000
FXUS63 KLOT 192018
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
318 PM CDT Fri Apr 19 2024

.KEY MESSAGES...

- Gusty west winds 35-40 mph persist this afternoon.

- Potential for patchy frost/temperatures briefly near freezing
  toward sunrise across our northwest cwa. Freezing
  temperatures more likely Sunday morning for the same area.

- Next chance for rain Monday night through Tuesday

- Another period of inclement weather possible towards the end of
  next week

&&

.DISCUSSION...
Issued at 318 PM CDT Fri Apr 19 2024

Through Saturday Night:

Deep upper level low pressure was evident in water vapor
imagery across Ontario this afternoon, with a fast westerly 125+
kt upper level jet streak along the southern periphery of the
low across the upper Midwest. At the surface, low pressure was
stacked beneath the upper low to our north, while a ridge of
high pressure extended from Montana to the lower Missouri and
Ohio river valleys. Within the gradient north of the ridge axis,
strong west to west-northwest flow was in place across the
forecast area, with otherwise dry but cool conditions across the
region. The surface ridge is forecast to eventually drift
eastward Saturday and Saturday night, allowing winds to
gradually ease with time.

In the near term, surface dew points in the 25-30F range were
producing relative humidities in the 30-35 percent range in
spots this afternoon, and may go a little lower still through
mid- afternoon. Combined with winds gusting as high as 35-40 mph
at times, this will continue to produce a somewhat elevated
fire danger through sunset. The cool, dry air mass will also
support temperatures lowering into the 30s across the area
tonight, with some potential for dipping near the freezing mark
across our northwest cwa for a brief time toward sunrise.
However, winds are expected to remain up a bit overnight within
the stronger pressure gradient, and those 7-12 mph will likely
prevent widespread freezing temperatures and frost formation for
our cooler northwest areas. At this time, in collaboration with
WFOs DVN and MKX have elected to hold off on any frost/freeze
headlines for tonight but later shifts will monitor for stronger
decoupling of boundary layer winds which could portend a
greater freeze risk. Winds will remain breezy, with 20-25 mph
gusts on Saturday, but will diminish more Saturday night with
the surface ridge edging east. Frost/freeze conditions thus look
to be more of an issue late Saturday night/Sunday morning.

Otherwise, a period of mid-level cloud cover is expected this
evening across our southern cwa, with mainly clear conditions
farther to the north. Forecast soundings suggest more of a
diurnal scattered to broken stratocu deck on Saturday, before
skies clear Saturday night. HRRR/RAP actually depict some light
convective precip during the day Saturday, though forecast
soundings suggest fairly shallow cloud layer and a stout
inversion around 5 kft which argues against carrying any
pops/precip mention in the forecast. Persistent low-level cold
advection will result in cooler temps through Saturday, with
low-mid 30s for lows tonight and highs from the upper 40s to
lower 50s Saturday. As indicated above, more widespread
lower-30s are possible across parts of northern IL away from the
metro late Saturday night.

Ratzer


Sunday through Friday:

High pressure will remain as the dominant weather feature for
Sunday and most of the day on Monday allowing dry conditions to
prevail. Winds are also expected to become southerly Sunday
night into Monday which will allow temperatures to gradually
warm towards more typical readings in the 60s. While these
southerly winds will also advect in higher dew points during the
day on Monday, forecast soundings do show that the atmosphere
may be able to mix into the base of a stout dry layer aloft. If
this is indeed the case then dew points may be slower to
increase and/or possibly decrease Monday afternoon leading to
lower RH values than forecast. Couple the potential for lower RH
readings with the expectation for winds to be gusting around 25
mph at times and there could be the potential for heightened
fire weather concerns during this period.

Our next opportunity for rain is expected to arrive Monday
night through Tuesday as a shortwave trough pivots through the
Great Lakes. Guidance has started to indicate that there could
actually be two periods of showers, one initially with the warm
advection ahead of the trough Monday night and the second with a
cold front Tuesday afternoon. While the impacts are nearly
identical with each round, there is the potential for some
thunderstorms Tuesday afternoon if sufficient instability can
materialize. However, low-level moisture does look to be
somewhat limited Tuesday afternoon which may limit the coverage
of thunder. Therefore, have decided to keep an isolated (around
20% chance) mention for now.

As the cold front exits into Indiana Tuesday night, showers and
any storms will come to an end as mid-level ridging begins to
build into the central CONUS. While this will allow for a period
of dry weather for the middle of the week, it will also
generate a period of slightly cooler temperatures (highs in the
50s) as colder air filters in behind the aforementioned front.
Unfortunately, the dry weather looks to be short lived as
guidance continues to show a series of troughs developing across
the western CONUS and ejecting towards the Great Lakes towards
the end of next week into next weekend. Obviously there is still
some uncertainties in exact timing and specifics, but it does
look as if we are in for another period of inclement weather.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1229 PM CDT Fri Apr 19 2024

The sole forecast concern for this TAF period is the gusty
west-northwest winds this afternoon. Gusts are generally
expected to remain around 30 kts this afternoon, but
occasionally higher gusts between 30 to 35 kts are possible.
While gusts will diminish somewhat this evening, forecast
soundings show that the atmosphere may remain mixed overnight so
expect at least periodic gusts around 20 kts to persist
overnight and through Saturday. Otherwise, expect FEW to SCT
VFR clouds this afternoon to gradually increase to BKN VFR
ceilings Saturday afternoon.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago