Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 200948
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
348 AM CST Sat Jan 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 342 AM CST Sat Jan 20 2018

Gaps in the cirrus showed that the spatial extent of stratus at 08z
across KS/OK/AR/TN/KY closely followed the RAP`s depiction of
condensation pressure deficits at 900 and 925 hPa. This stratus deck
is expected to continue spreading northward over the next several
hours. Weak lift within the saturated cloud layer may produce patchy
drizzle at times, and areas of fog are expected overnight as the
boundary layer continues to moisten.

Persistent S/SW winds around a high pressure center located
over the southeastern CONUS and ahead of a developing surface low
over southeastern CO will maintain the flow of warmer air into the
region today and tonight. Yesterday`s high temperatures reached the
50s areawide with only limited cloud cover noted on satellite
imagery. Similarly warm highs are expected again today even beneath
the stratus, which is consistent with the diurnal rise of ~10-15
degrees suggested by conditional climatology. If the stratus does not
spread as far northward as currently anticipated, then the going
forecast of highs in the low/mid 50s is likely 5-10 degrees too
cool. Tonight`s lows will likely be a few degrees warmer than last
night due to the cloud cover.

Kanofsky

.LONG TERM...  (Sunday through Friday)
Issued at 342 AM CST Sat Jan 20 2018

A deep upper trof will move east-northeast from near the 4
corners region through the central/southern Plains Sunday into
Sunday Night, continuing into the MS Valley on Monday, and then
weakening some as it moves into the Great Lakes/OH Valley Monday
Night-early Tuesday morning. In response, a surface low initially
in southeast CO will deepen and move into central KS by early
Sunday evening. The low will then continue deepen moving northeast
into southwest IA by early Monday morning and into the Great
Lakes by Tuesday morning.

Stratus/fog associated with shallow low level moisture will
blanket the region on Sunday morning in advance of the cyclone and
attendant cold front. Weak low level WAA should also result in
some drizzle. By Sunday afternoon the precipitation should become
more showery as low level flow backs resulting in deepening low
level moisture. Despite the extensive cloudiness, WAA should
contribute to above normal temperatures. And if we get breaks in
the cloud cover, then the temps could be even warmer than
currently forecast. It still appears the most active weather
should be centered on Sunday night and into Monday. Showers should
blossom in coverage on Sunday evening within the warm sector
ahead of the advancing cold front in response to increasing large
scale ascent associated with the upper system and the organizing
warm conveyor belt. Advective processes will also contribute to an
axis of weak and predominately elevated instability supporting
the potential for isolated to scattered thunderstorms, particularily
for central and eastern MO. The models vary slightly with the
eastward speed of the cold front across the region, and the model
consensus suggests it should stretch across eastern MO at 12Z
Monday. The main band of pre-frontal precipitation will be located
well ahead of the cold front, extending either across the eastern
CWA in IL over possibly even further east from eastern IL into
Indiana while the remainder of the CWA is located within a dry
slot. The cold front will then sweep across eastern MO and western
IL by early-mid afternoon with gusty and veering west-northwest
surface winds and CAA in the wake of the cold front and lifting
surface low. It still appears that light precipitation will move
back into central and northeast MO during the afternoon, and into
west central IL during the evening within the wrap-around region
of the deepening cyclone. This precipitation appears to be
associated with the cyclonic branch of the warm conveyor belt and
frontogenetic forcing. Intially it would be rain but as the cold
air deepens it should change to snow during the later part of the
afternoon and into Monday night as the system continues to pull
northeast.

Rather deep and broad cyclonic flow will be present in the wake of
the departing system Tuesday into Wednesday. Temps on Tuesday should
be pretty close to normal owing to lots of clouds early on and the
initial impacts of the post-frontal cold air mass, with some slight
moderation on Wednesday as surface high pressure dominates. The
remainder of the week however will feature a warming trend as low
level flow becomes southerly/WAA commences and as ridging builds
aloft.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1144 PM CST Fri Jan 19 2018

VFR conditions will prevail at the TAF sites thru about 10z, with
MVFR CIGs overspreading all sites from 10z thru 13z and then
likely persist for the remainder of the valid time. Mainly dry
weather will prevail thru the early part of the period, with
patchy drizzle entering the picture after 15z/Sat for areas north
of Interstate 70, but hard to necessarily add to TAF at this time
given low prob of occurrence at any one site. Otherwise, a steady
S-SW surface wind around 10kts is anticipated, backing SE by
Saturday night. LLWS conditions remain marginal and expected to
be just below thresholds and so have continued with the no
mention.

SPECIFICS FOR KSTL: Other than the above, wind directions should
remain below 210 thru the valid period, with best potential for
any drizzle currently expected to the north, so no mention at this
time.

TES

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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