Area Forecast Discussion
Issued by NWS St. Louis, MO

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS63 KLSX 190511
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1111 PM CST Fri Jan 18 2019

.SHORT TERM...  (Through Late Saturday Night)
Issued at 302 PM CST Fri Jan 18 2019

A strong winter storm is already beginning to take shape across the
four corners region of the desert southwest. Deterministic model
guidance has come into much better agreement with the large-scale
evolution of this storm system, especially compared to 24 hours ago.
Precipitation is already beginning to take shape near the mid-
Missouri Valley and is expected to really blossom over the next 6
hours as the low-level jet strengthens. Precipitation type is
expected to be mainly snow tonight across portions of northeast
Missouri and west-central Illinois where the winter storm warning
resides. The snow may mix in with or even change to rain for a brief
time as warmer air around 850 hPa advects northward. However, the
northern extent of any mixing within the warning area is expected to
stay just south of KUIN. For areas further to the south, mainly if
not all rain is expected through the night, though after about 0900
UTC the rain/snow line should begin to march southeastward as the
low-level flow backs to the northeast.

Deterministic model guidance is showing a much stronger midlatitude
cyclone compared to yesterday`s guidance, closing off to about 500
hPa south of the bi-state area. In fact, this midlatitude cyclone
exhibits many of the classic signatures for heavy snow cases for the
central portion of the LSX CWA, including but not limited to: 1)
deepening 850-hPa low tracking northeast near or just northwest of
the Ohio River, 2) deepening 500-hPa trough that becomes negatively
tilted with increasing height, 3) hints of a coupled jet streak
signature during the warm air advection phase with strong divergence
via the left-exit region of jet streak downstream of trough axis on
Saturday, and 4) surface low track just northwest of KMEM that
deepens with time. Model QPF looks quite low, and potentially
woefully low given the dynamic setup with this system. Boosted QPF
and associated snowfall amounts to account for what should be a
strong deformation zone that should track over the southeastern half
of the CWA from late Saturday morning through the afternoon hours.
The uncertainty remains exactly where and how high are the snowfall
totals. Model guidance at 1200 UTC also seemed too far southeast
with the heavier QPF axis, especially on Saturday afternoon. Leaned
closer to the typical 90nm to the left of the track of the 850-hPa
low, which previous research such as Browne and Younkin (1970)
showed was highly correlated to axis of heaviest snowfall. In
addition, given the deepening of this system, would not be surprised
if short-term guidance ends up shifting about 15-30 miles further
northwest with the track of this system. A similar northwest jog has
already happened on a few different occasions this winter including
the November 15, 2018 case.

The transition from rain to snow should continue from northwest to
southeast, and may occur faster than forecast if convection
materializes. Model guidance suggests convection may occur near the
transition zone as soundings show very steep midlevel lapse rates
and instability aloft as the dry conveyor belt punches into
southeastern Missouri Saturday morning. While the addition of
thunder was limited to the warm air advection precipitation tonight
across southeastern Missouri, would not be surprised to see a few
lightning strikes during the morning hours on Saturday as far north
as central portions of the CWA. If this were to occur that far north
and west, thundersnow would be possible. After the 850-hPa low
passes to the south of the region, all snow is favored for the
duration of the storm. Immediately after the changeover, expect
moderate to potentially heavy snowfall rates (up to 1"/hr) to last
anywhere from 6-9 hours. The short duration of accumulating snowfall
over the southeastern half of the CWA is the main limiting factor
for higher snowfall totals (6+"), though a general 2-4 inches are
expected. Locally higher amounts however near or slightly above 6"
are possible within heavier banding. Across northeastern Missouri
and west-central Illinois, the snow should wind down by mid to late
morning. Amounts in this area are still forecast to be in the 4-6"
range and there remains a high confidence in the snowfall amounts
for these locations.

The other main story during and in the hours after the conclusion of
the snowfall will be the very strong northerly winds (20-30 mph),
with gusts in the 30-40 mph range. This should reduce visibilities
due to blowing and drifting of snow and will act to prolong the
period of hazardous driving conditions as snow blows back over
previously plowed roadways. The gusty northerly wins combined with
falling temperatures will also yield the coldest wind chill readings
so far this season on Saturday night. Lowest wind chill values
across the CWA should reside in parts of northeast Missouri and west-
central Illinois where values of -5 to -10F are forecast.


