Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 132341

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
541 PM CST Tue Nov 13 2018

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 222 PM CST Tue Nov 13 2018

Surface ridge will continue to build into the region tonight. So
will have light and variable winds and clear skies. This combined
with snow cover over portions of central, east central and northeast
Missouri, as well as west central Illinois, will allow temperatures
to drop off into the teens for overnight lows.

On Wednesday, surface ridge begins to move off to the east while a
developing weather system over northeastern Texas begins to move
northeastward towards the Missouri bootheel. Will see increasing mid
and high clouds Wednesday afternoon. Moisture will be slow to return
to the region, but may see a bit of light snow over portions of
southeast Missouri and southern Illinois late in the afternoon.
Highs on Wednesday will be in the mid to upper 30s.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 222 PM CST Tue Nov 13 2018

(Wednesday Night - Thursday)

...Confidence Increasing in Significant Snowfall Across Parts of
the Area...

Deterministic guidance has continued to cluster together with how
the closed low across the Arklatex region will evolve over the next
48 hours. Some small differences still exist with respect to timing
and track, but the general consensus is for the closed low to slowly
lift northeast Wednesday night through the day Thursday. There is
nothing really steering this system at mid/upper levels, so it
appears that internal dynamics within the closed low itself will be
very important. Higher resolution guidance even shows this
explicitly as an individual vorticity maximum rotates cyclonically
from the southern portion of the parent circulation very quickly
into the northeastern quadrant. This appears to help the system jog
a bit more to the north from 0000 UTC to 1200 UTC Thursday.

Isentropic surfaces show a classic maturing midlatitude cyclone. The
moisture within the warm conveyor belt origins is actually more off
of the southwestern Atlantic Ocean compared to the Gulf of Mexico.
The cyclonic portion of the warm conveyor belt (TROWAL) is very well
defined, moving into southern portions of the CWA late Wednesday
night. The TROWAL is then expected to move north, even a bit to the
north/northwest, pivot, and then move off to the east/northeast.
This evolution may end up being similar to the heavy snow event of
March 4th, 2008. Wherever this pivot point occurs is where the
heaviest snowfall will likely occur. 320K isentropic surface also
shows a portion of the dry conveyor belt which wraps cyclonically
and ascends above the warm conveyor belt. This process is a common
feature of mature midlatitude cyclones and helps to increase
potential instability within the deformation zone.

Model soundings continue to show efficient snowfall processes within
the axis of heaviest snowfall. Maximum lift is centered within the
dendritic growth zone which is nearly supersaturated with respect to
ice. This is referred to as the crosshair signature which previous
research has shown to be a common feature for heavy snowfall (6+")
events. The dendrites should then aggregate as they head to the
surface as they fall in an atmosphere that is isothermal around -5C
up to near 700 hPa. Light winds throughout the column should
also prevent fracturing of these large aggregated dendrites.
Subfreezing surface temperatures and colder ground temperatures
compared to the past few snow events should also increase snowfall

Mean 850-hPa low track is from KMEM to near the Ohio/Mississippi
River confluence into southwestern Indiana. Browne and Younkin
(1970) technique would favor the heaviest snow axis very close to a
KFAM>>KSAR>>KSLO axis and still have the highest snowfall totals
near this axis. Due to increasing confidence in the track of the
system, did raise snowfall totals across much of southeastern
Missouri and southwest Illinois into the 3-6 inch range. These
totals still may not be high enough if the wetter ECMWF/NAM
solutions come close to verifying. Because of the potential for at
least isolated totals of 6+" of snow, the timing of the heaviest
snow near rush hour on Thursday, and concerns for tree damage and
associated power outages due to leaves remaining on many trees, went
ahead and issued a winter storm watch for parts of the area. The
watch is roughly to the southeast of the I-44 corridor in Missouri
and Interstate 70 in Illinois. An upgrade to a warning in addition
to a possible advisory further to the northwest is certainly
possible over the next 24 hours as confidence further increases.
Further to the northwest, lesser amounts of snow are possible
ranging from 1 to 4". This is where the main region of uncertainty
exists with respect to snowfall amounts: along and just to the
northwest of the existing winter storm watch, which includes the St.
Louis metropolitan area. A slight shift of the storm 20-30 miles to
the northwest could yield several more inches of snow whereas a
slight shift southeastward could yield little to no snow
accumulation. Fine tuning the exact path of this storm and its
associated deformation zone will likely be the primary challenge
with this event moving forward.

(Thursday Night - Next Tuesday)

Much quieter weather is in store after the winter storm moves off
into the Ohio Valley. Zonal to northwest flow aloft is likely for
the remainder of the forecast period. A cold front is expected to
move through Friday night into Saturday. A chance of rain/snow is
possible Saturday night into Sunday as a shortwave trough moves
through the mid-Missouri Valley into the Ohio Valley.

Temperatures will remain below normal for the duration of the
extended forecast. The warmest day is forecast to be on Friday ahead
of the cold front as highs reach into the 40s and 50s. Cooler
conditions are then likely for the weekend into early next week as
highs only reach the 30s and 40s.



.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 535 PM CST Tue Nov 13 2018

VFR flight conditions are expected to prevail through Wednesday.
Wind will become light and variable then shift to the northeast as
low pressure develops over the Tennessee Valley.


VFR flight conditions are expected to prevail at Lambert through
Wednesday evening. Wind will become light and variable then turn
to the northeast on Wednesday, and higher clouds will begin to
fill in toward the end of the period ahead of the next storm
system. Precipitation associated with this storm should hold off
until after 06Z Thursday.



Saint Louis     18  36  27  37 /   0  10  80  80
Quincy          15  37  21  41 /   0   0   5   5
Columbia        12  36  21  42 /   0   0  10  10
Jefferson City  12  37  23  43 /   0   5  20  10
Salem           19  37  28  34 /   0  10  80  90
Farmington      17  35  25  36 /   0  20  90  80


MO...Winter Storm Watch from Wednesday evening through Thursday
     afternoon for Iron MO-Jefferson MO-Madison MO-Reynolds MO-
     Saint Francois MO-Sainte Genevieve MO-Washington MO.

IL...Winter Storm Watch from Wednesday evening through Thursday
     afternoon for Bond IL-Clinton IL-Fayette IL-Marion IL-
     Monroe IL-Randolph IL-Saint Clair IL-Washington IL.



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