Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 090301

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
901 PM CST Mon Mar 8 2021

900 PM CST

The going forecast is in good shape. The main updates this evening
were to bump sky cover up, re-trend temperatures, and pare back
the patchy fog mention a bit.

A shortwave is pressing east this evening across Kansas and
Nebraska, and will continue to toss some additional high-level
moisture our way in the form of steadily increasing cirrus
tonight. The presence of this high cloud cover, combined with
steadily increasing flow in the 975-950 mb layer with time should
help hold temperatures up a bit, especially with southward extent
across the CWA where temperatures at this hour are still in the
lower 50s. Forecast soundings through the rest of the night also
show notable decreases in mixing ratios immediately off the deck
which taken with the above point a bit more away from fog
development. Have trimmed the areal extent of patchy fog wording
as a result, but have left this mention intact near the lingering
snow cover across our northwestern locales and closer to a diffuse
surface boundary near the Wisconsin state line. Updated products
have been transmitted.



200 PM CST

Through Tuesday night...

Currently, temperatures are some 20 to 25 degrees above normal and
in the upper 50s to middle 60s across northern Illinois and
northwestern Indiana. Breezy southwest winds gusting 25 to 30 mph
are making it feel a smidge cooler, but otherwise it sure looks and
feels like spring outside! Our stretch of above-normal temperatures
has also come with no precipitation, with the last instance of such
on February 28. In fact, never in our records has the first 8 days
of March featured no precipitation in Chicago! The same is not true
in Rockford as several years featured measurable precipitation on
the 1st of March.

Tonight, winds will become light and variable as a surface
pressure ridge builds into the western Great Lakes. Patchy ground
fog is likely across parts of north-central Illinois where at
least some melting of the snow pack has contributed to low-level
moisture. For example, surface dew points in Rochelle and Rockford
are in the lower 40s, while readings across central Illinois are
in the lower 30s. Lows tonight will fall toward the upper 30s
across the remaining snowpack to lower 40s elsewhere.

Tomorrow will be similar to today with much above normal
temperatures and breezy south winds. We do think there will be
modestly more cloud cover tomorrow compared to today as an upper-
level wave currently over Colorado makes it to our neck of the woods
scoots our way. Even so, high temperatures should still have no
problem climbing into the 60s, including where a snowpack remains.
In fact, we would not be shocked if a few locations hit 70,
especially south of I-80 where diurnal mixing may drop dew points
into the lower 30s making the air "easier" to heat up. Such a
scenario would become more likely should cloud cover end up less
than currently expected. Those planning on burning brush should do
so with care, as the dry and warm airmass with gusty south winds
will allow for fires to spread readily. Tomorrow night will be
downright mild with overnight lows in the lower 50s...which would
be otherwise be considered as "above normal" daytime highs! While
our stretch of warm weather is certainly welcome, it is not
unprecedented. Who can forget about the record-breaking warmth of
March 2012, which featured a week of daily highs in the upper 70s
to lower 80s and lows in the 60s?



300 PM CST

Wednesday through Monday...

After a prolonged quiet and dry stretch to begin March, we will
quickly transition into an active pattern mid to late week.

Pronounced upper-ridging that will have resulted in multiple days
of unseasonably warm conditions by mid-week will begin to shift
east of the area Tuesday night as a deep western CONUS long-wave
trough begins to breakdown. Surface low pressure currently
developing over the central High Plains will drape a cold front
from roughly Kansas to Lake Superior Wednesday morning as another
low forms over the central Great Plains.

Wednesday morning: A weak mid-level wave within strengthening
flow will lift NE from Kansas to Wisconsin Tuesday night into
Wednesday morning in conjunction with quickly increasing low-level
moisture. An associated area of showers with a few embedded
storms will likely brush at least northwest and north-central
Illinois late Tuesday night through Wednesday morning, with
decreasing coverage with southeast extent across the CWA.

