Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201740

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1140 AM CST Mon Feb 20 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 404 AM CST Mon Feb 20 2017

The synoptic pattern early this morning features a fairly sharp
and amplified trough across the Plains states. This trough will
progress eastward today into tonight, bringing some much needed
rainfall to portions of our region.

Latest radar imagery depicts a line of showers and embedded
thunderstorms stretching north to south from Iowa all the way into
portions of Texas. This line of convection was occurring within a
broad warm-air advection regime induced by the approaching ascent
of the upper-level trough. This trough will progress eastward
through the day today, although part of it will become cut off
over eastern TX by tonight, with another piece of it quickly
shifting eastward across the Upper Midwest within the better flow
aloft. This will allow the trough axis to take on a positive tilt
over MO/IL as it shears out and sags southeastward into the
region. Nevertheless, moisture transport/isentropic ascent along a
35-45 knot low-level jet should be sufficient for a broken line of
showers to move from west to east across the region mainly late
this afternoon into the overnight hours (a slightly slower
progression than the previous forecast). Mid-level lapse rates
and forecast elevated instability look fairly marginal for
thunderstorm potential, thus only expect a few isolated lightning
strikes as the line shifts through the region (best chances in
central/northern MO). Given the area is going to be split by the
better synoptic forcing which will cause a weakening of moisture
transport with time, rainfall amounts will not be overly
impressive (0.10-0.25" expected).

With the slower arrival of precipitation today, have bumped up
highs into the low to mid 70s which could once again threaten some
records. Lows tonight will continue to be mild ahead of the trough
axis and associated weak cold front, with readings only dipping
into the mid and upper 50s (perhaps as low as the upper 40s behind
the front in central/northern MO).


.LONG TERM...  (Tuesday through Sunday)
Issued at 404 AM CST Mon Feb 20 2017

Temperatures are expected to remain unseasonably warm through
Friday across most of the area before returning to near average
values over the weekend. Some record highs or record warm lows
could be tied or broken during this time (see climate section
below). There are few opportunities for widespread precipitation
until a strong low pressure system approaches the area later this

Wednesday looks like the warmest day of the upcoming week. A low
pressure system is expected to move through the western Great
Lakes on Wednesday. Favorable SSW-SW winds ahead of the system`s
trailing cold front will help push temperatures back into the 70s
across the area on Wednesday. However, condensation pressure
deficit plots depict an area of decreased H5-H3 CPD moving across
the area on Wednesday afternoon. The lowest values are at H4-H3,
suggesting that a thick cirrus deck could limit insolation and
temper the warm-up.

The trailing cold front then briefly stalls across the region on
Wednesday night before lifting northward as a warm front on
Thursday. Temperatures on the warm side of the front will once
again be unseasonably warm. Depending on how far north the
developing surface low can lift the warm front, there could be a
significant temperature gradient across the northern CWA on
Thursday, which means that parts of northeast MO and west central
IL could be significantly cooler than areas farther south. The
current forecast package reflects this scenario and has a roughly
10 degree difference between highs at KUIN and KFAM.

The maturing low pressure system then lifts across the area on
Thursday night and Friday. The tightening pressure gradient will
create breezy conditions during this time. Showers and
thunderstorms are possible on Thursday and Thursday night across
the northern CWA due to an increasing H85 LLJ impinging on the
boundary (possibly aided by additional ascent from favorable jet
dynamics, but this varies between models), but the best
precipitation chances appear to be north of the CWA at this time.

The system`s cold front is forecast to move across the area on
Friday afternoon/evening, ending the streak of unseasonably warm
weather. Despite the strong front, BUFKIT soundings show a stout
cap which may inhibit widespread SHRA/TSRA except over the
eastern CWA, where the cap may have weakened sufficiently by
Friday afternoon to allow more widespread convection to develop.

Another low pressure system could bring a chance of precipitation to
the area on Sunday/Monday.


.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1131 AM CST Mon Feb 20 2017

Showers associated with trof out ahead of main cold front have
diminished a bit this morning. So with slower movement and less
coverage, feel that showers will be slower to move into taf sites.
When rain does move in conditions to remain VFR or high end MVFR,
after 22z at KCOU, 01z Tuesday at KUIN and after 04z Tuesday for
metro area tafs. Otherwise, winds to be gusty from the south,
before diminishing this evening. Main cold front to slide through
towards daybreak on Tuesday, exiting taf sites by 15z Tuesday.
Will just see low end VFR/high end MVFR cigs and winds veering to
the northwest behind the boundary.

Showers associated with trof out ahead of main cold front have
diminished a bit this morning. So with slower movement and less
coverage, feel that showers will be slower to move into metro area after
04z Tuesday. Otherwise, winds to be gusty from the south, before
diminishing by 04z Tuesday. Main cold front to slide through
by 15z Tuesday. Will just see low end VFR cigs and winds veering
to the northwest behind the boundary.


Record Highs

      2/20    2/21    2/22    2/23

STL: 78/2016 79/1935 78/1995 77/1996
COU: 76/2016 76/1935 76/1995 73/1933
UIN: 72/2016 69/1930 70/1922 70/1922



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