367
FXUS63 KLSX 191925
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
225 PM CDT Sat Oct 19 2024

.KEY MESSAGES...

- Dry weather continues Sunday and Monday with warm days and
  seasonably cool nights.

- The best opportunity for rain comes Tuesday morning, but
  rainfall amounts will be light (<0.10) if it occurs. The long
  term dryness is likely to continue through next week.

&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 224 PM CDT Sat Oct 19 2024

The upper air pattern today features a broad ridge over the eastern
US and a cutoff low over the southwestern deserts. In fact, Phoenix,
Arizona is looking at a rare day in the 70s after spending most of
the month in the 100s. That trough will approach our area early next
week, but until then we remain under the influence of the broad
ridge and an associated broad area of surface high pressure. This
surface high has expanded back west into our area again leading to
lighter winds today than yesterday (and another cool start this
morning). Despite some mild temperatures today in the mid 70s, a few
degrees above normal, we`ll cool back down quickly tonight in this
dry air mass under clear and calm conditions. Without a significant
increase in moisture today, we`ll see another night of temperatures
in the upper 30s and low 40s. Valley locations will be vulnerable to
fog by morning as the crossover temperatures (upper 30s) become
attainable.

The surface high pressure axis nudges just a bit back to the east on
Sunday allowing for a bit more southerly flow and a couple more
degrees added to those high temperatures. A few locations,
especially in the western forecast area, approach 80 degrees. With a
bit more wind (and thus mixing) Sunday night we shouldn`t cool off
as much. Lows only reach the mid to upper 40s.

Kimble

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 224 PM CDT Sat Oct 19 2024

The first feature of note in the long term is that previously
mentioned cut off low over the desert southwest. After spinning
around in Arizona for a couple of days, it finally begins to nudge
eastward into the Plains on Monday in response to a trough moving
into the Pacific Northwest. Because of the persistent surface high
over the southeast US, moisture from the Gulf of Mexico has been
lacking across most of the US. The flow of moisture has been
westward into Mexico and then northward in a narrow plume through
New Mexico and the Texas Panhandle. So as the upper low moves east
it will initially have some decent moisture to work with, producing
rain in the western High Plains. However, as the low moves east it
opens up, loses its definition, and outruns its own access to that
limited moisture. So when the trough makes its closest pass through
our area late Monday night into Tuesday morning it will be losing
its forcing to trigger precipitation and only working with remnant
moisture aloft that it brought with it from Texas. So as the trough
moves east the shower activity will be decreasing in intensity and
coverage, eventually drying up altogether Tuesday afternoon.

This still represents our best chance of rainfall in this 7-day
forecast, with up to 90 percent of 12Z ensemble guidance producing
at least measurable rainfall in 24 hours in central and northern
Missouri. Our going PoP forecast is considerably less than that
owing primarily to the timing of this rain spanning the 12Z cutoff
between the Monday night and Tuesday time periods of our forecast.
So we`re still very confident that there will be showers around, but
PoP is held back by the question of whether it will be late Monday
night or early Tuesday morning. Either way, though, it won`t be of
much impact. Less than 30 percent of those same ensembles produce
0.1 inch of rain from this system. That`s not going to be enough to
put any real dent in our prolonged period of dry weather.

With the trough losing definition as it gets caught up in the larger
scale trough moving through the Upper Midwest, its passage won`t
have a significant impact on temperatures. In fact, temperatures
stay fairly warm, in the upper 70s to low 80s for highs, through at
least Monday and Tuesday, although cloud cover on Tuesday does add
some uncertainty and could hold temperatures back a few degrees. The
trough moving east through the Upper Midwest and Great Lakes will
keep the bulk of its forcing well to our north in the Great Lakes
and Canada. However, it will bring with it a more potent cold front
which passes through our area on Wednesday. Although moisture
available ahead of this front will be more expansive than with the
last trough, the lack of forcing to lift and condense that moisture
will lead to this being primarily yet another dry frontal passage.
There is some indication that showers and thunderstorms may form
along the front, but it`s likely to move through our area too early
in the day to generate the instability needed for that, so those
limited rain chances are focused well to our south.

The air mass behind Wednesday`s front is considerably cooler and
very dry. It`s not as cold as the air mass that arrived less than a
week ago, but it will be noticeable. There remains some variance in
the guidance on just how much of the cold air makes it into our area
as the bulk of the cold push heads more to the east along with the
relatively progressive trough. Variance in the guidance largely
centers around how deeply this trough digs into the eastern US.
Sources with a deeper trough bring cooler air (850mb temps around
+5C) while sources with a more shallow trough have a more mild
intrusion of cooler air (850mb temps around +10C). This leads to
greater uncertainty in the high temperature forecasts in the late
week time frame, with NBM interquartile ranges for high temperatures
reaching around 10F Thursday and Friday. On the cooler side, we
could be looking at highs in the mid 60s, a few degrees below
normal, but on the warmer side we could still see 70s, a few degrees
above normal. Low temperatures will depend more on how dry this air
mass is and whether we get any clear, calm nights. There`s much
greater variation in forecast dewpoints in this air mass, with the
GFS in particular quite dry. This has led to an even greater spread
in the low temperature forecasts with NBM interquartile range of 15F
or more. In the drier scenarios, and with at least one night with
the surface high nearby, subfreezing temperatures would be possible.
Most guidance, though, tracks this surface high to our northeast
through the Great Lakes so we may never really feel the coolest
potential from it.

The general consensus in guidance is that this cooler air mass won`t
linger for too long, as ridging of some sort moves back in behind
the rough. So long term projections are for a trend back toward
warmer conditions, at least for high temperatures. Although just how
warm depends on the position and strength of the ridge. NBM begins
to show some low probability (20 percent) of 80F in our region by
Friday.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1216 PM CDT Sat Oct 19 2024

Continued clear skies with light winds under the influence of high
pressure today and tonight. With the high nudging a bit closer, we
may be a bit more likely to see valley fog tonight with this most
likely at SUS. Otherwise VFR conditions are expected.

Kimble

&&

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

$$

WFO LSX