517
FXUS63 KLSX 180710
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
210 AM CDT Thu Jul 18 2024

.KEY MESSAGES...

- Dry weather with mild, below average temperatures will continue
  today through Saturday.

- A gradual transition to a more humid and unsettled pattern will
  occur early next week, with shower and thunderstorm chances
  chances building into the middle of the week.

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 210 AM CDT Thu Jul 18 2024

High pressure has settled into the region today behind yesterday`s
cold front, with northwest flow aloft and an abnormally dry and cool
airmass for mid-July. Yesterday evening soundings from upstream in
Illinois revealed PWATs well below 1 inch (.88 inches), which is
below the 25th percentile of climatology for this time of year. This
applies to temperatures as well, as observed temperatures today fell
3 to 5 degrees short of average, and are expected to dip even
further to around 5 to 8 degrees below average. The core of this
airmass has since moved even farther south and into our area, where
it will remain until late in the weekend. As such, expect several
days of mild and dry weather between today and Saturday, with high
temperatures in the upper 70s to low 80s, and morning low
temperatures in the upper 50s to low 60s. While there may be some
variation in temperatures, confidence is high that readings will be
below normal over those three days.

BRC

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 210 AM CDT Thu Jul 18 2024

Sunday through the middle of next week, the primary driver of local
weather will be a slowly approaching broad upper trough / cutoff low
from the central Plains. Ensemble guidance is in decent agreement
regarding this feature, surprisingly so considering that it is
weakly forced and features like this typically have a low degree of
predictability. In any case, as this feature very slowly moves
across the central Plains, the dry air currently in place will
gradually erode and precipitation chances will likewise slowly
increase.

At this stage, there remain many unresolved mesoscale features that
will dictate the specific day-to-day details. However, it will
likely take some time for the airmass to recover in the wake of our
currently dry air. This feature is not currently forecast to arrive
with strong dynamics that would help advect moisture northward
rapidly, so this is likely to be a gradual process. This is borne
out in the ensembles as well, with ensemble mean PWAT values
gradually increasing to near climatological averages sometime
between Monday and Wednesday. Keeping in mind that we are very close
to what is typically the climatological maximum of PWAT values, this
also aligns with gradually increasing precipitation probabilities
which peak between Tuesday and Wednesday and both the LREF and NBM.

Also noted in the ensemble data is a time-lag of some higher
outliers in precipitation to the Wednesday-Thursday timeframe, which
may be a reflection of the fact that PWAT and instability
projections continue to increase during this timeframe. This
introduces the potential for more variable convection but with a
higher ceiling for more intense rain rates/totals. Either way,
chances for showers and thunderstorms will exist between Sunday and
Thursday, with current probabilities peaking Tuesday/Wednesday.
Chances for strong or severe storms remain low considering the
modest dynamics, but this may slowly increase as instability builds
toward the middle of the week.

Meanwhile, the persistent presence of the upper low is also likely
to suppress temperatures heading into early next week, with values
remaining near to slightly below normal favored to continue.

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1037 PM CDT Wed Jul 17 2024

VFR flight conditions through the forecast period. Otherwise, will
see a few diurnal cu during the late morning through afternoon
hours. Light north winds will veer to the northeast by mid morning
on Thursday.

On another note, still confident that there will be some river
valley fog in parts of southeast Missouri, but well south of the
any terminals.

Byrd

&&

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

$$

WFO LSX