287
FXUS63 KLOT 190819
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
319 AM CDT Sun May 19 2024

.KEY MESSAGES...

- A few additional isolated showers and storms possible this
  afternoon and early evening, mainly south of the Illinois and
  Kankakee River Valleys.

- Summer-like warmth expected through Tuesday night,
  interrupted only by lake cooling Sunday; then during any
  showers/storms Monday and Tuesday.

- Scattered thunderstorms likely Monday afternoon and evening
  across portions of northern Illinois, with a low end threat
  for a couple/few isolated severe storms.

- Scattered (mainly non-severe) thunderstorms possible Tuesday
  morning and then after a break, thunderstorm chances return
  Tuesday evening/night, including some threat for severe wx,
  mainly western CWA.

&&

.DISCUSSION...
Issued at 319 AM CDT Sun May 19 2024

Through Tonight:

A cold front is passing over northeastern Illinois this
morning. It has just enough to lift and moisture for cloud
development to show up on satellite. However, pre-frontal low
layer dry air as seen on the 00Z DVN sounding (less than 45
percent surface RH) seems too have been too dry to overcome.
Given the convection on satellite, it seemed fit to keep PoPs at
a slight chance, below 20 percent, along the front as it
passes. And as dew point depressions continue to decrease into
the morning, perhaps there are some brief moments of light rain,
but confidence remains low and chances fizzle out after day
break.

No major changes to the forecast in that the cold front is
expected to gradually slow down and stall along and south of
I-80. Winds behind the front are out of the northwest and
gradually becoming east northeast through the morning.
Temperatures today will still increase and be unseasonably warm
in the 80s for most parts, but the onshore flow will help cooler
maritime air protrude deeper inland allowing Lake IL, Cook,
Lake IN, and Porter County have better chances to remain in the
70s with 60s close to the shore.

North of the pseudo stationary front will be lower dewpoints in
the 40s and 50s, while along and south of the boundary will be
dew points in the low 60s. In an uncapped environment this
afternoon, over 1000 J/kg of MLCAPE can be tapped into for some
showers and even thunderstorms to develop in the afternoon,
mainly south of the Illinois and Kankakee River Valleys. With
such weak wind shear, severe thunderstorms are not expected. The
next upper level trough moves over western Illinois after
midnight driving a reintroduction of PoPs into the western
forecast area before 6 AM ahead of the next weather system which
will be discussed in greater detail in the discussion below.

DK

Monday & Monday night:

Well defined vort max evident on water vapor imagery near Las
Vegas early this morning is progged to emerge out onto the central
High Plains later tonight and should be the impetus for strong to
severe thunderstorm development this afternoon over the central
Plains. This convection is likely to organize into a MCS, fed
by a 40-50kt low level jet tonight. Models continue to strongly
suggest that the incipient shortwave trough will become
convectively augmented/enhanced tonight as it moves into the
Corn Belt.

Decaying/weakening MCS is likely to move into WI and northwestern
IL Monday morning. Should see strong heating of moderately moist
air mass in advance of this weakening MCS. Guidance is supportive
of 1000-2000 J/kg of MLCAPE across the portions of our CWA that
aren`t convectively overturned or disturbed by whatever is left of
the morning MCS. Monday afternoon would anticipate either re-
intensification of the morning convection or new robust
development along remnant outflow boundary.

Shortwave trough is expected to provide a glancing blow of
synoptic ascent, mainly early in the afternoon. Shortwave and
associated speed max are both expected to quickly translate east
of our longitude during the afternoon, leaving quickly weakening
shear profiles in its wake over our CWA. Magnitude of the progged
instability would still support a pulse/isolated/brief type severe
weather risk Monday afternoon with more robust updraft. Convection
should dissipate rather quickly Monday evening with the loss of
daytime heating given the lack of synoptic support with heights
rising in the wake of the departing shortwave.

Tuesday & Tuesday night:

Strong to severe convection is once again expected to develop over
the central Plains Monday evening and probably congeal into an MCS
that will track toward the mid-Mississippi Valley. Cannot rule out
some weakening remnants of this MCS bringing a chance of rain to
mainly our northwest CWA Tuesday morning, however, a warm front is
progged to be surging rapidly northward Tues morning with
better support for convection quickly shifting well north of our
CWA by mid-late morning.

The GFS continues to be a quick outlier and consensus with
neighboring offices this morning was that the GFS solution is
likely too quick and has been largely discounted. The remainder
of the guidance (ECMWF, NAM, GEM) all would support a windy,
unseasonably warm (almost hot) day Tuesday afternoon with
minimal rain chances in our area.

Explosive severe thunderstorm development is likely over central
or western Iowa into northwest/north central Missouri Tuesday
afternoon. This convection likely wouldn`t reach our CWA until
probably mid-evening Tuesday, if not later. By the time it
arrives, boundary layer will be nocturnally stabilizing with
growing MLCINH and weakening MLCAPE. The most likely scenario
would be storms being in a weakening phase as they arrive Tuesday
evening. Depending on the extent of mesoscale organization that
takes place, certainly can`t rule out a threat for severe
convection, mainly in our western CWA Tuesday evening/night. If
there is a severe threat it would likely be in the form of
damaging winds and perhaps a QLCS tornado threat, but again, this
will depend heavily on strong mesoscale organization of convection
that allows it to overcome growing and increasingly large MLCINH
over our area Tuesday night.

Progged 925mb temps continue to support high temperatures making a
run at the lower 90s. Given uncertainties on potential debris
cloudiness, didn`t make any meaningful changes to the NBM`s mid-
upper 80s Tuesday, but quite plausible that some areas breaking
the 90 degree mark. If temps do overperform, that would increase
the chances of boundary layer moisture mixing out, potentially
lessening instability/increasing inhibition a bit more Tues night.

Wednesday & Wednesday night:

GFS and its ensembles continue to be on their own Wednesday with
respect to our weather, in particular the precip chances. A closer
look at the GFS reveals that it is forecasting Tuesday night`s
cold front to be more anafrontal in nature with the front at 850mb
still displaced a couple hundred miles northwest of the sfc cold
front. The GFS then seems to break out convection rooted between
825-875mb near the elevated front. The GFS is all by its lonesome
with this solution with ECMWF and GEM (and their respectable
ensembles) strongly supporting a cleaner fropa with breezy,
seasonably warm, and dry conditions Wednesday. Consensus with
neighboring offices was to remove/dramatically lower pops Wed into
Wed night.

Thursday and beyond:

Thursday looks to be sunny and seasonably warm with comfortable
humidity as the baroclinic zone moves well south of the area.
Next northern stream shortwave should result in increasing
chances of showers and storms again, perhaps as early as late
Friday, but more probable over the weekend. Timing, track,
intensity of this system still subject to change this far out,
but certainly gradually increasing pops offered up by the NBM
look very reasonable.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1251 AM CDT Sun May 19 2024

Weak cold front will move across the terminals early this
morning resulting in a brief (hour or two) wind shift to light
northwest to north-northwest before winds flip to northeasterly
by 12-13 UTC and remain northeast through the day. Winds will
become light this evening with gradual backing to southeast by
late tonight.

Otherwise, wx nil for this TAF with VFR conditions.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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