Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171749

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1149 AM CST Sun Dec 17 2017

212 AM CST

Through Monday...

Primary forecast challenges in the short term period of the
forecast revolve around stratus, fog, and drizzle potential and
the subsequent impacts on temperatures.

Have been monitoring a large bank of stratus over WI bleeding into
far northern IL using GOES-16 nighttime microphysics RGB, which
beautifully showed the stratus expanding south with 1 minute
resolution in meso-sector 2. Unfortunately, the leading edge
became obscured by high cloudiness around 0700 UTC, so flying a
bit blind this morning with respect to how far south the stratus
is making it. In terms of the public sky cover forecast, the
stratus location is somewhat irrelevant, since anywhere not
blanketed by stratus will still be OVC thanks to mid-high level
cloudiness streaming northeast ahead of southern stream
disturbance. Primary impact of stratus today will be to keep temps
from rising much at all and have lowered high temps northern CWA
as it would appear stratus could linger much of the day far
northern CWA.

Regarding the southern stream shortwave, guidance continue to show
this wave rapidly dampening today as it encounters confluent/convergent
flow aloft. Big question is how quickly the precip will dissipate
as it lifts north. The trend in model guidance has been strongly
toward a rapid dissipation of precip with many models now showing
little if any precip making it into our CWA today. Have trended
pops lower today, though generally keep pops higher than guidance
supports in deference to previous forecast and to be in better
collaboration with neighboring offices. Any precip that falls
should be light and in the form of liquid.

Tonight, what`s left of the weak front over the CWA will lift
north as a warm front with slightly higher low level moisture
advecting northeast into the area. Strong signal in available
guidance for development of widespread stratus deck this evening,
with some drizzle likely along with at least some threat for fog
development. Warm air advection and OVC skies should result in
nearly steady temps all night and have gone well above guidance
for low temps and suspect that low temp forecast for tonight is
still too cold over northern CWA. It`s unclear how dense the fog
will become tonight, since with 4" soil temps in mid-upper 30s
and some wind it is not a particularly favorable set-up for
stratus building down to widespread dense fog. Still some guidance
suggests that could happen, so maintained a mention of patchy fog
tonight, but suspect that will be fairly widespread light fog vs
dense fog.

Monday`s forecast is tricky thanks to the lingering stratus and
fog from Sunday night. Most guidance suggests that this stratus
will quickly clear out Monday, however given the time of year and
the expectation that it will be quite thick, it seems likely that
most of the area should stay socked in all day. Best chance of
clearing would be western CWA later in the afternoon, but even
that would likely be too little, too late for high temps Monday.
Given the expectation of stratus have lowered high temps from the
blended model guidance, however if stratus lingers as expected
then high temps Monday would likely be even cooler than what I`ve
got in for highs Monday.

- Izzi


318 AM CST

Monday Night through Saturday...

The main focus is the stronger storm system expected to impact the
Midwest and Great Lakes Thursday and Friday. As we`ve been
advertising, the above normal temperature regime will persist
much if not all of the work week. This will occur as the mid and
upper pattern across North America reshuffles, resulting in mainly
positive mid-level height anomalies. A dry cold front/trough
passage will occur Monday night into Tuesday. Steady to gusty
southwest winds ahead of the front Monday night will keep lows
above normal highs for mid December.

Guidance has backed off on the strength of the height falls and
CAA with this front, which will play a role in high temperatures
Tuesday. Assuming any lingering clouds clear out, mixing should be
fairly deep as a drier air mass moves in behind the front with
breezy west-northwest winds. Given well above normal thermal
profiles now expected behind the front, the drier air and deeper
mixing should enable temps to warm into the upper 40s to lower 50s
and wouldn`t be surprised at mid 50s readings especially south.
Again, the one fly in the ointment could be cloud cover which is
always tricky with such a low sun angle, but currently have decent
confidence in them clearing.

Tuesday night into Wednesday could very well be the coolest
portion of the work week and even it will be above normal. A broad
surface ridge axis will stretch across the region, so mostly clear
skies and diminishing winds will support lows in the 20s area
wide, followed by a pleasant Wednesday with highs in the mid 30s
to lower 40s under increasing mid and high clouds.

