Plainfield, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

884
FXUS63 KLOT 032022
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
222 PM CST Mon Feb 3 2025

.KEY MESSAGES...

- Areas of fog will continue this afternoon, primarily near and
  north of I-80. Dense fog will continue over portions of the
  Lake Michigan nearshore into early evening.

- Confidence continues to increase in a wintry mix including ice
  accumulations in our area in the Wednesday night to Thursday
  timeframe.

- Another chance for a wintry mix may materialize next weekend.

&&

.DISCUSSION...
Issued at 222 PM CST Mon Feb 3 2025

Through Tuesday:

Early this afternoon, a quasi-stationary frontal boundary
bisects our CWA from west to east. Quite the temperature
gradient has developed on either side of this boundary, as was
expected. Many of our northern sites are still stuck in the
upper 30s as of 1:30 PM while sites along the I-80 corridor have
made it into the lower and middle 50s. A couple of sites south
of I-80 have even tapped 60 already. This front has begun to
make gradual headway to the south, which will continue until the
front exits to our south this evening as high pressure advances
into the region from the northwest.

A bank of fog and low stratus has been extending north of the
front all morning, which has recently begun to propagate
southward as the front has. Widespread visibilities between a
half and one mile are now being reported across the interior
Chicago metro including in and around the city. This fog is
expected to continue spreading south down to about the I-80
corridor late this afternoon before dissipating. Despite diurnal
cooling, dry advection just off the deck behind this front
should do away with the fog not long after dark. The fog is
expected to linger over the lake a bit longer than over inland
areas, although improvements are already underway along the Lake
County (IL) shoreline. The southern bend of the lake is just
beginning to fog up now as the front has now made it into
northwest IN. A Marine Dense Fog advisory is in effect for our
nearshore zones through 6 PM this evening.

The cold airmass spilling over the region behind the front will
pull low temperatures down into the 20s area wide by late
tonight, possibly even upper teens near the IL/WI state line.
Some drying aloft tonight into tomorrow should offer up a mix of
clouds and blue sky during the day tomorrow. Continued cold
advection won`t allow temperatures to climb all that much during
the day tomorrow. Highs are forecast in the mid 20s across our
north up to the mid 30s in our south. Skies will cloud up again
during the evening and night ahead of our next system of precip
slated to arrive early Wednesday.

Doom

Tuesday Night through Monday:

Toward the middle of the week, an upper-level shortwave
originating from the central Pacific ocean will race across a
baroclinic zone draped across the central United States ahead of
a more pronounced upper-level wave traveling along the
US/Canadian border. Ensemble model guidance remains in fair
agreement that a weak surface low pressure system, if not
surface pressure trough, will accompany the upper-level
shortwave. With that said, spread remains on the exact strength
of the wave (ECMWF/EPS remains notably stronger than the
GEFS/GFS), as well as timing (the most recent iteration of
guidance depicts a slower evolution of the system as a whole).
Regardless, the "big picture" evolution of the system is more or
less consistent in that a broad region of warm-air advection-
driven precipitation will lift northeastward into the Lower
Great Lakes in the Wednesday afternoon through early Thursday
morning timeframe.

With easterly winds forced by a strong 1030+ mb surface high
positioned over New England providing a supply of relatively
dry low-level air as precipitation arrives, wet-
bulbing/evaporative cooling will cause initially above-freezing
surface temperatures to fall below freezing. Meanwhile,
increasing low-level warm air advection will cause temperatures
from about 1500 to 5000ft AGL to warm above freezing atop the
sub-freezing temperatures. The net differential low-level
thermal profile will support a messy wintry mix with
precipitation type at the ground starting as snow and sleet
before transitioning to freezing rain (liquid water that freezes
on contact with the ground). Generally speaking, the transition
will occur from south to north in tandem with northward-
spreading precipitation Wednesday evening. At some point,
surface temperatures should nudge above freezing at least along
and south of I-80, causing precipitation type to turn to a cold
rain. However, freezing rain may continue north of I-80 all the
way through daybreak Thursday. All precipitation should taper
from west to east Thursday morning within a few hours after
daybreak.

