Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 271746

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1146 AM CST Mon Feb 27 2017

341 AM CST

Through tonight...

After a quiet day and early evening, expecting showers and some
thunderstorms to develop across the region late this evening into
overnight. The severe weather threat though is low.

The early morning satellite water vapor imagery depicts an upper
level short wave traversing northeast over the Ohio River Valley,
with some of its cloud cover over the southeast forecast area.
This wave will depart east by later this morning though some
patches of clouds may continue today as a generally broad lower to
mid-level tropospheric warm advection regime encompasses the
region. Based on upstream temperatures yesterday under 925mb
temperatures of 3-4C, expecting highs in the lower 50s across most
of the area, although this could be modulated by clouds.

Upper level west-southwest flow increases sharply tonight as a
western U.S. longwave shifts eastward. An associated low
amplitude short wave is expected to eject northeast with a low-
level jet of 40-45 kt below this pointing into the eastern
Iowa/northern Illinois/southern Wisconsin region. Increasing
moisture transport and gradually steepening mid-level lapse rates
within this forcing for ascent should allow for showers to
increase in coverage fairly quickly between 10 pm and 2 am given a
consensus of convective-allowing model solutions. The northwest
half of the area is more favored for coverage, as well as the
isolated to scattered thunder. The magnitude of instability, as
well as relative to the forecast -10C to -30C range does not look
supportive of a noteworthy hail risk.



341 AM CST

Tuesday through Sunday...

The base of the western U.S. long wave trough will shift eastward
fairly quickly during Tuesday and Tuesday night with a strong 140
kt upper level jet. This jet looks to impinge over the mid-
Mississippi Valley into the forecast area late in the day into the
evening Tuesday. At the surface, guidance still shows variance
with how the upper and mid-level energy translates to a surface

There will very likely be two surface lows tracking northeast,
one earlier in the day Tuesday into Wisconsin, and then a second
one with the main upper jet sometime Tuesday evening/night over
the forecast area. The first of these will be weaker but should at
least steer surface warm advection into the area with surface
temperatures probably climbing into the upper 50s to lower 60s
even with anticipated cloud cover. While winds presently look to
be offshore in northeast Illinois, cannot rule out some onshore
component given uncertainty in the pressure pattern and strength
of the south-southwest winds. A secondary warm front with the
second low, more of an effective warm front with higher dew point
air (possibly at or above 60F), is favored on a blend of guidance
to be entering the southern forecast area late in the day, though
again there is fairly high spread on this, with the NAM further
north in its 00Z and 06Z solutions, and the GFS more southeast
with its 00Z.

Ingredients are there for likely thunderstorms late in the day
Tuesday or even more so Tuesday evening. Depending on how far
north the higher theta-e air reaches should dictate how far north
the potential for severe thunderstorms is. Potent dynamics
combined with MUCAPE values of 700-1500 J/kg warrant concerns for
large hail and possibly a wind threat closer and south of the
effective boundary. If surface to mixed-layer CAPE can return far
enough north, then a higher potential for a larger range of severe
weather types will be in play in the forecast area. One concern
if that will happen is what could be a thick cirrus shield during
the day Tuesday. Both NAM and GFS soundings show fairly solid
moisture from 20-30kft streaming over from the southwest. Again
that would likely have little to no effect on some convection
rooted above the boundary layer, but could keep the surface-based
threats south of I-80 or even just south of the CWA. Dynamics with
70 kt of 0-6 km shear and strong low-level shear generated by
winds just off the deck warrant plenty of attention on this time
period, as this could present a decent early season severe weather

As is typical with a strong synoptic system this time of year, the
backside will bring some stout cold advection and the potential
for snow, although the system`s fairly quick progression continues
to somewhat limit the snowfall accumulation potential.
Temperatures are likely to be dropping quickly Wednesday morning
given the forecast northerly winds behind what will be a sub-1000
mb surface low. Mid-level frontogenesis along the tightening
baroclinic zone aloft, as well as at least a transient area of
deformation, should provide precipitation across part of the area
Wednesday morning and possibly into the afternoon. Precipitation
type would favor a rain/snow line shifting southeast during the
day Wednesday. While confidence in the cold advection pattern
Wednesday is high, it is much lower in if and how much of the area
will be favored for a period of light to possibly moderate snow.

Northwest flow aloft looks to prevail Thursday and Friday with at
least one, possibly two, low amplitude waves forecast to pass
near the area by global guidance on Thursday and Thursday night.
These could bring some light rain/snow chances, likely to trend
toward light snow if they do produce a swath of precipitation. The
cold advection pattern strengthens into Thursday night and the
more northerly low-level wind field could bring a short lived
period of lake effect snow into Porter County and possibly Lake
County Indiana.

Beyond, the pattern looks to warm into the weekend with 850mb
temperatures forecast into the double digits on Sunday, before a
potentially strong system passes over the region to start next



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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