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fxus63 klot 171903 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
103 PM CST sun Dec 17 2017

Short term...
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 Illinois 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 overcast 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 County Warning Area
as it would appear stratus could linger much of the day far
northern County Warning Area.

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 County Warning Area 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 County Warning Area 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 overcast 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 County Warning Area. 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 County Warning Area 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


Long term...
101 PM CST

Monday night through Sunday...

The extended features generally a milder pattern through mid to
late week, a rain to snow type system Thursday night into Friday,
then a return to colder temperatures for the weekend and even
colder air into next week. Some snow chances to appear near the
tail end of the forecast cycle near the Christmas Holiday.

Mild southwest flow will continue Monday night into Tuesday ahead
of a cold front that will also largely remain to our north, but it
will graze the area Tuesday. The main question will be how much of
the stratus will erode Monday night. The mesoscale NAM suggests
some lingering stratus overnight into Tuesday under an inversion,
while global guidance generally suggests clouds do fade. If the
global models are right and the NAM is exhibiting its typical high
bias of low level rh, it could be a fairly warm day like this
past Saturday. Still high spread in ensemble guidance to say one
way or the other, but either way we will remain above normal.
High pressure will build in Tuesday night into Wednesday, and
will bring a brief cooler shot on Wednesday behind the cold front.
There will be a weak wave to our north Wednesday, but just expect
an increase in mid to higher clouds.

The Reservoir of cold air locked up well north in Canada will get
dislodged later this week as low pressure in the Gulf of Alaska
shifts into the western United States. This low will eventually
dig and close off (if only briefly) near The Four Corners region
of the southwest. Deep southwest flow ahead of this low will
bring the last gasp in the the mild pattern across our region
Thursday into Friday.

The next precipitation maker for the region will be driven by this
system, as a surface low in Lee of the central rockies will eject
toward the Midwest. As outlined fairly well the overnight shift
discussion, the general trend with this system has been for the surface
to track either over the area or farther westward. This would act
to slow the cold air arrival and leave northeast Illinois and northwest
in on the mild and windy side of things for most of the event.
There is still much up in the air with this system with timing and
p-type concerns, but it looks like we will remain on the liquid
side of this one for most of it until maybe the very end. A with
such a dynamic and mild system this time of year, there is a non
zero thunder chance.

There is a decent signal from the long range guidance on return to
a colder pattern, and possibly a fairly potent one for early next
week. A deep upper level longwave trough will shift into the
Continental United States, while the southeastern ridge will also
strengthen which will initially slow the arrival of the coldest
air. Confidence on colder temperatures is medium-high, but the
magnitude of the cold may be mitigated by the lack of snow cover
across the region also.

For those dreaming of a white Christmas, all hope is not lost, as
several mid to long range models do advertise energy building
around the base of the trough and then move this forcing northeast
sometime in the Christmas eve/early Christmas day time frame. At
this point there have still been plenty of model run changes to
say much more than that.



for the 18z tafs...

Under light northeast flow IFR cigs had developed over nrn Illinois
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 Illinois 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.


101 PM CST

Marine concerns center on several gale force wind
episodes. The first is associated with low pressure tracking
across central Canada Monday night ahead of a cold front.
Forecast guidance still depicts the 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 30 kt winds south with the best
chance for prevailing lower end gales over the central and
northern portion of the lake. There is a little better agreement
on the system for late in the week, and the associated
strengthening surface low will likely pass over Lake Michigan.
The strength of the low is still a bit in question, but there will
be gusty south winds ahead of it, followed by a shot of west-
northwest winds. Either time could pose at least a brief gale



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