Considering the wintry precipitation will be largely WAA-driven
(which is showery/streaky in nature rather than stratiform/
widespread), wintry accumulations may be somewhat uneven across
our area. Even more complicating is the threat for legitimate
convection at times, given forecast soundings from both the GFS
and ECMWF depict 100 to locally 500 J/kg of MUCAPE centered from
around -10 to locally -40C. Not only would this lead to even
more streaky precipitation amounts, but there may very well be
instances of non-damaging hail and lightning strikes embedded
in sleet and freezing rain. Taken altogether, this leads to an
expectation for highly variable ice accumulation amounts across
our area, ranging from a few hundredths near the Wisconsin state
line to perhaps locally 0.25 to 0.33" in the wake of convective
cores (near/south of the I-88 corridor?). Regardless of the
eventual uneven ice amounts, we will have to issue a Winter
Weather Advisory for some if not all of our area for the
Wednesday evening into early Thursday morning timeframe for
hazardous travel conditions, downed tree limbs, and localized
power outages. (Note that the criteria for an Ice Storm Warning
is 0.25" of ice accumulation. While such amounts may materialize
on a local basis in our area, would prefer to see winds
stronger than 10 mph during and after the period of greatest ice
accretion as well as a forecast for widespread accumulations
>0.25" to warrant the issuance of a Winter Storm Watch and
eventual Ice Storm Warning).

One final thing with regard to the mid-week period - There
remains a threat for a narrow band of snow with surface
accumulations at some point on Wednesday in northern Illinois or
southern Wisconsin prior to the arrival of the wintry mix. The
band of snow would be associated with a subtle lead wave acting
upon the northward-moving baroclinic zone to force a narrow zone
of frontogenesis. While forecast soundings depict a stout wedge
of dry low-level air from the surface to around 5000 ft,
relatively steep lapse rates of 6-7K/km in the DGZ and in the
upward-moving portion of the frontogenetical circulation may
allow for snow rates to overcome the dry air and "break through"
to the ground in a very localized band. Far too often have we
seen a band of virga translate into snow (with accumulations)
in spite of little to no signal depicted by modeled QPF output
in these sorts of regimes. For this reason, did opt to maintain
the inherited 20% chances for snow areawide on Wednesday prior
to the arrival of the wintry mix.

Continued warm-air advection will allow for surface temperatures
to climb into the mid 30s areawide on Thursday, allowing for
ice to begin melting. Ensemble model guidance advertises the
next threat for precipitation, which may very well be a wintry
mix, sometime around Saturday.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1132 AM CST Mon Feb 3 2025

Key Aviation Messages for the current TAF period:

- IFR vis/cigs through the early afternoon

- Winds switch to the northeast with a period of wind gusts up
  to 25 knots this afternoon

A warm front is draped over northern Illinois with southerly
winds in Central Illinois and northeasterly winds in southern
Wisconsin. The Chicago terminals remain in the in between with
winds currently (at the time this discussion was published) in
the process of going from northwest to northeast as the front
sags southward. IFR cigs and vis are being observed along and
behind the front. As a reinforcing front moves southward this
afternoon, a period of lower vis down to 1 mile is possible at
area terminals. Visibilities are expected to improve behind the
front as it moves south. Winds will prevail out of the northeast
behind it and gust between 20 to 25 knots through the
afternoon/early evening. IFR cigs will linger through the
afternoon before lifting to MVFR levels this evening.

While wind gusts are expected at terminals through the evening,
there is lower confidence on the timing of when they will
diminish. However, guidance has a signal that they could flip
over to the northwest for a brief pre-dawn period. Winds will
then return to the northeast after day break. VFR conditions
are expected at the end of the TAF. A few isolated gusts to 20
knots are possible, but not for a long enough duration for a
formal mention in the current TAF.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 6 PM CST this evening for the IL and
     IN nearshore waters.

     Small Craft Advisory until 3 PM CST Tuesday for the IL
     nearshore waters.

     Small Craft Advisory until 6 PM CST Tuesday for the IN
     nearshore waters.

&&

$$

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