Area Forecast Discussion
Issued by NWS St. Louis, MO

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
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 211912

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
112 PM CST Tue Nov 21 2017

Issued at 720 AM CST Tue Nov 21 2017

Added in sprinkles mention to portions of the northern and
northeastern CWA for late this morning thru mid-afternoon. Banded
pcpn over IA behind the cold front is strongly associated with
frontogenesis region at H850-700 as well as H900-950 convergence and
has produced not just sprinkles but also some sleet. Models take
this region and either move it mainly eastward or weaken it as it
moves south but probably not before it at least is able to produce
some light and brief pcpn for the northern CWA late this morning.
Depending on how well it is supported later today aloft by a
strong shortwave (dependent on exact track which will be close),
pcpn reaching the ground may be able to continue thru into central
and southern IL this afternoon.


.SHORT TERM...  (Through Late Tonight)
Issued at 315 AM CST Tue Nov 21 2017

A  upper TROF was located over the Mississippi River Valley early
this morning, with several imbedded shortwave disturbances.  Three
are of note:  the first was directly over our region, helping to
create an area of mid and hi level cloudiness to its east in IL; a
second, stronger disturbance was over northern MN and eastern ND;
and a third was over western MT.  At the surface, a cold front
extended from western WI thru central IA and into southeast
Nebraska.  SW surface winds existed ahead of the front across our
region and have kept temperatures in the 40s for much of the area.
The atmospheric column was much too dry and lifting mechanisms too
weak to produce any pcpn anywhere near our region.

The first shortwave disturbance over our region will exit before
12z/Tue.  The strong second disturbance over MN/ND will be the main
driver in deepening the upper TROF and its southern periphery will
come close to edging our northern and northeastern CWA during the
afternoon hours, but much of the lift from it will remain outside
the CWA.  The third disturbance over western MT should track well
south of our region during late this afternoon and early evening.
Focusing on the second disturbance to edge our N and NE CWA this
afternoon, this will also track behind the timing of the passage of
the surface cold front which should make it thru areas N and W of
STL metro this morning, STL metro during midday, and areas S and E
of STL metro during early-mid afternoon.  Both low level moisture
will be limited due to presence of a blocking RIDGE over the Gulf of
Mexico and aloft due to a very dry initial column and the main lift
missing our area and it tracking too fast to saturate anything below
the mid levels.  Taking all of this into account, despite a cold
FROPA, it should be dry today but with plenty of cloud cover.  The
strength of the cold front today will result in falling afternoon
temps, especially for areas near and north of Interstate 70, along
with gusty NW winds of 25-30mph.  Max temps will be slightly above
average ranging from the upper 40s in northern MO and central IL, to
the 50s elsewhere.

A cold Canadian high pressure will build in and dominate our region
tonight with clear skies and below average temps, as readings bottom
out in the lower 20s for most areas, or about 10 degrees below


.LONG TERM...  (Wednesday through Monday)
Issued at 315 AM CST Tue Nov 21 2017

The tranquil, dry weather pattern will continue through the
forecast period with dry, cold frontal passages.  Despite full
sunshine it will be colder on Wednesday with northwesterly mid-upper
level flow, and a surface ridge moving through the forecast area.
Warm advection, mid level cloudiness will drop southeastward through
our area Wednesday night with a warming trend beginning on Thursday.
More pronounced warming can be expected on Friday as surface/low
level winds strengthen from a southwesterly direction.  Highs on
Friday will be around 10 degrees above normal for late November.  A
cold front will drop southeastward through our forecast area Friday
afternoon and night as an upper level trough moves eastward through
the Great Lakes region.  It appears that any precipitation will be
northeast of the forecast area closer to the better upper level
forcing, although could not totally rule out a few sprinkles in
northeast MO and west central IL Friday afternoon and evening. Will
see cooler temperatures beginning Saturday and continuing through
the weekend as a large surface ridge moves slowly through our area
with continued northwesterly upper level flow, although the coldest
air should remain northeast of the forecast area.  Will see a
warming tend beginning on Monday as the surface winds become
southerly as the surface ridge shifts east of the region.


.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 106 PM CST Tue Nov 21 2017

As cold front moves through winds to veer to the northwest to
north and become gusty. Winds will then diminish after 02z
Wednesday. Otherwise, mid and high clouds associated with front to
scatter out by 03z Wednesday.

As cold front moves through winds to veer to the northwest to
north and become gusty. Winds will then diminish after 03z
Wednesday. Otherwise, mid and high clouds associated with front to
scatter out by 03z Wednesday as well.




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