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888
FXUS63 KLOT 191600
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1100 AM CDT Mon Apr 19 2021

.UPDATE...
1053 AM CDT

No major changes to the forecast through this afternoon. The cold
front continues to move southward and is moving across northwest
Indiana and Iroquois and Ford Counties at this time. West-east
corridor of rain continues between the I-88 and I-80 corridors
and will spread southward with time this afternoon. May see some
redevelopment along and just north of the I-88 corridor early
this afternoon as a shortwave is expected to quickly move east
across that area. Otherwise he bulk of the precip will be to the
south. Areas behind the front have seen their highs for the day
while temps of around 60 ahead of the front will fall into the
40s.

MDB

&&

.SHORT TERM...
335 AM CDT

Through Tuesday...

All eyes remain on the potential for a late snowfall across the
area beginning late tonight and continuing into Tuesday night. The
southeast trend noted in guidance 24 hours ago continued with the
00Z guidance, with a bulk of the precipitation across the CWA now
expected to occur during the day Tuesday. These changes have
further diminished the risk of any substantial snowfall in the
forecast area (though any accumulation this late could be
considered substantial) primarily due to lower precip rates and a
more diurnally-unfavorable period for snow to accumulate.

Today and Tonight: The cold front continues to progress
southeastward toward the CWA, stretching from near Green Bay to
Dubuque and into north-central Missouri as of 3am. As guidance has
noted for a couple days, a faster trend in this frontal passage
across especially the northern CWA continues to favor a colder day
today than was anticipated early this past weekend. Temps will
struggle to reach 50F north of I-80 where cloud cover will
dominate through the day. A fairly prominent swath of low-level
f-gen developing along the front will foster a narrow band of rain
late this morning through this afternoon. The initial focus will
be centered from Mendota to Aurora to Chicago around mid-morning,
then translate south to an area centered between I-88 and slightly
south of the Kankakee/Illinois River Valley for much of the
afternoon and into this evening. An overall lull in precip is then
expected late this evening into the overnight hours north of a
Pontiac to Valparaiso line as the favored axis of low-level fgen
drifts farther south. However, increasingly colder low-level
thermo profiles will support a transition over to snow where
precip does continue.

Tuesday: An axis of precip within the moist conveyor belt of a
developing surface low will quickly spread northeast into the area
beginning around daybreak. A neutral to slightly negatively-
tilted trough rotating into the mid-Mississippi River Valley under
the right-entrance of a departing upper jet streak will provide
continued ample forcing for widespread precipitation across much
of the CWA through the day. However, the bulk of the deepening of
the surface low over the Ohio River Valley is now progged to occur
further east, resulting in a broader and lighter precip shield
across the area. Thermo profiles will continue to support mainly
snow mixed with some rain during this time, but accumulations will
be suppressed by lower snowfall rates and the increasingly strong
late April sun providing enough insolation to maintain melting at
the surface. With storm total precip trending toward under 0.5"
and effective SLRs likely staying well below 10:1 for a bulk of
the event, slushy accumulations of 1-2" on primarily non-paved
surfaces appears likely for areas south of I-80 and especially
east of I-55.

Tuesday night: Mid-level moisture will quickly advect northeast
of the area by mid-evening, effectively ending any appreciable
widespread synoptically forced precip. Guidance has continued to
indicate the potential for a secondary piece of mid-level energy
closely following the primary wave across the area Tuesday night.
Decreasing mid-level temps will begin to increase low-level lapse
rates amid residual moisture below 700 hPa. This will support the
potential for lingering snow showers through the night for
portions of northeast/east-central Illinois into northwest
Indiana.

while cloud cover may be more abundant than previous expected due
to the factors noted above, confidence was high enough to prompt
the issuance of a Freeze Watch for the entire area. With temps
starting in the 30s for much of the area in the early evening, it
will not be hard to cool into the low 30s and upper 20s. With the
potential high impact of a hard freeze on budding vegetation after
2+ weeks without sub-freezing temps for most of the area, felt it
was prudent to issue the watch on this shift to provide two full
days notice to complete any necessary preparation. This is
especially the case with the expected snow falling on the
vegetation.

Kluber

&&

.LONG TERM...
306 AM CDT

Wednesday through Sunday...

Rain/snow showers will likely linger Wednesday beneath cold upper
trough, before dry and somewhat milder weather develops Thursday.
Medium range guidance depicts the potential for some additional
rain showers Friday into Saturday as another frontal system
approaches and transits the forecast area. Sunday looks to be
largely dry at this distance. With the exception of cool
temperatures Wednesday, readings through the period are expected
to moderate back to near average levels for late April.

