Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191624

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1124 AM CDT Sun Mar 19 2017

1121 AM CDT

Some lower clouds will hang on near the lake and extending from
northwest Indiana into eastern Illinois where some low level
moisture lingers and southeast winds advect this northwestward.
This along with the cooler east flow will keep these areas a tad
cooler today.

Otherwise, latest guidance suggest the conditional/marginal risk
for some hail overnight will be focused in our southwest on the
nose of the low level jet and better instability axis. Latest EHWO
graphics will be updated shortly.



330 AM CDT

Through Monday...

The main concerns during the period will be the chances for
thunderstorms over the area tonight into Monday, with a few
storms even capable of producing marginally severe hail.

For today, surface high pressure will shift east of the area
today allowing the low level flow to turn southerly. This should
result in a warmer day across the area, with the exception of
areas along the lake shore, where an onshore east-southeasterly
flow is likely to persist. Overall, expect highs of 50 to 55
degrees inland, and 40s near the lake.

An amplified upper level ridge axis is expected to shift eastward
into the area later today. However, as this occurs, a strong
northern stream storm system shifting eastward across Canada will
deamplify the ridge axis by tonight. This will process will result
in strong westerly upper level flow across the northern 1/3rd of
the central CONUS atop the upper ridge over the southern states.
Within this pattern, a mid-level short wave impulse is likely to
shift eastward across the Midwest tonight into early Monday. As
this impulse approaches tonight, it will help focus the nose of a
40+Kt low-level jet into eastern Iowa and northwestern Illinois
overnight. The associated warm and moistening conditions will
also occur beneath a region of steepening mid-level lapse rates,
and will likely set the stage for a period of thunderstorms over
the area late tonight into Monday. MUCAPE values of up to 1,000
J/KG are expected to develop over the area, and given the stronger
westerly flow aloft, some of these storms could result in a
marginally severe hail risk, especially given the likely presence
of steepening mid-level lapse rates. At the present, it appears
the highest threat for any hail producing storms would mainly be
confined around the I80 corridor and points south.

Showers and thunderstorms are likely to linger into Monday as a
cold front begins to move into northern Illinois. This cold front
will push over northern Illinois during the morning, then over my
southern counties during the afternoon. The threat for
thunderstorms will shift southward with the frontal boundary.
While conditions are likely to start out on the mild side on
Monday morning, temperatures are likely to drop during the
afternoon. This will be especially true over northeastern
Illinois, where cold flow off the lake will allow the colder air
to gradually ooze inland through the afternoon.



345 AM CDT

Tuesday through Saturday...

Colder conditions are in store for the area for the middle of the
week as the strong storm system shifting eastward over Canada
begins to dig a trough over the eastern lakes region and over New
England. This will support a 1035+ MB surface high building in
over the Western Great Lakes region by Wednesday. While the heart
of this much colder airmass will shift over the eastern lakes
region into New England, it does appear that our area will get a
glancing blow of this colder airmass. The lower-level baroclinic
zone is expected to sag southward over the area during the day on
Tuesday, and as this occurs, some model guidance suggests that
another mid-level impulse could help strengthen a band of
frontogenesis across portions of northern and central Illinois.
This could induce a band of precipitation over portions of the
area on Tuesday. Currently, it appears that it may be just warm
enough still to support mainly rain from any precipitation, though
if some colder air moves in quicker, there could be a period of
rain or snow. Currently, we have mentioned a chance for some
showers with this disturbance on Tuesday, mainly over the
southern half of the area.

Wednesday looks like a dry but a rather cold day across the area.
Onshore northeasterly flow will likely result in temperatures
holding in the low to mid 30s for many near the lake. Farther
inland if likely wont be much better, with some areas perhaps
approaching 40.

Later in the week it appears temperatures will warm again in
advance of a developing storm system over the central and southern
PLains. The storm system may end up tracking northeastward over
the western Great Lakes Region by next Friday night. As a result,
a period of much warmer weather, along with increased chances for
showers and thunderstorms will be possible over the area by
Friday. Temperatures on Friday could warm well into the 60s ahead
of an eastward moving cold front. Cooler weather is likely by
Saturday following the cold frontal passage.



For the 12Z TAFs...

Lake stratus had been impinging on NE IL, however recent 1 min
high resolution GOES-16 imagery shows this stratus dissipating
over land and now hugging the shore. Cannot rule out brief MVFR
CIGS early this morning, but given recent satellite trends, flow
likely too light to advect this stratus inland, and the time of
day not anticipating MVFR CIGS at MDW/ORD.

Light winds this morning should transition to a light south-
southeasterly gradient this afternoon which will allow a lake
breeze to form. Gradient suggests the lake breeze should surge the
most over the northern suburbs arcing southeast back to near the
lake near GYY. Best chance of lake breeze passage appears to be
GYY and ORD, with a slightly lesser chance at MDW.

Any lake breeze will wash early this evening and attention will
turn to SHRA/TSRA chances tonight. Convective allowing models are
strongly supportive of showers and thunderstorms breaking out over
eastern IA later this evening then moving across northern IL
overnight. Given how far out we are, only included a VCTS in the
TAFS, but does look like a pretty good shot at a period of TSRA at
most of the terminals overnight. Later TAFS will probably need to
include a TEMPO or short prevailing TSRA group once timing can be
better pinned down and confidence increases a bit.



156 AM CDT

While the pattern will be relatively active this week, not
anticipating significant marine issues. Will need to monitor
Tuesday night for a period of near gale force northerly winds.
Otherwise, a strong storm system could develop late in the week,
though there remains significant model differences on the
potential track and intensity of late week system.






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