Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191922

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
222 PM CDT Wed Apr 19 2017


1121 AM CDT

Weak surface cold front is dropping south across the area, with
expected northeast wind shift and falling temperatures occurring,
most pronounced near the lake. A large temperature contrast will
exist this afternoon northeast to south and southwest. The main
concern is convective trends and extent of any isolated severe
threat this afternoon and evening. Lead short- wave and associated
disjointed convective complex is currently moving over the area.
IR imagery generally exhibited a warming trend of cloud top temps,
though there is enough MUCAPE and effective shear for cores to
pulse up on the leading edge of the complex, for a threat for
isolated small hail.

After this first wave departs this afternoon, there will be a
period of subsidence in its wake, though scattered convection
could fester near the stalled out frontal boundary well south of
I-80 in east central IL into Indiana. This area will need to be
watched for any pulsing cores and a limited hail/wind threat.
Attention then turns to the next shortwave producing convection
over western IA. Trajectory of this wave which can be seen on W/V
imagery and lightning trends with the convection will be more
northeasterly. Thus, forcing/lift from this wave will be most
likely to clip our western and northwest CWA, where there may be a
threat for elevated hail producing storms north of the boundary
and an isolated surface based severe threat in the vicinity of the
stalled frontal boundary. As mentioned in AFD below, the front
will lift back north of the area as a warm front tonight, but at
this time the focus for convection along and ahead of the cold
front trailing from low pressure appears limited, with the best
forcing passing north of the area into Wisconsin. Overall
continue to favor the idea of severe threat within the CWA being
limited, but a bit higher in the western/northwest CWA.



246 AM CDT

Through Thursday...

Lots to talk about in the short term with convective potential and
some accompanying severe threat as well as some truly wild temp
swings the next 30 hours over portions of the CWA.

Temps today:

Early this morning a cold front extends from near KGRB southwest
into southern IA. As the associate low continue to move northeast
toward James Bay, look for this trailing front to basically sag
southward into northern IL this morning. Initially, not looking
for a terribly impressive temp gradient with this weak front so
only slight drop off in temps expected behind the front. Winds
behind the front will eventually veer to NNE, albeit fairly
lightly, so there will be a somewhat bigger temp drop behind the
front near the lake, likely into the 50s this morning. By this
afternoon, low pressure organizing and moving off the central high
plains will result in some pretty significant pressure falls over
IA, which will result in a strengthening easterly gradient and a
more pronounced lake breeze front with even chillier air spreading
farther inland late this afternoon into this evening. To the
south of the strengthening and eventually lake enhanced quasi-
stationary front look for high temps to climb into the upper 70s
or lower 80s this afternoon.

Precip Today:

Precip-wise today, there will remain a threat of some isolated
light showers with the weak cold front oozing south this morning,
but any precip looks to be light, brief, and or not much
consequence. Of greater interest, is shortwave trough most evident
on GOES-16 6.9 micron channel (mid-level water vapor) tracking
east across northern KS/southern NE. Convection has been
blossoming in advance of this feature over NE KS and eastern NE
with guidance tracking this shortwave ENE into eastern IA/northern
IL by early afternoon. Would anticipate a gradual uptick in
coverage of showers and thunderstorms in advance of this shortwave
from mid morning into the early afternoon. Majority of this
activity is likely to occur north of strengthening quasi-
stationary front this afternoon suggesting a pretty good
likelihood of t-storms remaining elevated. The new GOES-16 7.3
micron channel (low level water vapor) shows a substantial warming
and drying in an area where RAP soundings depict an healthy
elevated mixed layer (EML)/steep mid level lapse rates. The EML is
forecast to continue advecting ENE today and while convection
will mostly be well ahead of the advancing EML, some of the storms
developing toward the western extent could enjoy some of the
steeper lapse rates aloft and when combined with improving
effective bulk shear this afternoon, cannot rule out an elevated
marginally severe hail threat with storms north of the boundary.

Timing of this shortwave would suggest it moves east and places
most of our CWA in the more capped and in a region of subsidence
in its wake late this afternoon into early this evening. Timing of
this shortwave is not ideal for surface based development along
the boundary in our southern CWA, as peak heating/max
destabilization will tend to be achieved as the wave is exiting.
There could be a short window early to mid afternoon near the
front where surface based convection could develop before lake
enhanced boundary catches up with synoptic front and shoves the
newly combined lake/synoptic front south undercutting any SB storm
that manages to develop near the boundary. Overall, threat of
storms rooted in the boundary layer with any appreciable wind
damage or tornado threat looks very low as the window of time for
such storms to occur would be very short.

