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Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
417 am CDT Fri Apr 28 2017

Short term...
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 Continental U.S.. 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.



Long term...
411 am CDT

Saturday through Thursday...

The main focus for the long term period is the large storm system
expected to impact the region through the weekend. The main
local impacts with this storm system looks to be the good
potential for some impressive rainfall amounts, especially late
Saturday through Sunday, and this could result in flooding
potential on area streams and rivers. We also cant rule out the
possibly of a few strong to severe thunderstorms on Sunday south
of I-80.

The upper-level system is expected to dig into a closed low over
The Four Corners region by early Sunday morning. As this occurs,
a low-level southerly jet will strengthen and begin to transport
deep Gulf moisture northward up the lower and mid-Mississippi
Valley. This should help gradually shift the focus for heavier
precipitation and thunderstorm chances northward into our area
through the day on Saturday as the 850 mb warm front shifts
northward. While there are signs of this being a heavy rain event
for the area, there are still questions as to where the heaviest
precipitation rates will occur. Convection occurring to our south
tonight and on Saturday may adversely impact the moisture feed
into northern Illinois on Saturday, and because of this
possibility, there are still questions as to where the heaviest
rain axis and highest rainfall rates will end up. For this reason
no Flood Watch is planned at this time. We will, however, continue
to message of heavy rain and river flooding threat with the esf.

The main storm system is expected to shift over the area by
Monday. With the slow movement, the surface warm front is likely
to remain south of the area on Saturday, which will result in cool
northeasterly flow over northern Illinois. This in combination
with periods of rain, will likely result in very small diurnal
temperature changes Saturday. As a result, temperatures during
much of the day Saturday could struggle to reach 50 degrees,
especially north of I-80. These cool conditions are also likely
to continue over far northern Illinois on Sunday as well.

While far northern Illinois will remain on the cold side of the
warm front during the day Sunday, it appears that my southern
counties in central Illinois and into northwestern Indiana may end
up getting into the warm sector of the system for a period Sunday
afternoon as the storm system begins to shift northeastward over
northern Missouri. As a result, we could end up with a significant
temperature gradient across the area, with 40s north and near the
lake, and mid to upper 70s to the south of the warm front. The
other concern would be the threat of surface based strong
convection later Sunday into Sunday evening as the systems cold
front moves towards the area. At this time, the main threat of
severe weather looks to remain south of the area. Lapse rates do
not look impressive over the area, but given the dynamics of the
storm system, with very impressive kinematics, I cant rule out
some strong to severe storms impacting my southern counties late
Sunday. The threat for very heavy rain and possible flooding will
also continue on Sunday.

The heavy rain threat should end by Sunday evening as the frontal
boundary shifts across the area. As the flow is expected to shift
southwestward across the area Sunday night as the surface low
occludes to our northwest, temperatures across northeastern
Illinois may end up warming overnight as the flow shifts offshore.
Periods of showers are still expected on Monday as the cold upper
low shifts overhead. This system is likely not to exit the region
until Tuesday.

Cool but mainly dry weather is expected from Tuesday Onward.



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.



416 am CDT

As low pressure to the north of the lake lifts further away and
as high pressure builds over the lake, speeds across the lake will
continue to diminish this morning. However, in the near term,
speeds to 30 knots may occur briefly this morning over the north half
while winds of 15 to 25 knots may occur over the south half. Once
again, speeds will diminish over the entire lake today into this
evening. This diminishing trend does end later tonight into
Saturday as high pressure moves into Ontario and while low
pressure develops over the Southern Plains. This will allow winds
to turn more northerly and increase, especially on Saturday.
Expect hazardous conditions for small craft to return late tonight
and continue through much of the weekend. Speeds up to 30 knots
likely across much of the lake Saturday and Saturday night, and am
still monitoring the potential for a period of gales Saturday
night across the south half. At this time, it appears that these
gales will be occasional but it is possible for more prevailing
gales. Attention turns to the north half by Sunday where there is
slightly higher confidence of prevailing gales occurring. Will
need to continue to monitor both periods.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...4 am Saturday to 4 PM Sunday.



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