Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 301750

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT Thu Mar 30 2017

1250 PM CDT

Continue to monitor for the threat for strong to severe
thunderstorms south of I-80 later this afternoon, and particularly
south of the Kankakee River. The convection currently lifting
north-northeast is associated with a short-wave disturbance ahead
of the closed mid-upper low over Missouri. Given strong moist
transport flowing over warm front and PWAT values around or over
1", main threat next few hours is heavy rainfall and localized
ponding/flooding on some roads. This is especially the case with
saturated grounds due to what already fall overnight. Will be
monitoring for potential short-fused Flood Advisory issuance.
Otheriwse, can`t rule out some small hail to pea or dime size
(especially south of I-80) for stronger cores with cold
temperatures aloft/700 mb temps near or below 0 Celsius resulting
in fairly low freezing levels and wet bulb zero heights.

The main risk for strong to severe thunderstorms looks to evolve
over or just south of our southern CWA counties this afternoon,
particularly south of the Kankakee River. Good speed shear will be
in place with strong lower and mid-level flow, but directional
shear from 850-500 limited given meridional nature of closed low.
As low pressure just north of STL lifts northeast this afternoon,
surface warm front will continue to progress north and likely
reach areas south of the Kankakee River, namely portions of Ford,
Iroquois, Benton, and Japser and Newton Counties.

Temperatures in the 60s and dew points approaching 60 topped by
relatively steep lapse rates could yield 500-1000 j/kg of SBCAPE
near and south of warm front. In presence of supportive bulk
shear, low topped supercell structures are favored. With earlier
mentioned parameters in favor of hail, threat for marginally
severe hail could extend northward to near I-80 with elevated
MUCAPE north of surface warm front. Where warm sector reaches,
anticipating backed (southeast/east-southeast) surface winds to
result in decent veering in lowest 1km, which could be enough low-
level shear/SRH for an isolated tornado risk. In addition,
downdrafts from strongest warm sector storms would pose a risk for
damaging winds. Will continue to monitor observational trends



320 AM CDT

Through tonight...

Main forecast concerns/challenges will remain with continued
shower and thunderstorm chances this morning through early this
evening, with strong to severe storms still appearing highly
probable this afternoon thru early evening south of I-80. Although
the highest threat for severe weather will be for areas southeast
of a Pontiac to Kankakee to Valparaiso IN line, can`t rule out
isolated stronger storms further to the north across the Chicago
metro area.

Large upper level system continues to slowly progress east
through the central Plains at this time. With this in place,
continued stream of shortwave energy is lifting northeast over the
region. This support along with strong/focused isentropic ascent
over northern Illinois has allowed the scattered showers to become
more widespread early this morning. Instability has already
worked its way north across the area with MUCAPE in upwards of 500
J/KG already situated over the southern portions of the CWA. This
has allowed scattered thunderstorms to occur over much of the
CWA. With this continued forcing and instability in place, expect
the more widespread nature of this precip and thunder chances to
continue through much of early this morning. Focused ascent then
lifts north closer to mid morning, however, given large scale lift
still present, most locations will likely still observe isolated
to scattered showers. Thunder threat may also lower for the time
being but instability remains in place so can`t still rule out
isolated thunder. As PWAT axis of an inch or more continues to
slide overhead, periods of heavy rainfall will remain a
possibility anytime this morning. Don`t anticipate any widespread
severe weather through anytime this morning, but with instability
likely to continue to increase and with high potential for
stronger forcing in place, can`t rule out an isolated stronger
storm and small hail.

Any lull with any precip looks to be short lived later this
morning, as additional upstream energy along with lifting surface
reflection and boundary support continued precip development to
lift north. This should bring another period of showers and
thunderstorms to much of northern Illinois and northwest Indiana
by late morning into midday. By early afternoon, guidance
indicating a substantial increase in instability over the area,
with instability axis situated from east central Illinois and
northwest Indiana up into the southern part of the lake. Its
during the early to mid afternoon time frame that I think hail
could be possible across the eastern two thirds of the CWA, mainly
areas east/southeast of a Peru to Chicago line. Confidence to the
extent of this hail is lower, but guidance indicating the
possibility for very steep mid level lapse rates to be in place
during this time and think the threat of hail remains. Any surface
based instability should still be lacking, so a wind threat at
this time would be low to nonexistent. However, any storms will
continue to have a chance to produce heavy rainfall.

