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FXUS63 KLSX 270351
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1051 PM CDT Thu Sep 26 2024

.KEY MESSAGES...

- The remnants of Tropical Cyclone Helene will impact much of the
  region beginning Friday. Widespread rain amounts of 1-3" are expected
  in parts of southeast Missouri and southwest Illinois, with a
  very sharp cutoff in totals to the northwest.

- Strong and gusty northerly winds are also forecast late
  Friday/Friday night, particularly in parts of south-central
  Illinois. Sustained wind of 20-30 mph with gusts of 35-40+ mph
  are expected, though exactly how strong the winds will get is
  heavily dependent on how far west the remnants of Helene track.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 205 PM CDT Thu Sep 26 2024

Isolated to widely scattered showers and weak thunderstorms are
expected early this evening in parts of southeast Missouri and
southwest Illinois. This is where weak surface-based instability
(MLCAPE 150-300 J/kg) resides beneath the closed mid/upper level
low, with weak surface convergence near an inverted trough. Any
convective elements should cease by late evening as instability
diminishes nocturnally. The rain will become more stratiform in
nature, and become widespread overnight as low-level moisture
advection strengthens. Light, to occasionally moderate, rain will
gradually spread from east to west from south-central Illinois
into east central and southeast Missouri toward daybreak. This
east to west progression should continue Friday morning before
stopping as the remnants of Tropical Cyclone Helene continue to
interact with the closed mid/upper level low across the mid south.
The area of vorticity associated with Helene "slingshots" to the
northwest on Friday around the closed low due to the Fujiwhara
effect. The remnants of Helene become fully enveloped/merged with
the closed mid/upper level low by Friday evening. However, the
strength of the surface cyclone will not weaken much from late
Friday afternoon through early Friday night due to some upper-
level support via enhanced divergence beneath the left-exit region
of a 70+ knot jet streak. The surface low is then expected to
rotate cyclonically across much of the mid/lower Ohio Valley, with
the center making its closest pass to our neck of the woods
Friday evening. This is when northerly winds/gusts should be the
strongest on the northern/northwestern periphery of the center of
the low. The rain shield is expected to become less organized and
begin to push off to the south and west Friday night with the
stronger forcing for ascent also pushing out of the region.

Temperatures on Friday will really struggle to move much in areas of
steadier rain (and lower clouds). Highs in the mid to upper 60s are
forecast for parts of southeast and east central Missouri as well as
southwest Illinois. These readings are very near the ADJMET
guidance, which tends to perform well in these expected conditions.
Further northwest, gradually warmer temperatures are forecast due to
lessening rain chances and higher cloud bases.

Significant rainfall and strong, gusty northerly winds are the main
concerns with the remnants of Tropical Cyclone Helene, and are
further elaborated upon below:


RAINFALL: While some additional rainfall is likely this weekend, the
widespread significant rainfall should conclude Friday night. Storm
totals of 1-3" are forecast across parts of southeast Missouri and
southwest Illinois, with a very sharp cutoff in totals the further
northwest you travel. There is the potential for locally higher
totals as high as about 4", which is supported by the LPMM of the
12Z HREF. These high event totals are also supported by extremely
anomalous 850-hPa flow out of the east. The NAEFS has record u-
component (west-east) wind magnitudes. Anomalous easterly flow at
850-hPa has shown a high correlation to heavy precipitation events
within deformation zones from previous research.

The chances for flash flooding however continue to look very low,
with 6-hr flash flood guidance values on the order of 2.50-3.50".
These values are generally at or above our forecast totals for the
entire event, with the expected rain falling for the most part over
a 18-24 hour time span for most locations. In addition, the rain
will be stratiform in nature, with little/no convective elements.
This really helps limit the rainfall rates and any flash flood
potential.

The odds of river flooding also look quite low, with the only
forecast point with probabilities of minor flooding in our HSA near
10% being the Black River at Annapolis (9-10%). Some ponding of
water is expected along with some rises on area
streams/creeks/rivers, but water should mostly stay within banks.


WIND: As mentioned above, the strongest winds and gusts are expected
to be late Friday afternoon/Friday evening as the remnants of
Tropical Cyclone Helene make their closest approach to the area.
Sustained wind speeds of 20-30 mph with gusts of 35-40+ mph are
expected, particularly across portions of south-central Illinois.
There remains a very strong signal for anomalous winds/gusts, with
last night`s 0Z EPS displaying EFI values near 1.0 and SOTs of 2-3.
The EFI near 1.0 illustrates the near-uniform agreement amongst the
50 members of the EPS suite and the SOT above 1 is indicative of the
potential for fairly extreme winds for this time of year. The key is
for this time of year as late September climatologically speaking is
not known for high winds, which is in part why those EFI/SOT values
are so high. One factor pointing to increased vulnerability is the
predominant wind direction (out of the north). Very strong northerly
winds are rare this time of year, aiding stress to tree limbs. In
addition, leaves are still on trees which adds to the weight load.

