Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 122327

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
627 PM CDT Thu Oct 12 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 320 PM CDT Thu Oct 12 2017

The surface ridge over the Ohio Valley will continue to slowly shift
eastward through the period.  The warm air advection on the backside
of this high has helped to erode the stratus from southwest to
northeast today, a trend which should continue into this evening.
However, with the loss of sunshine, this clearing may slow a bit
which could allow clouds to hang in through portions of the early
overnight across IL. With the clouds clearing late, temp/dewpoint
spreads will remain fairly small across portions of eastern MO and
IL.  Thus, expect some fog to develop overnight into Friday morning
in these areas.  Some may be locally dense, especially in IL.

Otherwise, Friday will feature warming conditions as winds increase
out of the SSE.  There may be some passing clouds due to a weak
shortwave passing through the area, but temperatures should still
warm into the lower 80s in most locations.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 320 PM CDT Thu Oct 12 2017

The synoptic pattern at the start of the long term period will
feature amplifying flow aloft as a trough digs into the
Intermountain West, encouraging southwesterly flow over the Midwest.
This trough will move through the region on Sunday, bringing a
northwesterly upper-level flow regime early next week, before the
flow goes more zonal by the end of the period.

The main focus of the long term period continues to revolve around
Saturday/Saturday night.  The approach of the trough will foster
surface cyclogenesis Saturday.  This low will quickly deepen and
move northeast into the Great Lakes by Saturday evening. Ahead of
it, temperatures will get rather warm. With the 850mb thermal ridge
settling over the area, gusty southerly winds, and enough sunshine,
temperatures should warm into the mid/upper 80s and low 90s. This
will be close to record values at STL, and likely a couple degrees
shy at COU/UIN.

1000-2000 J/kg of MLCAPE will develop Saturday afternoon/evening
ahead of a cold front across portions of west-central and northeast
MO. Surface-based or near surface-based convection will develop
Saturday afternoon across western MO and move into central/northeast
MO and western Illinois Saturday evening. Given deep-layer shear
around 50 knots oriented largely parallel to the front, expect a
quick transition to a linear storm mode. Given the expected
kinematic and thermodynamic parameters, severe convection will be
possible. It appears the main threat will be damaging winds given
around 50 knots of low at 5k feet.  However, there will also be the
chance for an isolated tornado or two as mainly speed shear in the
lowest 3km contributes to sufficient helicity values.

As the front moves southeast across eastern MO and southwest IL, the
severe threat should decrease.  This is due to less expected
instability, plus the suggestion within the guidance suite that the
surface cold front will begin to undercut the convection.  In fact,
it appears by Sunday morning the front will have cleared the
majority of the area.

Behind the front, conditions will quickly cool down and dry out as
surface high pressure builds into the Central Plains. Given fairly
light winds and clear skies expected Sunday night and Monday night,
lows will be fairly chilly (in the upper 30s/low 40s).  We may have
to monitor for some patchy frost potential each of these nights in
some of the typically cool locations.

Otherwise, temperatures will warm up by the middle of next week as
the high shifts off to the east and return flow commences.  Highs
will climb back into the 70s by Tuesday of next week.



.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 614 PM CDT Thu Oct 12 2017

Low clouds continue to push to the northeast of all terminals. VFR
conditions for all sites. UIN may have some scattered low clouds
lingering but should not stay much past 00Z. Period will be dry
for all sites with winds from southeast btw 5 and 10 kts.
Overnight there is a possibility of some fog for all TAF sites
but COU. Left the fog out due to some uncertainty but will
reassess with new model data.


VFR conditions for the TAF period. Period will be dry with light
southeast winds. Possibility for some fog to develop overnight
for STL but did not include due to uncertainty. Will reassess
with new model runs.



Saint Louis     55  83  64  90 /   0   0   5  10
Quincy          53  81  65  84 /   0   5  20  40
Columbia        55  84  65  87 /   0   5  10  10
Jefferson City  54  86  66  89 /   0   5  10  10
Salem           52  79  59  87 /   0   0   0   5
Farmington      52  82  61  87 /   0   0   0   5




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