As we move forward into the mid to late afternoon time frame, 21-
01Z, do think an increased threat for severe weather will occur.
At this time, this threat appears to be for areas southeast of a
Pontiac to Kankakee to Valparaiso IN line. During this time
surface low and boundary really lift north into the southern
portions of the CWA. Guidance indicating the possibility for any
capping inversion to erode, and surface/ML instability to increase
for these areas. This will present a damaging wind threat along
with a hail threat for any storms during this time. As renewed
forcing arrives over the region with the upper level low finally
swinging through, should see scattered storms fire up across
central Illinois. Steering flow will be to the east northeast,
with these storms possibly getting into this area anytime around
21z or there after. Guidance is still varying to the northern
extent of the strongest development, with the higher threat of
more widespread severe still just to the south of the CWA.
However, with this area getting into the warm sector, surface
based instability present, and with bulk shear averaging out
around 35-45 KT, still think there is a threat for a period of
more organized development this afternoon into the early evening.
Once again, hail and damaging winds are the main threats. Some
uncertainty with the mode of convection though, but with the
expected bulk shear, cannot rule out the potential for some
isolated supercellular structures. If this scenario were to occur,
an isolated tornado threat could briefly present itself. Will
continue to monitor this potential, as well as the potential for a
slightly northward shift to the threat area this afternoon. Any
threat of severe weather should come down by early this evening.



320 AM CDT

Friday through Wednesday...

Upper level low and surface reflection expected to still slowly
move east through the region on Friday and with these features
still in place, chances for showers will continue. These chances
should diminish by the afternoon, with the final departure of this
system. A period of dry weather appearing likely going into
Saturday as surface high moves overhead. A low chance for some
light precip late Saturday night, but higher chances for showers
don`t return until Sunday and Sunday night. Confidence higher for
the precip potential but do have lower confidence to the extent of
it. Guidance looks to be struggling with handling northern and
southern stream waves across the central CONUS. Will keep an eye
on this period as one solution could provide some chances for
thunder. Similar pattern continues going into next week with
another brief lull in the activity expected, and then with chances
for precip once again returning by midweek. Still aways out, but
guidance fairly consistent with bringing a large upper level
trough and surface low across the region during that time. Details
to fall out later, but setup would favor thunder, and so have
continued thunder message for Wednesday.



For the 12Z TAFs...

Showers will continue on and off throughout the day today across
northern Illinois and northwest Indiana, and there could be a few
embedded thunderstorms at times. Area of low pressure near Kansas
City early this morning will move ENE across central Illinois this
afternoon and evening and across northern Indiana overnight. Showers
will continue throughout much of the day out ahead of the low.
Modest instability will overspread the terminals by early this
afternoon and remain in place into the evening and could result in a
few embedded thunderstorms. Continue to think the best chance at the
terminals will be early to mid this afternoon as some of the better
forcing moves across the region, and this timeframe is highlighted
with a tempo for TSRA, but cannot rule out isolated thunderstorms
later into the day. With the low passing south of the terminals
this evening, winds will back to the north and eventually north-
northwest and we will begin to lose some of the deeper moisture
resulting in precipitation tapering to drizzle. Ceilings are
already IFR over most of the area early this morning, and outside
of some variability in the near term, mainly at MDW/GYY where VFR
level ceilings are lingering, expect IFR to prevail throughout the
forecast period. LIFR will become increasingly likely this
evening and overnight.



307 AM CDT

Low pressure will lift from the Central Plains early this morning to
the eastern Great Lakes Friday. Moderately strong easterly flow to
around 30 kt is in place ahead of the low this morning with a few
gale force gusts possible mainly across the south half of the
lake possible through the morning hours. Winds will gradually back
to the north Friday as the low passes south of the lake. Winds
should diminish over the weekend as high pressure builds across
the region. Another low is expected to move from the Central
Plains to the Ohio Valley early next week which will result in
another period of moderate flow, easterly late Sunday backing to
the north Monday into Tuesday.



LM...Small Craft Advisory...Winthrop Harbor to Burns Harbor until
     4 AM Saturday.

     Small Craft Advisory...Burns Harbor to Michigan City IN until 3 PM Thursday.




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