Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010532

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1132 PM CST Tue Feb 28 2017

.UPDATE...720 PM

Looking ahead for later evening into overnight...

Upper jet of 145-160 kt as captured by the 00Z RAOB network is
punching northeastward into Missouri and western/central Illinois
through the next few hours. Convection has started to blossom
with this forcing for ascent ahead of the surface cold front in
Missouri, and expect growth of supercell/multicell structures
further northeast ahead of the front into western Illinois during
the next 2-3 hours. Deep layer shear at or in excess of 60 kt
favors these storms to organize, and the slightly more aligned
850-500mb flow by late evening would favor some propensity to
organize into clusters or a quasi-liner structure. Convective-
allowing guidance generally supports this, passing organized
storms across the southern forecast area between 10 pm and 2 am.

The southeast forecast area...generally along and along and south
of a Peru to Gary line...has been more untapped. Surface
observations in this area indicate continued gusting southerly
winds ahead of the 998 mb surface low near the Quad Cities, and
RAP analysis indicate the lowest 100mb of moisture is still high
in this area. This indicates that CAPE from the boundary layer, or
just above it, is present, with the 00Z ILX sounding indicating
near 1,300 J/kg of CAPE for a modified sounding near Pontiac. So a
tornado threat will be highest in this area south of a Peru to
Gray line, especially south of any outflow boundaries from
ongoing activity. Cannot rule out a threat further north given the
surface low placement as far north as it is and if the boundary
does shift further north in the wake of ongoing convection (if it
weakens more). However, the 00Z DVN sounding and RAP soundings
going forward indicate instability to be mainly rooted aloft
further north. Certainly enough for hail, but possibly shrinking
the tornado threat further north.




The earlier MCV has weakened and now moving over the Chicago
area, with stronger wind signatures having weakened into Cook and
Will Counties. The overall morphology of the storm line from the
south metro southward to northern Ford County has been slightly
downward, though there was a mesonet site in eastern Grundy County
that reported a 49 mph gust about 20 minutes ago. These gusts are
likely in fingers/pockets, and not widespread along that area.

Further southwest, along and now just east of I-55 in Livingston
County, the nature of the storms is a bit more robust, namely in
the form of deeper cores. The low-level air mass continues to
support near surface buoyancy on RAP soundings in the far southern
CWA ahead of these storms, with instability just off the deck of
over 1,000 J/kg. So certainly a hail threat, and some wind and
isolated tornado potential with any QLCS-like structure, though
the storms are becoming less aligned perpendicularly with the 0-3
km shear vector and this may indicate a lowering threat of
embedded tornadoes. Nonetheless, will be watching closely
along/south of U.S. Highway 24.

Further northwest, part of the Tornado Watch will be cancelled
shortly as parameter space for severe weather shifts east. The
rest of the watch will likely be cancelled prior to end time as
the storms move out.



218 PM CST

Through Wednesday...

Isolated thunderstorms are starting to fire, which is a little ahead
of schedule, but the latest RAP analysis features upwards of 1000-
1500 J/kg of CAPE along the MS River.  CAPE values will continue to
climb through this evening to around 1500 J/kg. RAP analysis already
features of 60 kt of 0-6km shear and 25-35 kt of 0-1km shear.
Expecting scattered storms to continue to form this afternoon mostly
along the MS River and then shift eastward late this afternoon.
Steep upper level lapse rates will allow for hail. The strongest
storms will be capable of producing golfball size hail or larger
due to strong, possibly rotating updrafts.

The big question is where exactly will the low over Iowa track?
Current analysis is further north than most guidance members so I`m
leaning toward a more northerly path. That should translate in
increased shear and an even better chance of severe weather. Storms
should increase in coverage as the upper level vort streamer pushes
over the region late this afternoon into this evening. CAMs have
been pretty consistent as far as how the storms will evolve, but
still only have medium confidence in timing and mode of storms.
Storms through this evening will have mainly a hail risk as
mentioned above. Tornadoes will be possible but most likely tied to
discrete supercells. Supercells are possible given steep lapse
rates, shear, and CAPE values, but unsure about how many will be
able to form. Low level wind shear increases as the low nears
this evening and near the warm front which is just north of the
IL/WI state line.

Storm mode should change to more linear structures nearing midnight
as the cold front begins to approach the region.  A strong low level
jet should keep the low levels well mixed allowing for damaging
winds at the surface.

If storms train, flooding will be possible. PWAT values are about
0.8-1 inch, but storms will be moving toward the northeast at 60-65
MPH, so individual storms will be moving very quickly.

Overall, the message is about the same as this morning. Scattered
storms will develop this afternoon and increase in coverage and
intensity this evening.  All severe weather hazards are possible.
Severe storms come to an end late tonight as the cold front moves

Gusty northwest winds are expected tomorrow with gusts of 30-35 MPH.
Temperatures will be falling through the day so daily highs will
likely happen early in the morning. Temperatures fall into the 30s
by late Wednesday afternoon. Precip on the backside of the low
spreads over the region and should start as rain. Expecting rain
to change over to snow as temperatures drop through the profile.
Only expecting accumulating snow along the IL/WI state line where
up to an inch is possible through Wednesday afternoon. Given
recent warmth and rain, only expecting snow to stick to grassy



156 PM CST

Wednesday night through Tuesday...

