Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230827
AFDLOT

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

.UPDATE...
805 PM CST

Made some adjustments to the forecast, especially with respect to
temps tomorrow.

Expect gradient to remain strong enough ahead of the approaching
weak cold front to keep fog threat to a minimum in the warm
sector. Certainly can`t rule out some of the more sheltered low
spots fogging up, but widespread fog appears unlikely. Gradient
does weaken some in the cold frontal trough, which could allow for
some fog to develop, but cold should be progressive enough that it
really doesn`t seem to favor a solid dense fog deck developing.
Planning to maintain mention of patchy fog in grids/forecast for
tonight.

Behind the weak cold front moving through after midnight, it looks
like a strong lake enhanced back door front will come barreling
in off Lake Michigan during the early-mid morning hours Thursday
resulting in a sharp drop in temps near the lake. Low clouds and
some fog may immediately follow the frontal passage, but any fog
looks unlikely to persist. Moderate northeast winds off the still
cold lake should cause temps behind the front near the lake to
fall into and hold steady in the upper 30s with wind chills
dropping to near 30! Impacts on temps from this back door front
will be felt fairly far inland, however the impact on temps will
lessen as distance from the lake grows.

Izzi

&&

.SHORT TERM...
244 PM CST

Through Friday...

Active weather is expected across the region to close out the
remainder of the week. This afternoon, low pressure is analyzed
over southern Minnesota and is forecast to move east across the
Great Lakes this evening and overnight. Out ahead of the low,
unseasonably warm and moist conditions persist with temperatures
in the mid 60s and dew points into the low to mid 50s. With fog
and low clouds hanging on so late into the morning and early
afternoon for much of the CWA, and still in place across portions
of east-central IL and northwest IN, dewpoints have not yet been
able to mix out and AMDAR soundings indicate a strong inversion
based around 920-940mb should limit additional mixing the rest of
the afternoon. This should allow for fog to redevelop this evening
and overnight, possibly dense once again. A weak trailing cold
front is expected to move across the area during the predawn hours
Thursday turning winds out of the northwest and eventually winds
will swing around to the northeast tomorrow morning as a modest
ridge builds over the Upper Midwest. Northeast flow will keep
temperatures cooler tomorrow, especially close to Lake Michigan,
but still seasonably mild. Farther Inland, expect highs into the
50s, and 60s are possible as you near central Illinois.

Meanwhile, a strong upper level disturbance is progged to dig
across the intermountain west tonight and shift east across the
Central Plains Thursday night into Friday becoming negatively
tilted as it nears the Midwest. At the surface, low pressure will
lift from western KS midday Thursday to Iowa midday Friday. Broad
southerly flow will transport moisture rich air up the Mississippi
Valley through the day Friday with 1.0+ inch PWats lifting into
the local area tomorrow evening. There are some modest timing
differences but thinking the CWA will remain dry through at least
mid afternoon with chances for precip increasing from the
southwest through the late afternoon. Most of the area could stay
dry through 00Z but eventually expect precipitation to become
widespread as the high PWat air moves in along with 300-500 J/kg
MUCAPE that would support a few embedded thunderstorms. The focus
of the LLJ and strongest moisture convergence will shift to the
IL/WI state line shortly after daybreak Friday morning with mid
level dry slot wrapping into the local area.

Details in the evolution of the surface pattern remain a bit
murky for the day but in general expect and occluding low shifting
east across Iowa and northern Illinois through the day Friday
with the surface warm front draped over or just north of the local
forecast area. Changes in frontal position will impact temps but
in general expect 40s north of the boundary and 60s south with a
tight gradient possibly over our northern tier or two of counties.
Very steep 700-500mb lapse rates of 8-9C are expected to
overspread the area Friday resulting in a corridor of 600-800 J/kg
MLCAPE. Strong capping inversion around 750mb during the first
half of the day will begin to erode during the afternoon as broad
ascent overspreads the region associated with mid level height
falls and left exit region of the upper jet. An eastward advancing
surface cold front should be able to kick off thunderstorms
within a weakly to modestly buoyant environment, but strong shear
will result in a threat for severe weather. The primary hazard
will be strong straight line winds, but cannot rule out tornado
development as well. Threat for severe weather should start to
shift off to our east around 00Z and also begin to diminish with
loss of daytime heating.

Deubelbeiss

&&

.LONG TERM...

Friday night through Wednesday...

201 pm...Main forecast concerns include much colder air this
weekend with the potential for snow Saturday morning...and then
another storm system Tuesday/Wednesday next week.

By Friday evening...the bulk of the thunderstorm activity is
expected to be east of the cwa with some lingering showers
possible...mainly in the eastern cwa. Colder air will be spreading
into the cwa and its likely there will be a lull in the precip as
the dry slot moves across the area. By Saturday morning...the
main shot of colder air will arrive and with it a period of light
snow or snow showers is expected and likely across most of the
area. But the best chance of any accumulation appears to remain
across far northern IL and confidence as to how much may
accumulate is fairly low from this distance. The ground will be
warm and what snow does accumulate may end up just on elevated and
grassy surfaces. However...did bump pops up some and with the qpf
amounts approaching a tenth of an inch in the far north...snow
amounts near an inch are possible...again low confidence with
amounts diminishing to just a few tenths between I-88 and I-80.
This activity should be ending by early afternoon but be replaced
with strong/gusty northwest winds. Still some differences with
regard to the pressure pattern/surface low location but gusts into
the 30-35 mph are certainly possible.

