Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200246

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
946 PM CDT Mon Apr 19 2021

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 310 PM CDT Mon Apr 19 2021

Key messages through the short-term forecast period:

1) Confidence in measurable snow is increasing further south to near
the St. Louis metropolitan area. Warm surface temperatures will
likely prohibit accumulations on most surfaces, but grass and
elevated surfaces may see accumulation.

2) Potentially-historic cold temperatures are all but certain
tomorrow night. Century-old record low temperatures could fall, and
damaging freeze is expected areawide. The Freeze Watch in effect for
tomorrow was upgraded to a Freeze Warning.

A climatologically-impressive cold front is slowing down, now
approaching the St. Louis metropolitan area. Strong southwesterly
flow and very warm temperatures ahead of the front across a
majority of the CWA have held the front back, but with loss of
daytime heating, the front should advance rather quickly to the
south this evening and overnight tonight. Temperatures continue to
trend cooler with this historically-cold air mass, and now the
potential for frost exists in portions of north-central MO behind
the front. However, with precipitation likely beginning around the
time temperatures would bottom out overnight and elevated winds
providing some mixing, I opted not to issue a Frost Advisory for
tonight. That being said, isolated areas of frost cannot be ruled
out, and precautions should begin for protecting tender vegetation
against the upcoming unprecedented cold weather.

After the front sags to the south tonight, a very robust upper-level
shortwave will traverse the central Plains and enter the Mid-
Mississippi Valley through the morning tomorrow. Deterministic
guidance continues to depict strong PVA associated with the trough
as it moves through the region, which will provide ample upper-level
support for precipitation Tuesday. A very stout band of 850-700mb
frontogenesis continues to be depicted among the deterministic
global models and CAMs, and this band of enhanced lift spends
residence time in the DGZ. All of this points to the potential for
measurable snow across portions of the CWA, primarily in
northeast/central MO and west-central IL. The saving grace for
accumulations in this area would be if antecedent temperatures are
too warm to allow snow to stick. Seeing that recent temperatures
have been above freezing, and today in particular was quite warm,
widespread accumulation on pavement appears unlikely. That being
said, a sloppy morning commute is in store for those in
northeast/central MO and west-central IL.

A large, important area of uncertainty is the point at which
precipitation changes over from snow to rain as the stratiform band
pushes south through the day. Guidance for surface temperatures
continues to trend colder late Tuesday morning into the afternoon,
especially within the band of precipitation. While guidance doesn`t
go as far as suggesting freezing/subfreezing temperatures as far
south as St. Louis, locally-high precipitation rates could cool
temperatures to the point where minor accumulations could occur
during a brief window in the afternoon. Unfortunately, the ability
to forecast the magnitude by which precipitation rates cool the air
and effect accumulations is quite low until the event materializes.
Given the potential for sufficient cooling to allow for periods of
snow during the afternoon in east-central and southeast MO, minor
accumulations were expanded south compared to the last forecast.
That being said, the scenario of a rain/snow mix this far south
appears more likely given the marginal temperatures and high sun
angle of mid/late April.

The lovely temperatures we experienced today will be a distant
memory 24 hours from now when a highly-anomalous, potentially
historic cold air mass takes hold of the region tomorrow night. In
addition to 100 year old minimum temperature records for April 21st
remaining in serious jeopardy of being broken, a damaging freeze
appears all but certain areawide. The Freeze Watch that was issued
yesterday for tomorrow night was upgraded to a Freeze Warning as a
part of this forecast package. While temperatures continue to trend
slightly colder on Thursday as well, no watch or warning is in
effect for that time period as of yet.


.LONG TERM...  (Wednesday through Next Monday)
Issued at 310 PM CDT Mon Apr 19 2021

While attention was given to the forecast concerns through Wednesday
morning, here are the key messages in the extended period:

1) Another round of very abnormal cold is expected Wednesday night/
Thursday morning, with renewed concerns for a damaging freeze.

2) Other than a chance for precipitation Wednesday afternoon with a
moisture-starved clipper, the forecast remains dry through Thursday

3) Rain chances appear more likely to finish the week, but the
combination of timing uncertainty and a complicated system evolution
kept PoPs relatively muted.

Deterministic guidance continues to hint at a quick clipper system
affecting the region Wednesday afternoon. Moisture availability
appears very limited, but sufficient forcing for very light
precipitation across northeast MO and west-central IL does exist.
Given the nature of the system, particularly its low-QPF potential,
the NBM does not show any mentionable PoPs. Using a blend of
deterministic guidance, slight-chance PoPs were added to account for
this potential.

The historically-cold air mass that sets up tomorrow will remain in
place areawide Wednesday night into Thursday as well, though
temperatures may be ever-so-slightly warmer. Regardless,
temperatures continue to trend lower for this time period, and
renewed potential for widespread freeze exists. The only aspect
working against a second night of freeze would be cloud cover
inhibiting radiational cooling. While there are some clouds in the
forecast for Wednesday night/Thursday morning, the air mass may be
cold enough in its own right to cause a freeze without help from
robust radiational cooling.

The forcing for the rain chances later this week remains pretty
complex, but the chance for measurable precipitation is slowly
increasing. Three separate streams of energy have the potential to
phase as they enter the region: one from the north, one out of the
central Rocky Mountains, and a third from the south. Deterministic
and ensemble guidance continues to differ regarding evolution and
phasing of the shortwaves. WPC Cluster Analysis shows consent among
the guidance regarding some precipitation Friday, but varies
regarding the amounts and location. Given the reasonable uncertainty
with the system and more immediate concerns in the short-term
period, PoPs from the NBM were used for this period.



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 938 PM CDT Mon Apr 19 2021

A cold front has moved into southeast MO and southwest IL
this evening with north-northwest winds at the taf sites. Post
frontal rain and snow will spread into UIN and COU late tonight
with the ceilings and visibilities dropping into the MVFR category
late tonight, and then into the IFR category early Tuesday
morning. Rain will move into the St Louis metro area Tuesday
morning, briefly changing to snow in the afternoon. Conditions
will deteriorate to IFR by late morning in the St Louis metro
area. The precipitation will end by late afternoon in UIN and COU,
and end in the St Louis metro area by early evening, followed by
improving ceilings and visibilities.

SPECIFICS FOR KSTL: A cold front has moved into southeast MO and
southwest IL this evening with north-northwest winds at STL.
Post frontal rain will spread into STL Tuesday morning around 15Z,
briefly changing to snow in the afternoon. The ceiling and
visibility will deteriorate to IFR by late morning. The
precipitation will end by early evening, followed by improving
ceilings and visibilities into the VFR category by 04Z Wednesday.



Issued at 310 PM CDT Mon Apr 19 2021

An potentially-historic airmass is expected to threaten low
temperature records across our area for the middle of the week.
Here is a look at record min temps and record low max temps during
that time.

                  TUE 4/20           WED 4/21         THU 4/22

St. Louis (KSTL)   28/1904            32/1904          31/1936
Columbia (KCOU)    28/1953            29/1907          29/1986
Quincy (KUIN)      27/2013            26/1904          27/1986

St. Louis (KSTL)   42/1918            46/1893          43/1884
Columbia (KCOU)    38/1918            42/1992          48/1893
Quincy (KUIN) 39/1918 37/1918 44/1927


MO...Freeze Warning from 1 AM to 9 AM CDT Wednesday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-
     Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Shelby MO-Warren MO-Washington MO.

IL...Freeze Warning from 1 AM to 9 AM CDT Wednesday for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Randolph IL-Saint Clair-Washington IL.



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