Deepening low pressure will be pulling away from the region
Wednesday evening, with the tail of the mid-level deformation zone
transiting the far east/northeastern parts of the forecast area
early. Lingering precipitation is expected to be primarily snow
based on thermal profiles, though there may be some rain/snow mix
early in the evening across the far east/southeast counties where
surface wet-bulb temps start out above freezing and 1000-850 mb
thickness values are just under 1300 meters. Forecast soundings
and time-height sections depict mid-level drying occurring from
the northwest during the late evening and overnight hours,
depicting an end to any significant precip generally by midnight.
QPF from various guidance is generally less than 0.05" (though the
NAM is a little more excited over northwest IN), which combined
with snow to liquid ratios approaching 12-15:1 would yield a
couple of tenths to perhaps a little under an inch of additional
snow accumulation with the highest additional amounts over
northwest Indiana. Otherwise, blustery northwest winds 25-35 mph
early in the evening will gradually diminish overnight as the low
moves away and the pressure gradient relaxes. Overnight lows fall
into the 20`s, with partly to mostly cloudy skies.

Attention the turns quickly to a clipper system digging southeast
from the Dakotas Thursday morning. Guidance has been pretty
consistent with this feature, which tracks southeast across
northern IL and into northern/central IN Thursday afternoon and
evening. NAM appears a bit too far north with the track of the
surface low, based on the path of the vort max, and is thus
appears to be too warm in low levels and a little slow with the
arrival of precip. Prefer a blend of GFS/ECMWF with respect to
track, low level temperatures and with the QPF axis across the
northeast third of the cwa. This blend also maintains coldest
temps/thicknesses across that area, with areas farther south more
likely to see a mix of rain/snow or all rain while the far
north/northeast tier of counties remain primarily all snow, with
a quick accumulation of up to an inch near the IL/WI border by
early evening. This could have some impact on the afternoon rush
hour based on current expected timing. The short wave associated
with this system continues to dig southeast into the mid-upper
Ohio Valley Thursday night, aiding in amplification of the larger
scale upper trough across the eastern CONUS.

As the trough to our east deepens and moves away late Thursday
night and Friday, upper ridging develops across the Mississippi
Valley, with surface high pressure moving across the forecast
area. Dry weather is short lived however, with global guidance
bringing another quick-moving mid-level short wave across the
area Friday night into early Saturday and producing another
period of light snow which may initially mix with rain early
across our southern counties. Upper ridging then strengthens
across the area into the weekend as upper troughing moves ashore
out west. This portends a warm-up for the Midwest as southerly low
level flow becomes established, with temperatures in the 50`s and
60`s over the weekend and into early next week. Some differences
become evident in the global guidance by days 6-7, though it
appears rain chances (and potentially thunder chances too) will
ramp up by that time as the deepening upper trough moves out
across the region.



For the 06Z TAFs...

Thunderstorm concerns will diminish in the coming hours, though this
organized sw-ne oriented line of storms will move through
northwest Indiana through the overnight hours as the main upper
level lift shifts the focus east and southward. There will be some
rain and infrequent embedded thunder behind it. The cold front
will act as the the next source for shower and embedded thunder,
but this would likely impact areas south of the Chicago terminals.
This next batch would occur in the 9z-12z time frame.

Concern will shift to low ceilings and reduced visibility behind
the cold front currently over southwest Wisconsin. Expect somewhat
light and variable winds, anywhere from se to nw in the next few
hours as low pressure will move over the terminals through 9z- 10z
or so. Winds will generally shift to west and then northwest on
Wednesday. Low dewpoint depresssions behind the front and the nw
winds would tend to suggest the low cigs and vsbys should filter
in later tonight. Confidence on IFR is high, though confidence on
LIFR is not as high, but it is certainly possible. Timing is
medium confidence.

The actual front will arrive in the morning with some chance for
showers, and then the upper trough will shift through in the
afternoon with additional chances for showers in the afternoon. Low
level mixing should promte ceilings to lift to MVFR and possibly
VFR. Thermal profiles in the afternoon tend toward a rain snow mix
in the mid afternoon north of Chicago, then snow dominating late
afternoon even near Chicago. Shower chances will be diminishing as
snow becomes more favored such that accums south of I-88 would be
limited. Expect the gusty NW winds to continue and likely increase
in the afternoon as strong pressure rises occur into the early
evening hours.



317 PM CST

Headlines...Upgraded the gale watch to a warning for early Wednesday
morning through late Wednesday night. Made no changes to the current
small craft advisory, but added another small craft advisory beginning
mid Wednesday morning and going into Thursday morning.

The low over Iowa will pass over the southern end of the lake
tonight. Winds are below small craft advisory criteria, but gusts
up to small craft criteria are possible so will keep the small
craft going. The low continues northeast tonight through Wednesday
and winds become north to northwest behind the low. Gale force
winds are expected early Wednesday morning through Wednesday
evening. Thinking gales will not be observed in the nearshore
waters. Northwest winds decrease through Thursday and another
weak low passes south of the lake Thursday evening. High pressure
moves over the lake Friday and continues east over the weekend.
Winds become south behind the high Saturday. A surface low travels
east over southern Canada early next week and then another
surface low is progged to form over the plains Sunday night or



LM...Gale Warning...LMZ777-LMZ779...9 AM Wednesday TO MIDNIGHT

     Small Craft Advisory...NEARSHORE WATERS
     UNTIL 4 AM Wednesday.

     Small Craft Advisory...LMZ744-LMZ745...10 AM Wednesday TO 10 AM

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...10 AM
     Wednesday TO 4 AM Thursday.




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