Gosselin

.LONG TERM...  (Sunday through Next Friday)
Issued at 302 PM CST Fri Jan 18 2019

Cold air, the coldest of the season, continues to move into the area
through midday Sunday with 850 temps near -10. As surface ridge
continues to move off to the east SE flow will return Sunday night
into Monday keeping the region not as cold as previously thought.
Highs on Sunday should be in the teens and low twenties with a low
Sunday night in the teens. Temps will rebound to the upper twenties
to mid thirties Monday. Wind chills on Sunday could reach the
negative single digits for northern parts of the CWA.

Sunday and Monday look to remain dry, with the exception of an upper
level disturbance skirting the NE portion of the CWA Sunday night.
GFS/Euro both have the disturbance weakening as it approaches the
CWA so have kept PoPs low. Precip chances ramp up again Tuesday
ahead of the next system with low level moisture and large scale
ascent ahead of upper level trough and associated low. The Euro/GFS
are in good agreement with the placement and timing of this system
while the Canadian is much slower. I went with the faster solution
which will be a mainly rain event at the onset. Southerly flow and
plenty of WAA will push daytime temps into the 40s ahead of a cold
front that will make it through the region late Tuesday night/
Wednesday morning. The cold air behind the front will change the
precip to snow and drops temps into the high teens and low twenties.

The next system will affect the area Thursday and Friday as the next
upper level trough is expected to move into the region system. There
is high variability with the global models for this system so I have
left PoPs low at this time.


Walsh

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1048 PM CST Fri Jan 18 2019

Precipitation will continue to move through the taf sites late
tonight and Saturday ahead of an upper level disturbance over the
central and southern Plains and northeast of a surface low over
eastern OK late this evening. This surface low will move east-
northeastward through northern AR late tonight and into the TN
Valley region Saturday morning. Precipitation type should be
mainly snow in UIN with rain changing to snow in COU late tonight
and in the St Louis metro area early Saturday morning as colder
air advects into this region. The cloud ceiling will remain low,
around 300-600 feet late tonight and Saturday morning with the
visibility lowering as the rain changes to snow. Easterly surface
winds will back around to a northerly direction Saturday morning
as they become very strong and gusty due to a tightening surface
pressure gradient leading to blowing snow which will further
reduce the visibility on Saturday. The snow will taper off from
the northwest to southeast Saturday afternoon with improving
visibilities. The cloud ceilings will also rise into the MVFR
category Saturday evening.

SPECIFICS FOR KSTL: Precipitation will continue to move through
the STL area late tonight and Saturday ahead of an upper level
disturbance over the central and southern Plains and northeast of
a surface low over eastern OK late this evening. This surface low
will move east-northeastward through northern AR late tonight and
into the TN Valley region Saturday morning. Precipitation type
will change from rain to snow early Saturday morning, around 13Z
Saturday as colder air advects into this region. The cloud
ceiling will remain low, around 300-600 feet late tonight and
Saturday morning with the visibility lowering as the rain changes
to snow. Easterly surface winds will back around to a northerly
direction Saturday morning as they become very strong and gusty
due to a tightening surface pressure gradient leading to blowing
snow which will further reduce the visibility on Saturday. The
snow will taper off from the northwest to southeast Saturday
afternoon with improving visibilities. The cloud ceilings will
also rise into the MVFR category Saturday evening, and into the
VFR range late Saturday night as the surface wind gradually
diminishes.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory from 6 AM to 9 PM CST Saturday for
     Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO-
     Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Washington MO.

     Winter Weather Advisory until 6 PM CST Saturday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-
     Lincoln MO-Moniteau MO-Montgomery MO-Osage MO-Saint Charles
     MO-Warren MO.

     Winter Storm Warning until 3 PM CST Saturday for Knox MO-Lewis
     MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...Winter Weather Advisory from 6 AM to 9 PM CST Saturday for Bond
     IL-Clinton IL-Fayette IL-Madison IL-Marion IL-Monroe IL-
     Randolph IL-Saint Clair IL-Washington IL.

     Winter Weather Advisory until 6 PM CST Saturday for Calhoun IL-
     Greene IL-Jersey IL-Macoupin IL-Montgomery IL.

     Winter Storm Warning until 3 PM CST Saturday for Adams IL-Brown
     IL-Pike IL.

&&

$$

WFO LSX


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.