744
FXUS63 KLOT 170803
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 AM CDT Fri May 17 2024

.KEY MESSAGES...

- Areas of dense fog this morning will quickly dissipate a couple
  hours after sunrise this morning
- A few spotty showers & perhaps a t-storm this afternoon near &
  south of the Kankakee/Illinois River Valley
- Very warm temperatures expected Saturday through Tuesday with
  highs well into the 80s, though some lake cooling possible
- Active Monday and Tuesday with scattered thunderstorms with
  the threat for strong/severe storms and flooding.

&&

.DISCUSSION...
Issued at 303 AM CDT Fri May 17 2024

Through Saturday:

Light winds and a moist boundary layer with dewpoints depressions
nearing zero has allowed for ground fog to develop. As of this
writing (0730 UTC), fog is still in the process of developing,
but already have a few sites down to 1/4SM. See no reason that fog
won`t grow more widespread and dense through sunrise, so plan to
hoist a dense fog advisory for about the southern half of the CWA
through early/mid morning. The high sun angle should help quickly
burn off the fog this morning, despite fairly light winds.

Pretty substantial moisture gradient expected across our CWA today
with low levels in particularly still moist over southern 1/3 or
so of the CWA. Heating of this moist boundary layer will result in
modest afternoon instability with forecast soundings showing
SBCAPE values 500-1000 J/kg and minimal CINH. Forcing will be
lacking with weak height rises and no obvious surface/low level
focusing/forcing mechanism. RAP/HRRR forecast soundings show
evidence of mid level subsidence, which could help keep coverage
sparse and might make it hard for updrafts to achieve charge
separation and develop into thunderstorms. Measurable precip
chances today of 15-20% south of the Illinois and Kankakee River
Valleys seems reasonable.

The light winds should allow for formidable lake breeze to charge
well inland this afternoon. Most substantial cooling will be
closer to the lake where shoreline readings will probably spend
the afternoon in the lower 60s ranging to the lower 80s across
most of the CWA.

Saturday looks to be several degrees warmer than today with
guidance progged 925mb temps in the lower 20s Celcius. These
temps are warmer than the 90% percentile values for mid-late May
and should easily support high temps at least into the mid 80s.
More of a southerly gradient should result in far less
pronounced of lake breeze Saturday afternoon, likely confined
mainly to the IL shore, particularly the north shore
communities.

- Izzi

Saturday Night through Thursday:

After quieter conditions on Saturday night, warm temperatures
are expected to creep back into the 80s on Sunday. A weak cold
front will then move across northern Illinois on Sunday
afternoon with a lake enhanced push of the front farther inland.
The front itself can act as the forcing mechanism to create
some shallow clouds and even the slight chance for showers. Even
the NBM is now trying to resolve small areas where isolated
showery activity may develop. Given the lower confidence in
projecting the exact placement of shower development, most of
the area was kept dry at this time.

On Monday, an anomalously deep long wave trough will begin to
eject out over the Rockies and move eastward toward the Plains
with cyclogenesis occurring at the surface. A small upper level
wave will move northeastward out of the Rockies sending a lobe
of vorticity over Illinois. Stronger forcing will likely be
better to the northwest under the right entrance region of the
upper level jet. And while there is still expected to be enough
instability for showers and a few storms to develop Monday
afternoon, the projected lack of shear should keep the severe
risk suppressed for the time being. Southerly winds will
advect moisture northward resulting in precipitable water well
over an inch. Localized ponding and flooding will be the
primary risk keeping the northwestern portion of the forecast
area in a level 1 out of 5 for heavier rainfall.

By Tuesday, an upper level ridge builds over Texas reinforcing
the southwesterly winds aloft, lifting the sub-tropical jet
northward and sending the next surface cold front through the
area. Model soundings are suggesting 40 to even 50 knots of
deep layer shear, steep mid level lapse rates, and sufficient
instability that would lead to Tuesday being the better chances
for a set up that could lead to potentially strong to maybe even
severe thunderstorms. Additionally, the persistent southwest
flow will continue to push higher moisture content into the
region allowing for a renewed chance for localized flooding. So
it is no surprise that once again the forecast area is within a
level 1 out of 5 for excessive rain. While it might be easy to
get excited over Tuesday, the details still need to be ironed
out as models are still disagreeing on how the low forms over
the Southern Plains as well as its track (and speed) to the
northeast over the area which influences timing and overall
severity of storms.

On Wednesday the surface low is expected to continue its
journey northeastward away from the area, but models are on
different sides of the coin on the "when". The GFS has it
becoming slight more diffuse and taking a slight north-
northeasterly track, keeping it closer to the area and thereby
supporting storm redevelopment Wednesday morning and early
afternoon. However, the Euro has it moving much quicker through
the area, allowing much of Wednesday to be dry.

Temperatures will finally cool behind the front with high
temperatures on Wednesday and Thursday expected in the 70s. The
next weaker upper level trough moves east of the Rockies on
Thursday providing another chance for showers and storms to end
the work week.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1244 AM CDT Fri May 17 2024

Key Aviation Impacts for the current TAF period:

- Potential for lower cigs and vis development early Friday
  morning

- Lake breeze provides northeast winds Friday afternoon for
  terminals closer to the shoreline

With sufficient low level moisture present and southerly winds
becoming light and variable, there is a chance for reduced vis
and even some lower cigs Friday morning. Dense fog is not
currently expected, but vis could reach 3 to 4 SM for a brief
window between 09Z and 13Z, though potentially lower vis in
more rural areas away from the lake.

After day break, there should be enough mixing to scour out any
lingering low clouds. VFR conditions are expected with a lake
breeze developing in the afternoon. As temperatures increase in
the afternoon, there is a slight chance for showers to develop
closer to the front exiting to the east. While there could be a
small window for showers near KGYY, for now it was kept out of
the TAF.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.

&&

$$

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