Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 191134

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

.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 12z TAFs through 12z Wednesday Morning)
Issued at 625 AM CDT Tue Sep 19 2017

Dense fog is reducing visibilities and ceilings at the terminals
to LIFR or VLIFR early this morning. Expect these conditions to
improve to VFR by 15Z once mixing starts. There are a few
thunderstorms which could affect KCOU early in the period.
Otherwise dry and VFR conditions are expected the rest of the
period.  Winds are expected to be light through the period.


Fog is reducing visibilities and ceilings at the terminals
to LIFR or VLIFR early this morning. Expect these conditions to
improve to VFR by 15Z once mixing starts. Otherwise dry and VFR
conditions through the period with light winds.



MO...Dense Fog Advisory until 10 AM CDT this morning for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-
     Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Louis
     City MO-Saint Louis MO-Shelby MO-Warren MO-Washington MO.

IL...Dense Fog Advisory until 10 AM CDT this morning for Adams IL-
     Bond IL-Brown IL-Calhoun IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Monroe IL-Montgomery IL-Pike IL-Saint Clair IL.



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