Area Forecast Discussion
Issued by NWS Chicago, IL

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403
FXUS63 KLOT 240513
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1113 PM CST Thu Feb 23 2017

.UPDATE...
844 PM CST

Primary change to going forecast this evening was to sharpen up
the temperature gradient across the area tomorrow. Strong warm
front is looking increasingly likely to make it as far north as
about I-80, with areas north of the front likely to see low
clouds, fog, drizzle, and occasional showers with stiff northeast
winds likely to hold temps in the upper 30s or lower 40s. GFS and
NAM have been consistent their past few runs with this, though
RAP and HRRR are a bit more gung-ho on bringing the front a bit
farther north, especially given the tendency for the cold lake to
retard northward advancing warm fronts this time of year.

To the south of the boundary temperatures should climb well into
the 60s with 70 not out of the question far south if the sun
breaks out. Big question convectively for tomorrow is how far west
will initiation occur. Impressive EML advecting into the area
should provide for a strong capping inversion that will take some
work to overcome, though strong forcing and heating in the warm
sector should eventually cause cap to break, just a matter of
whether it is over our eastern CWA or just to our east. Unusually
strong instability for this time of is progged tomorrow with very
strong shear as well, leading to the potential for supercells and
damaging wind producing line segments. At this distance, too hard
to say exactly where initiation will take place, so no changes
made to that aspect of the forecast this evening.

Continued isentropic ascent tonight should lead to continued
development of scattered showers and eventually some thunderstorms
too as steeper lapse rates advect north and lead to increasing
elevated instability. Overall, forecast for tonight appears to be
in good shape. Did add some fog to the forecast for late tonight
and especially Friday morning north of the warm front, where some
locally dense fog could form, especially north of the warm front
close to the surface low.

Izzi

&&

.SHORT TERM...
230 PM CST

Tonight through Friday...

Several forecast concerns through the short term period, including
large north-south temperature gradient, locally heavy rainfall,
timing/extent of thunderstorms, and the potential for severe
weather Friday afternoon across the southeastern parts of the cwa.

Deepening surface low pressure was over far southwestern Kansas
this afternoon, with a nearly stationary frontal boundary
extending northeast into central IL and IN. This low is expected
to propagate to near far southern Lake Michigan by mid-afternoon
Friday, in response to a strong mid-level short wave ejecting from
the central Rockies. Isentropic upglide was already developing
into the region this afternoon, as indicated by rain spreading
into western IL per regional radar mosaic. Increasing south winds
above the warm frontal inversion into this evening will support
strengthening moisture transport, and an increase the coverage
and intensity of showers/rain. Guidance focuses the greatest pops
and rainfall amounts generally along/north of the I-88 corridor
across northern IL, with high-res members eventually shifting
focus mainly across and north of the IL/WI border area early
Friday morning. Forecast soundings depict precipitable water
values approaching 1.00" late tonight, with blended guidance QPF
of 0.75-0.95 inches north of I-88 indicated through Friday.
Thunderstorm potential appears fairly high for late February, with
a plume of steep mid-level lapse rates (700-500 mb >7.5 C/km)
atop 40-50 kt southwest low-level jet late this evening into early
Friday morning. Forecast soundings depict anywhere from 150-450
J/kg of elevated mucape which should support embedded thunder
especially across the aforementioned area overnight.

The surface low is progged to be over far northeast MO/western IL
near Quincy by 12Z Friday, with general model agreement in taking
it across the southern parts of the Chicago metro/far southern
Lake Michigan during the mid-late afternoon hours. With the mid-
level dry slot spreading across the cwa from the southwest during
the afternoon, the focus for surface-based thunderstorm
development is across the southeast parts of the cwa, especially
east of I-55 and along/south of I-80 (with much of the high res
CAM guidance highlighting areas from I-57 east). SPC has
appropriately tightened up the western part of the day 2 slight
risk a little further east of their previous outlook. WRF and GFS
soundings indicate fairly substantial capping around 750 mb,
though large scale height falls associated with ascent with
approaching mid-level wave, divergent left exit region of upper
jet and surface cold front should be enough to force a narrow
band of convection along/just ahead of the front during the
afternoon. While mlcapes are fairly week (<500 J/kg for the most
part), 40-50 kts of effective shear will support storm
organization. While some supercell structures are possible, the
primary severe threat should be damaging winds from convection
within gusty southwesterly 25-35 kt background wind field. Storms
should exit the cwa prior to sunset. Back to the west, the
southeastern periphery of the synoptic deformation precip should
spread in during the afternoon, but should remain all liquid
through sunset. Occasional light rain/drizzle is possible earlier
in the day as well across much of the area as low levels remain
moist beneath the mid-level dry slot and increasing deep layer
ascent.

Temperatures will continue to depict a large north-south variance
across the forecast area. Cool northeast winds north of the warm
front and low track will produce temps mainly in the 30s and 40s
this evening, but will slowly rise into the 40s-50s by Friday as
the low approaches. Farther south 50s overnight will warm to the
60s Friday before colder air surges in late.

Ratzer

&&

.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 06Z TAFs...

Multiple potential weather related aviation problems over the next
24 hours. First, scattered SHRA and TSRA will continue to develop
and move across the terminals overnight. IFR CIGS also likely to
persist into Friday, likely building down to LIFR. Best threat of
TSRA appears to be west of the Chicago terminals based on latest
radar trends, though additional development is possible, so opting
to maintain tempo TSRA for a few hours to start the TAF.

Continued moistening of the low levels and slowly approaching
warm front favor lowering CIGS and VSBY, with cold wind off the
lake further favoring very low CIGS and potential for dense fog
Friday morning as warm front attempts to lift north. For now, kept
the denser fog potential in a PROB30 group due to inherent
uncertainties in forecasting fog, however synoptic pattern quite
favorable for dense fog north of a strong warm front and MOS
guidance is hitting the threat fairly hard as well.

Latest guidance would support surface low tracking just south of
the terminals, except perhaps GYY could pop into the warm sector
Friday. North of the surface low track, very little improvement in
CIGS expected and dense fog potential could be maximized as
surface low draws closer and gradient weakens late in the morning
into early afternoon. Expect improvement to CIGS and VSBY as winds
back to northwest and somewhat drier boundary layer air advects
into the area. Winds should swing around to the west or northwest
late in the afternoon or early in the evening, but stronger and
potentially gusty winds will likely hold off until later in the
evening with the better push of cold air. Guidance also continues
to show a north-south oriented band of snow moving across the
terminals after midnight, likely resulting in IFR vsby at times.

Izzi

&&

.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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