Area Forecast Discussion
Issued by NWS St. Louis, MO

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLSX 191758

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1258 PM CDT Thu Oct 19 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 334 AM CDT Thu Oct 19 2017

Early this morning, a RIDGE of surface high pressure extended from
the Mid-Atlantic states, thru the mid-South, and back into eastern
Texas, resulting in a light southerly flow for our region.  A
surface cold front was beginning to push into southeastern Iowa.
Northwest flow prevailed aloft.  The atmospheric column remains
quite dry with the only cloud cover in our region being ice crystal
cirrus clouds to the north and south, with clear skies directly
overhead.  Temperatures in most locations were in the upper 40s and
lower 50s with cooler temps in some sheltered valley areas.

The surface RIDGE axis of high pressure will remain nearly
stationary to our south as the cold front to our north expected to
reach as far as far northern MO and central IL before stalling this
morning and dissolving this afternoon.  It is anticipated to have no
impact.  The result will be another day of south to southwest flow.
The regions of cirrus clouds to our north and south are expected to
make little inroads into our region during the daylight hours,
giving us full or nearly full sunshine.  Aloft, the northwest flow
will give way to RIDGE development overhead and resultant increased
thicknesses.  With a warmer start to the day and the above
conditions, max temps should be higher than persistence by a cat or
two, and preferred 75 to 80 for most locales, with some low 80s

The building RIDGE aloft will slide east tonight and should
effectively open the way with the southwest flow for the large area
of cirrus clouds to our south to then move into our region for late
tonight and beyond.  Additional high clouds late in the period and
otherwise similar other conditions should edge min temps slightly
higher than persistence once again, with readings bottoming out in
the low-mid 50s for most areas with pockets of 40s possible in


.LONG TERM...  (Friday through Wednesday)
Issued at 334 AM CDT Thu Oct 19 2017

Friday still looks warm and dry in southerly flow on the west side
of the high pressure system over the Mid Atlantic Region.  850mb
temperatures rise to the 15-16C range across the region.  While
forecast soundings only mix to around 900mb, still think the high-
side of guidance is the the way to go for Friday which yields upper
70s and low 80s across most of the area.

Models are in good agreement regarding the strong trof and cold
front passage this weekend.  30-40kt low level jet early Saturday
morning ahead of the trof is forcing 850mb moisture convergence over
Missouri.  Models are also showing a subtle shortwave moving
northeast across the area around 12Z Saturday which could produce
some sunrise surprise showers.  However...forecast soundings are
pretty dry so will keep PoPs below mentionable thresholds during the
day Saturday morning with slightly increasing PoPs in the afternoon
over northeast Missouri as moisture increases and the front
approaches the area.  All guidance is now showing the upper trof
moving into the eastern Plains by 12Z Sunday with the cold front in
Central Missouri.  Rain and thunderstorms continue to be likely
along and ahead of the front.  GFS is forecasting maximum 500-700
J/Kg MUCAPE over our area ahead of the current thinking
is that severe weather threat will be limited Saturday night and
Sunday morning.

GFS and ECMWF now both cut off an upper low by 06Z Monday. However,
the GFS is faster in moving the low east across the Mississippi
Valley.  The slower ECMWF develops a surface low over northern
Louisiana and this low moves rapidly northeast into Michigan by 12Z
Tuesday.  The ECMWF prints out 2-3 inches of precip over our area in
a strong deformation zone Monday through Tuesday morning as this low
moves up the Mississippi Valley and into the Ohio Valley.  In
contrast, the GFS is bone dry during this period.  Needless to say,
the Monday and Tuesday time frame is a very low confidence period in
the forecast.  All medium range guidance shows a highly amplified
pattern by by Wednesday the a strong longwave trof and/or cut off
low over the eastern CONUS and northwest flow aloft over the
Mississippi Valley.  This pattern indicates below normal
temperatures which is reflected in cool guidance number in the 50s.


.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1240 PM CDT Thu Oct 19 2017

VFR conditions to persist through forecast period with south to
southwest winds.



WFO LSX is the U.S. government's official web portal to all federal, state and local government web resources and services.