Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230459

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1059 PM CST Wed Feb 22 2017

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

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.



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.




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 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


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.




201 pm...Areas of dense fog extend across much of the open waters
of Lake Michigan this afternoon. A cold front will move south
across the lake tonight and this should end the fog threat across
at least the northern portions of the lake but dense fog may
persist into the overnight hours tonight across the southern parts
of Lake trends will need to be monitored this

Low pressure will move across northern portions of the lake this
evening with a trailing cold front moving south across the lake
which will shift winds northeasterly and these winds will increase
to 30 kts on Thursday as low pressure over the central plains
moves northeast to the western lakes by Friday night. Winds are
expected to increase to low end gales Thursday night as the low
approaches and the gradient tightens. Once the low moves east of
the lake Saturday...a period of northwest gales will be possible
Saturday into Saturday evening. cms


LM...Gale Watch...LMZ777-LMZ779...MIDNIGHT Friday TO 9 AM Friday.




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