Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161110

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
610 AM CDT Wed May 16 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 311 AM CDT Wed May 16 2018

Early this morning, a weak NW flow aloft pattern existed over our
region with a weak upper LO over northern Nebraska embedded
within this flow. The surface cold front has since moved to the
south of the forecast area, but much of the low level moisture
remains, struggling to get scoured out by the weak NE surface
flow. A wave of low pressure existed over far southern IL along
the cold front and its associated convergence has been
occasionally finding weaknesses in the convective inhibition to
create isolated thunderstorms in this area. There are indications
that moisture convergence is also increasing over central MO and
latest satellite data supports this with increasing cloud cover
based at 6000ft. Otherwise, a thin veil of high cloudiness was
over much of the forecast area, but is also slowly exiting to the
east. Temperatures were in the 60s, with minimal dewpoint

There is good model agreement for early this morning that develops
isolated to scattered showers and thunderstorms from the moisture
convergence over central MO by 12z and continues this thru about
15z as it slowly propagates eastward towards STL metro and is
anticipated to weaken. Otherwise, a minimal PoP will exist from
roughly Sparta to Salem, IL for some convection in southern IL
that may spill over into this area.

Heading into this afternoon, low level wind fields will remain
quite weak and the abundance of low level moisture for areas
roughly from I-70 and southward should remain. Models continue to
also advertise some weak moisture convergence for much of the area
south of I-70 stemming from the area of low pressure to our
south, and with peak heating we should also see convective
inhibition significantly reduce. This continues to support
potential for isolated to scattered showers and thunderstorms for
areas south of I-70 this afternoon and into early this evening.
The small upper LO over northern Nebraska is expected to track
into northwest MO early this evening and stay in far western MO
overnight. This track should be too far west for any influence for
rain chances overnight, especially with continued drying of the
column into central MO by this time.

Despite a cold front having moved thru yesterday, temperatures
will follow very close to persistence, demonstrating along with
its lack of scouring out moisture, the weak strength of the front.
Max temps today should be in the upper 70s and lower 80s. Low
temperatures tonight will be more reflective of a longer period of
effective low level drying, with mins forecast from the mid 50s
to the lower 60s.


.LONG TERM...  (Thursday through Tuesday)
Issued at 311 AM CDT Wed May 16 2018

As some of the previous forecast discussions have noted, a
pseudo-Rex blocking pattern (also known as a high-over-low
blocking pattern) is forecast to develop over the central and
eastern CONUS late this week. The significance of a Rex block is
that the circulations around the high and the low centers act to
reinforce each other. This reinforcement makes it difficult for
the pattern to progress, which in turn tends to keep the weather
fairly similar from day to day until the pattern finally breaks
down. Models are often too quick to break down large-scale
blocking patterns, so it will be interesting to look back in
several days and see how they handled this smaller-scale blocking

In general, temperatures look warmer than average across most of
the area for Thu through Tue. Daily high temperatures will
generally range from the upper 70s to the upper 80s each day from
Thu through Tue. The warmest day is likely to be on Saturday after
a warm front lifts through the region. Lows will generally range
from the upper 50s to upper 60s, with some variation to be
expected depending on cloud cover and precipitation.

Isolated to scattered thunderstorms are possible on Thu across the
southern CWA, and isolated storms are possible again on Fri across
the extreme eastern CWA which will be in closest proximity to a
surface low. Otherwise, E/NE flow around the southern side of a
surface high pressure center will generally maintain dry flow into
the region. By Saturday, the axis of the blocking pattern is
forecast to have shifted far enough eastward for the developing
trough over the western CONUS to start affecting the region,
primarily in the form of increasing precipitation chances from Sat
through Sunday. Temperatures on Saturday will also warm into the
mid/upper 80s due to the passage of a warm front. The chance of
rain will persist through Sunday night with a break expected on
Mon before precipitation chances increase again on Mon night and
Tue after a southwesterly branch of the LLJ begins interacting
with a lifting frontal boundary.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 610 AM CDT Wed May 16 2018

Patches of low clouds exist early this morning underneath what is
mainly a mid level cloud deck over the TAF sites. These low clouds
range from IFR to low-end MVFR category and should dissipate by
mid to late morning. Some thunderstorms have developed over
portions of northeast and central MO early this morning and these
should hold together until late morning when the main rain threat
will be to the south of the TAF sites. For now, this early morning
thunderstorm threat should be clear of any TAF sites but will
monitor. The best chance of any TAF sites for being affected this
afternoon will be KCPS, with the other sites currently deemed too
low of a probability to mention. Otherwise, look for light NE
surface winds and predomiantly VFR conditions and dry weather to
prevail thru the valid period.

SPECIFICS FOR KSTL: Small cluster of thunderstorms over northeast
and central MO should dissipate before getting too close to the
terminal to mandate a VC or TEMPO mention. The next round of
thunderstorms should be to the south of the terminal this
afternoon and likewise precludes mention in the TAF. Otherwise,
look for VFR conditions, dry weather, and light NE surface winds
that will straddle crosswind 030 values at times.





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