Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240824

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
324 AM CDT Tue Oct 24 2017

221 AM CDT

Through tonight...

A deep trough is centered over the upper Mississippi Valley this
morning. Water vapor imagery reveals a strong wave rounding the
base of the trough over the lower Ohio Valley while a second wave
is lifting north across the eastern Great Lakes region. The
southern wave is progged to dig across the Tennessee Valley today
helping to further deepen the upper trough into a cutoff low
allowing the northern wave to pivot westward back over the western
Great Lakes region. Meanwhile, cold air mass will continue to
spill across the Midwest today with 850mb temperatures cooling
down to -1C to -2C midday. Cold advection will help result in
steep low level lapse rates and a deeply mixed boundary layer
today which coupled with the tight pressure gradient from a
deepening surface low meandering over northern Michigan will
produce strong gusty winds. GFS soundings support wind gusts in
the low to mid 30 kt range late this morning through the
afternoon. This is just below wind advisory criteria (gusts GTE
39kt), but top of the channel winds are closer to 40 kt which
would be just above criteria. Given the overcast skies expected
through the day would think that these higher end gusts may be
very sporadic and more likely tied to any any showers in the area,
so leaning against any wind headlines at this time. Bottom line,
though, is that today will be very windy and colder with
temperatures holding steady in the low to mid 40s through the day.
Showers will be in place throughout much of the day under the
influence of the developing upper low and pinwheeling vort lobes.
May see some drying in the west mid to late this afternoon, then
overnight expect the CWA to be dry as the upper low/trough axis
shifts to our east and a corridor of strong mid level height rises
overspreads the area in its wake. Should see partial clearing
overnight which will help to allow temperatures to fall into the
low to mid 30s. Since we maintain a bit of a gradient keeping a
light breeze overnight and cloud cover increases again from the
northwest late, widespread frost seems unlikely despite the cold



221 AM CDT

Wednesday through Monday...

Main upper trough axis shifts east to the eastern Great Lakes
Wednesday as another vort max rides the northwest flow into the
western Great Lakes. Relatively shallow layer of saturation will
occur in the mid levels with this wave and should be able to
produce a few light showers. Low level temperatures and temps just
aloft are a big question mark for now. NAM guidance, for instance,
cools the column below freezing while the GFS is considerably
warmer with a 3000 ft+ near surface warm layer. This certainly
would impact p-type, along with modest timing differences, with
earlier timing being more favorable for a few snow flakes. For
now, will leave mention of snow and rain/snow mix early Wednesday
morning transitioning to all rain by mid morning. Not all areas
will see precipitation on Wednesday and those areas that do see
precipitation will likely not see much, maybe a tenth or two
(primarily liquid -- no snow accumulation expected) on the high
end. In addition, the system moves pretty quickly across the
region and by early afternoon it appears precip will shift
southeast of the CWA. Temperatures will moderate slightly
Wednesday and expect highs near 50F.

Meanwhile, another strong upper level disturbance is progged to
dig across the Canadian Rockies on Wednesday and across the
Dakotas into the Upper Midwest Thursday. A weak surface reflection
will develop over Minnesota Thursday and track across southern
Wisconsin Thursday night. Locally, out ahead of the low
temperatures should briefly warm up Thursday as southwest flow
advects 850mb temperatures of around 10C back over the area which
will contribute to afternoon highs in the upper 50s to low 60s.
For now, the low track keeps most of the precipitation to our
north Thursday and early Friday. A trailing cold front will sweep
across the region Friday and will kick off some afternoon showers
primarily east of the I-55 corridor. For the western half of the
CWA, with early fropa timing, expect morning highs with falling
temperatures in the afternoon and by Friday night expect
temperatures back near the freezing mark across the CWA.

Late Friday into the weekend, confidence in the forecast details
diminishes as upper pattern becomes more amplified with another
closed upper low developing and meandering over the region. There
will be periodic chances for precipitation and cannot rule out
wintry precip at times as -5C to -7C 850 temps overspread the
region. Guidance suggests the upper low will shift east late in
the weekend and into early next week allowing temperatures to
moderate and resulting in primarily dry weather.



For the 06Z TAFs...

1233 am...Several forecast concerns this period including cigs...
wind speeds/gusts...and rain end time.

Light rain over northern IL early this morning will continue to
expand in coverage through the morning with a period of more
moderate rain expected after sunrise. This area of rain will
slowly shift to the southeast this afternoon with an end time
first at rfd and then across the Chicago terminals. Continued
shower mention through late afternoon but this will need some
refinement with later forecasts as the rain may end sooner than
currently expected...but low confidence.

Cigs will lower into low mvfr early this morning and remain low
mvfr through early afternoon. Its possible that cigs may dip into
ifr during the mid/late morning but confidence is too low to
include a bkn/ovc ifr cig at this time. Cigs will gradually lift
through mvfr later this afternoon...become vfr this evening and
scatter out after midnight. However...a fast moving wave will
bring increasing clouds by daybreak Wednesday...with some mvfr
potential after sunrise.

Westerly winds will remain in the 10-15kt range early this morning
with some periodically higher gusts. But as the gradient tightens
later this morning...west/northwest winds will steadily increase.
Confidence regarding how strong speeds/gusts will become is only
medium. The strongest winds are likely to be across far northern
IL and into southern WI. But gusts may still approach 30kts over
all the terminals this afternoon. cms



324 am...Previous forecast in good shape with no major changes
this morning. Strong low pressure near the Straits of Mackinaw
will remain nearly stationary through this afternoon while
deepening a bit more. This low will lift northeast tonight and
Wednesday as it begins to weaken. Gales already occurring over the
northern part of Lake Michigan and these will increase to storm
force later this morning with gales eventually spreading across
the southern part of the lake. Still some limited potential for
storm force gusts on the southern end of the lake and maintained
occasional gust mention. Speeds will steadily diminish late
tonight into Wednesday morning as the gradient weakens.

A weak ridge of high pressure will move across the lake Wednesday
night into early Thursday morning with northerly winds becoming
light then shifting southerly Thursday morning. Low pressure will
move across the northern plains Wednesday as another low develops
over IA/IL Thursday. These two lows will merge over the western
lakes region and lift north to the upper lakes Saturday while
slowly weakening. However...there still remains uncertainty with
this next system and small track differences could make for big
wind direction and speed changes. cms


LM...Small Craft Advisory...IN nearshore waters until 1 PM Tuesday.

     Gale Warning...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...1 PM
     Tuesday to 4 AM Wednesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...7 AM Tuesday to 1
     PM Tuesday.

     Gale Warning...LMZ777-LMZ779...9 AM Tuesday to 9 AM Wednesday.




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