Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 201740
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 PM CDT Mon Mar 20 2017
1148 AM CDT
Continue to monitor an area of strong to severe thunderstorms over
east central Illinois and northwest Indiana shifting to the east
southeast. At this time, the focus for the strongest
thunderstorms will be for areas southeast of a Pontiac IL to
Wheatfield IN line. This is where strongest ascent owing to
passing shortwave energy, and lower level/surface convergence is
situated. Stronger instability owing to rather steep mid level
lapse rates is still present, aiding in a continued severe threat
over the next 1-2 hours. Instability remains elevated, without any
surface/ML/0-3KM CAPE present. With this setup, large hail will
continue to be the primary threat. Expect the thunderstorm threat
to diminish by mid afternoon.
312 AM CDT
Abbreviated discussion this morning due to ongoing strong to
Ongoing nocturnal convection expected to continue pushing
southeast with weakening trend expected toward sunrise as storms
move out of the greater instability and LLJ begins to veer/weaken.
There is a pretty good signal with the various CAMS that we will
see convection re-develop near the cold front later this morning
or early this afternoon, primarily over our eastern CWA.
Instability will be weaker and not expecting any appreciable
severe threat today after the nocturnal hail threat subsides in
the next couple hours.
Winds will shift to the north behind the front, except northeast
near the lake. Temps will slowly fall behind the front, except
near the lake where a more rapid and significant drop into the 30s
is expected this afternoon. Guidance had been previously pretty
gung-ho on low clouds behind the front today, though more recent
runs have backed off somewhat. Not seeing much of anything
upstream yet, so confidence is low, but increasingly looking like
we could see more sun than previously expected today.
312 AM CDT
Tuesday through Sunday...
Secondary push of cold air expected Tuesday afternoon. Earlier
guidance had been hitting a band of post frontal f-gen driven
precip across our southern CWA, though latest model runs have all
trended dry or at least much drier Tuesday afternoon. Given the
unanimous signal in the guidance, have lowered pops for Tuesday
afternoon. Could see some lake effect clouds and perhaps a spit or
two of lake effect snow grains later Tuesday night over NE IL near
the lake. Chances of anything measurable look close to zero, so
just planning to beef up sky cover a bit in the grids Tues night
into early Wed, but no pops or precip mention planned ATTM.
Wednesday should be a blustery and fairly cold day by late March
standards with a stiff northeast wind off the lake and temps
likely not reach 40 in many areas, especially near the lake.
After a quick hit of cold air Wednesday upper ridging quickly
builds east Thursday as strong upper low plows into the southwest
CONUS. Temps will begin to rebound on Thursday, but the more
significant warming will take place Thursday night as a warm front
moves across the region likely resulting in rising temps Thursday
night. Strong low level theta-e advection should promote shower
and thunderstorm development Thursday night into early Friday
morning with the warm front. As it looks now we should get
solidly into the warm sector Friday with highs likely to climb
well into the 60s, with 70F not out of the question depending on
extent of cloud cover in the warm sector.
Shower and thunderstorm chances will ramp up again Friday night
with at least showers remaining possible into the weekend
depending on how progressive this system ends up being. Still some
sizable differences between the medium range models in the
evolution of this upper low, so confidence is low. It does on the
latest runs at least we could transition into a strong NE low
level flow Saturday, which would be raw and fairly cold, whether
or not precip continues into the weekend.
For the 18Z TAFs...
1240 pm...Main forecast concern are the winds which have become
light and variable at many locations...with light southerly at
mdw/gyy. Over the next 1-2 hours winds are expected to shift to
the north/northeast with speeds under 10kts...but these speeds are
expected to increase to 10-15kt during the mid/late afternoon and
some gusts are possible. Winds are expected to remain mainly
northerly tonight...possibly shifting north/northwest Tuesday
morning and then turning back to the northeast Tuesday afternoon
with speeds 10-15kts and some gusts possible Tuesday afternoon.
Mvfr cigs will continue this afternoon and begin to scatter and
lift mid/late this afternoon. Current timing to scatter out mvfr
cigs may be a little too early and trends will need to be
Bulk of the showers/thunderstorms will remain well south of the
tafs this afternoon...though short term guidance does suggest some
shower activity is still possible late this afternoon across
northwest Indiana as winds shift northerly. Confidence is fairly
low at this time...but trends will need to be monitored for shower
potential this afternoon for gyy. cms
227 AM CDT
A cold front will shift southward over the lake through the
morning. Once this front front passes, expect a prolonged period
of northerly flow over the lake through Wednesday evening.
Initially, the northerly winds will be up around 20 kt later today
into early Tuesday. However, a stronger surge of northerly winds
up around 30 KT is expected to develop over the lake Tuesday and
Tuesday night in response to a strong surface high building
southeastward over south central Canada. This period of stronger
winds will likely result in hazardous conditions to small craft
over the southern Lake Michigan near shore waters late Tuesday
through Wednesday morning.
Once this surface high shifts over the eastern lakes by early
Thursday morning, the winds over the lake will shift southward
for a period again during the day Thursday. However, another cold
front is likely to shift down the lake Friday night into early
Saturday. Following this frontal passage, another potent surface
high is likely to build over the Upper Great Lakes and southern
Canada as a storm system tracks over the southern lakes region.
These two pressure patterns could result in a period of strong
northeasterly winds again on the lake for a period this weekend.
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