956
FXUS63 KLSX 281049
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
549 AM CDT Tue May 28 2024

.KEY MESSAGES...

- A few showers and a thunderstorm or two will be possible across
  northern Missouri and west-central Illinois this afternoon.
  Thunderstorms are not expected to be strong or severe, and most
  areas will remain dry today.

- Dry, mild conditions persist tomorrow and Thursday.

- The chance for rain and a few thunderstorms returns late
  Thursday through Saturday, but the chance for strong or severe
  storms remains low.


&&

.SHORT TERM... (Through Late Thursday Afternoon) Issued at 257 AM
CDT Tue May 28 2024

Benign conditions exist throughout the area early this morning as
deep northwesterly flow prevails. Water vapor imagery reveals a
broad upper trough located across the Great Lakes, with an embedded
mesoscale vorticity maximum nearing the greater Chicago area and
another weaker vort max on its heels in Minnesota. Locally, dry air
aloft has allowed skies to largely clear, and with some hints of
valley fog beginning to form across Ozark river valleys where
decoupling as allowed surface winds to weaken. This patchy fog will
likely expand slightly through the night, but persistent
southwesterly breezes outside of protected valleys will likely
prevent this from becoming widespread.

During the day tomorrow, the trailing shortwave described above will
pivot into the Great Lakes region along the path of its predecessor,
which will tighten the pressure gradient at the surface and usher in
a reinforcing weak cold front. While this feature will be working
with a modified airmass and marginal moisture, some shallow
convection will be possible due to a combination of cooling aloft
and deep mixing. Thermodynamic profiles suggest shallow
convection (equilibrium levels of only 15kft) and limited
instability (maximum of 1000 J/kg, but more likely 250-500 J/kg),
which will limit the ceiling. However, there will be sufficient
effective shear within the storm depth to allow showers to persist
and perhaps support a weak thunderstorm or two. These isolated
showers are not likely as strong as yesterday`s activity, though,
and most would likely remain north of I-70.

Meanwhile, deep mixing will also support efficient momentum
transport, leading to noticeably gusty surface winds in the
afternoon. This will be especially noticeable across Illinois and
northern Missouri, where gusts to 25 mph will not be out of the
question. Otherwise, expect comfortable temperatures and humidity,
with highs in the mid 70s to low 80s.

Overnight, a surface ridge of high pressure will move into the area,
bringing with it cooler, drier air and weakening winds. Some patchy
shallow fog may redevelop in low-lying areas, but this isn`t
expected to be widespread. High pressure and dry air will continue
to dominate area weather Wednesday and Thursday as well, with mild
temperatures generally in the 70s to near 80 and mostly light winds.

BRC

&&

.LONG TERM... (Thursday Night through Monday) Issued at 257 AM
CDT Tue May 28 2024

A relatively active pattern is likely to return to the region by
the end of the work week, as a slow-moving shortwave attempts to
push through a blocking ridge aloft. As this shortwave and its
attendant surface low moves east, a compact low-level jet streak
will transport moisture northward, although model projections
suggest that warm temperatures aloft and weak lapse rates will
limit the potential for noteworthy instability. When coupled with
only modest wind shear aloft, the potential for strong or severe
thunderstorms appears to be low at this time.

However, relatively high confidence exists that at least some
precipitation will develop between Thursday night and Saturday
afternoon, likely due to a combination of modest warm air advection
and elevated instability. This high forecast confidence may not be
immediately apparent in the gridded forecast, but this is because
the bulk of the uncertainty is in the timing and precipitation
amounts. The primary driving feature will be up against a blocking
ridge aloft, and this limits the predictability of such a
feature. This is evident in ensemble members and cluster analysis,
which depict this feature and precipitation timing with a high
degree of variance. This explains why the current gridded forecast
features 48 hours of widespread 30 to 50% precipitation chances
throughout the period, but the chance for at least some rain
falling sometime during this timeframe is much higher.

As we head into next week, zonal flow aloft is expected to return
with gradually building heights aloft, along with the potential for
additional passing shortwaves and associated precipitation.
Temperatures, moisture content, and instability are also expected to
be on the rise, increasing the potential for thunderstorms as
shortwaves pass through the ambient flow. However, the timing of
these features remains highly uncertain, and the northerly
displacement of stronger winds aloft will likely limit available
wind shear, and may inhibit the potential for stronger
thunderstorms. Likewise, CIPS and CSU guidance also do not show a
strong signal for such storms in the extended period, which appears
reasonable based on current available data.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 543 AM CDT Tue May 28 2024

VFR conditions are expected with limited exceptions throughout
the 12Z TAF period. At the start of the period, some shallow
patchy fog has developed in local valleys, but is not expected to
impact any TAF sites. Isolated showers may impact UIN during the
afternoon today, with a stray lightning strike possible as well.
Probabilities for lightning are too low to include in this
iteration of the TAF, but may need to be included in future
updates. This activity is not likely to impact other terminals.
Otherwise, breezy northwest winds can be expected during the
afternoon today, along with a scattered VFR cu field. Overnight,
some shallow patchy fog will again be possible in sheltered
valleys, but not likely to impact local terminals.

BRC

&&

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

$$

WFO LSX