This evening temps/precip:

This evening should start out dry and quite chilly over most of
the CWA with most areas north of I-80 only in the 40s, while front
should be stalled out over our far southern CWA with 70s south of
the front. By late this evening, typical diurnal strengthening of
the low level jet could result in some showers and storms
developing over northern IL in the WAA north of the front. As the
sfc low heads toward WI late tonight, look for the surface front
to jump rapidly northward sending any organized rain/storms north
into WI and sending temps over northern IL rapidly skyrocketing
back into at least the 60s if not near 70 prior to sunrise
Thursday morning.

Overnight/Thursday Temps/precip:

Thunderstorms are likely to develop along a cold front over the
corn belt to our west this evening, but this convection should be
weakening and could completely dissipate before it even reaches
our CWA late tonight/early Thursday morning as we head into the
nocturnal min for cold frontal convection. Cold front will move
across the CWA after sunrise Thursday and maintained some low pops
in case any convection can develop along the front, but at this
point it looks like a better chance of seeing little/no precip
along the front for most of the CWA. Better chance of storms along
the cold front will be over NW IN midday into early afternoon
Thurs as airmass destabilizes ahead of the front. In the wake of
the front, fairly stout shot of cold air advection should result
in another period of atypical diurnal temp trends, with falling
temps behind the front in the afternoon.

- Izzi


246 AM CDT

Thursday night through Tuesday...

Much less to talk about in the long term thankfully. We should
enter an extended period of below average temps accompanied by the
familiar cold winds off the lake. This pattern will start Friday
and probably peak on Saturday as low pressure tracking across the
TN Valley tightening the northeasterly gradient. Friday and
Sunday temps near the lake could approach 50, but on Saturday its
likely temps will only top out in the mid-upper 40s along the
lake. Medium range guidance continues to track that Valley low far
enough south to ensure most of our CWA misses any rain with it.
Temps look to moderate heading into Tuesday as northern stream
shortwave moves into the Upper MS Valley and turns our winds
southerly allowing for quickly moderating temps.

- Izzi


For the 18Z TAFs...

Main concerns: winds through the period, CIGS/VSBY into this
evening and limited threat for additional SHRA/TSRA through the

A cold front has sunk well south of the terminals, which will
result in steady northeast and east winds north of the boundary
through the evening with sustained speeds of around 10-15 kt. CIGS
in the MVFR range are likely much of the time at ORD/MDW/GYY,
though there may be occasional variance. The threat for additional
direct TSRA impacts at the terminals appears low through the early
evening hours, with a slightly higher chance at GYY this
afternoon as an upper disturbance continues to progress east.
Lower VSBY may occur at times as well in the marine influenced air
mass north of the front, though have lower confidence in
prevailing occurrence.

The next upper disturbance of note moving northeast from IA could
result in a threat for SHRA/TSRA being near the RFD area during
the evening and early overnight hours. It appears the best focus
will pass to our north into Wisconsin, so confidence is even lower
in impacts at the eastern terminals overnight. Opted to hold onto
a VCSH and PROB30 for TSRA for now. Low pressure will pass to the
northwest tonight, lifting a warm front north of the area, with
gusty southwest winds in its wake. A cold front will sweep across
the area Thursday morning. Winds will shift west and increase
further to sustained speeds near/around 20 kt and gusts
near/around 30 kt. Cannot rule out another period of MVFR CIGs
right behind the front.



220 PM CDT

Active weather is expected across the Great Lakes as a series of
lows move across the country. Low pressure over the Central Plains
this afternoon will lift to central Lake Michigan Thursday
afternoon. Moderate south flow to around 25 kt will be in place
across the south half of the lake overnight into Thursday while a
window of easterly gales is expected across northern Lake Michigan
north of the low track. West to northwest winds to 30 kt will
overspread the lake behind the departing low Thursday night and
could be near gale at times as colder air overspreads the lake.
Moderate north flow persists Friday into Saturday as a strong high
builds into the Upper Midwest. Winds over northern and central
portions of Lake Michigan should briefly diminish later Saturday
and Saturday night as the ridge axis settles over that area, but
an area of low pressure moving east across Ontario Saturday night
will push a cold front across Lake Michigan Sunday night resulting
in winds picking up once again late in the weekend.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...3 AM Thursday to 11 AM Friday.




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