Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/26/23

Area Forecast Discussion...Updated

National Weather Service Saint Louis MO

 820 PM CST Sat Nov 25 2023

.UPDATE...
Issued at 819 PM CST Sat Nov 25 2023

The forecast in terms of placement, timing, and accumulation of the
upcoming snowfall remains on track this evening. RAP-based analysis
shows a wedge of dry air starting around 600 mb down to the surface,
increasing confidence that the light returns noted over portions of
central and northeastern Missouri are virga emanating from the mid-
levels. As precipitation starts tonight and saturates the low
levels, this dry air will cause temperatures through the lower
atmosphere to cool to the wetbulb and approach or drop just below
freezing. Our thinking is still that this cooling trend will be
offset by warm air advection for areas along and south of I-70 via
southerly to southwesterly low-level flow noted in the 00z SGF
sounding and recent ACARS soundings out of KSTL, keeping precip in
this portion of the CWA mostly rain overnight into Sunday morning. I
can`t rule out that a couple of wet snowflakes could mix in at times
along the I-70 corridor, but the primary focus for snow remains
across northeastern Missouri and west-central Illinois where low-
level thermal profiles will be more conducive for snowfall.

Elmore

&&

.KEY MESSAGES...

- Snow is expected to begin overnight tonight across northern Missouri
  and into western Illinois. The bulk of the snow will fall early
  Sunday morning with 1 to 3 inches likely. Travel impacts are
  expected across this region.

- Snow chances along I-70 and southward continue to decrease as warmer
  air is expected. Precipitation along I-70 could begin as
  snowflakes before changing to rain as the temperature warms
  overnight. Minimal travel impacts are expected.

- A cold front on Sunday brings cold and dry weather behind it,
  with the temperature gradually rebounding through the coming
  week. The next chance for rain comes late in the week.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 243 PM CST Sat Nov 25 2023

Our much anticipated trough is emerging out of the Rockies this
afternoon and out into the Plains. Gulf of Mexico moisture pushing
northward through Texas and Oklahoma and is running up into and over
a frontal boundary leading to some significant snowfall in Kansas.
As the upper trough moves east tonight this area of precipitation
will spread northeastward with the mid level southwesterly flow. The
system will lose its direct access to Gulf of Mexico moisture as it
exits Oklahoma with the precipitation in our area being driven
primarily from mid level moist advection from the southwesterly
flow. This will result in lighter precipitation than what is
observed further west, but still a rather widespread light
precipitation is expected across the entire region. Timing still
looks like right around midnight for entry into our forecast area
with the last of the precipitation exiting to the east around midday
on Sunday.

Atmospheric profiles will experience top down moistening in an
overall cold air mass resulting in snowflakes being generated at
least aloft. What falls at ground level will depend on near surface
temperatures when precipitation does arrive. With the low level warm
advection overnight, we expect temperatures to be coldest at the
beginning to middle of the night with a steady or slowly rising
temperature after midnight as precipitation is falling. Compared
to prior forecasts, temperatures are a little bit warmer with more
widespread above freezing temperatures expected especially south
of Interstate 70. This translates to more melting of snowflakes as
they fall and less potential for any impactful accumulation in
the Interstate 70 corridor. It is possible that precipitation
starts as snowflakes but transitions to rain as the temperature
warms in this region. Further north, though, snow will be the
dominant precipitation type with temperatures just cold enough for
accumulations. The Winter Weather Advisory area highlights this
area of greatest potential for significant snow accumulations and
travel impacts.

