Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191700

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1200 PM CDT Tue Sep 19 2017

Issued at 1004 AM CDT Tue Sep 19 2017

Dense fog is lifting with stratus covering the northern half of
the CWA. Have decreased expected highs by a few degrees across
northeast Missouri and into west central Illinois as they will
have a few hours less of insolation to work with this afternoon.
Otherwise, just a slight chance of a shower or storm across the
eastern Ozarks this afternoon as warm front begins to lift
northeast through the area.



.SHORT TERM...  (Through Late Tonight)
Issued at 256 AM CDT Tue Sep 19 2017

GOES-16 imagery and surface observations continue to show that
dense fog is expanding eastward across the area in the wake of
where clouds are moving out of the area early this morning. Just
extended the dense fog advisory into the St. Louis metro area
where there was rain last night and dewpoint depressions are at or
near zero. KALN and KSET are already at 1/4SM and KSUS just fell
to 3/4SM. An increasing number of webcams are showing fog in the
metro area, so expect this trend to continue through sunrise. Will
continue to monitor areas east of the current fog advisory to see
if further expansion is needed.

Otherwise expect a mainly dry conditions the next 24 hours as the
RAP shows the trough moving east of the area and an upper ridge
begins to build over the region tonight. Temperatures will be
above normal today as 850mb temperatures climb into the 16-19C
range. Dewpoints again tonight will be above normal lows.


.LONG TERM...  (Wednesday through Monday)
Issued at 256 AM CDT Tue Sep 19 2017

The overall forecast scenario is very similar to both 12 and 24
hours ago. The upper air pattern will become highly amplified this
week and then remain so through early next week. A stout upper
ridge over the eastern half of the CONUS will bring several
consecutive days of unseasonably warm temperatures to MO/IL along
with very few opportunities for widespread precipitation.

Early morning water vapor imagery depicted a large mid/upper
disturbance with several embedded PV anomalies located over the
northwestern CONUS. This feature had already started to induce
surface cyclogenesis across the northern plains and into the
Canadian Rockies as per early morning METARs. A stronger PV
anomaly is forecast to reach the PacNW coast by 12z Wed, and this
feature will eject the initial disturbance northeastward through
Canada while reinforcing and deepening a developing longwave
trough over the western half of the CONUS. Subsequent downstream
height rises will allow an elongated upper ridge axis to stretch
from Mexico through the central CONUS and into southern Quebec.
There is very good agreement on the overall upper air pattern over
the central CONUS despite some variations between models.

High temperatures will be in the mid/upper 80s to low/mid 90s
beneath the stout upper ridge. These values are 10-15+ degrees
above average for mid/late September but a few degrees below the
record highs at St. Louis, Columbia, and Quincy for September
20-25. Southerly to southwesterly surface flow will also provide a
steady stream of low-level moisture, and the resulting higher
humidity will yield heat index values a few degrees warmer than
the air temperatures.

The active frontal boundaries are expected to remain north and
west of the CWA for the next several days, and the upper ridge
axis nearly overhead will shunt any weak shortwaves away from the
area. Those frontal boundaries are forecast to edge closer to the
CWA on Wed night/Thu morning and again early next week, which
could bring a slight chance of rain to parts of central and
northeastern MO. Most locations will probably remain dry for the
next several days.

A few strong thunderstorms are possible across northeastern MO and
extreme west central IL on Wed evening ahead of a stalled cold
front. The amount of instability (especially considering moisture
pooling and steep H7-H5 lapse rates) may compensate for the
limited shear, but the forcing from a weak LLJ does not appear to
be particularly strong. Any storms which do develop could also
produce heavy rainfall considering that the forecast PW values of
around 1.7 are nearly +2SD for September.



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1159 AM CDT Tue Sep 19 2017

Dense fog from this morning has dissipated with a scattered deck
of cumulus taking hold across the area for the afternoon. Could
still see an isolated thunderstorm given proximity to warm front,
but most locations will remain dry. Light southeast wind will
continue until tomorrow when wind direction will become southwest
and increase.

VFR expected with a light southeast wind becoming southwest





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