Area Forecast Discussion

000
FXUS63 KLSX 191144
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
544 AM CST Tue Nov 19 2019

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 324 AM CST Tue Nov 19 2019

The surface low associated with the clipper system early this
morning was located over western Illinois with a trough axis
extending nearly due southward along the Mississippi River. Shower
activity has waned over the past couple of hours and limited
slight chance PoPs early this morning along/east of the
aforementioned trough axis. The remainder of the day should be
characterized by decreasing cloudiness from west to east along
with seasonable temperatures. Coolest readings are forecast to be
across south-central Illinois due to lingering clouds into the
early afternoon. High temperatures will only reach the upper 40s
in these locations. Further southwest, an earlier clearing should
lead to highs reaching the upper 50s to near 60 degrees for
portions of central and southeastern Missouri.

Fog should develop late tonight, mainly along/east of the
Mississippi River. These areas are favored for the development of
fog due to the expected timing of the departure of stratus today,
as well as light and variable winds expected near a surface ridge
of high pressure. The current expectation is for visibilities to
stay largely above 1 mile, but if clearing is delayed a few hours
than currently forecast, some patchy dense fog may be possible in
west-central and/or south-central Illinois. Outside of fog
development, a mostly clear sky along with temperatures near to slightly
above normal for the date are forecast. Lows tonight should be
mainly in the 30s across the bi-state region.

Increasing clouds are expected on Wednesday ahead of the next
weather system ejecting out of the Rocky Mountains. The timing of
the onset of rain has continued to slow down compared to a couple of
days ago, with the daylight hours on Wednesday likely staying dry
areawide. Increasingly southeasterly winds will help yield
moderating temperatures compared to those of today. Look for
highs to reach the upper 50s to mid 60s across the area which is
fairly close to the warmer MAV MOS numbers.


Gosselin

.LONG TERM...  (Wednesday Night through Monday)
Issued at 324 AM CST Tue Nov 19 2019

(Wednesday Night - Friday Night)

Widespread shower activity remains very likely Wednesday night,
mainly ahead of the approaching cold front in a region of strong low-
level moisture advection. Rainfall amounts do not appear too high
due to the relatively short duration of rainfall. Amounts generally
should be around a quarter to a half inch for most locations. Also
cannot rule out a rumble of thunder as the most aggressive model
guidance suggests some low MUCAPE values and Showalter indices near
0 late Thursday night. However, still feel that the probability of
occurrence is too low to explicitly mention with this forecast
package.

The cold front will slowly progress through the CWA on Thursday with
diminishing chances of any shower activity behind it. Another round
of rain appears increasingly likely Thursday night perhaps into
early Friday north of the slow-moving cold front. Added likely to
low categorical PoPs as far north as the I-44/I-70 corridors in
Missouri and Illinois respectively. This area of rainfall will be
supported by strong synoptic forcing via low-level moisture
advection, mid/upper level diffluence, and vorticity advection
ahead of the closed low entering the south-central Plains.

Uncertainty in the forecast increases beginning on Friday as the
deterministic ECMWF progresses the cold front further
southeastward than the GFS/CMC. In addition, the closed low
ejecting out of the Rockies into the south-central Plains is
weaker. Consequently, very little precipitation is depicted from
Friday through Friday night on the ECMWF. This solution was not
favored with this forecast package, but also cannot be discounted
at this time. The more realistic and likely scenario is for
continued light rain early on Friday focused across southern
sections of the forecast area. Additional precipitation may then
spread northeastward associated with the approaching closed low.
This precipitation would start off as rain, but as the
temperatures cool in the lower portions of the atmosphere the rain
should mix with and changeover to snow. This should occur from
parts of central Missouri northeastward through west-central
Illinois. Farther southeast, the vast majority of any
precipitation should be in the form of a cold rain. Light
accumulations of wet snow still could be in the offing for
northwestern sections of the forecast area if the stronger GFS/CMC
solutions are on the right track.

Temperatures will start off this period very mild ahead of the cold
front. Lows Wednesday night should be in the upper 40s to low 50s.
Highs on Thursday should warm nicely into the low to mid 60s as
well. For reference, these temperatures would be about 10 degrees
above normal for the date. Behind the cold front, a return to below
normal temperatures is expected. Lows both Thursday and Friday night
will mostly be in the 30s and highs on Friday are only forecast to
be in the low to mid 40s.


(Saturday - Monday)

A fairly quiet, benign weather pattern appears in the offing for
this upcoming weekend into early next week with northwest flow
aloft. Dry weather is favored along with moderating temperatures.
After initially starting off on the cool side on Saturday/Saturday
night, return flow around a departing surface ridge of high pressure
should lead to temperatures climbing back into the 50s for Sunday
and Monday.


Gosselin

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 538 AM CST Tue Nov 19 2019

A period of MVFR ceilings with bases between 1000 and 2000 feet
above ground level are likely this morning as stratus advects from
north to south. An improvement is forecast heading toward midday
and just beyond, with VFR conditions for this afternoon and early
overnight. Fog is expected to develop along and east of the
Mississippi River late tonight, so added MVFR visibilities at KUIN
and KSUS/KCPS. IFR visibilities may be possible, especially if
clouds clear today a bit later than currently forecast. Northwest
winds will become gusty late this morning and continue into the
afternoon hours. By late this afternoon and early this evening,
wind speeds will quickly diminish as a ridge of surface high
pressure quickly approaches from the west.


SPECIFICS FOR KSTL:

A period of MVFR ceilings with bases between 1000 and 2000 feet
above ground level is likely roughly between 1400 and 1700 UTC
this morning. This area of stratus should move off to the east
with VFR cumulus/stratocumulus into early this afternoon.
Northwest winds will increase in speed toward midday with gusts to
around 20 knots through mid/late afternoon. Wind speeds will
quickly diminish by early this evening as a ridge of surface high
pressure approaches the terminal. Could see some fog development
late tonight into early Wednesday morning but confidence too low
to put in the TAF at this moment in time. If fog were to occur,
visibilities would likely dip into the 4-6SM range.


Gosselin


&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion