Scientific Forecaster Discussion
fxus63 klot 181926
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
226 PM CDT Wed Oct 18 2017
138 PM...only forecast concern is a weak cold front Thursday
morning and possible lake breeze Thursday afternoon.
Gusty southerly winds will diminish with sunset with high temps
expected to reach the lower 70s most locations this afternoon.
Temps will quickly fall into the 50s this evening and then only
slowly drop overnight and probably not into the 40s until near
daybreak as a weak cold front moves into the northern Illinois. The
gradient will become quite weak by Thursday afternoon and winds
may end up light and variable for a time and this will allow a
lake breeze to form. Though confidence of how strong and how far
inland this lake breeze may move is low. High temps will be
similar to today...though cooler near the lake depending on the
timing of the lake breeze. The front is expected to become
stationary across the far southern cwa Thursday afternoon with the
potential for some higher dewpoints there as moisture pools near
the front. Cms
225 PM CDT
Thursday night through Wednesday...
Quiet/dry weather will continue at the start of the period, as
boundary in place not anticipated to support any precip
development across the area. Upper level ridging and backing
low/mid level flow will help this boundary lift back north through
the region Thursday night into Friday. This along with sunny
skies on Friday will support above normal temps well into the 70s.
Dry and above normal temps continue into Friday night but still
expecting a system to move across the region this weekend and
provide the potential for precip mainly Saturday night into
Doesn't look to be any large changes with this anticipated system
and possible precip. However, there does appear to be some slight
timing differences, with most locations staying dry during the
day Saturday. Given a steadily increasing south/southwest flow,
can't rule out some warm air advection showery activity late in the day primarily
over northwest/north central Illinois. Strongest forcing
associated with approaching upper level trough and surface
trough/boundary, doesn't arrive until Saturday night. This forcing
along with a likely strong low level jet focused across the region, should
support a decent precip shield to move across the County Warning Area and have
continued higher pops during this period. Some lower confidence
with the thunder potential though. However, an axis of higher
dewpoint air and weak instability along with the previously
mentioned forcing, will support some possibility of thunder and
have maintained mention of thunder over the entire area. With the
mid/upper level trough axis still pushing through the area on
Sunday, could see precip linger across at least the eastern half
of the County Warning Area Sunday morning but with thunder chances to be on the
low side. On the backside of this system, slightly cooler air mass
expected but also will vary through early next week. This is
until a large upper level trough is expected to move across the
region, and bring even colder air and the potential for showery
activity through midweek.
for the 18z tafs...
1233 PM...only forecast concern this period are the winds.
Southerly winds have been slower to increase but are now starting
to become gusty. Low level winds continue to increase through the
afternoon into this evening so speeds/gusts will increase through
mid afternoon. Speeds/gusts are expected to diminish with sunset
but the stronger winds in the low levels will continue and added
low level wind shear mention for this evening. These low level
winds begin to diminish overnight. A weak cold front will move
across the area toward daybreak Thursday morning. Winds will shift
to the northwest but speeds are expected to be light. There could
be a period of light and variable winds midday/early Thursday
afternoon. Winds may turn southeasterly by late Thursday afternoon
and there will likely be a lake breeze...but confidence on how
far inland this may move as well as wind directions is low.
Otherwise...dry/VFR through the period with passing cirrus this
evening through Thursday. Cms
225 PM CDT
No significant changes to the forecast this afternoon into this
evening, which includes the Gale Warning and Small Craft Advisory.
Southerly winds have increased across the entire lake today,
ahead of strong low pressure to the northwest. At this time, gales
are likely over the north half, with speeds up to 30 kt now in
place over the south half. In the nearshore waters, southerly
winds of 20 to 25 kt are being observed. Expect these trends to
remain in place for the rest of the afternoon into this evening,
with an increasing trend likely becoming more of the trend over
the open waters. This will increase the gales more towards higher
end gales, up to 45 kt. While gales are still not expected over
the south half, occasional gale force gusts are a possibility. As
this low lifts further to the north and an associated frontal
boundary moves across the lake tonight, expect speeds to diminish
across the entire lake especially after the midnight time frame.
However, speeds approaching 25-30 kt may still persist for a time
on Thursday across the north half.
Lm...Small Craft Advisory...nearshore waters
until 3 am Thursday.
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