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FXUS63 KLSX 181015
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
415 AM CST Fri Jan 18 2019

.SHORT TERM... (Through Late Saturday Afternoon)
Issued at 414 AM CST Fri Jan 18 2019

Latest winter storm coming into better focus, but still some
uncertainty on when and how long areas that start as rain will
become mixed with then change over to all snow.

For today, surface ridge will continue to slide off to our east with
strong low level WAA ahead of next weather system as well as
moisture surging into the region ahead of this system. Temperatures
today will range from the low 30s far north to the low 40s far
south, a bit below MOS guidance, as lingering SC deck will keep
temperatures from rising too much. Precipitation will develop over
northwestern MO early this afternoon and track east, increasing in
coverage as it moves into the forecast area. It will be all snow for
the warning area at onset, while a rain/snow line sets up along a
Mexico, MO to Jerseyville, IL to Vandalia, IL line and will remain
in this area through the overnight hours, with all rain south of
this line.

The timing of the transition is still questionable, though 00z
models are a bit faster and further north with the 850mb low track,
which would have all snow everywhere by Noon on Saturday as the
colder air aloft filters in. Then the system will begin to lift
northeast away from region, with some deformation zone snow but
amounts will be rather light. As for temperatures, will have
midnight highs in most locations with temperatures falling
throughout the day.

Overall, expecting 4 to 6 inches of snow by Saturday afternoon for
portions of northeast Missouri and west central Illinois in the
warning area. South of the warning, expect between 2 and 4 inches of
snow in the advisory area, with less than 2 inches for far southeast
portions of forecast area. This will be a very wound up system with
winds picking up from the east then back to the north as it lifts
out with sustained winds between 15 and 25 mph with gusts near 35
mph at times. So will see some blowing and drifting of the snow
during the day on Saturday.

Byrd

.LONG TERM... (Saturday Night through Thursday)
Issued at 414 AM CST Fri Jan 18 2019

By Saturday night, precipitation will be winding down as both the
GFS/NAM are showing the main 500mb trough moving off into the Ohio
Valley. A secondary weaker trough will be moving across the CWA at
00Z Saturday evening, but the NAM/GFS has it moving quickly off to
the east, so will only keep chance PoPs with little if any
additional accumulations to the south and east of St. Louis.

Cold air advection will continue across the CWA into midday Sunday
when the GFS/ECMWF is showing a surface ridge moving across Missouri
and Illinois. Model 2M temperatures are still showing Sunday
morning lows in the single digits and teens with wind chills from
-10 to +10. Highs on Sunday will not recover to much as 850mb will
be around -10C. Highs should only be in the teens and 20s. Lows on
Monday morning will not be as cold as winds turn out of the
southeast and increase in the wake of the surface ridge.

Dry weather is expected Sunday through Monday. Chances for
precipitation will increase on Monday night as low level moisture
and large scale ascent increase ahead of the next trough and
attendant cold front move across the area midweek. The 00Z runs of
the global models have come into better agreement with moving a
northern stream trough quicker through Missouri and Illinois while
holding a southern stream trough back over the Southern Plains. For
now will go with the faster solution start with some slight chances
of snow up over northeast Missouri and rain/snow mix over southeast
Missouri on Monday night. Then we will transition to mainly rain on
Tuesday as warmer air moves into the area, then back to snow as a
cold front moves across the area late Tuesday and Tuesday night.

There is some more uncertainty about another storm that will affect
the CWA by Wednesday night and Thursday. All of the global models
are showing the aforementioned Southern Plains upper trough lifting
out and moving eastward by mid-late next week, but only the Canadian
has the northern periphery moving across southern and and eastern
parts of the CWA. Will maintain only low PoPs for snow at this time
until there is more agreement between the models.

Other than Tuesday when temperatures will climb up above normal in
the warm sector, highs next are expected to stay below normal (lower
to mid 30s) as 850mb temperatures stay between -5 and -10C.

Britt

&&

.AVIATION... (For the 06z TAFs through 06z Friday Night)
Issued at 1101 PM CST Thu Jan 17 2019

A surface ridge extending from eastern ND and MN south-southeast
through MO will move eastward late tonight and Friday. Surface
winds will be light late tonight. The stratus cloud deck ranging
from 600 feet in height in STL to 1900 feet in UIN will gradually
lower late tonight with fog developing due to the light wind and
moist surface/boundary layer aided by daytime melting of snow. The
lowest visibility will likely be in COU, possibly as low as
1/2-1SM late tonight/early Friday morning. The visibility will
improve late Friday morning as an easterly surface wind
strengthens due to a tightening surface pressure gradient as the
surface ridge shifts east of the area, while surface low pressure
deepens over the TX Panhandle region. The cloud ceiling will rise
to around 1000-1500 feet in the afternoon, then fall again Friday
evening. Light rain will spread eastward into COU and the St
Louis metro area Friday evening with light snow moving into UIN.

SPECIFICS FOR KSTL: A surface ridge extending from eastern ND
and MN south-southeast through MO will move eastward late tonight
and Friday. Surface winds will be light late tonight. The stratus
cloud deck at 600 feet in height will continue to lower late
tonight with fog developing due to the light wind and moist
surface/boundary layer aided by daytime melting of snow.
The visibility will improve late Friday morning as an easterly
surface wind strengthens due to a tightening surface pressure
gradient as the surface ridge shifts east of the area, while
surface low pressure deepens over the TX Panhandle region. The
cloud ceiling will rise to around 1000-1500 feet in the afternoon,
then fall again Friday night. Light rain will spread eastward
into STL Friday evening.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory from midnight tonight to 6 PM CST
Saturday for Audrain MO-Boone MO-Callaway MO-Cole MO-
Crawford MO-Franklin MO-Gasconade MO-Jefferson MO-Lincoln
MO-Moniteau MO-Montgomery MO-Osage MO-Saint Charles MO-
Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
Genevieve MO-Warren MO-Washington MO.

Winter Storm Warning from 3 PM this afternoon to 6 PM CST
Saturday for Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
Ralls MO-Shelby MO.

IL...Winter Weather Advisory from midnight tonight to 6 PM CST
Saturday for Bond IL-Calhoun IL-Fayette IL-Greene IL-Jersey
IL-Macoupin IL-Madison IL-Monroe IL-Montgomery IL-Saint
Clair IL.

Winter Storm Warning from 3 PM this afternoon to 6 PM CST
Saturday for Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX
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