Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280826

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
326 AM CDT Fri Apr 28 2017

325 AM CDT

Through tonight...

The main forecast concerns today and tonight will be with the
timing and chances for some light rain showers.

GOES-R water vapor imagery early this morning indicates the
presence of a strong northwesterly upper-level jet across the
western CONUS. This is carving out a rather extensive upper trough
across the western half of the country. Ultimately, this large
system will produce inclement weather across the region through
the weekend.

While the main storm system will remain west of the area today and
tonight, a series of lead upper level disturbances, currently
generating some showers over portions of western Missouri and
Kansas early this morning, is expected to move across the area
this afternoon and evening. This looks to result in some light
showers across the area later today and this evening. The main
threat for thunderstorms will remain well south of the area
through tonight in closer proximity to nose of a developing the
low-level jet.

With a good amount of cloud cover and the threat of some rain,
temperatures today will remain on the cooler side, though warmer
than Thursday. Overall, expect temperatures to top out in the 60
to 65 range for areas inland from the lake. Along the lakeshore,
onshore flow will result in cooler conditions.



308 PM CDT

Friday night through Thursday...

An overview of the extended is we are forecasting for a lot of rain
of upwards of 3-4 inches through Sunday night. In addition, strong
to severe storms are possible Sunday and Sunday night. Temperatures
will be slightly below normal with mainly cloudy conditions. While I
am confident that we will see a lot of rain through the weekend, I
am still hesitant to issue a flood watch at this time. Thinking the
flood risk will not begin until Saturday, so I elected to wait on a

Weakening showers may linger Friday night as an upper level vort
streamer shifts north over Wisconsin and a surface boundary.
Thinking the better chance of measurable precip will be closer to
the vort streamer over Wisconsin, but kept a chance of showers in
the forecast.

The other area of showers over central IL and central IN rotates
north and spreads from south to north through Saturday.  The next
surface low organizes over Missouri/Arkansas. Rainfall is expected
to increase in intensity Saturday night into Sunday ahead of the
low`s warm front.  PWAT values increase to 1.5-2.0 inches.  PWAT
values this high are extremely rare this early in the season.
Sounding climatology for ILX and DVN suggests that the average PWAT
values are 0.6-0.7 inches, and the daily  maximum is only 1.2
inches. So a lot of moisture is coming our way Saturday night along
with an upper level wave that will force widespread showers.  Some
showers may be heavy at times. The only question is will convection
to our south rob the area of some of the moisture leading to less
rainfall than currently forecast?

Thunderstorms will certainly result in enhanced rainfall rates, and
there is a least a chance of embedded storms Saturday night across
the forecast area. Forecast soundings feature elevated CAPE, but a
stout inversion will likely lead to embedded thunder.

The warm front lifts through the forecast area Sunday while the
surface low shifts into southern Iowa late Sunday afternoon.  I have
concerns about how much CAPE we will have to work with given mostly
cloud skies Sunday, but forecast soundings feature 500-1000 J/kg of
CAPE will higher values south of I-80.  Shear values vary from 40-80
kt, so strong to severe storms do look probable especially since the
low`s cold front moves through in the evening and overnight. Model
QPF suggests a line of storms forms ahead of the cold front Sunday
evening in eastern IL and shifts east through the evening.

In addition to strong/severe storms, more rain is expected. Rainfall
amounts Sunday and Sunday night are forecast to be 1.25-2.00 inches
with locally higher amounts associated with thunderstorms.

The low moves from Iowa to Lake Superior through Monday with wrap
around showers expected through Monday night. High temps will also
be lower with highs only in the 50s Monday.  Gusty southwest winds
to 30-35 MPH are possible.

While we are still under the upper level trough Tuesday, we could
see a glimpse of the sun and a break in the rain. Guidance has light
QPF values over the forecast area, but thinking there is only a
slight chance of light rain Tuesday afternoon. High temps may be a
bit warmer and get into the low 60s.  Dry and slightly below normal
conditions continue Wednesday, but another low pressure system may
bring rain to the region Thursday.



For the 06Z TAFs...

Dry and VFR conditions expected across the terminals this morning,
while lower clouds continue to shift to the northeast. Although
there are some scattered VFR ceilings, the majority of any cloud
cover this morning will come from mid and high clouds. Do
anticipate a gradual lowering of ceilings throughout the day
today, as the next system approaches from the southwest. This
system will also support scattered showers to move across the
terminals later this afternoon through the early evening. At this
time, have not included much of any vis restriction but could see
including some MVFR vis with later forecasts. Ceilings will
continue to fall through the evening and overnight hours, with low
end MVFR and even IFR definitely a possibility. With a light
gradient in place early this morning, winds may be variable for a
time. However, do expect the current light westerly wind to
gradually transition over to a southeast wind this morning. Winds
may initially be light with this transition, but should see an
increase in speed through the morning, with winds eventually
turning more easterly in the afternoon.



308 PM CDT

South to southwest winds are gusting to 45 kt over the northern end
of the lake, and expecting winds to diminish below gale force this
evening.  Brief periods of gales are possible Saturday night through
Monday night, and a small craft advisory will probably be needed
Friday night through Sunday and again Monday and Tuesday.

Looking at an active pattern. The low over Lake Superior continues
north tonight and a weak high moves over the southern end of the
lake tonight. The next low forms over the southern plains Saturday,
track to Iowa Sunday night, and then continue to Quebec by Tuesday.

Southwest winds become north Friday evening and increase to 30 kt.
North to northeast winds continue until the low moves through Sunday
night/Monday morning.  The low`s cold front slides across the lake
Monday evening. Northwest winds weaken Tuesday afternoon through



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...4 AM Saturday to 4 PM Sunday.




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