Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 080829
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
329 AM CDT Sat May 8 2021

.SHORT TERM...
328 AM CDT

Through Sunday...

Clear skies and calm/light and variable winds have allowed
temperatures to readily fall into the low to mid 30s across the
region early this morning away from the heart of Chicago and near
the immediate lakeshore where temperatures are still in the upper
30s and lower 40s. The Frost Advisory handles things very well,
with even a few spots likely to dip to and just under freezing for
a brief time this morning. Temperatures today should be fairly
similar to Friday`s readings, although likely a few degrees cooler
across our lakeside locales as a freshening lake breeze looks to
get a move on pretty early.

Our next weather-maker--a robust PV anomaly which can be seen
pinwheeling eastward into Montana on moisture channel imagery--
continue to drag an elongated trough axis east across the Great
Basin, and this will eventually emerge east of the Continental
Divide later today. Enhanced upper divergence within the left exit
region of a developing 100 kt jet streak will foster cyclogenesis
across western Kansas through this afternoon and evening as
everything gradually shift east into this evening. An extremely
impressive frontogenetic circulation is forecast to develop with
time this evening and into Sunday morning as a loosely-coupled jet
materializes overhead. Resulting forcing for ascent ramps up
quickly, and it looks like we`ll manage to saturate up the the
lowest 10 kft or so pretty readily through this evening. Do
envision that most of the light reflectivity values in the CAM
guidance this afternoon is mostly virga/sprinkles given the dry
sub-cloud layer, however and have limited lower PoPs to the
southwestern half of the forecast area.

The main change/addition with this forecast issuance was to expand
the snow mention (yep...snow...) a bit across interior portions of
northern Illinois late tonight and very early Sunday morning. Any
precipitation change-over will essentially be tied to system
dynamics, and likely in a small corridor (and brief window) under
the maximized region of 700 mb frontogenesis. Guidance has been
all over the map in this regard, and there continues to be some
degree of uncertainty on the northern terminus of the better
precipitation chances as mesoscale subsidence to the north of the
f-gen circulation and a drier low-level airmass will chew away at
things.

The 00z guidance suite seems to have at least started to rectify
some of these spatial differences, with the deterministic ECMWF
jogging a bit north (as well as its ensemble system) and is now
mostly in line with the rest of the global output and short
range/hires solutions. The UKMET remains a bit farther south, but
the guidance trends tonight have at least helped increase
confidence in spreading some higher PoP chances to the Wisconsin
state line. As the aforementioned f-gen circulation peaks in
intensity (roughly 2 AM tonight to 8 AM Sunday), mid-level lapse
rates are forecast to steepen appreciably, with theta-e lapse
rates trending solidly negative in the 700-550 mb layer. Did even
contemplate adding some isolated thunder to the grids into
northern Illinois as a result, but held off for the time being--
there`s probably a slightly stronger signal for this into central
Illinois where MUCAPE values are forecast to inch past 100-200
J/kg. The result of this intense forcing and steepening lapse
rates will be to augment precipitation rates such that the column
is forecast to cool perilously close to the freezing mark, and
internal Bourgouin-based energy grids do support the expansion of
a snow mention for locales north of I-80 and away from the lake
where strengthening onshore flow will probably keep total column
positive energy high enough to fully melt any frozen hydrometeors.

We continue to suspect that the remarkably high snowfall output
being advertised by the HRRR is considerably overdone and taken
too close to the Lake Michigan shoreline. Perfect-progging its
sounding output really only supports 2 to maybe 3 hour window
where all snow would be favored, and Cobb-based snow-to-liquid
ratios are probably being impacted to the high side by the
extremely large vertical velocities present through the column.
Atmospheric moisture content will undoubedtly be high, and snow is
certainly looking possible across northern Illinois, but the
current look of things suggests that this should generally remain
limited to a rain/snow mix in spots--maybe briefly all snow where
precipitation rates maximize. With the current forecast, we`ll
continue to advertise temperatures that are just a bit too warm to
start accumulating snow amounts, but do note that some minor
slushy accumulations on grassy surfaces are possible roughly
around the I-88 corridor and away from the lake.

