Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 282149

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

349 PM CST

Convection has been developing and steadily intensifying over
eastern IA and northwest IL. Both ILX and DVN afternoon special
soundings indicated a formidable capping inversion. Continued low
level theta-e advection combined with increasing ascent has been
gradually eroding the cap and given intensity trends, its likely
storms are solidly surface based.

Over our western CWA, surface obs show slightly cooler temps and
slightly lower dewpoints, so current environment is possibly a bit
less favorable for surface based storms now. Short range guidance
continues to forecast environment to quickly improve and will
likely grow increasingly favorable for maintaining storms over
western IL as they move east. Given their fast movement, it is
possible they could eventually outrun the better air as they get
east into NE IL, but that is still a couple hours out and we`ll
need to closely monitor.

Given the favorable instability and extremely strong deep layer
shear profiles would anticipate a continuation of the transition
of this activity to supercellular. Strongest low level shear
profile is over central and north central IL, with the current
best co-location of most favorable low level shear and instability
over north central IL, mainly south of I-88. This would suggest
that in the next couple hours the highest tornado threat would
exist with the storms developing from Bureau Co southwest to near
Burlington, with the highest short term threat likely to spread
into LaSalle and Livingston Counties.

Large hail, damaging wind, and perhaps a tornado risk cannot be
ruled out north of this area into Ogle and Lee Counties in the
next couple hours, but the greatest area of concern appears to be
LaSalle and into Livingston Counties. Extrapolation of storms
moving into Bureau County suggest the threat should move into
LaSalle County toward 430 PM.



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 18Z TAFs...

IFR cigs spread over the region as a warm front lifted north
through the region this morning. South winds will continue to
gust arnd 20 kt into this evening. Scattered storms are expected
to form around 22Z this afternoon with periods of thunderstorms
through most of the night. Have high confidence in storms and
lightning impacting the terminals, and there may be multiple
periods of storms. However, have medium confidence in timing and
exact location of storms. Some of these storms will be strong and
possibly severe. The main threats would be large hail and damaging
winds, but there is a chance of tornadoes.

Decided to keep cigs in the IFR to MVFR range throughout the
forecast since almost all sites in the region are MVFR or lower.
Some guidance members suggest cigs could go VFR late this evening
before the cold front arrives, but do not have enough confidence
to go VFR at this time. The cold front moves through after
midnight with winds becoming northwest and cigs lowering to IFR.
There should be a brief break in precip behind the cold front but
then more precipitation is expected Wednesday morning. The precip
should start as rain, and then transition to snow as temperatures
fall. Have low confidence in when rain will become snow so only
have -RASN in the TAFs.



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.




WWW.YOUTUBE.COM/NWSCHICAGO is the U.S. government's official web portal to all federal, state and local government web resources and services.