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000
FXUS63 KLOT 010528
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1128 PM CST Thu Nov 30 2023

.UPDATE...
Issued at 820 PM CST Thu Nov 30 2023

Regional water vapor imagery depicts a compact upper-level
shortwave moving across Oklahoma behind a region of broadening and
cooling cloud tops indicative of diffluent upper-level flow.
Low clouds continue to steadily stream northward into the Lower
Great Lakes ahead of the approaching wave, which will stick
around for the foreseeable future. Current temperatures, which are
in the mid to upper 40s and already running above model guidance,
appear poised to remain more or less steady overnight as clouds
prevent radiational cooling and weak winds limit meaningful
advection. As the aforementioned upper-level wave ejects
northeastward overnight, rain will spread from south to north
across the area making way for a wet Friday morning commute.
Steady rain should transition to drizzle and showers with time and
continue into Friday evening.

While increasing northeasterly winds should facilitate the
advection of drier, and perhaps cooling, temperatures along the
Wisconsin state line tomorrow morning, prospects for a transition
to wet snow across northwestern Illinois at least during the
morning and afternoon appear to be dwindling. For one, the
already warmer start to temperatures suggests more "positive
energy" to melt hydrometeors prior to reaching the surface, even
with wet-bulb cooling of the low-level thermal profile. Speaking
of which, the somewhat fractured nature of frontogenetical forcing
into two components (one tomorrow morning and another tomorrow
evening) and little to no evidence for "surprise" CSI suggests
relatively limited forcing to afford precipitation rates heavy
enough to facilitate efficient wet-bulb cooling of the column. It
is for these reasons that the latest available guidance, which is
trying to catch up to the observed warmer temperatures, is
trending away from really any snow across our northwestern
forecast area tomorrow morning and afternoon.

With that said, an opportunity still exists for snowflakes to mix
with rain tomorrow evening as the second component of the
fractured wave moves overhead. Thanks to the passage of the
surface low across central Illinois, winds along the Wisconsin
state line will adopt a more northerly component, allowing for
drier air more susceptible to wet-bulb cooling to drift southward
into far northwestern Illinois. So, snowflakes may yet mix with
rain tomorrow evening near Rockford.

All things considered, we will maintain a mention of rain/snow
mix near and around Rockford tomorrow evening but remove any
mention of snow accumulations.

Updated products will be sent soon.

Borchardt

&&

.SHORT TERM...
Issued at 316 PM CST Thu Nov 30 2023

Through Friday night...

Key Messages:

- A widespread soaking cold rain is expected (for most) late
  tonight through Friday morning. Rainfall amounts of 0.5" up to
  an inch are forecast.

- Conditions continue to favor the rain to mix with, or even
  change over to a period of wet snow for period Friday morning
  across interior sections of far northern Illinois, primarily
  near/west of the Fox Valley.

- There remains a chance (currently 40%) of some slushy wet snow
  accums up to a inch or so (mainly on non paved surfaces) within
  the narrow zone generally along and north of a Dixon to McHenry
  line. Closer to the WI border, there remains a low (~20%) chance
  of amounts around 2" if the wet snow comes down at a heavy
  enough rate to overcome marginal temps and mild ground conditions.

- Periods of light precipitation, primarily in the form of light
  rain and drizzle will persist through Friday afternoon (60-70%
  probs), with some wet snow probably (50-60%) continuing to mix
  in over portions of far northern Illinois.

- Precipitation will reinvigorate (thought with continued
  primarily light rates) Friday evening with 80-90% probabilities
  near and north of the Kankakee River. A similar zone of far
  northern Illinois to Friday morning will likely (>60% chance)
  have at least a mix with wet snow, but no additional accumulation
  is forecast.

Castro/KJB

Discussion:

A compact and strong Pacific impulse (and its associated surface
low) currently noted in the water vapor imagery over the southern
High Plains will eject east- northeastward into the lower Great
Lakes Region into Friday. As it does so, the flow aloft will
become strongly diffluent as a loosely coupled upper-jet structure
sets up across the Lower Great Lakes region. This in combination
with a northward influx of unseasonably deep Gulf Moisture (PWATs
at or above 3/4") will set the stage for a widespread
precipitation event across the area late tonight into Friday
morning. 12z forecast guidance remained in good agreement with the
northern periphery of the system`s expansive precip shield
reaching into southern Wisconsin. As a result, the entire CWA
remains on track to see precipitation from this system (peak PoPs
of 100% pre-dawn Friday through Friday AM). Total liquid
equivalent amounts of a half to as much as an inch are expected.

Much of the precipitation tonight into Friday will be in the form
of a cold rain, especially along and south of a Waukegan to Peru
line. However, chances increase with northward extent towards the
WI state line (and inland from Lake Michigan) that the column
will dynamically cool enough into Friday morning to support a
change over to a period of wet snow, thanks to a strengthening
low-mid level f-gen circulation in addition to the strong large
scale ascent. The 12z forecast guidance did generally trend a bit
warmer with temperatures and dew points during the highest
precipitation rates. In addition, pavement temps will likely have
a longer lag to cool off tonight following the over- performing
high temps this afternoon. Given the marginal thermal set-up and
mild ground conditions, it`s uncertain if snow rates (where p-type
does go over all snow) will be heavy enough to result in any
pavement accums.

The above being said, some accumulations up to an inch remain
possible (up to ~40% chance), especially for areas north of a
Dixon to McHenry line. Probabilities for accumulations of 2" (or
just upwards of 2") came down to only 15-20% over the far northwest
CWA (roughly north and west of Rockford). All in all, at this
point it appears that travel impacts will be limited due to
marginal surface temperatures, which would favor most of the wet
accumulations (where slushy accums occur) on non-paved and
elevated surfaces. We will continue to message this possibility in
and around the Rockford area.

