Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 011117

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
517 AM CST Mon Mar 1 2021

238 AM CST

Through Tuesday...

We`re in for quite the change today as northwesterly breezes which
have been persistent through the night will continue to drive a
at least a portion of a renewed cP airmass into the region today.
With lingering cold advection through the day, temperatures should
only manage to claw their way into the low to mid 30s, about 15
to, in some cases, 20 degrees cooler than yesterday. Winds won`t
be as gusty, but a fresh breeze around 15 to 20 mph will hold wind
chill values in the 20s.

A fast-moving disturbance--caught up in the renewed broad cyclonic
flow aloft--is currently pressing southward out of the Minnesota
Arrowhead Region. While the overall magnitude of forcing with this
clipper will gradually wane with time as it continues its
southeasterly journey (surface reflection should diminish to an
open wave by early this afternoon), model guidance/soundings
depict a bit more in the way of mid-level moisture coincident with
some impressively steep lapse rates as ascent spreads across
northern Illinois by mid-late morning. Surface dewpoints dropping
in the teens does indicate the presence of a much drier lower
troposphere today, but the combination of lingering modest ascent
through the dendritic growth zone and sufficient saturation down
to around 850 mb in the latest soundings justifies a brief mention
of flurries across northern Illinois this morning along with some
beefed up sky cover.

Surface-850 mb delta Ts around 10 C and the quick arrival of this
much drier airmass does look to greatly limit any lake effect
potential into northwest Indiana this evening and overnight,
although envision we`ll hang onto at least some lake effect cloud
cover for a time this evening. High pressure glides overhead
tonight, sending winds nearly calm area-wide. Given the prevalence
of single-digit dewpoints upstream, think we`ll be set up with
a nearly ideal radiational cooling potential tonight. In addition,
while the 40-50 degree dewpoints and warm temperatures yesterday
essentially obliterated the snowpack across central and northeast
Illinois, last light visible satellite imagery did show lingering
snow across northwest Illinois. With this in mind, have dropped
temperatures tonight into the lower teens (maybe some single
digits possible in our favored cold spots over the lingering
snowpack) and 20s.

Tuesday will be a bit warmer as warm advection re-develops while
our high pressure departs off into the Ohio River Valley.
Increasing flow just off the surface spells the potential for some
gusty southwesterly winds as well. Momentum transfer profiles
support gusts towards 30 to maybe 35 mph across northern Illinois
and a bit lower with southward extent.



238 AM CST

Tuesday night through Sunday...

A reinforcing cold front is slated to arrive into the area
sometime late Tuesday night and into Wednesday. While the NAM is
possibly suffering from its typical boundary layer moisture
problems, also starting to see some signs in the guidance favoring
more cloud cover as flow turns off the lake. The GFS and CMC have
seemingly also trended a bit cooler/pessimistic in this regard
while the ECMWF/ECE continues a much warmer tune. For the time
being, have trended temperatures near the lake cooler and spread
this a bit farther inland, with mid 40s to low 50s well inland and
into central Illinois, but there could be a bit of a bust
potential here with this latest frontal passage.

The main story then through the rest of the week looks to be one
of colder temperatures near the lake with persistent light flow
and the development of afternoon lake breezes. While a
considerable degree of spread exists, there`s a weak signal for a
shortwave to drop almost due southward out of Wisconsin on
Saturday with sufficient mid-level saturation to at least toss
some additional cloud cover our way. This evening`s ECMWF and GFS
are trying to squeeze out some light precipitation in the region
of nearby. Ensemble output suggests this remains a low potential
and have left the forecast precipitation-free at this time.

With the persistent north to northeasterly flow, did also adjust
high temperatures this weekend down below the offered blended NBM
guidance which came in towards the 75th percentile (or higher)
among the entire guidance envelope. Thereafter, extended guidance
suggests we may turn a bit more active towards next week with
some ensemble chicklets showing some non-zero values of surface-
based CAPE...



For the 12Z TAFs...

Northwest winds, at times gusting up to 20 kts, will be the norm
this morning. Broken mid-level clouds will stream in after
daybreak from the northwest followed by a secondary cloud deck
based near 3000-35000 feet by early afternoon. A few flurries
will be possible mainly from 17-20Z with no visibility
restrictions of accumulations expected. Clouds are expected to
clear by late afternoon as winds become light. As a surface high
pressure system slides overhead tonight, winds will drop to 3 kts
or below at all terminals. A south to southwest wind direction
will become established toward the end of the TAF period.



LM...Small Craft Advisory...LMZ743 until 9 AM Monday.

     Small Craft Advisory...Gary to Michigan City IN until 6 PM Monday.



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