Surface low pressure will be east of the area by Wednesday morning,
with associated mid-level short wave and more widespread
precipitation also moving away from the region. The larger scale
upper trough is progged to hang back across the western Lakes and
upper Midwest however, with a couple of weaker vorts expected to
translate through the flow. Cold mid-level air aloft within the
trough (-17 to -19C at 700 mb) is expected to result in a layer of
fairly steep lapse rates up through about 675 mb, which will likely
support the development of convective snow/rain showers during the
day and especially with diurnal "heating". The thermal column
appears cold enough to support snow showers during the morning,
which would eventually transition to a rain/snow mix then all rain
as surface temps warm by midday/afternoon. Some minor snow
accumulations can`t be ruled out during the morning when surface
temps are expected to be near the freezing mark. Given the expected
thermodynamic profiles, have leaned toward the cooler end of the
guidance spectrum for surface temps and undercut blended NBM highs
by a few degrees into the low-mid 40s.

Scattered showers may linger into Wednesday evening, before they
dissipate Wednesday night as the upper trough and its cold pool
begin to move off to the east of the region. Mid-level height rises
on the order of 60-70 meters at 500 mb are noted later Wednesday
night into Thursday as short wave ridging develops aloft in the wake
of the departing trough. Surface high pressure, centered over the
lower Missouri and Ohio valleys, spreads east on Thursday with low
level winds becoming more westerly. Sunshine and moderating temps
aloft should allow daytime temps to warm back into the 50s, with the
breezy west flow limiting lake cooling to the immediate northwest
Indiana shore areas.

Guidance is in reasonable agreement in bringing another deamplifying
mid-level short wave across the region Friday, with low level return
flow focused mainly along and west of the Mississippi River during
the day. ECS and GEFS ensemble means maintain some potential for
precipitation to spread into the forecast area on Friday, though
with a bit of a slowing trend noted and with the 00Z operational
runs of the ECMWF and GFS focusing lower chance pops for showers
mainly west and later in the day/Friday night. Guidance shows more
spread into Saturday with the degree of phasing of two additional
mid-level waves. Despite these differences, the general consensus is
that a frontal system will move through the region Saturday with an
accompanying chance of additional showers. Ensemble temps continue
to moderate ahead of the front Saturday, thought still in the
average range for late April.

Global guidance depicts surface high pressure building in Saturday
night into Sunday behind the departing upper trough, supporting
generally dry conditions to end the weekend. Broad ridging is
depicted aloft across the central CONUS, with the fast/flatter flow
however potentially brining additional low-amplitude short wave
energy toward the region later Sunday or Sunday night. This could
support some precip chance as warm/moist advection returns into the
region on the back side of the departing surface high. For now
though, the bulk of Sunday looks to be dry if not the entire day.

Ratzer

&&

.AVIATION...
For the 12Z TAFs...

Aviation Forecast Concerns:

* Cold front moves through terminals early this morning with a
  northwest wind shift and gusts around 20kt.

* Period of light rain/showers develops behind the front. MVFR
  ceilings possible in precipitation.

* Precip sags south of terminals later this afternoon/evening,
  with period of VFR conditions expected into overnight.

* Precip redevelops into area early Tuesday morning, in the form
  of a snow/rain mix. MVFR ceilings and some IFR visibility
  possible.

Surface cold front was pushing into northern IL at 5 AM/10Z, and
will move southeast into the Chicago metro terminals through
14-15Z. Southwest winds ahead of front have started to gust near
20 kts, and this should continue for a time behind the front as
well after winds shift northwest. Scattered showers behind the
front are expected to fill in across the terminals for a few hours
behind the front this morning, with MVFR ceilings developing. Rain
should sag south of KRFD, KDPA and KORD by midday, lingering a
little longer at KMDW and longer still into the afternoon at KGYY.
Precip eventually pushes south of all terminals this evening, with
VFR conditions and lighter northwest winds overnight, eventually
shifting north and then northeast early Tuesday morning.

Precipitation is expected to spread back into the area early
Tuesday morning, likely in the form of a snow/rain mix. Ceilings
again lower to MVFR, with visibility into the IFR range possible
as precip increases. Precip may change over to all snow at times,
with some minor wet accums on grassy surfaces possible.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Freeze Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108...9 PM
     Tuesday to 9 AM Wednesday.

IN...Freeze Watch...INZ001-INZ002-INZ010-INZ011-INZ019...9 PM Tuesday
     to 9 AM Wednesday.

LM...None.
&&

$$

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