Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 220834

Area Forecast Discussion
National Weather Service Saint Louis MO
334 AM CDT Tue May 22 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 329 AM CDT Tue May 22 2018

Satellite imagery and surface observations indicate that low clouds
and fog have developed early this morning across parts of central,
east central and southeast Missouri.  Some of this fog will be dense
at times until mixing begins after sunrise, then it will begin to
dissipate.  A surface ridge will move across Missouri and Illinois
the rest of today into tonight keeping skies mostly clear and winds
generally light.  Temperatures today will climb back above normal as
temperatures mix above 850mb.  Lows tonight will only fall into the
lower to mid 60s even with clear skies and light winds given the
dewpoints in the 60s.


.LONG TERM...  (Wednesday through Monday)
Issued at 329 AM CDT Tue May 22 2018

An upper level ridge will continue to strengthen over the
Mississippi Valley on Wednesday...shifting slightly east Thursday.
Meanwhile, easterly flow from a surface high centered over the
Great Lakes should continue to push relatively dry low level air
from the Ohio Valley into the Mid Mississippi Valley. The
combination of dry low levels and subsidence aloft should keep the
forecast area dry at least through Thursday night...and likely
through Friday as well. The upper ridge will be breaking down on
Friday as a shortwave moves up through the Great Plains and into
the Upper isolated convection is possible over parts
of central Missouri...but still think the low level moisture will
be lacking for widespread rain. 850mb temperatures are forecast to
be in the 16-17C range...pushing 18C on Friday. This should be
good for highs in the mid to upper 80s. This agrees well with MOS
guidance through Friday. Relatively dry air and few clouds should
allow for good diurnal falls each night into the 60s.

Medium range models continue to trend wetter as a Gulf system moves
north into the Mississippi Valley.  The weather pattern is reminiscent
of the pattern from last week with the low drifting up the
Mississippi Valley Saturday through Monday.  Like last week, a
blocking upper ridge builds north of the low causing it to stall.
Increasing low level moisture associated with the low will likely
help to produce diurnal convection over the weekend.  Temperatures
should remain warm in the mid to upper 80s...potentially even warmer
as 850 mp temperatures move up into the 19-20C range.  Will stick to
the mid to upper 80s though as afternoon highs will be affected by



.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1047 PM CDT Mon May 21 2018

Predominantly VFR conditions, dry weather and light winds will
prevail at the TAF sites thru the valid period. The main item of
concern is fog potential overnight. A weak cold front remains
effectively stalled in the western portion of STL metro with
dewpoints near 70 and depressions of only a few degrees the
standard across the area. Anticipating little movement or effect
from this front until daytime Tuesday. The key will be how much
additional cloud will form near front with so far much of it on
the cool side away from the TAF sites, allowing them to cool to
near the crossover temp values. Model guidance has been lowering
dewpoints too aggressively and making this adjustment should
result in more fog for STL metro sites, especially with respect to
IFR VSBYs. Lower crossover temps and better mixing and drying at
COU and UIN should lead to a more brief occurrence of fog.

SPECIFICS FOR KSTL: Upgraded fog mention slightly to prevailing
and lowering to the cusp of IFR category. Number of ways it can go
wrong with frontal clouds keeping temps up, but confident in
sticking with higher dewpoints overnight. If clouds stay away and
allow temps to cool to crossover temps or below, IFR should be
easy to attain.



Saint Louis     85  66  87  67 /   0   0   0   0
Quincy          82  62  84  64 /   0   0   5   5
Columbia        83  64  86  65 /   0   0   5   5
Jefferson City  84  63  86  65 /   0   0   5   5
Salem           83  61  84  62 /   0   0   0   0
Farmington      84  62  84  61 /   5   0   5   0




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