly to even northeasterly at ORD/MDW. This trough
appears a bit weaker, hi-res guidance is still holding winds N-NW,
so have trended that way, though with medium confidence. Forecast
soundings depict quick drying this evening. There is a weak
gradient on the northeast side of high pressure that will maintain
a west wind component through the night.

Only concern for Wednesday is lake breeze development. Current
guidance keeps the lake breeze out of ORD and MDW and confined
just east as the lower level southwest winds will increase ahead
of developing low pressure across the plains.

KMD

&&

.MARINE...
407 AM CDT

Brisk west-northwest flow will be in place across the lake today.
A cold front trailing from weak low pressure over the northern
Lakes this morning will shift winds to more north-northwest this
afternoon, earliest on the north half. Have issued a Small Craft
Advisory for the entire Illinois and northwest Indiana nearshore,
starting at 10am for Illinois and already in effect for Indiana
given hazardous waves observed at the MI City Buoy. Winds are
expected to shift north-northeast by the early evening in the
nearshore, but speeds will likely come down fairly quickly. High
pressure will move over the region on Tuesday, with the high
center moving over the southern portion of the lake and then
shifting east late in the day. Lighter west-northwest winds than
Monday will shift to southerly in the afternoon.

Focus then turns to the next period of strong wind concern. This
will be Wednesday through Wednesday night. As unseasonably strong
low pressure moves toward the northwestern lakes region by
Wednesday evening, southerly winds will increase to 30 kt. With
the approach of the trailing cold frontal zone Wednesday night,
the strongest winds will focus into the central and southern
portion of the lake with direction becoming southwest. Brisk
west-southwest winds will continue on Thursday, especially south,
with uncertainty on wind direction north given potential for a
wind shift. Wind direction uncertainty increases on Friday, though
speeds are likely to be lighter. A weak low pressure may pass
near or over the southern tip of the lake. Finally, a few periods
of fairly widespread thunderstorms are probable in the Wednesday-
Friday timeframe. Wednesday in particular could also see a risk
for strong to severe thunderstorms reach the lake.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement...INZ002 until 4 AM Tuesday.

     Beach Hazards Statement...INZ001...4 PM Monday to 4 AM Tuesday.

LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 10 PM
     Monday.

     Small Craft Advisory...Gary to Michigan City IN until 4 AM Tuesday.

&&

$$

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