575
FXUS63 KLSX 101753
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1253 PM CDT Wed Jul 10 2024

.KEY MESSAGES...

- Other than a persistent threat for weak, scattered
  thunderstorms through Friday, a generally-calmer weather pattern
  takes hold of the region into the weekend.

- Temperatures gradually rise from the mid-80s (near normal) today
  into at least the mid-90s (nearly 10 degrees above normal) by
  early next week.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 248 AM CDT Wed Jul 10 2024

The surface low that was once Tropical Depression Beryl is now
northeast of the bi-state region, placing the forecast area in
generally easterly flow. Aloft, the extratropical transition of
the system has formed an amplified trough over the Mid-
Mississippi Valley that is bolstering cloud cover generally along
and east of the Mississippi River. While this wave is forecast to
lift northeast along with the surface low it attends, another weak
shortwave is evident in GOES-East mid-level water vapor in the
northern Great Plains. As the sun rises today, elevated dew points
from uninterrupted Gulf moisture and increased evapotranspiration
stemming from a nearly-saturated 0-10cm soil layer (per NASA
SPoRT analyses) will combine to increase MUCAPE across most of the
region. The wave diving south amidst cyclonic flow aloft into the
region looks strong enough to spark weak convection during the
afternoon from north to south. Exactly how far south any
thunderstorms get is not completely clear, but the highest chances
(upwards of 40%) exist north of the I-70 corridor. Otherwise,
temperatures will return closer to the mid-80s we generally expect
for July.

Rain chances will wane with sunset tonight, but not end completely
with continued shortwave impulses in the stalled cyclonic flow
interacting with elevated instability. With more limited
instability, the intensity of any thunderstorms will be even lower
still overnight. Fog will also be a concern again where skies are
clear amidst weak surface winds. I`m not confident in many impacts
outside of river valleys at this point, mainly because cloud cover
and the presence of convection may throw a wrench in radiational
cooling. As the temperatures rise again Thursday, the instability
will become more surface-based and threatens persistent 15-35%
chances for thunderstorms peaking during the afternoon and early
evening. The shear parameter space looks very weak and not favorable
for a tangible severe weather threat, though gusty winds would be
possible with any thunderstorm that develops. The one difference
between today and Thursday is that the forcing aloft Thursday looks
weaker, which will further hinge any convection on instability. With
heights remaining neutral/weakly rising, and a ridge settled over
the western CONUS expanding east gradually on Thursday, I am less
confident in convection by then. Regardless, with most guidance
having some degree of convection and plenty of instability,
scattered thunderstorms remain in the forecast. Otherwise,
temperatures tick up ever-so-slightly into the mid/upper 80s for
Thursday.

MRB

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 248 AM CDT Wed Jul 10 2024

Seasonable temperatures and humidity persist into the end of the
work week under gradually weakening but persistent cyclonic flow
aloft, which will allow for convection chances to linger
accordingly. Again, while instability continues to increase through
the week as both temperatures and humidity gradually rise, shear
parameters are largely unimpressive for stronger thunderstorms.
Friday night into Saturday, most ensemble guidance advances a weak
warm front through the Mississippi Valley as the upper-level ridge
further expands north and east overhead. While some showers and
thunderstorms can be expected as it lifts north and east, once it
departs there is potential that we see little to no activity for
Sunday into Monday with strong capping and warm mid-levels in place.
At the same time, temperatures rise above normal (roughly 90
degrees) under persistent 850mb/925mb warm air advection. There`s
agreement among the global-scale ensembles and the NBM that above-
normal temperatures are very likely (90%+ chance) early next week.
These probabilities coincide well with 850mb temperatures exceeding
the 90th climatological percentile during that period, with more
extreme values further to our north and west.

At some point on Monday, global ensembles show a persistent signal
for an amplified mid-level wave to track southeast from central
Canada into the Great Lakes region by Tuesday. While this feature`s
strength and position is far from certain, it has potential to
weaken the ridge and draw a slow-moving cold front south into the
Mid-Mississippi Valley on Tuesday. Rain and thunderstorms along the
front itself represent the most widespread potential for rain in the
forecast. While current modeled shear parameters do not favor much
organized, stronger convection, this may change as the exact pattern
aloft comes into greater clarity. The front`s southward progress is
also in question, with some scenarios having it successfully pass
through the region and others stalling it over us. If the former
occurs, largely dry conditions and below-normal temperatures (as low
as the mid-80s) would take hold in its wake. If the front stalls,
rain chances will persist and temperatures would vary across the
region. While the distribution of the probabilistic temperature
guidance trends cooler (closer to normal low 90s) by Wednesday, its
interquartile ranges anywhere from 10-15 degrees. In light of this
uncertainty, the NBM temperature output, which roughly represents
the median of the guidance in this case, was used as the forecast.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1248 PM CDT Wed Jul 10 2024

Isolated to scattered thunderstorms are beginning to develop across
northern Missouri and Iowa and are expected to continue to move
southeast through the afternoon and early evening. Confidence is
highest that KUIN, KCOU, and KJEF could see convection through the
early evening. Thunderstorms will remain scattered so low confidence
in direct impacts to terminals, thus went with VCTS at this time. At
the St. Louis Metropolitan terminals, confidence remains too low in
the thunderstorms reaching east-central Missouri, so kept out of TAF.

There is a chance of fog in river valleys and low lying areas
overnight. The uncertainty in the development of fog is how
widespread cloud cover will be. Should cloud cover diminish more
than what is currently forecast, more fog will develop with
potentially low MVFR/high IFR visibilities. Otherwise, there is
another chance of thunderstorms Thursday but confidence in impacts
to terminals is too low to include in TAF.

MMG/Britt

&&

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

$$

WFO LSX