Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 190831

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

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.



320 PM CDT

Sunday night through Saturday...

The surface high shifts east Sunday night while a strong low reaches
Hudson Bay. The low`s cold front will slide across the region
Monday, and precip is expected ahead of the front. Showers spread
northwest to southeast Sunday night, and I have high confidence in
showers.  Expected isolated to scattered thunderstorms as there are
steep lapse rates aloft. Not expecting severe weather as CAPE is
very minimal.  Unsure how long isolated lightning strikes will
continue, but have a slight chance of thunder along and east of I-55
Monday morning.  Winds turn north behind the front so temperatures
will range from the mid to upper 60s away from the lake to the upper
40s along the lake.

The GFS features a band of precip along the IL/WI state line Monday
afternoon and evening, while other models are dry. Decided to keep
the forecast dry behind the front.  A secondary cold front moves
down the lake Tuesday ahead of a large high pressure. Winds become
north behind the front, and cooler air aloft spreads across the
midwest through Tuesday night. High temps will be coolest Wednesday
and range from the upper 30s along the lake to the low to mid 40s

Confidence is low regarding the forecast late next week as guidance
differs greatly in how the next low will evolve. The GFS has a low
forming over the plains Thursday, which then passes over Wisconsin
Friday evening.  The ECMWF, on the other hand, has the low forming
Thursday night and passing over the mid Mississippi Valley Saturday.
 The models are consistent that the system will bring precip and
probably storms to the region so had enough confidence to keep high
PoPs in the forecast.



For the 06Z TAFs...

High pressure moving into the region will result in light winds
which will become fairly light south-southeasterly Sunday. This
should allow a lake breeze to develop and probably reach ORD late
in the afternoon and possibly approach MDW. Given the SSE synoptic
gradient, confidence is lower at MDW and kept lake breeze wind
shift out of MDW. Otherwise, look for some showers and
thunderstorms to develop Sunday night, though still a wide range
in model solutions with regards to placement, so only included a
PROB30 for now.



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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