Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232014
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
214 PM CST Thu Feb 23 2017

.SHORT TERM...
326 AM CST

Through tonight...

A backdoor cold front will slide down the lake and move through the
forecast area this morning. Guidance seems rather consistent with it
arriving at 9-10AM, but sometimes these types of fronts are quicker
than forecast. The main impact from the front will be much cooler
temperatures. Highs along the lake will be in the low 40s, and could
be even lower. Locations along the lake will likely see their highs
very early this morning with temperatures falling into upper 30s by
the afternoon.  Locations away from the lake will warm until the
front moves through. Highs away from the lake will vary from the
upper 40s near RFD to the low to mid 60s south of I-80, and then
temps will drop in the afternoon.

Precip ahead of the next low forms over the region this evening and
overnight. The main question is how far north will the warm front
get tonight. Guidance seems rather consistent with the front
reaching I-80 before stalling Friday morning. This is further south
than models were predicting 24-hours ago.  Models feature enough
elevated CAPE to warrant a chance of embedded, non-severe
thunderstorms throughout the forecast area tonight.

The benefit of that warm front is temps are expected to rise
overnight.

JEE

&&

.LONG TERM...

Friday night through Thursday...

214 pm...Main forecast concern is the potential for accumulating
snow early Saturday morning...mainly across the northern half of
the cwa...along with much colder temps. Active pattern next week
with various systems but with low confidence.

Much if not all of the convection is expected to be east of the
area by early Friday evening as colder air spreads across the
region and a dry slot moves across the area in the evening. Wrap
around moisture on the back side of the departing low will move
across the area late Friday night into Saturday morning. Qpf
amounts by Saturday morning could approach a tenth of an inch
across the north...lowering to trace amounts across the south.
With the colder air arriving a bit faster...precip should
transition to mainly snow by late evening and continue through mid
morning on Saturday. While snow amounts have gone up...generally
around an inch north of I-88...trailing to a few tenths along
I-80...there is still quite a bit of uncertainty as to how much
snow may accumulate on pavement. While the ground will be warm...
air temps will be falling into the mid/upper 20s by sunrise
Saturday. If the snow is able to fall quickly and heavy enough...
there could be some accumulation on pavement but confidence is
low from this distance. Northwest winds will become strong/gusty
Friday night and continue into Saturday but there remains
differences as to how strong winds will become. Gusts into the
30-35 mph look reasonable currently with speeds/gusts diminishing
with sunset Saturday evening as a ridge of high pressures moves
across the area Saturday night.

Southerly winds could become breezy/gusty Sunday as the gradient
tightens between the departing ridge and a trough moving across
the western Great Lakes with low pressure moving from the southern
plains Sunday to the Ohio Valley Monday...as it weakens. The Ecmwf
remains dry with system while the Gfs lifts precip into the
southern cwa and maintained chance pops for this potential. And if
precip did materialize...it could be a mix of rain/snow but
confidence remains low.

Another ridge of high pressure moves across the area Monday night
as the next low moves from the central plains Monday night to the
upper Great Lakes Wednesday though the Gfs favors a weaker low
tracking through the Ohio Valley. Trended more toward the Ecmwf
which would suggest mainly rain Tuesday/Wednesday with precip
ending before colder air arrives. Confidence remains low with this
system as well. Both models then show a clipper system moving
across the area Thursday night and from this distance...are in
fair agreement. Thermal profiles would support mainly snow with
this clipper. cms

&&

.AVIATION...
For the 18Z TAFs...

Complex aviation weather scenario over the next 24-30 hours, with
low clouds, fog, rain, embedded thunder and gusty winds all being
forecast concerns.

In the near term, a secondary cold frontal trough has sagged
across the area this morning, with high pressure building across
the upper Midwest. MVFR/patch IFR deck has developed southward
behind this boundary with winds northeast off the lake, and cigs
1200-1800 foot expected to linger with some patch IFR possible
this afternoon.

Deepening surface low pressure system over southeastern Colorado
will lift northeast into the mid-Mississippi Valley by Friday
morning, with a warm front extending just south of Chicago.
Showers already developing along the IA/MO border in region of
warm/moist advection ascent ahead of the low, and this rain is
expected to expand northeastward across the forecast area later
this afternoon and evening. Forecast soundings depict steepening
mid-level lapse rates above the warm frontal inversion overnight,
with the potential for clusters of embedded thunderstorms from
roughly late evening through early Monday morning before the focus
for heavier precipitation shifts north of the WI state line. Rain
and cool northeast surface winds north of the surface warm front
are expected to produce widespread IFR/LIFR cigs across the area,
which will likely lower Friday morning as the surface low
approaches and flow weakens.

The surface low is forecast to track just south of Chicago Friday
afternoon, with redevelopment of thunderstorms expected primarily
southeast of Chicago ahead of the trailing cold front and near the
surface warm front. High-res guidance suggests a few isolated TS
may develop near Chicago during the morning, before focus shifts
southeast of a CMI-IKK-GYY line in the afternoon. Winds shift
west-southwest behind the low later in the afternoon with gusts
increasing to 25-30 kts.

Overall, confidence medium-high in overall forecast trend. Lowest
confidence in timing of thunderstorms, and will likely be able to
add greater timing detail as things set up later this
afternoon/evening.

Ratzer

&&

.MARINE...

214 pm...Low pressure over the south/central plains will move
northeast to the southern tip of Lake Michigan Friday evening and
then into Quebec Saturday. Strong northeasterly winds will
increase to gale force tonight as the gradient tightens ahead of
this approaching low. While gales will diminish Friday evening...
speeds will remain in the 30kt range and then as colder air
spreads across the region and the gradient tightens...northwest
gale force winds are possible from early Saturday morning through
Saturday afternoon. A ridge of high pressure will move across the
southern lakes region Saturday night with winds shifting southerly
Sunday and a period of 30 kts is possible especially on the
southern half of the lake. Another strong area of low pressure may
move across the upper midwest and northern lakes region Tuesday
into Wednesday next week. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE WINTHROP TO WILMETTE HARBORS
     UNTIL 3 PM Friday.

     Small Craft Advisory...LMZ741-LMZ742-LMZ743-LMZ744 UNTIL 9 AM
     Friday.

&&

$$

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