Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172001

Area Forecast Discussion
National Weather Service Saint Louis MO
301 PM CDT Mon May 17 2021

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 257 PM CDT Mon May 17 2021

The boundary that has been responsible for our rain the last couple
of days is currently positioned along the I-70 corridor and attached
to a surface low spinning over southeastern Kansas and southwest
Missouri. Warm air advection ahead of the low is aiding in
maintaining cloud cover CWA-wide and scattered showers north of the
boundary. As the low slowly progresses northeastward this evening
and overnight, and subtle shortwaves ripple over the CWA, the
boundary will gradually pivot and lift northward with convection
spawning along it. This convection will be primarily focused over
northern and eastern portions of the CWA, but isolated convection
is still possible area wide. Instability will be limited, but I
can`t rule out an occasional rumble of thunder overnight.

On Tuesday, the shortwave and surface low will move north of the
CWA, slowly pushing convection out of eastern portions of the CWA in
the afternoon. Our attention then turns to eastern Kansas and
western Missouri, where convection is forecast to spawn during the
afternoon and evening along a remnant outflow boundary from
overnight Plains convection. There is much uncertainty in where
exactly this convection will spawn, as can be seen in the spread
of solutions in the CAMs. However, this environment will be
characterized by MUCAPE of 1,000-1,500 J/kg per the SPC HREF and
around 20-30 kts of 0-6 km bulk shear, so storms that do develop
may become strong and capable of gusty winds and hail. Relative to
our CWA, current indications are that portions of central
Missouri have the greatest chance of seeing these storms if they
do develop.


.LONG TERM...  (Wednesday through Next Monday)
Issued at 257 PM CDT Mon May 17 2021

From Wednesday through early next week, the upper level pattern will
become increasingly amplified as a trough builds across the eastern
United States and a trough digs southward across the Rockies. This
will keep the Middle Mississippi Valley locked in southwest flow
aloft, with subtle shortwaves within the flow passing over or near
the CWA. Southerly surface flow will also remain in place over the
CWA, continuing to advect warm, moist air into the area. This will
provide an ideal environment for rain and storm chances as
shortwaves interact with this air mass. However, confidence remains
low on the exact timing and placement of rain chances, and the
seemingly wet forecast is a reflection of that uncertainty. However,
there will still be plenty of dry time. As we move into the weekend,
ensemble clusters all show the eastern ridge building westward into
the Middle Mississippi Valley, reducing our rain chances to some
degree. With sustained southwest flow aloft and southerly flow at
the surface, our temperatures will start the period around normal
before climbing nearly 10 degrees above normal. While how warm we
get each day will depend on convection and associated cloud cover,
ensemble cluster analysis and CIPS analogs support this general
upward trend in temperatures.



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1225 PM CDT Mon May 17 2021

A rather difficult forecast is in store for all local terminals this
period. Moisture moving northward ahead of a low pressure system
along the Kansas-Missouri border has been brining waves of showers
and low ceilings across all terminals this morning, and will
continue to do so through the forecast period. The best rain
coverage currently extends from central Missouri northeastward into
west-central Illinois, impacting the KCOU and KUIN terminals. Model
guidance has not been handling this rain well, but given current
radar trends, confidence is high that the first part of the period
will be pretty wet at these terminals. Rain coverage is expected to
expand later this afternoon and impact the KSUS, KSTL, and KCPS
terminals. For all local terminals, heavier rain cores within the
broader are of lighter rain may reduce visibilities at times to 2-3
miles or lower, but these cores are difficult to predict outside of
an hour of lead time or even less. There is a low possibility of
thunderstorms as well, but given their low probability and isolated
nature, confidence in impacts to terminals is low. As we get later
into the period, confidence in where the more consistent rainfall
will be decreases, so while VCSH is used currently at all local
terminals, more consistent and impactful rain may be possible.

As for ceilings, IFR flight conditions are currently present from
central Missouri northward through west-central Illinois, impacting
the KCOU and KUIN terminals. Model guidance at the very least
maintains these ceilings over these terminals through much of the
period, and also wants to bring similar ceilings over the KSUS, KSTL,
and KCPS terminals later this evening. However, guidance has been
overly aggressive with low ceilings as of late, so confidence in this
occurring is not high at the moment, so I have maintained MVFR flight
conditions through the period.





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