Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220239
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
939 PM CDT Sat Oct 21 2017

.UPDATE...
Issued at 918 PM CDT Sat Oct 21 2017

A cold front was moving southeastward through northwestern MO
this evening. A line of convection was along this front with
trailing stratiform precipitation behind it. Isolated showers had
recently developed across parts of northeast and central MO well
ahead of the cold front. The line of convection will gradually
weaken as it moves into northeast and central MO around midnight
due to waning instability, but could still generate brief heavy
downpours along with strong gusty winds. Looking at the latest
HRRR model runs this convection will not likely get into the St
Louis metro area until very late tonight and early Sunday morning,
and by this time should have weakened quite a bit and be mainly in
the form of disorganized scattered showers. Unseasonably warm
temperatures can be expected tonight ahead of the cold front,
particularly across southeast and east central MO and southwest
IL due to the cloud cover, gusty southerly winds, and relatively
high surface dew points. Lows in St Louis metro area tonight will
be around 15 degrees above normal.

GKS

&&

.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 00z TAFs through 00z Sunday Evening)
Issued at 526 PM CDT Sat Oct 21 2017

Cold front extending from MN south through extreme northwestern
MO into western OK will move southeastward through the taf sites
late tonight and Sunday morning. There was a line of thunderstorms
along this front which will move into UIN and COU late this
evening near midnight, and eventually into the St Louis metro area
by early Sunday morning. The convection should slowly weaken in
intensity late tonight as the instability wanes. There may also be
isolated showers ahead of the cold front which could impact the
taf sites this evening. Lingering light showers may continue in
UIN and COU during at least much of Sunday morning and in the St
Louis metro area Sunday afternoon due to a slow moving upper level
disturbance behind the cold front. MVFR cigs will advect into the
taf sites late tonight and early Sunday morning along and behind
the cold front. The ceiling should gradually improve to VFR
conditions Sunday afternoon. The southerly surface winds will veer
around to a northwesterly direction after fropa in UIN and COU
towards early Sunday morning and in the St Louis metro area by
early Sunday afternoon. There will be a strong southwesterly low
level jet tonight which will lead to some LLWS conditions at the
taf sites, but it appears that there will be enough nocturnal
mixing to keep the surface wind up so it will not likely be
strong enough to include in the tafs.

SPECIFICS FOR KSTL: Cold front extending from MN south through extreme
northwestern MO into western OK will move southeastward through
the STL area Sunday morning. There was a line of thunderstorms
along this front which will move into the STL area by early
Sunday morning. The convection should slowly weaken in intensity
late tonight as the instability wanes. There may also be isolated
showers ahead of the cold front which could impact STL this
evening. Lingering light showers may continue in the STL area
Sunday afternoon due to a slow moving upper level disturbance
behind the cold front. MVFR cigs will advect into the STL area
late tonight and early Sunday morning along and behind the cold
front. The ceiling should gradually improve to VFR conditions
Sunday afternoon. The southerly surface winds will veer around to
a northwesterly direction after fropa by early Sunday afternoon.
There will be a strong southwesterly low level jet tonight which
will lead to some LLWS conditions, but it appears that there will
be enough nocturnal mixing to keep the surface wind up so it will
not likely be strong enough to include in the tafs.

GKS

&&

.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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