Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181205

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
605 AM CST Mon Dec 18 2017

330 AM CST

Through tonight...

The main concerns are with areas of fog and drizzle this morning
and then cloud cover trends this afternoon along with the impact
of these trends on temperatures. A warm front is lifting north
across far northern Illinois this morning, with light to moderate
southwest winds. Patchy to areas of fog and drizzle (with even
pockets of steadier drizzle) are in place early this morning, with
RFD reporting 1/4 SM visibility at this hour. Based off observations
just to the west and southwest of the RFD area, it appears that
the dense fog is likely to be short-lived, so not planning any
headlines for it. Any lingering fog should erode by the mid to
late morning hours, with patchy drizzle gradually sliding east,
and a completely dry afternoon favored.

Returning to cloud cover trends, GOES-16 advanced nighttime
microphysics RGB indicates a hole in the low stratus poking into
northeast IA, but otherwise solid coverage of it extending north
of the hole and south of it into northern MO. Cloud cover trends
are as usual tricky. Have overall leaned a bit more pessimistic
than much of the guidance. Boundary layer flow driving the
movement of the stratus is not expected to significantly pick up
until after dark, remaining out of the west-southwest 10-20 kt
range through that time. Thus, anticipate a slow movement of the
cloud deck, keeping much of the daytime hours OVC. Temperatures
have gradually risen through the night, so the mild starting point
will still enable low 40s highs along the WI border counties and
mid to upper 40s everywhere else. Should the thinking on clouds be
incorrect, a quicker clearing could result in going highs being a
bit too cool.

Southwest winds will pick up tonight ahead of a cold front and
become gusty at times, along with some cloud cover. Therefore,
temperatures are unlikely to drop much. Leaned closer to raw model
guidance for lows with this in mind, and a bit above previous
forecast, with mid 30s to around 40 indicated. Would not be
surprised if some locations remain above 40 through the night.



129 AM CST

Tuesday through Sunday...

Big changes on tap during the long term period in the hemispheric
circulation over the north western hemisphere with a transition
from a more zonal split flow, to a more phased and highly
amplified meridional flow by the weekend. The result will be a
change from unseasonably mild conditions this week to a colder
pattern heading into the weekend, with a potentially wet, stormy
period as we make the transition. As is always the case, the
devil will be in the details, which get murkier and more uncertain
by Friday into the holiday weekend.

Tuesday, a cold front trailing southwest from a powerful cyclone
tracking across central Canada will move across the area during
the day. Moisture and forcing will be lacking with this front, so
not anticipating much more than a wind shift, with even the colder
air lagging behind the front. Should start the day Tuesday
unseasonably warm, so given the expected sunshine, it looks like
many areas could make a run at reaching/climbing above 50 degrees
Tuesday. Behind the weak cold front, look cooler temperatures
Tuesday night through Wednesday night, but even those cooler temps
still look to be a few degrees above average.

Pattern should get more interesting heading into the end of the
work week as strong shortwave digs into the southwest U.S. and
is progged to close off a rather deep tropospheric circulation.
Lee side cyclone will develop Thursday with surface warm
frontogenesis taking place east from that low into the mid MS
Valley Thursday. Initially, looks like most of our CWA could be
north of the warm front during the day Thursday.

By Thursday night, the sfc cyclone will move northeast to the
southern Iowa/northern Missouri with the associated warm front
progged to lift north across the CWA Thursday night. Expect rising
temperatures Thursday night behind the warm front with most areas
likely well into the 40s if not lower 50s by Friday morning. Cold
front should move across the area Friday afternoon, but not before
most (if not all) of the CWA warms into the 50s Friday. Could see
a few showers Thursday night, mainly northern CWA ahead of the
warm front and then again Friday into Friday evening ahead of the
cold front.

Forecast confidence decreases substantially at this point as
medium range models have been trending farther west with the axis
of the highly amplified long wave trough. The latest 00z
operational runs have continued this trend and that has
potentially big implications not only on temperatures over the
weekend but also precip chances. Unlike earlier runs which had
the heart of the arctic air mass spreading into the area in the
wake of the Thursday night/Friday morning`s shortwave, it now
holds the heart of the Arctic air farther west. While that would
keep temperatures more moderately cold this weekend, the bigger
implication is that it would keep us far closer to the baroclinic
zone and potential storm track. If we do end up close to the
baroclinic zone over the holiday weekend, that would bode well for
those dreaming of a white Christmas, as any shortwave energy
riding up the eastern side of the deep long wave trough could
easily lay down some accumulating snow. Important to keep in mind
that medium range models have yet to really lock into a solution
and until they do, further refinements to the forecast could be
needed. If you have travel plans across the Midwest over the
holiday weekend, it would be advisable to pay attention to later
forecasts as there is potential for wintry precip making travel
dicey, but too soon to say to what extent and where.

- Izzi


For the 12Z TAFs...

Main concern is CIG/VSBY trends today. IFR CIGs are in place
across most of the area this morning, with a few holes in spots.
The most prominent hole is across parts of eastern IA to the MS
River, though it`s smaller than it was a few hours ago. The low
cloud shield will gradually shift east through the day. Had been
monitoring LIFR CIGs that made it into ARR/DPA, increasing concern
for LIFR CIGs and IFR VSBY due to lower tower VSBY at ORD and
MDW. Latest trends have reduced confidence in these occurring, so
have backed off to a TEMPO in the TAF through 14z. May be able to
remove this TEMPO if there`s no signs of lower CIGs and VSBY
through 13z.

With the exception of some of the small holes in the IFR CIG
coverage per current obs (such as ARR and CO9), expecting slow
improvement in CIGs. Due to low-medium confidence in trends,
maintained timing of improvement to MVFR and scattering to VFR
from previous TAF. The timing of CIG improvement will likely need
refinement once trends become more clear, with the possibility of
earlier or later improvement still in play. Any lingering patchy
drizzle should shift east by the mid morning.

Winds will be west-southwest today, shifting to southwest tonight
ahead of a cold front and then west behind the cold front on
Tuesday, along with occasional gustiness in 15-20 kt range.
Strong west winds of around 45 kt at about 2000 feet AGL tonight
will create marginal LLWS conditions, which was maintained from
previous TAFs.



416 AM CST

West-southwest winds will increase through today, reaching 30 kt
on the north this afternoon. As a cold front trailing from strong
low pressure near James Bay approaches the lake this evening,
southwest winds will increase to lower end (35 kt) gale force on
the north half of the lake. There may be a very brief lull in the
gales as the front sweeps across the lake early Tuesday morning,
but they will return quickly out of the west-northwest by the late
morning and continue through Tuesday evening. Have issued a Gale
Warning to cover this. While a few/occasional gale gusts can`t be
ruled out on the south half tonight and Tuesday, prevailing gale
speeds/gusts appear less likely, so again the warning is only for
the north half.

A Small Craft Advisory is in effect for the nearshore waters for
the brisk southwest winds ahead of the front tonight and the west
winds behind the front on Tuesday. Gusts to 30 kt are possible
along portions of the Indiana shore late tonight into Tuesday
morning. Conditions hazardous for small craft are again possible
for the Indiana shore Tuesday night as winds turn northwest and
then north late before diminishing with high pressure moving over
the region.

The next period of concern for strong winds over the lake is with
an area of low pressure late in the week. Confidence is still low
in the track and strength of this system, with gale force winds
most likely Friday night into Saturday behind it as much colder
air returns to the region.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...6 PM Monday
     to noon Tuesday.

     Small Craft Advisory...LMZ744-LMZ745...6 PM Monday to 3 PM




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