Wednesday afternoon: Precip chances beyond Wednesday morning will
hinge on the amount of low-level cloud cover lingering under a
replenished EML above 800 hPa. This will effectively cap the
potential for any deeply forced precip. However, the residual low-
level cloud cover may be sufficiently thick to foster either
pockets of very light rain or diurnally enhanced light showers as
the area remains within broad low-level isentropic ascent.
Moreover, uncertainty in the coverage of low-level clouds raises
some questions on both high temps and the potential for strong
winds during the afternoon. While a strong pressure gradient alone
will result in gusty winds during the afternoon, any enhanced
mixing owing to less cloud cover will tap into 40-45 knot winds as
low as 3kft. Will continue to message possible SSW gusts to 40
mph Wednesday afternoon, though stay tuned to forecast updates as
advisory-level winds (gusts 45mph+) may be reached for at least
some of the CWA.

Wednesday night through Thursday: Mid to upper-level forcing will
increase across the area during this time as the aforementioned
boundary to the north drifts south toward, then across, the CWA.
PWATs increasing to as much as 1.2" from deep moisture transport
within a dynamically strong wind field will result in the
development of widespread rain across the area overnight Wednesday
through much of the day Thursday. Low-level flow somewhat
parallel to the boundary will support an axis of moderate rain
with some embedded storms somewhere across the northern 2/3 of
Illinois and north half of Indiana during this time. The lower
PWATs compared to what would be more common in a few months will
limit the heavy rain potential, but a swath of 1-2" across the CWA
is possible. At this time, the favored area is across the
southern CWA. Additionally, as noted in previous forecasts, a
slower progression of the boundary will increase the small risk
for isolated strong storms across the far southern CWA given the
low-CAPE/high-shear set-up.

Friday through next weekend: Consensus guidance suggests the
boundary will progress far enough south that the final breakdown
of the original western CONUS trough will remain south of the area
through the weekend. Temps will return to seasonable levels in
the 40s.



300 PM CST

A very mild air mass will be over the region through Wednesday.
Here is a look at record temperatures at Chicago and Rockford for
Tuesday, March 9 and Wednesday, March 10:

Tuesday March 9th
Record High for Chicago: 69 in 1974 (forecast is 67)
Record High for Rockford: 68 in 1977 (forecast is 62)

Wednesday March 10th
Record High Min for Chicago: 52 in 1876 (forecast is 53)
Record High Min for Rockford: 44 in 2010 (forecast is 53)



For the 00Z TAFs...

Aviation Forecast Concerns:

* Light/variable winds develop this evening, then turn southerly
  Tuesday morning.

* Strong south winds Tuesday become gusty near 30 kt at time in
  the afternoon.

* South winds ease slightly but likely remain gusty Tuesday
  evening, with LLWS potential once again.

Weak surface cold frontal boundary was sagging south across
southern WI late this afternoon, and will stall across northern IL
later tonight. Winds will become light and variable in the weaker
gradient in the vicinity of the boundary, and with weak high
pressure developing across southern WI/northern IL overnight.
Parts of north central IL where some snow cover lingers may see
some patchy fog development overnight, though a fairly extensive
area of high clouds will spread across the region and will likely
help limit the extent of this fog. Mainly KRFD would be at risk
here, with some snow cover and added moisture from snow melt

Deep low pressure will lift northeast across the Northern Plains
Tuesday, with surface pressure falls across the Plains and upper
Midwest leading to the development of strong gusty south winds
across the forecast area especially by early afternoon. Forecast
soundings support gusts approaching 30 kts. Models are likely too
quick to stabilize the boundary layer and decrease winds Tuesday
evening, with south winds likely easing slightly but remaining
gusty. Southwest low level jet of 45-50 kts edges into the region
by late evening, suggesting a period of LLWS through the night
even with some surface gustiness persisting.

Other than any patchy shallow fog away from the Chicago metro
terminals tonight, mainly mid and high level VFR cloud cover is
expected through the period.



300 PM CST

There is increasing concern for southerly gale force winds across
the Illinois and Indiana nearshore on Wednesday into Wednesday
evening. Prior to that, expect periods of hazardous southwest to
southerly wind speeds/gusts Tuesday-Tuesday night. A fairly deep
low pressure will track northwest of the lake on Wednesday,
tightening the pressure gradient. Despite clouds and some morning
rain on Wednesday, the higher speeds aloft are fairly low to the
surface. Increasingly stable conditions are likely out beyond 3
miles or so, but as is typical in these setups, the 35+ gales can
extend at least a couple miles into the nearshore waters. Given
uncertainty in the frequency of gales due to upstream cloud cover
on land, held off on the issuance of a Gale Watch with the
afternoon forecast.





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