Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280309

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

909 PM CST

No big changes to going forecast, though did hit thunder harder in
the grids tomorrow night.

A warm front is situated across MO and downstate IL this evening,
poised to move northward tonight as cyclogenesis occurs over the
central plains. Strengthening low level flow will begin to quickly
advect higher theta-e air mass north tonight, with strengthening
isentropic ascent likely to result in some showers and
thunderstorms developing overnight. Guidance in fairly good
agreement on lifting this warm front rapidly north into Wisconsin
Tuesday morning. WAA driven showers and thunderstorms should lift
north out of the area with the warm front, however impressive
elevated mixed layer (EML) will be in place setting the stage for
potential elevated weak convection during the day in the warm
sector. Fairly extensive stratus deck already exists behind the
warm front across the Arklatex and this stratus deck should
rapidly advect north and blanket the area Tuesday.

The presence of stratus should prevent any substantial insolation,
so warming of the boundary layer will be driven largely through
advection. The stratus deck and impressive EML above the stratus
should provide for a formidable capping inversion limiting the
threat of any surface based convection through much of the day.
Secondary warm front/moisture discontinuity boundary is forecast
to develop over northern IL during the afternoon and some of the
convective allowing models (CAMs) do develop convection near this
feature. That scenario is certainly plausible, however the more
substantial threat of threat of surface based convection should be
delayed until very late in the afternoon or more likely during the
evening hours when guidance suggests that strong synoptic ascent
will result in a weakening MLCINH and an increasing chance of sfc
rooted convective development.

Continued transport of unseasonably warm/moist air mass (Tds in
the upper 50s to perhaps lower 60s) beneath seasonably cold air
aloft should result in unusually strong SB based instability for
this time of year. Assuming synoptic ascent is indeed enough to
lift through the low level inversion/inhibition, then potential
will exist for robust boundary layer rooted convection to develop.
Given the very strong shear profiles, if instability gets as
strong as guidance suggests it could, then potential would exist
for sizable severe weather risk. This would certainly be an out of
season event and there are many modes of failure, but there are
indications that we could be entering into a severe weather
parameter space in which significant severe weather occurs. CAMs
are offering a wide variety of solutions with respect to
convectivedevelopment and storm mode, so confidence isn`t
terribly high in how things will unfold. However, many ingredients
do appear to be coming into place for non-trivial severe weather
risk across the CWA, in particular southern CWA where confidence
is highest in instability being sufficient. The primary threat
should be large hail/damaging winds, but very strong low level
shear profiles and low LCLs could support a tornado risk well into
the evening hours if any sustained supercells can develop or with
any well developed QLCS.

Worth stressing that this is far from a high confidence set up,
however there does appear to be a non-trivial threat of
potentially respectable out of season severe weather event. For
this reason alone, heightened awareness of the weather is
encouraged tomorrow. Also, given the majority of the low level
destabilization is expected to be driven by advection rather than
insolation, the severe threat may not wane during the evening
hours, and in fact could increase at night as some models forecast
instability to increase during the evening hours. In summary,
while confidence isn`t yet great, there is some potential for a
night time, out of season severe event tomorrow evening/night.



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 00Z TAFs...

Forecast concerns focus on wind direction at issuance time and the
on 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 day tomorrow.

Broad region of modest south winds have developed across the
region, with a shift to sely with the passage of a weak lake
breeze boundary moving inland. Expect a short period, likely less
than 1 hour where wind speed increases to 8-10 kt and drops off to
6 kt or less.

Focus will then shift to shra/ra/isold tsra chances after
midnight. Strong warm/moist advection will develop aloft
overnight as low pressure 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 should lifts
out to the northeast by 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 by mid-day. Expect some improvement to at least 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 tomorrow evening. Will maintain the
prob30 forecast for the late afternoon and early evening hours,
given some uncertainty in timing of TS impacting the terminals.


337 PM CST

Low pressure over Colorado will reach southern Lake Michigan Tuesday
evening. Southeast winds become south and increase to 15-20 kt ahead
of the low overnight. Wind gusts may increase to near small craft
advisory conditions, but do not have enough confidence to issue a
small craft at this time.  Winds become southwest behind the low
Tuesday night 15-25 kt. While winds may be close to small craft
criteria again, waves will probably be hazardous to small craft
Wednesday. This is especially true for the IN nearshore waters. Low
end gale force gusts are possible across the lake late Tuesday night
into Wednesday morning. Did not have enough confidence to issue a
gale watch at this time.

The low continues northeast, and another weak low passes near or
south of the lake Thursday.  A high pressure ridge passes over the
western Great Lakes Friday, and south winds increase to 30 kt behind
the low Friday night into Saturday. Guidance then differs greatly in
the path and strength of the low early next week so have low
confidence in the forecast that far out.






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