St. Peters, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

562
FXUS63 KLSX 150427
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1027 PM CST Thu Nov 14 2024

.UPDATE...
Issued at 932 PM CST Thu Nov 14 2024

The back edge of the stratus cloud deck roughly paralleled the
Mississippi River prior to 04z this evening. The west-to-east
clearing has panned out slightly quicker and farther east than
previously expected. As a result, efficient radiational has
already dropped temperatures into the mid-30s to the low to
mid-40s over central/northeastern sections of Missouri.
Hourly temperatures were adjusted to be more representative of
latest trends. This includes a broader spread of morning
temperatures in the 30s for Friday morning lows. The more
impactful change is the eastward shift in the dense fog potential,
which will generally line up along and west of the clearing line.
The original dense fog advisory was extended eastward with an
effective time of 06z-15z.

Maples

&&

.KEY MESSAGES...

- Dense fog will blanket much of central and northeast Missouri
  and parts of east-central Missouri overnight into Friday
  morning.

- A more active weather pattern will impact the area during the
  first half of next with with widespread showers (80-90% chance)
  in the Monday afternoon-early Tuesday time frame.

- A cooler weather pattern that could included a showers (20-50%)
  is forecast to evolve during the later part of next week.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 230 PM CST Thu Nov 14 2024

The stratus has been rather persistent so far today and clearing
has begun across parts of central and southeast MO. Given the
current trends, I have been slow on clearing tonight and used a
combo of the GLAMP and NAM/RAP low-level RH fields which would
bring the clearing line to the vicinity of the MS River later
tonight and then hold it somewhat steady until after daybreak
Friday. With surface high pressure settling into the heart of the
CWA overnight, very effective radiational cooling is expected in
the wake of the departing clouds. Fog should develop across
central/northeast MO and portions of east-central MO in
association with the radiational cooling, which is supported by
cross-over temperatures and CAM visibility guidance indicating a
high probability of dense fog. Accordingly I have issued a dense
fog advisory. If clearing is more aggressive tonight then it`s
quite possible the dense fog advisory may need to be expanded
farther east.

The fog should dissipate by 15-16z as the associated saturated
layer is rather shallow. The region will be experiencing rising
heights aloft and ridging in response to the upper trof departing
across the eastern U.S. and amplification associated with the
upstream deepening western trof. Despite the rising heights, the
area will remain under the influence of the slowly retreating
surface high, and this will keep temperatures in check. Highs
Friday will be warmer than today, owing to the return of sunshine,
and near to slightly above normal.

Glass

&&

.LONG TERM...  (11Saturday through Next Thursday)
Issued at 230 PM CST Thu Nov 14 2024

By Saturday we will be within southwest flow aloft as the upper
ridge progresses eastward and a new closed low develops across the
Desert Southwest in association with continued deepening and
evolution of the western longwave trof. The attendant continued
eastward retreat of the surface/low-level high will bring the
onset of a WAA regime, however much of the initial warming will be
aloft as evident in soundings showing a huge temperature
inversion. Thus while high temperatures will be warmer and above
normal, they will be nowhere near the magnitude that might be
implied strictly viewing the 850 mb temps in the teens. The warm-
up will continue into Sunday as low-level winds become more
favorable with highs averaging 10-15 degrees above normal.

The latest LREF ensemble guidance suite is in good agreement and
consistent with previous runs indicating the southwest upper low
will become negatively tilted as it ejects through the
southern/central Plains Monday and into the mid/upper MS Valley
Monday night-early Tuesday. Moisture transport in advance of the
system and forcing with the deep layer cyclone should result in a
swath of pre-cold frontal rain/showers spreading west-east across
the area late Monday afternoon through Monday night/early Tuesday
morning. As mentioned yesterday, the progressive nature of the
system should limit rainfall amounts. In fact the latest
exceedance guidance from the 100 member LREF has shown diminishing
probabilities of amounts over 1 inch (highest 40-50% in the
western CWA). There remains some question with regards to thunder
and severe potential. Virtually any formidable progressive upper
trof from now through May will be accompanied by strong deep layer
shear, and getting sufficient instability for thunder is the
first step with increasing magnitude and stratification through
the boundary layer needed to bring a severe threat. The most
aggressive solutions have MUCAPE in the 150-350 J/KG range Monday
afternoon-evening, not to dissimilar from yesterday when there
was some embedded isolated storms for a time in the rain band.
However the more critical surface/ML CAPE needed for any severe
threat appears to be lacking at this time with only the 90th
percentile showing values creeping over 100 J/KG. We will need to
keep an eye on how the instability evolves with newer guidance,
but the current consensus lacks enough for even the mention of
thunder.

From next Wednesday through the end of next week there continue to
be large differences in the evolving upper air pattern across the
Nation`s midsection. Ensemble guidance and deterministic models
are consistent showing the development of a large slow-moving upper
low however the position varies greatly in the Wednesday-Friday
period. The LREF cluster analysis is almost equally dispersive
with its probabilities of four clusters depicting an upper low
position anywhere from over the southern Rockies to the upper
Great Lakes on Wednesday. These differences persist through the
end of the week and there is a 7-9 degree spread between the
25th-75th percentile for the 850 mb temperature at 18z Thursday.
Thus there is a high degree of uncertainty in both temperatures
and sensible weather during this period.

Glass

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1027 PM CST Thu Nov 14 2024

MVFR clouds have cleared from west to east more quickly that
previously expected. While MVFR ceilings are now east of all
terminals with VFR conditions for the time being, it may not last
long. Radiational cooling and calm to light and variable surface
winds are expected to support fog development farther east than in
the last update. While all sites have the potential to have some
extent of fog late tonight into early Friday morning, KCOU, KJEF
and KSUS stand the best chance to see dense fog (1/4SM) with IFR
conditions for a few hours.

There is some question with the westward development near
KCOU/KJEF with the noted eastward shift. For now, prevailing
groups were continued until there is enough confidence to remove
the conditions. The remainder of the site may see MVFR fog
development given the later clearing and slightly milder
temperatures. VFR conditions are forecast to return by mid-morning
Friday.


Maples

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CST Friday for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Shelby MO-Warren MO-Washington MO.

IL...Dense Fog Advisory until 9 AM CST Friday for Monroe IL-Randolph
     IL.

&&

$$

WFO LSX