Mesoscale models continue to come in a bit higher on the total
precipitation amounts relative to the global scale models. This is
likely due to the higher resolution of smaller scale features within
the dominant band of precipitation. At the moment, forcing still
looks pretty weak as the limited moist flow ahead of the boundary
will be out of the southwest. This limits the high end potential
to right around 0.25 inch of liquid precipitation. With
climatologically low snow ratios and mild surface temperatures,
this translates to roughly 2 to 3 inches of snowfall as a
reasonable high end amount. This is represented fairly well by the
HREF LPMM, although the area which sees these higher end amounts
will likely be less widespread than indicated by this product. If
greater frontogenetical forcing is able to materialize
particularly in the snow growth zone, then higher amounts would be
possible in a more narrow band. But at the moment the forcing
looks primarily diffuse in an area of mid and upper level lift and
low level moist advection ahead of the advancing trough resulting
in a fairly widespread light precipitation scenario.

As the trough moves east, surface high pressure pushing down through
the High Plains will push a cold front eastward through our area
during the day Sunday. This will shift winds to the west and bring
in much drier air in at all levels, shutting off the
precipitation. Temperatures will warm solidly above freezing in
the well-mixed air mass behind this front although it will
certainly feel chilly. Another front associated with a northern
stream low tracking through southern Canada will move in from the
northwest Sunday night bringing even colder air with it. Despite
a well-mixed post-frontal environment, temperatures will fall
into the 20s and teens by early Monday morning.

Kimble

.LONG TERM...  (Monday through Next Saturday)
Issued at 243 PM CST Sat Nov 25 2023

Surface high pressure moves through the Plains in the wake of the
departing trough leading to dry and cold conditions on Monday.
Temperatures begin to rebound as early as Tuesday as the surface
high moves east. Greater uncertainty shows up in the temperature
forecast from Tuesday onward as we will be in the transition zone
between the cold air mass to our north and east and a building
ridge over the southern Plains to the east of the next
southwestern US trough. The general trend of gradually warming
temperatures is clear although just how quickly this occurs or how
warm it gets is the main source of uncertainty. At the low end of
the interquartile range, temperatures next week could be near or
slightly below average, while on the higher end we could warm into
the 60s, some 10 degrees above average.

Our next chance of rain will come with the trough moving through the
southwestern US and out into the Plains on Thursday. There is rather
strong agreement in the guidance on timing of this wave, with the
most likely period for rain in our area on Thursday night into early
Friday. Despite some uncertainty in the track this should be an all
rain event for us as temperatures will have warmed well above
freezing by then. 00Z ensemble guidance suggests only low
probabilities for significant rainfall (more than 0.5 inch), but
many of the newer 12Z deterministic models are showing a better
moisture connection and greater potential for 0.5 inch or more.
These newer runs are also more solidly drying things out behind
this wave as well, so some greater clarity in the timing and
intensity of the late week rainfall is likely starting to emerge.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 537 PM CST Sat Nov 25 2023

Impacts will be experienced at all local terminals tonight into
tomorrow afternoon as a weather system moves through the area.
First, precipitation will spread across the local terminals from
west to east later this evening into the overnight hours and
continue into tomorrow morning. Light snow is generally expected
at KUIN, with warmer temperatures south of that terminal causing
rain to be the primary precipitation type at KCOU, KJEF, KSTL,
KSUS, and KCPS, though snow may mix in with the rain at KCOU and
KJEF more than what is currently indicated in the TAFs. I also
can`t rule out heavy snow at KUIN producing lower visibilities
than what is currently indicated, though confidence in heavy snow
directly impacting the terminal is low at this time.

Increasing low-level moisture due to the onset of precipitation
will also cause ceilings to lower overnight, with high confidence
in IFR ceilings at KUIN, KCOU, and KJEF. I can`t rule similar
impacts at KSUS, KSTL, and KCPS during the late morning, but
confidence in this occurring is low at this time. A front moving
over the terminals will cause winds to become westerly and
strengthen tomorrow, which in turn will help push low-level clouds
and precipitation out of the area.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until 9 AM CST Sunday for Knox MO-Lewis
     MO-Marion MO-Monroe MO-Shelby MO.

IL...Winter Weather Advisory until 9 AM CST Sunday for Adams IL-Brown
     IL.

&&

$$

WFO LSX