Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231808
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
108 PM CDT Thu May 23 2019

.UPDATE...
Issued at 1100 AM CDT Thu May 23 2019

Latest surface analysis placed the surface front from southwest
Missouri to just south of St. Louis to near I-70 in south central
Illinois. This boundary separates relatively cool and dry air and
the warm and very moist air to the south. There continues to be a
band of rain with embedded thunderstorms just north of the front
over south central and southeast Missouri. Expect this band to
gradually decrease in areal coverage into early afternoon as the
latest runs of the RAP continue to show an upper level ridge
building over the CWA this afternoon. The front will begin to
move slowly north during the afternoon. Additional development
for thunderstorms along the front will be determined by the
strength of the ridging, so have just maintained a chance of
thunderstorms based on the the strength of the convergence and
instability along the front. There will be the potential for a few
strong storms given MLCAPES between 2000-3000 J/kg and deep layer
shear close to 40kts.

The front will continue to move north tonight. CAMS are still
depicting the potential for storms to move into northeast Missouri
and west central Illinois late this evening and overnight which
will bring another risk for strong storms.

Britt

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 417 AM CDT Thu May 23 2019

Ongoing broken line of strong to severe thunderstorms from
Vandalia, IL through STL Metro to near Springfield, MO continues to
slowly push southeastward, where the effective surface boundary
currently is.  It is on portions of this leading edge where the
Flash Flood threat, if there is one, has also shifted.  Some
additional non-severe thunderstorm development continues to lag
further northwest through central MO thanks to WAA aloft.
Temperatures were in the 70s ahead of the effective surface
boundary, and have fallen off into the 60s and even 50s behind it
and the slightly lagging synoptic cold front.

There is enough dynamic and thermodynamic support for severe storm
threat to persist a couple hours after sunrise as it pushes more
into southern IL and southern MO.  This may mandate a temporal
extension of WW215 as a result.

Otherwise, overall coverage of storms looks to decrease heading
later into the day and into tonight as it gradually shifts back
northward into northern MO and central IL late tonight, with the
effective boundary shifting into more capped air as the upper RIDGE
attempts to build back into our area.

The upper RIDGE looks to have one good day where it is well
established, and should result in a mostly dry forecast for Friday
but will also result in a hot day as well, with max temps well into
the 80s all locations with low 90s possible in the STL metro area.

The strength of this RIDGE after Friday looks wobbly to the point
where convective development just to the northwest of our region
looks likely and as/if this persists, will likely shift the main
activity back into our region again anyway.  If this happens, this
is also probably to be accompanied with a severe storm and excessive
rainfall threat building in from northeast MO and possibly for areas
further southeast.

TES

.LONG TERM...  (Friday Night through Wednesday)
Issued at 417 AM CDT Thu May 23 2019

This pattern looks set to then continue well into next week with
rain chances shifting northwestward for a time if the RIDGE can
regain dominance and then back into our area as it weakens.  There
is low confidence on any one particular day being dry or wet and
being this is May going into June, the two wettest months
climatologically, the likelier solution is on the wetter weather
until a higher confidence pattern emerges.

When the RIDGE is more dominant, temperatures will surge well into
the 80s and into the low 90s in spots, whereas wetter weather will
tend to keep them in the lower 80s or even 70s.

TES

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1225 PM CDT Thu May 23 2019

Broken to overcast ceilings have lifted around the St. Louis area
terminals but remain at KCOU. Expect these ceilings to rise to VFR
in the next few hours. Band of rain continues to move just south
of the terminals over central and east central Missouri which may
bring a brief sprinkle at either KCOU or the St. Louis area
terminals. There will be a slight chance for additional
thunderstorms this afternoon at the terminals as a front moves
slowly north across the area, but the coverage of the storms will
be too low to include in the the TAF at this time. Additional
chances for storms will be from mid evening into the overnight
hours when additonal storms move into the area from northwest
Missouri. Here again questions on timing and coverage prevent
adding TSRA to the TAF at this time.

SPECIFICS FOR KSTL:

Ceilings have improved to VFR and should remain VFR the remainder
of the period. Only exception maybe if there is any thunderstorms
that affect the terminal. There is a slight chance this afternoon
as a front moves slowly north across the area, but the coverage
of the storms will be too low to include in the the TAF at this
time. Additional chances for storms will be from mid evening into
the overnight hours but here again questions on timing and
coverage prevent adding TSRA to the TAF at this time.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX


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