Total precipitation amounts may near or exceed 2 inches in spots
south of I-80, but given the very dry antecedent conditions, we`re
not overly concerned with a flood threat although some minor
ponding could result where rainfall rates do maximize.
Precipitation should taper from west-to-east through Sunday
morning and afternoon with just some lingering northeasterly
breezes through the day.

Carlaw

&&

.LONG TERM...
232 AM CDT

Sunday night through Friday...

The persistence of an unseasonably cool weather pattern through
the middle of next week, continues to be the main weather story
during the period.

A blocky upper level weather pattern across the northern CONUS and
Canada will persist through Sunday before breaking down into a
more progressive pattern into early next week. The pattern shift
will allow the upper low (and the associated colder airmass) over
Ontario to migrate south-southeastward into the Great Lakes on
Monday as a surface Canadian high drops southward into the Upper
Midwest. This will essentially drive a reinforcing shot of
unseasonably cool conditions into the region on Monday.

While a generally dry pattern is expected across the area
following Sundays precipitation event, I cannot rule out
a few scattered showers during on Monday as a quick moving mid-
level impulse shifts across the area. For now, I have kept Monday
dry, but we may need to considering adding in a small chance for
light showers/sprinkles. Otherwise, expect cool conditions again
on Monday, with highs only in the 50s (coolest near the lake). A
downright chilly night is then likely Monday night, as skies clear
and winds ease. Overnight lows look likely to drop into the low
to mid 30s outside of Chicago, so this will be another night of
potential widespread frost.

Surface high pressure moves overhead on Tuesday. Mostly sunny
skies should help push inland temperatures back into the low 60s,
but a lake breeze will keep temperatures in the lower 50s near
the lake. Temperatures then look to fall into the 30s again
Tuesday night. This may support another night favorable for
frost development, but we will have to monitor the possibility
for more high level cloudiness.

Temperatures will continue to moderate back into the middle 60s
for inland areas for Wednesday and Thursday. Both days are likely
to be dry across the area, but I cannot rule out a few showers on
Thursday as another mid-level disturbance looks to move overhead.
However, limited moisture could allow this disturbance to move by
with no more than some cloud cover.

The turn towards a more progressive upper level pattern should
allow another disturbance to approach the area late in the week.
The potential for better moisture return with this feature may
result in better chances for showers and potently some storms late
in the period. Otherwise, expect temperatures to warm closer to
normal by the end of the forecast period.

KJB

&&

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

There are no major aviation weather related concerns through this
afternoon and early evening. For the rest of the overnight hours,
winds will trend light and variable (except at GYY where a
northeast wind will likely prevail) as high pressure continues to
build in overhead. Dewpoint depressions have fallen a bit at DPA,
and with the clear skies and light winds, some very shallow/patchy
BR remains possible just before daybreak. At this time, don`t
anticipate this being anything under some light VFR BR, but will
continue to monitor observations and guidance trends tonight.

Winds at ORD/MDW may start off this morning (after 12z) light
northwesterly, but a freshening lake breeze is expected to pretty
quickly move across the terminals later this morning and
afternoon, switching winds to the east and east/northeast. The
inherited 18z timing looks good and have made no changes there,
with this wind shift a bit earlier at GYY and a bit later at DPA.

Forecast confidence begins to drop off notably overnight tonight
and into Sunday morning as our next storm system approaches. Model
guidance continues to indicate a fairly sharp northern cutoff in
both precipitation and lower cigs/vsbys as this system develops.
At this extended range, elected to favor a more optimistic tune,
indicating a period of light/VFR rain developing this evening as
the top-down saturation process begins, with a lowering to MVFR
late tonight/early Sunday morning. If some of the current guidance
is correct, cigs/vsbys may be a category or so lower at MDW than
ORD, but this level of detail won`t be added at this range. We`ll
continue to keep an eye on guidance trends over the next several
TAF iterations, as some current guidance suggests that much lower
cigs/vsbys are possible at the Chicago area terminals late
tonight, even with the potential for some snow to mix in at times.
This latter potential remains on the outer ends of the
probabilistic envelope, and have refrained from a mention of
wintry precipitation in the ORD/MDW extended TAFs as a result at
this time.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Frost Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039-ILZ103-ILZ105-ILZ106-ILZ107-ILZ108 until 8 AM
     Saturday.

IN...Frost Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 until 8 AM
     Saturday.

LM...None.
&&

$$

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