A ridge of high pressure will move across the area Saturday night
with winds turning back southerly on Sunday and then a weaker
system moves from the central plains Sunday to the Ohio Valley
Monday. Bulk of the qpf with this system remains mainly south of
the cwa. If precip does materialize in the southern cwa...it might
hold off until after sunset Sunday evening and by then precip
type could be a mix of rain/snow changing to all wet snow. Still
quite a bit of uncertainty from this distance.

Another weak high passes across the area Monday afternoon/evening
and then the next system arrives Tuesday with still quite a bit of
uncertainty as to how this system evolves. Low pressure is likely
to develop over the central plains Tuesday and move northeast
across the western lakes Tuesday night into Wednesday. This should
keep precip mainly rain across the area as the warm front lifts
north across the area Tuesday but given the low confidence...large
changes will be possible over the next few days though best chance
of precip currently appears to be in the Tuesday afternoon/evening
time period. Another period of colder temps then looks possible
behind this system Wednesday into Thursday. cms

&&

.AVIATION...
For the 06Z TAFs...

Still some potential exists for fog overnight, probably light fog
if it even develops at the terminals, but confidence in fog not
becoming a problem is increasing. Weak cold front will move
through the terminals overnight with a wind shift to northwest,
but the more important front will be a lake enhanced front that
develops and coming ripping in off the lake during the morning
hours. Some of the more recent guidance has slowed the arrival of
the front, but more often than not these fronts move quicker than
expected, so opted to leave the timing from the previous TAFs for
now. Once front develops and is on the move, should be able to get
a better timing later tonight. Behind the lake front, a period of
low CIGS is possible, especially closer to the lake, with either a
slow rise in CIGS during the afternoon if not an outright
scattering out of the CIGS.

Conditions will likely deteriorate again Thursday evening as
showers and perhaps some thunderstorms move into the region. For
now have kept thunder out of the TAFs, but this is something that
will need to be considered in later updates. Increasing low level
moisture should set the stage for a re-development of low CIGS,
likely IFR Thursday night.

Izzi

&&

.MARINE...
227 AM CST

The main forecast concerns continue to focus on the significant
storm system expected to impact the region Friday into Saturday.

An area of low pressure currently shifting over northern Lake
Michigan, will drive a cold front southward down Lake Michigan
this morning. This will set the stage for increasing northerly
winds, likely up around 25 KT during the afternoon. These
northerly winds will allow waves to build above 4 feet over
southern lake Michigan this afternoon, so a small craft advisory
will be issued for the near shore waters this afternoon into
tonight.

The main storm system expected to impact the lakes region, will be
shifting out over the central Plains later today, then begin to
shift east-northeastward towards the lake late tonight into
Friday. As this occurs, winds will increase more and shift
northeasterly in response to strong pressure falls occurring over
northern Illinois and eastern Iowa. As a result, it appears
likely that northeasterly gales of 35 to 40 kt will develop over
the northern 2/3rds of the lake later tonight, and persist on
Friday over the northern half of the lake. The strongest winds
will likely be on the western half of the lake. For this reason,
we have issued a gale warning for all but the southern 4 open
water zones of Lake Michigan for late tonight through much of
Friday. Note that this warning does not include the Illinois or
Indiana near shore waters, as the stronger winds should be north
of these areas.

Expect the surface low to shift over the lake on Friday
afternoon/evening. As this occurs, a period of lighter winds may
setup over portions of the lake. However, as the low continues to
the northeast, expect a period of strong west-northwesterly winds
to setup over the lake late Friday night and on Saturday. These
west-northwest winds could result in the need for another gale
warning for the lake during this period.

Gale force winds should ease over the lake Saturday night as a
surface ridge of high pressure shifts eastward from the Plains to
the Lower Mississippi Valley by Sunday morning. This high is then
expected to continue towards the southeastern Atlantic coast by
Sunday evening. Ultimately, this pressure feature will allow the
winds over the lake to back southwesterly over the lake by Sunday
afternoon. While wind speeds will likely ease early Sunday
following the northwesterly gale event, southwest winds will be on
an upward trend Sunday afternoon as a fast moving clipper system
tracks eastward over the Upper Midwest. Therefore, a period of ~30
KT southwesterly winds are possible late Sunday into Sunday
evening.

While forecast uncertainty increases in the extended period, it
does appear that another storm system could take aim on the Great
Lakes region next week, either on Tuesday or Wednesday. As a
result, this could set up another period of strong winds over the
lake.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Gale Watch...LMZ777-LMZ779...MIDNIGHT Friday TO 9 AM Friday.

&&

$$

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