Turning to later in the work week, significant re-amplification
of anomalous poleward ridging up to and north of Alaska will set
the stage for the likelihood of the return of a colder pattern
Christmas weekend. The late week storm system continues to be
highly uncertain with respect to its magnitude, track and
evolution from the Plains toward the Midwest and Great Lakes as it
affects the region Thursday and Friday and heralds the return of
the colder pattern. A swath of accumulating snow will be likely
with this system to the north of this path. It is quite early yet
to be mentioning trends in any meaningful sense given
predictability issues this far out, but a majority of the guidance
and ensemble members currently favor keeping the snow swath mostly
north and west of the CWA.

Adding some more detail, the mid/upper pattern will be complex
with this setup, with none of the major players being sampled by
the RAOB network for the next few days. Thus expect continued
model/ensemble variability until everything gets sorted out. This
is important to note, because while much of the latest guidance
exhibited a decided shift to a warmer outcome for the CWA, we`re
still in the time range where significant shifts are possible if
not likely.

It appears that some energy in the western trough will dive into
the southwest CONUS and could get even cut off or take on a
negative tilt, slowing the overall progression of the system.
Thus, there was also a slowing trend noted in the 00z guidance
suite. Where the surface low ejects northeast from in the Plains
will ultimately dictate its track and impacts to the CWA later
Thursday through Friday, with again the majority of the 00z
guidance favoring a warmer/rainier outcome due to downstream mid-
level heights over our area being pumped higher. In fact, the
warmest models, namely the 00z operational ECMWF (and a
substantial majority of its ensemble members), support a surface
low track that would surge unseasonable warmth and moisture into
parts of the area Friday and a non-zero thunder threat. On the
other end of the spectrum, the 00z GFS and some of its ensembles
still kept open possibility of a change to snow for parts of
northern half of CWA.

Taking the previous few paragraphs into account, have trended
grids to more rain mention Thursday night and Friday, but still
feature a rain/snow mix for the northwest half of the CWA given
the inherent uncertainty/lower confidence in this time range. This
includes bumping up temperatures from previous forecast. As the
much colder air arrives behind the cold front later Friday into
Saturday, may need to monitor for a short-wave riding along the
front for any snow possibly getting back into parts of the CWA.
Finally, confidence is fairly high in below normal temperatures to
start Christmas weekend, but low confidence in the magnitude of it.
As we`ve seen this month, the overall lack of snow cover has
modified the cold air masses that have visited the area. A shot of
even colder air is possible Christmas Eve into Christmas Day.



For the 18Z TAFs...

Under light northeast flow IFR cigs had developed over nrn IL
this morning, with a warm front just to the south of the terminals
and withing a very weak pressure trough. Winds are beginning to
turn more sly as the warm front inches northward. There should be
a gradual transition to more sly-swly flow through the early
afternoon and MDW has already shifted to 220/04. Expect that cigs
will improve to vfr as the thin stratus deck dissipates with the
shift to more sly winds. The weak pressure gradient will remain in
place through the remainder of the afternoon and overnight, so
wind speed should remain arnd 5 kt or less. Latest guidance
suggests that some of the pcpn over cntrl IL may spread north and
overcome the relatively lower sfc dewpoint air, with some showers
possible this evening. So, have added the vcsh group at 02z.
Moisture profile guidance still suggests a saturated lower level
with drier air moving in abv 08-10 kft, so confidence remains
relatively high in cigs/vis dropping to ifr in br/dz. Expect that
IFR conditions will persist to at least mid-day tomorrow or early
afternoon until winds begin to pick up in advance of a cold front
dropping south through the Upper Mississippi Valley.


212 AM CST

Only real marine concern through the middle of next week is with
powerful low pressure tracking across central Canada bringing a
period of strong, potentially gale force, winds Monday night ahead
of a cold front. Forecast guidance has been slowly backing off on
the magnitude of the winds with this event and it now looks like
the best shot of gales is over the northern portions of the lake
with the southwest winds Monday night. Behind the front Tuesday
afternoon into Tuesday evening, it is looking more like 30kt
winds south with the best chance for prevailing lower end gales
over the central and northern portion of the lake. Thus have
trended the forecast winds down slightly. Still looks like the
next system will impact the region late in the week, but the
forecast track is still highly uncertain and will play a big role
in eventual wind speeds/direction.

- Izzi





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