020
FXUS63 KLSX 031728
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1228 PM CDT Thu Jul 3 2025

.KEY MESSAGES...

- Subtle increases in temperatures are expected each day through
  Saturday. Despite increasing moisture, much of the forecast
  includes heat index values at warm, but manageable levels.

- Largely dry conditions will hold through Friday, July 4th, and
  potentially Saturday, as chances for showers and thunderstorms
  remains low (25% or less). Better chances (30-50%) arrive
  Sunday into early next week.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 336 AM CDT Thu Jul 3 2025

Surface to mid-level ridging continues to dominate the pattern this
morning and will maintain dry conditions through Friday, July 4th.
IR satellite shows mid-level clouds ranging from few to broken in
spots overhead, which are remnants of yesterday`s thunderstorms over
eastern Nebraska and southern Iowa. If not for the cloud cover and
slight increase in dewpoints (mid-60s/low-70s), it would be nearly
identical to last night with calm/very light surface winds and
ridging overhead. Clouds just may be the greatest mitigating factor
in repeating an episode of river and valley fog, but it`ll be
something to monitor through sunrise. If clearing can be sustained
for any real duration, it`d be the typical suspects (KJEF/KSUS/KCPS)
impacted by fog in the few hours leading up to sunrise.

The next couple of days will continue somewhat of a wash, rinse,
repeat forecast as nighttime dewpoints are scoured out by deep
mixing during the daylight hours. Surface to mid-level riding will
gradually shift east in conjunction with the upper level ridge
currently positioned over the Plains. This brings a shift in surface
flow in terms of modest warm air advection that will push
temperatures slightly higher today and again Friday. Much like
yesterday, additional vort lobes round the northern periphery of the
ridge bringing thunderstorm potential to parts of Iowa, which sends
some cloud cover into the local area. However, it doesn`t look to
throw a wrench into the forecast with the ridge maintaining the dry
forecast. Considering the warming temperatures and holiday weekend,
heat index values will be of interest. That said, dp/dt shows
guidance has not only backed off on the magnitude of warmth, keeping
peak heat indices in the mid/upper 90s. While not comfortable, these
values should be manageable if preventative measures (e.g. hydration
and avoiding overexertion) are put into practice.

Temperatures climb subtly through the Independence Day holiday with
highs in the upper 80s to low-90s today and low to mid-90s Friday.
Lows will be in the mid-60s to low-70s.


Maples

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 336 AM CDT Thu Jul 3 2025

Largely dry conditions are expected Friday night into Saturday with
the ever-so-slight eastward shift in the synoptic pattern. Southerly
flow at the back side of the surface/mid-level ridge gradually draws
warmer temperatures and higher moisture into the region Saturday.
Recall that just a few days ago, low chances were drawn into the
western sections of the CWA late on July 4th. The delay has
continued with low end chances (15-25%) very slowly progressing
west to east Saturday into Saturday night. These chances are
confined to the central and northeast Missouri into west-central
Illinois. Even if thunderstorms do not materialize over the
northwest quadrant of the CWA, some cloud cover will be introduced
from thunderstorms further west. Therefore, the warmest
temperatures (mid-90s) are favored over roughly the southeast half
of the CWA.

A weak cold front trails a corresponding surface low that moves into
the Great Lakes Region in the later half of the weekend. This front
sinks southward by Sunday, bringing better chances (30-50%) for
showers and thunderstorms. Dewpoints in the 70s and PWATs of 1.5-2"
hug the back side of the ridge, pooling along and ahead of the front
in a southwest/northeast orientation. This leaves little doubt that
moisture will be the issue. LREF mean SBCAPE values climb to around
2,000 J/kg with some individual deterministic guidance pushing 3000
J/kg Sunday afternoon/evening. However, mean shear values remain at
or less than 15 knots, which will prevent well-organized convection.
LREF shows high probabilities (60-70%) for measurable precipitation,
but quickly drops off (20-30%) for QPF around 0.10". There is one
reservation, however, as high PWATs will support efficient rain
production on a localized scale, mainly through the diurnal peak
into Sunday evening.

The upper level ridge is expected to flatten over the Midwest as
several upper level shortwaves keep the northern CONUS busy. The
upper ridge then builds back in over the Dessert Southwest with
upper vorticity rounding the top of the ridge into the Plains.
Meanwhile, deterministic guidance shows surface high pressure
attempting to build southward from the northern Plains into the Mid-
Mississippi Valley by Monday. Long range guidance isn`t exactly in
lockstep with this feature, creating some spread in the
precipitation potential. Where, exactly, the surface high tracks
will determine the difference between a largely dry Monday or
another round of scattered thunderstorms best support during the
diurnal peak. NBM chances (30-40%) seem reasonable at this point,
but may eventually prove to be too widely dispersed in a
geographical sense.

This pattern will continue to support episodes of showers and
thunderstorms over the course of the week. Though NBM temperatures
quartiles have narrowed slightly since yesterday, the caveat will be
precipitation potential with any given location cooled by afternoon
thunderstorms and leftover cloud cover. Generally speaking,
temperatures remain fairly steady and near seasonable norms through
the end of the forecast period.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1228 PM CDT Thu Jul 3 2025

Dry and VFR flight conditions will persist through the TAF period
with largely diurnal cumulus overhead, and nighttime conditions less
favorable for river valley fog. Winds will remain light but become
more southerly on Friday behind a departing surface high pressure
center.

Pfahler

&&

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

$$

WFO LSX