The 12Z HREF has very high probabilities (>80%) for wind advisory
criteria (gusts of 45+ mph) in parts of southwest and south-central
Illinois. The HREF is however comprised of only 4 CAMs and their
time-lagged components (8 members total). Coarser model guidance is
not nearly as high regarding sustained wind speeds or gusts. The EPS
had 40-50% probabilities for advisory criteria gusts Friday evening
across parts of south-central Illinois. For sustained wind speeds,
the LREF showed 20-30% chances for sustained wind speed criteria
(30+ mph). These speeds and gusts however are highly contingent on
the track of the retrograding surface low, and a slight shift east
(west) would decrease (increase) the chances for significant
winds/gusts. Another potential limiting factor is the timing (after
dusk for the most part), where it becomes harder for gusts to mix
down to the surface. In addition, both the temporal (~0Z-05Z) and
spatial (S/C IL) "windows" for advisory criteria are both
limited/small. Have therefore held off on the issuance of a wind
advisory for south-central Illinois at this time.


Gosselin

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 205 PM CDT Thu Sep 26 2024

(Saturday - Sunday)

While the steady, widespread rain should be over by late Friday
night, isolated to scattered rain showers remain possible through
the weekend. The closed mid/upper level low will be slow to depart,
and continue to hang around the lower Ohio Valley. Subtle areas of
vorticity will continue to rotate around the closed low helping
produce the shower activity, mainly across parts of southeast
Missouri and southwest Illinois. The activity likely will be at
least in part diurnally driven, with the greatest coverage likely in
the afternoon each day.

Temperatures will remain variable across the area, with coolest
readings in the south and east (where more clouds are expected) and
warmest conditions further to the north and west (where more
sunshine will exist). Low 70s are forecast each day in parts of
southeast Missouri and southwest Illinois with upper 70s to near 80
degrees forecast in parts of central and northeast Missouri as well
as west-central Illinois.


(Sunday Night - Next Thursday)

Rain chances depart by Sunday night as the closed mid/upper level
low finally moves eastward into the mid Atlantic. Mid/upper level
shortwave ridging moves in from the north and west, leading to dry
and warmer (upper 70s to low 80s) conditions for Monday afternoon.

Medium range guidance is in pretty good agreement that a strong
trough will move through southern Canada just north of the border
late Monday night into Tuesday. This feature will help bring a cold
front through the mid-Mississippi Valley, bringing cooler
temperatures in its wake. There is little/no chance of measurable
rain associated with this feature due to weak surface convergence
along the frontal boundary, stronger mid/upper level forcing well
north of our area, and weak moisture return ahead of the front.
Probabilities for measurable rain from the LREF over a 24-hour
period are only 10-15% ending Tuesday night.

The coolest night is expected to be on Tuesday night/Wednesday
morning with lows generally in the 40s. Exactly how cool it gets
will depend in large part to the timing of the trailing surface
anticyclone (and sky cover of course).

By late next work week, building mid/upper level heights and 850-hPa
mean temperatures on the EPS/GEFS climbing well above +15C portend
to a quick moderating trend, with above normal temperatures
appearing likely.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1039 PM CDT Thu Sep 26 2024

Rain moves into the region by morning along with some lower
ceilings. The St Louis metro is the most likely of the TAF sites
to be affected by this. Quincy will likely stay north of the rain
and lower ceilings, while central Missouri might see some rain but
is unlikely to get the lower ceilings. This rain lasts for the
bulk of the day before tapering off in the afternoon to evening.
Dry air moving in from the north will help erode the ceilings and
give way to VFR area wide by tomorrow evening.

Northerly winds are light at the moment, but will increase Friday
morning and become gusty through the day. The strongest gusts,
30KT or more, come Friday evening mainly in the St Louis metro and
potentially at Quincy as well. The biggest uncertainty regarding
winds is the degree of mixing that occurs in the lowest part of
the atmosphere near the ground. With warm, moist air circulating
into the area from the east that will promote a stable layer and
poor mixing of these winds. In that case, lower wind gusts will be
observed at the terminals, but wind shear would become noticeable
just above the surface where winds ramp up to 50KT or more at
2000FT. One complication is dry air being pulled in from the
north. This dry air would promote clearing in the low levels and
some mixing which would enhance the ability of stronger wind gusts
to reach the surface. In that case, wind gusts over 40KT are
possible at the terminals with less of a factor for wind shear.

Kimble

&&

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

$$

WFO LSX