Heavier rain rates and any accumulating wet snow will abate by
mid-day. Thereafter, it appears we will continue to see waves of
drizzle or light rain at times through the afternoon as broad
ascent continues through a deep saturated layer from the low
stratus base up to about 600 mb. Looking at a pretty murky
afternoon as a result, with any persistent drizzle capable of
knocking visibility down into the 1-2 mile range. In addition, as
alluded to in the key messages section, sufficient saturation of
the DGZ may remain vs. farther southeast to keep some wet snow
mixed in over parts of the far NW CWA, including Rockford. High
temperatures will range from the upper 30s over interior far
northern Illinois to the lower-mid 40s for the rest of the area.

In the wake of tonight/Friday`s system, a weak secondary upper-
level wave will cross the forecast area Friday evening and
overnight as the weakening low-level low pressure drifts from near
STL to near DTW Friday afternoon through Friday night. Modest
low-level isentropic ascent ahead of the highly NW-tilted low with
height will support a re-invigoration of a broad shield of light
precip across much of the CWA. Thermal profiles support primarily
all rain except for areas northwest of a Dixon to McHenry line
(roughly the 850 hPa low track) where snow should mix with and
possibly change over from rain. Surface temps look to be marginal
for any notable accumulations, but a few tenths of an inch cannot
be completely ruled out on grassy surfaces (confidence was too low
for any formal accums in the grids).

The main change with this afternoon`s forecast issuance for the
Friday evening/night period was to increase PoPs up to 80-90% for
areas near and north of the Kankakee River based off ensemble
PoPs jumping into that range. The steady light precip will wind
down after midnight Friday night. Low temps will be in the mid-
upper 30s for most of the area, except locally low 30s far NW and
around 40 near/along the Chicago shore.

Castro/KJB/Kluber

&&

.LONG TERM...
Issued at 219 PM CST Thu Nov 30 2023

Saturday through Thursday...

Key Messages:

* Multiple chances for rain and rain/snow mix through Tuesday night

* Seasonal temperatures through mid week, but the potential for
  anomalously warm temperatures build late in the week

Mostly cloudy skies are expected on Saturday with temperatures in
the 40s. Mid-level subsidence and cold air advection will likely
keep the area drier as the previous short wave moves east. However,
the 500 mb long wave trough over the western Texas panhandle is
projected to move northeastward toward the Great Lakes through the
day on Sunday. Overnight low temperatures Saturday into Sunday are
expected to be above freezing for most of the region, but best
chances for subfreezing temperatures at the surface is projected
northwest of a Dixon to Woodstock line. Precipitation is likely (55%-
65%) to be a cold rain, with a wet snow for northwest Illinois
Sunday morning, through Sunday evening. A secondary shortwave
traversing around the long wave keeps confidence low on the exact
timing of precip ending and light rain/drizzle could linger into the
early part of Monday morning. Northwest flow behind the trough will
provide additional cold air advection for colder, drier conditions
Monday morning.

Yet another upper level trough is expected to drop down out of
Saskatchewan on Tuesday and deepen as it approaches Lake Michigan
providing another chance for rain or rain/snow mix. There is still a
lot of uncertainty with the timing and speed of the system, but it
looked like the blend was going too low with PoPs in comparison to
the suite of ensemble models. So PoPs were nudged upwards to have a
chance (30% to 40%) be displayed in the forecast. Lastly, the Euro
has a much broader negative tilt to the upper level trough, while
the GFS has a more meridional dip to it with a tighter gradient.
Winds were maintained with gusts around 20 mph, but if models trend
toward the GFS it is possible that the winds are underdone.

An upper level ridge over the intermountain west will grow and
strengthen as it drifts eastward over the Northern Plains.
Temperatures will be around seasonal normals for much of the week.
However as the ridge drifts east, higher pressure will grow and
there is a signal for the potential for temperatures to warm to
almost 10 degrees above normal for the end of the week.

DK

&&

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

Forecast concerns include...

Lifr cigs Friday/Friday night.
Periods of rain through Friday night.
Rain/snow mix at RFD Friday evening.

Main change for this forecast is to remove snow mention at RFD for
Friday morning. Much of the latest guidance is warmer with rain as
the main precip type. Rain is still expected to spread across the
terminals during the predawn hours and become moderate, perhaps
briefly heavy, after sunrise and then should quickly taper off to
light rain by late morning. Light rain or drizzle will likely
continue in the afternoon with another period of light rain Friday
evening. It`s during this time that a rain/snow mix will be
possible across northwest IL and at RFD but confidence remains low
for any possible snow accumulations. This precipitation is
expected to end by late Friday evening or very early Saturday
morning.

Mvfr cigs have become prevailing across much of the area and will
lower through low mvfr and then quickly drop into ifr as the rain
arrives. Cigs are expected to settle into lifr by mid morning and
will likely remain lifr through early Saturday morning.
Visibilities may remain in the 2-3sm for much of Friday, perhaps
improving a bit by early afternoon and then lowering again early
Friday evening.

Light and variable winds will turn light northeasterly over the
next few hours and then slowly increase in speed to 10-15kts by
daybreak as directions turn more easterly. Winds may become
southeast for several hours at MDW/GYY through mid morning, before
turning back easterly Friday afternoon. Directions will slowly
turn northeasterly Friday evening and more northerly by early
Saturday morning. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...Winthrop Harbor to Wilmette Harbor IL...4
     AM Friday to 10 PM Friday.

&&

$$

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