Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 120306
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
906 PM CST Sat Nov 11 2017

.UPDATE...
Issued at 905 PM CST Sat Nov 11 2017

Radar mosaic shows a broad N-S swath of light pcpn across western
MO and is starting to edge into central MO currently. Out ahead
of this, a batch of heavier pcpn has developed around the UIN-HAE
area. Examination of RAP vertical temp profiles and the high
potential for evaporational cooling and seeing some higher pockets
of reflectivity on radar, there does appear to be a small window
of opportunity for sleet to mix in with the rain. It should not be
persistent enough to dominate to where a mention is necessary and
with relatively warm ground, it should not be a factor on
impacts. Temps ranged from the upper 30s in parts of southern IL
to lower 50s in central MO. A rather substantial 15-20F dewpoint
depression exists. Satellite has cloudy skies areawide.

For the most part the previous forecast is on target and made few
changes, with pcpn chances, light as they will be wrt QPF,
ramping up to at least likely for a 2-3 hour window later tonight.
Much of the pcpn that falls will be light rain, but moisture will
become more shallow with time and by daybreak Sunday and into much
of Sunday morning, enough shallow moisture with weak convergence
from a cold front moving thru will justify a switchover to drizzle
for a short time before pcpn ends.

Persistent clouds thru the day on Sunday nearly everywhere and a
cold FROPA will limit any temp rises and should keep most areas in
the 40s.

TES

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 250 PM CST Sat Nov 11 2017

Thick cirrus will quickly give way to lower clouds this evening in
response to broad warm advection ahead of an approaching shortwave
from the Plains. Rain is expected to develop this evening in central
Missouri where low level moisture convergence is maximized and then
spread/expand east toward the Mississippi River by midnight and into
Illinois through daybreak on Sunday. DPVA from the approaching
shortwave will also aid in synoptic lift across the area. While we
are not expecting a signficant amount of precipitation with this
system, measurable precipitation will be widespread enough to
warrant likely and categorical PoPs tonight. Thickening cloud cover
and a southerly wind will keep temperatures from falling too much
tonight and have given the nod to a blend of short term model
guidance for minimum temperatures Sunday morning.

Surface low and cold front, associated with the shortwave, will move
across the CWA on Sunday. Have maintained a slower solution, with
best chances of rain along and east of the Mississippi River. Light
wind, due to the proximity of the surface low, will become northwest
during the afternoon in the wake of the surface cold front. Did
shave a few degrees off high temperatures for Sunday given thick
cloud cover and lingering precipitation.

CVKING

.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 250 PM CST Sat Nov 11 2017

Active pattern this week as two upper level trofs will impact the
region. The first trof will move into the Upper Plains Tuesday.
Associated cold front will drop into the CWA early Wednesday. Ahead
of the front decent moisture transport from LLJ and WAA should allow
for some widespread showers. Cannot rule out an embedded
thunderstorm with the system.

The second system moves into the area Thursday night into Friday. As
a surface low moves into southern MO a warm front is draped across
the CWA before low moves east into IL and pushes associated cold
front through the CWA. Models lack agreement with the onset of
precip with this system. GFS is more agressive with precip
associated with the warm front and the ECMWF keeps the period dry
until the cold frontal passage Friday morning. Both models agree
with pre frontal precip but are not consistent with timing, The
ECMWF seems to lag behind GFS by 12 hours.

Temps for the period will be close to seasonal normals with some
slightly above normal temps Wednesday through Friday.

Walsh

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 548 PM CST Sat Nov 11 2017

CIGs will lower overnight into at least low-end MVFR category at
all TAF sites as an area of light rain overspreads the area. There
will be a higher likelihood of IFR at COU and especially UIN and
have included in those forecasts. The rain may become drizzle
towards the end of the event heading into Sunday morning, and if
realized, will contribute to VSBY reductions into IFR, but short
of that, VSBYs should stay MVFR or better. A cold front will drop
thru Sunday morning taking light/variable winds and becoming NW
but speeds should remain less than 10kts. A region of low clouds
should hold together, especially around UIN, into Sunday afternoon
with this core region then dropping southeastward in the wake of
the front. With that in mind, could see low CIGs continue at STL
metro sites well into Sunday night, but should clear out of UIN
and COU shortly after nightfall Sunday evening.

SPECIFICS FOR KSTL: Deterioration of CIGs into MVFR by late
tonight but rain onset should occur around midnight. Rain
intensities should be light and may end as drizzle after sunrise
Sunday. Low-end MVFR should continue thru Sunday and into Sunday
night with the potential of IFR late Sunday afternoon and evening
depending on how it evolves, but this time period will be more
favorable for IFR to occur. Some clearing may occur by late Sunday
night.

TES

&&

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

$$

WFO LSX



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