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FXUS63 KLSX 271724
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1224 PM CDT Fri Sep 27 2024

.KEY MESSAGES...

- The remnants of TC Helene will bring rain to much of the area
  through tonight. 1 to 3+ inches of rain are expected where the
  steadier rain falls across parts of east central and southeast
  Missouri into southwest and south central Illinois, with lesser
  amounts to the west and northwest.


- Strong and gusty northerly winds are also forecast this
  afternoon and this evening. Sustained wind of 20-30 mph with
  gusts of 35-40+ mph are expected primarily across south central
  and southwest Illinois into parts of the eastern Ozarks in
  Missouri. A Wind Advisory has therefore been issued for these
  areas.


&&

.SHORT TERM... (Through Sunday)
Issued at 344 AM CDT Fri Sep 27 2024

The remnants of TC Helene will begin impacting our area today. The
regional radar mosaic is already showing a stream of moisture
wrapping around a closed upper low over southwest Tennessee that`s
producing rain across much of the eastern half of the forecast
area. This moisture is loosely associate with Helene, but the
hurricane is still well to the southeast of the are over Georgia,
and is not forecast to move into the Tennessee Valley until this
afternoon when it will be wrapped into the north side of the
aforementioned upper level low. The GFS and NAM still have the
pressure at the center of the low down to between 980-990mb this
afternoon into early evening. The pressure gradient between the
low to our east-southeast and a 1015+ mb surface high over the
Upper Midwest will produce strong and (at times) gusty winds. The
strongest winds will be across south central into south west
Illinois where the pressure gradient is the tightest. The NBM does
not do well in strong wind situations, so I relied on a blend of
short range ensemble guidance which yielded sustained winds
between 25-30ks and gusts up to 40kts. While the eastern Ozarks
aren`t as likely to see the sustained winds that Illinois will,
there will likely be some wind gusts to 40kts or a bit stronger
given the higher terrain. HREF probabilities of winds of this
strength are high (>70-80%) so I have high confidence in these
values. Have therefore issued a Wind Advisory for parts of
southeast Missouri into southwest and south central Illinois for
this afternoon and evening. While the rest of the area will likely
see some breezy conditions, it`s unlikely the wind will make
advisory criteria as the low will continue to fill and the
pressure gradient will slacken.

The other significant impact from TC Helene will be rain.  I have
less confidence in the western/northwestern extent of the rain
shield associated with the remnants of Helene.  While it looks like
areas along and east of I-44 in Missouri and along and east of I-55
in Illinois are nearly 100% sure to see a fairly steady rain over
the next 24-36 hours with this system, points west are much less
certain. There will likely be a very sharp cut off between areas
that receive the steadier rain through Saturday morning, and areas
that get intermittent rain or none at all. If the low wobbles 30-50
miles it will likely make a huge difference in where that cut off
line will be at any given time.  This also makes a huge difference
in accumulations which range from 0 in northeast Missouri and west
central Illinois, to more than 3 inches across the eastern Ozarks
and parts of southwest Illinois.  As with wind I believe the NBM was
too broad and PoPs were not high enough across the eastern Ozarks
into southwest Illinois, so a blend of short range guidance was used
for PoPs yielding a tight gradient between likely/categorical PoPs
and low chance PoPs across much of east central Missouri into west
central Illinois just north of the St. Louis Metro area.  While
rainfall amounts exceed 3 inches in parts of southeast Missouri and
southwest Illinois, these areas mostly missed out on the highest
rainfall from our last event, and flash flood guidance remains high
in the 2-4 inch range for 1-6 hours respectively.  The 3+ inch
values are storm totals through Sunday (with the heaviest rain today
and this evening), but any 6 hour period barely exceeds an inch.
While this may cause creeks and streams to rise, widespread
flooding/flash flooding does not appear to be likely so have held
off issuing a Flood Watch at this time.

All guidance shows the combined upper low and remnants of Helene
continuing to spin over the Tennessee Valley on Saturday and Sunday.
 While the wind will still be relatively strong on Saturday, the
pressure gradient will have slackened by then to keep winds below
advisory criteria.  There will still be a significant amount of
moisture wrapped into the system both days, and all guidance shows
precip lingering through the weekend along and east of the
Mississippi River to a greater or lesser extent, with the greatest
chances in south central Illinois.  Clouds and showers will tend to
suppress temperatures for the weekend with highs generally in the
low to mid 70s.

Carney

&&

.LONG TERM... (Monday through Thursday)
Issued at 344 AM CDT Fri Sep 27 2024

Briefly, in the long term:  The combined low and remnants of Helene
move east across the central Appalachians on Monday as a northern
stream short wave dips into the Upper Midwest.  This short wave
drives a cold front through the Mid-Mississippi Valley on Tuesday,
with a cool high pressure system settling into Missouri behind it on
Wednesday.  Temperatures briefly fall a few degrees below normal for
Tuesday night into Wednesday under the influence of the high.
However, the flow aloft becomes zonal quickly after the short wave
passes which moves the surface high east of our area allowing return
flow to warm temperatures back up above normal for the latter half of
the week.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1214 PM CDT Fri Sep 27 2024

Moisture associated with TC Helene, now centered over the southern
Appalachian Mountains, continues to stream into the region and
interact with an upper-level low. The resultant rain and MVFR CIGs
at the St. Louis metropolitan area terminals will continue into
the early evening before a break through the evening and early
overnight. Another round of rain with lowering CIGs returns a few
hours before sunrise and gradually lifts by the early afternoon.

Of additional concern are wind speeds - as Helene`s remnants
approach, generally north/northeast winds will continue overnight
into Saturday sustained at 15-20kts with gusts around 30kts. The
system will gradually weaken, resulting in a slackening of wind
speeds and gusts by late morning into the afternoon. While rain
will still be a threat in the St. Louis region, the best chances
are east of the Mississippi River.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Wind Advisory until 1 AM CDT Saturday for Iron MO-Madison MO-
     Reynolds MO-Saint Francois MO-Sainte Genevieve MO.

IL...Wind Advisory until 1 AM CDT Saturday for Bond IL-Clinton IL-
     Fayette IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX