Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272038

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
238 PM CST Mon Feb 27 2017

237 PM CST

Tonight through Tuesday...

Short term forecast concerns focus on timing of shower and
thunderstorm periods late tonight through Tuesday. While storms
tonight will be elevated above the boundary layer, the potential
for strong/severe near-surface based convection ramps up Tuesday
afternoon and evening.

Early afternoon surface analysis depicts low pressure developing
in the lee of the Rockies across northeastern Colorado, and 3-4 mb
surface pressure falls to the east across Nebraska and Kansas.
These pressure falls were occurring downstream of a strong upper
level speed max translating across the central Rockies. The
sheared mid-level short wave associated with this speed max is
progged to move quickly east-northeast to the western Great Lakes
region late tonight, enhancing a broad region of southerly low-
level flow across the Midwest. Low level moisture return is
already underway across the southern Plains, and a developing 45
kt southwesterly low level jet is expected to pull this Gulf
origin moisture northeastward into the forecast area through
Tuesday. Guidance is in good agreement in developing precipitation
across Missouri and Iowa early this evening, and expanding it
northeastward into northwest/north central IL by midnight and
across the remainder of the cwa during the overnight hours.
Forecast soundings indicate weak (generally <200 J/kg) elevated
instability developing as saturation occurs on the nose of the low
level jet, which indicates potential for embedded thunderstorms,
and various convection-allowing guidance appears to support this
as well based on simulated reflectivity data. Mid-level wave moves
through around sunrise, with heavier showers/thunderstorms
lifting northeast of the area around that time with strong mid-
level drying noted in forecast soundings. Low levels remain
saturated however, which may produce a period of lingering light
rain/drizzle through mid-morning.

This dry mid-level air, and a lingering inversion near/north of
the warm front, looks to suppress surface/near-surface based
convection across the area during the morning/early afternoon
hours, though large scale height falls develop from the west
during the mid-late afternoon hours. This works to erode capping
present in the 850-800 mb layer per forecast soundings, with the
approach of another mid-level short wave expected to result in
development of stronger surface/near-surface based storms across
the area by late afternoon. High-res convective guidance depicts a
broken line of strong/potentially severe storms ahead of the cold
front by evening, which then propagate east across the forecast
area Tuesday night. Strong deep-layer wind fields and MUCAPEs
increasing to 1000-1500 J/kg toward evening highlight severe
potential, especially over the southern cwa. Damaging winds, large
hail and an isolated tornado threat are included. SPC has
continued to nudge the Day 2 slight risk northward to about I-80,
with the remainder of northern IL in the marginal risk area.

After a brief evening cool-off, temperatures should become steady
or rise slowly overnight as south-southeast winds push warmer air
northward. Most of cwa should warm to the 60`s Tuesday afternoon,
though far northeast (IL lake shore north of Chicago) could hold
onto a southeast component to the wind and lake cooling.



237 PM CST

Tuesday night through Monday...

Tuesday night through Monday...

The surface low over the forecast area deepens as it continues
northeast. Showers and storms continue Tuesday night, and the low`s
cold front swings through late Tuesday night/early Wednesday
morning. Gusty northwest winds move in behind the front, and
temperatures drop through the day into the 30s north of I-88.
Lingering precip may change over to snow Wednesday afternoon, but
expecting little to no accumulation north of I-88. Areas south of I-
88 will likely be too warm to see all snow.

Cool air aloft continues to funnel in Wednesday night leading to
lows in the mid to upper 20s.  Northwest winds diminish so wind
chills will be around 20 above zero.

High temps will be in the 40s Thursday, and another lobe of upper
level vorticity and a surface low move over the region in the
afternoon. Have medium-high confidence in precip occurring, but
medium-low confidence in precip type. Rain or snow is expected north
of I-88 while all rain is expected south of I-88.  Forecast
soundings have saturation through the DGZ, so not expecting sleet or
freezing rain at this time.  Soundings also feature minor amounts of
CAPE, but not confident enough to include thunderstorms in the

High pressure builds over the Mississippi River as the low heads
east. Lake effect snow may persist through Thursday night for NW
Indiana.  Gusty winds become northwest, and lows will be in the low
20s.  Wind chills will be in the low teens to possibly upper single
digits above zero.

The next vort streamer passes over the region Friday night. Guidance
varies on how far south the precip will get so kept chances of
precip along the IL/WI state line.  Gusty southwest winds move in
behind the vort streamer. Warm air also moves in leading to highs in
the 50s Saturday.  Guidance then differs greatly with the low early
next week. The ECMWF features a deep low moving over southern WI
Monday, while the GFS has an elongated trough moving over the lake
Monday night. The surface low stays over south central Canada. Both
models feature precip so kept high end chances for rain.  Above
normal temperatures also look likely early next week.



For the 18Z TAFs...

Forecast concerns focus primarily in timing a period of shra/tsra
into the terminals toward/after midnight tonight through the pre-
dawn hours of Tuesday morning. Also of concern is the potential
for a period of IFR cigs during the morning as a warm front lifts
across the area, and expected redevelopment of tsra late in the

Broad region of modest south winds have developed across the
region this morning, as high pressure shifts off to the east.
Surface analysis depicts a subtle warm front lifting across
northern IL/northwest IN with some patchy sct-bkn cu developing
just south of the terminals at mid-day, though expect
raising/scattering of this over the next few hours. Farther west,
surface low pressure will develop in the lee of the Rockies this
afternoon, and this will help back winds more southeasterly by
evening. Strong warm/moist advection will develop aloft overnight
as this low moves east across the Plains, and a 45 kt low-level
jet develops. Showers/thunderstorms are expected to blossom west
of the Mississippi River this evening, spreading east across the
terminals overnight. Forecast soundings depict modest elevated
instability developing after midnight as well, with the potential
for at least isolated embedded thunder and possibly greater TS
coverage. This first wave of shra/tsra lifts out to the northeast
around sunrise with mid-levels drying behind it. Some patchy light
rain or drizzle may linger, possibly with a period of IFR cigs,
until a more distinct warm front lifts across the area through
mid-day. Some guidance indicates IFR cigs persisting through a
good portion of the day, though would expect some improvement at
least to MVFR after passage of warm front. High-res convection
allowing models then depict redevelopment of shra/tsra during the
late afternoon hours, with the greatest concentration along a cold
front which will push across the terminals Tuesday evening. Have
indicated a prob30 late for ORD`s 30 hour TAF, though tsra will be
fairly likely just beyond that time period.



225 AM CST

Much of the lake has observed varying winds for the last several
hours as high pressure is situated to the south/southeast, and
while low pressure moves east through southern Canada. Over the
north half, a southward moving front has turned winds to the
northwest. With this northwest wind, colder air has allowed some
slightly stronger winds to 30 KT to be observed. Over the south
half, southwest winds and slightly more stable conditions across
the lake have kept speeds more in the 10 to 20 KT range. As both
of these surface features move further away from the lake, expect
the gradient and winds to further diminish over the entire lake
through early this morning. However, gradient becomes more
organized by midday with winds turning more southerly and
increasing back to 10 to 20 KT. Southerly winds then expected
tonight into Tuesday, as developing low pressure over the central
Plains moves towards the lake. Speeds will likely further increase
to 30 KT across the open waters Tuesday, while winds of 15 to 25
KT are observed over the nearshore. At this time, conditions
hazardous for small craft are not expected. However, speeds could
trend slightly higher for this period, and will continue to
monitor this period. This low is then expected to move across the
lake Tuesday night, with a period of stronger northerly winds
likely Wednesday into Wednesday night.






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