Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 132345
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
545 PM CST Tue Feb 13 2018

.SHORT TERM...
155 PM CST

Through Wednesday...

Quiet conditions are in place with high pressure overhead. We are
getting some breaks in the mid level cloud deck, with more
clearing expected late this afternoon and tonight. The high will
shift east of the area tonight, which will open the door to
southwest winds. This pattern tends to favor warmer air advancing
into the area, but with gradually increasing dewpoints also favor
lower cloud development also. There is a leading band of clouds
over Missouri, and near term guidance generally thins this out as
it shifts northeast into central Illinois. In spite of the
clearing skies favoring another temperature drop, southwest winds
and higher dewpoints will keep readings a bit warmer than past
nights.

Above freezing dewpoints will arrive south of I-80 tomorrow mid
morning, and spread into the remainder of the area in the
afternoon. Dense fog is not a concern initially as the dewpoint
surge is only modest at first. Expect stratus to at least impact
areas along and southeast of I-57 in the afternoon. Moisture depth
appears to be shallow suggesting any drizzle would be confined to
this axis also. Areas south should reach the 40s, and with GEFS
ensembles holding readings near or slightly below 40 closer to
Chicago.

&&

.LONG TERM...
305 PM CST

Wednesday night through Tuesday...

Multiple periods of active or impactful weather setting up for the
long term forecast period.

For Wednesday night into Thursday, main concerns will be drizzle and
fog.  As high pressure pushes east to New England and low pressure
develops over the Central Plains, a return flow of low level warmer,
moist air will set up, overspreading the fresh snow pack that covers
all but the far southern portions of the CWA.  The warm, moist
advection will lead to fog development and as the saturated layer
deepens slightly by early evening, some patchy drizzle should mix in
with the fog.  There is some concern for freezing drizzle over the
far nwrn portions of the CWA where temperatures could dip down to
32F for a short time early Thursday morning.  Another concern for
Thursday will be snow melt and ice breakup jamming on area rivers.
By Thursday afternoon, a cold front will push into southern
Wisconsin and eastern Iowa.  With dewpoints climbing to arnd 40F
along I-80 and middle to upper 30s dewpoints all the way to the
Wisconsin border, in advance of the front. The warmer, high dewpoint
air should be an efficient snow melt scenario, so area rivers will
need to be monitored for ice breakup. Some rainfall is likely in
advance of the front in a zone of deep layer isentropic lift. The
rainfall and warm/moist advection should help accelerate the snow
melt, which could lead to sloppy conditions on roadways and rising
water levels on area rivers and streams. The cold front is still
expected to push through the region late Thursday night and Friday,
bringing temperatures back into the lower 20s over the Rockford area
to arnd 30 F for the far sern portions of the CWA.  Any lingering
pcpn should transition from rain to snow through the night, but no
accumulation is expected as pcpn amounts should be low after
midnight Thursday night.  The greater concern for Friday morning
will be the chances for the slushy areas to freeze as persistent
cold advection following the frontal passage should allow for little
diurnal warming on Friday, with temperatures remaining fairly steady
through the day, limiting any melting potential.  Another area of
high pressure is expected to build across the region Friday night,
bringing at least a short period of clearing skies and dry weather
conditions through Saturday.

By Saturday night, the longer range models begin to diverge in
handling the impacts of the next northern stream system to impact
the region.  There is general agreement on low pressure moving
across Canada and dragging a cold front across the Upper Mississippi
Valley while a southern stream system develops over the Lower
Mississippi Valley.  The main model differences are related to the
southern stream system, with the GFS suppressing the system much
farther south than the ECMWF solution, which is, not only farther
north into the Ohio Valley, but also phases somewhat with the nrn
stream system and spreads pcpn across nrn IL/IN, which could be at
least a rain/snow mix, depending on temperatures.  Have gone with
slight chance PoPs for Saturday afternoon, given some uncertainty to
how the system will evolve, but given the large differences in the
guidance, there is low confidence in forecast details for the local
area for the weekend.  Significant model differences in the
progression of the long wave pattern aloft continue into next week,
with lower than normal forecast confidence continuing through the
remainder of the forecast period.  The general trend is for
southwest flow aloft developing as upper troughing develops over the
wrn CONUS with general ridging over the ern 2/3 CONUS.  This would
suggest a general warming trend into next week with periodic
chances for pcpn as srn stream energy lifts through the swly flow
aloft.

&&

.AVIATION...
For the 00Z TAFs...

Quiet weather expected in the near term through the overnight
hours. Ridge axis will shift east away from the terminals this
evening causing winds to shift to the southwest where they will
remain through the remainder of the forecast period. Mid and high
clouds will continue to gradually clear from the west leaving
mostly clear skies and VFR conditions overnight.

Main forecast concern is the development of low stratus and fog as
southwest flow advects higher dewpoints into the region atop our
dense snow pack. Most model soundings show development at some
point during the day Wednesday from early morning through the
afternoon. MOS guidance on the other hand maintains VFR
conditions through the period with lower conditions not arriving
until Thursday. Have pretty good confidence that IFR or lower
conditions will develop but very low confidence in the timing.
Right now, 00Z timing is a bit of a dart throw, much later than
raw model soundings would suggest but much earlier than supported
by MOS guidance. Thinking that MOS may be a bit slow to bring up
dewpoints which are progged to be in the 36-38 range by 00Z
tomorrow by NAM/GFS/ECMWF/GEM. In addition, NAM/GFS 950/975mb
moisture fields initialize well on the low ceilings in TX/OK and
bring that up over the terminals late afternoon/early evening.
Hopefully timing comes into better agreement by the next model
cycle.

BMD

&&

.MARINE...

305 pm...High pressure is moving off the New England coast as a
series of lows track across Canada and another low develops over
the western plains. The moderate gradient over the lake will lead
to south to southwest winds to 30kts through Thursday morning.
There could be some periodic gale gusts mainly over northern
portions of Lake Michigan tonight into Wednesday morning. Low
pressure will move across the upper lakes Wednesday night into
Thursday. A trailing cold front from this low will move south
across the lake on Thursday with winds shifting northerly and
increasing back to 30kts for Thursday night into Friday. Another
strong high will build across the northern Plains Thursday night
and then move east across the southern Great Lakes region Friday
night.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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