Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272009

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
309 PM CDT Thu Apr 27 2017

249 PM CDT

Through Friday...

The 992mb surface low just north of Sault Ste Marie, Michigan is
pulling away from the region with its cold advection having kept
afternoon temperatures in the 40s area wide. The southwest wind
gusts of 25 to 30 mph will ease through mid-evening as the
pressure gradient relaxes. The cyclonic flow should keep clouds
much of the night given the far reach westward on current visible
satellite imagery. Temperatures should only drop slightly thanks
to the expected blanket of clouds. However, under any holes that
were to develop, temperatures would likely drop into the lower to
mid 30s, and if such scattering were to persist, there would be a
frost threat. Confidence in the clouds remaining though is fairly

Moist advection aloft begins Friday morning as the next system
takes shape in the southern Plains. The 850-925mb flow is forecast
to increase to 25-35 kt on Friday afternoon, advecting moisture
northward through the warm advection regime. In tandem with this,
an initial short wave ahead of the trough will assist in lift,
likely supporting light showers spreading across the area.
Confidence is somewhat low on what shower coverage will be, but
enough to still warrant likely wording in the forecast. With the
expected warm advection clouds, and some showers, have undercut
the guidance blend on forecast highs and could still be a little
too warm.



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 18Z TAFs...

The summary of the aviation forecast is gusty winds this
afternoon with very gradually lifting cloud bases, and then
gradually lowering clouds Friday afternoon with likely some rain
showers spreading in.

The surface low over northern Lake Michigan is pulling away from
the area with gusty west-southwest winds and continued cloud
cover. The cold advection pattern will support the gusts through
the afternoon and some occasional lower gusts early this evening.
Confidence is minimal in whether low clouds will continue
tonight, but even if they do they should lift to VFR.

The next weather system approaches on Friday causing clouds to
lower in the afternoon as moisture streams aloft. Support for rain
showers should increase in the afternoon, with an area spreading
northeast from northern Missouri/western Illinois in the morning.
This may be more of scattered showers, but do think there will be
some around. The lowering of cigs to MVFR or IFR is likely to not
be until after dark on Friday.



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...nearshore waters
     until 7 PM Thursday.




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