Area Forecast Discussion
Issued by NWS St. Louis, MO

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496
FXUS63 KLSX 212008
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
308 PM CDT Sat Oct 21 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 308 PM CDT Sat Oct 21 2017

Main issue continues to be the cold front that will slide east
across region tonight through midday on Sunday. Models have decent
sfc based CAPEs thru this evening, then diminish quickly after 06z.
With slower onset of precipitation and weakening instability, feel
that as line of storms approach CWA, they will weaken and expect
mainly widespread showers and isolated thunderstorms with passage of
front. Still not ruling out isolated strong to severe storms for
portions of central/northeast MO late this evening.

Models now more in agreement with upper level trof deepening and
cutting off late in the day on Sunday. This will slow exit of
activity on back side of system, especially for eastern half of
forecast area.

As for temperatures, will see mild lows for one more night, in the
low 50s to mid 60s. Then on Sunday, morning highs in the low 60s to
low 70s, then steady or falling temperatures during the afternoon
hours.

Byrd

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 308 PM CDT Sat Oct 21 2017

Period begins with amplified flow as a deep trof becomes
cut off across the region. Mdls are still struggling with this
feature. However, the 12z guidance is in much better agreement than
prev cycles and confidence is building. Still, have kept low PoPs on
Mon to account for a potential slower soln.

After a brief warmup on Mon, another cdfnt pushes thru the region
Mon afternoon as a deep low develops over the Great Lakes region.
The area shud remain seasonable cool with reinforcing shots of cold
air. A couple of clippers will help to warm temps as well. Mdls
differ late in the period with exactly how much cold air will arrive
Fri into Sat as another cdfnt pushes thru the region. The GFS/ECMWF
both point to low chances of snow Fri night into Sat, perhaps even
longer into Sat morning than currently forecast. A lot can change
between now and next weekend, but will be an event to monitor.


Tilly

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1253 PM CDT Sat Oct 21 2017

Showers ahead of boundary to remain fairly isolated, so kept tafs
dry til late this evening as main cold front approaches region. As
for winds, to persist from the south and gust to near 25kts at
times. Have showers moving in between 04z and 07z and persist
through rest of forecast period. As for thunderstorm chances,
models showing instability waning as system approaches, so for now
just kept vicinity thunderstorm mention til daybreak for taf
sites. South winds to veer to the west to northwest behind cold
front between 10z-11z for KCOU and KUIN and around 16z for STL
metro tafs.

SPECIFICS FOR KSTL:
Showers ahead of boundary to remain fairly isolated, so kept taf
dry. As for winds, to persist from the south. Have showers moving
in by 07z and persist through rest of forecast period. As for
thunderstorm chances, models showing instability waning as system
approaches, so for now just kept vicinity thunderstorm mention til
13z Sunday. South winds to veer to the west to northwest behind
cold front by 16z for STL.

Byrd

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     64  66  50  70 /  90  90  30  10
Quincy          57  61  45  68 / 100  70   5   0
Columbia        55  61  44  70 / 100  60   5   0
Jefferson City  57  62  43  71 / 100  70   5   5
Salem           64  72  50  67 /  60  90  60  40
Farmington      63  67  47  70 /  90  90  40  20

&&

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

$$

WFO LSX



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