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000
FXUS63 KLSX 142335
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
635 PM CDT Sun Apr 14 2024

.KEY MESSAGES...

- Anomalous warmth will continue on Monday, with high temperatures
  well into the 80s once again.

- Isolated to widely scattered thunderstorms are possible (20-30%)
  very late Monday afternoon into Monday evening. A few of these
  thunderstorms could produce quarter sized hail and gusty winds.

- Strong to severe thunderstorms are possible areawide Tuesday,
  mainly during the late afternoon/early evening hours with all
  hazards possible. Portions of northeast and central Missouri
  continue to have the best chance at seeing severe weather, with
  more uncertainty further to the southeast Tuesday night.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 222 PM CDT Sun Apr 14 2024

A weak cold front was located near the Missouri-Iowa border early
this afternoon. This boundary has made very little progress
southward through the day, but may get a bit more of a push as a
midlevel shortwave trough slides through the Great Lakes. By 1200
UTC Monday, this frontal boundary is likely to stall out somewhere
along/near the I-70/I-64 corridors in Missouri and Illinois
respectively. Little to no movement is expected on Monday until very
late afternoon/early evening when leeside cyclogenesis commences in
western Nebraska. This is when it will begin to progress back
poleward as a warm front. This frontal boundary will be a focus for
the potential for isolated to widely scattered convective
development beginning late afternoon or early evening. There remains
some uncertainty regarding convective initiation, but latest HREF
>40 dBZ ensemble paintball show most CAMs by or just after 0000 UTC
Tuesday do have at least some thunderstorms in the vicinity of the
frontal boundary. Surface convergence along the boundary appears
fairly weak, but most guidance now does suggest a midlevel shortwave
trough coming out of the south-central Plains by early evening. This
feature seems to be key to helping initiate activity by early
evening. The NAM/hi-res NAM has this feature but still fails to
develop convection. Looking at soundings, this is due to a stronger
cap in place than other guidance. In other words, there is
uncertainty with respect to two features: 1) the strength, timing
of a midlevel shortwave trough and 2) strength of a cap. Based on
the preponderance of evidence though, it does seem like at least
some initiation is likely by early evening near the frontal
boundary. Coverage likely will be isolated to widely scattered in
nature. A few of these storms may be strong to severe through the
late evening hours, with hail and wind both threats. Severe hail
would be most likely in transient supercell structures given deep-
layer shear approaching 35 knots as the midlevel shortwave trough
approaches. Some sporadic wind damage also appears possible due
to dry midlevels of the atmosphere (entrainment) and lots of dry
air below the cloud base.

Temperature wise, no significant changes are anticipated over the
next 24-36 hours. Some slightly cooler (5-8F) low temperatures are
forecast overnight tonight in parts of northeast Missouri and west-
central Illinois behind the cold front. The combination of a clear
sky and light/variable winds also should yield cooler lows across
area river valleys. High temperatures on Monday are expected to be
on average a couple of degrees cooler (low to mid 80s), mainly due
to a more easterly component to the surface winds and an uptick in
cloud cover compared to this afternoon. Lows Monday night are likely
to be very mild, mostly in the low to mid 60s as surface winds turn
more out of the south and sky cover continues to increase. To put
these lows in perspective, not only would they be about 20 degrees
above normal for the date, but in most locations are actually very
close to normal high temperatures for the date.


Gosselin

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 222 PM CDT Sun Apr 14 2024

(Tuesday - Tuesday Night)

The focus on Tuesday continues to be on the chances of severe
weather, mainly during the late afternoon through the mid/late
evening hours. Model guidance continues to show increasing chances
of showers and thunderstorms during the late morning/early afternoon
hours, mainly in parts of east central and southeast Missouri as
well as southwest Illinois. It is a bit difficult to discern if this
is a continuation of earlier morning activity or showers and storms
that move northeastward out of Arkansas. Regardless, this activity
likely will not be particularly organized with limited instability
and broad mid/upper level forcing for ascent southeast of the
midlevel closed low. A few strong to severe storms cannot be ruled
out, but the much better threat will be later in the day beginning
further to the northwest.

The main focus is later on in the afternoon with the approach of a
pseudo-dryline from western Missouri. Scattered supercells are
likely to develop along this dryline and race northeastward into
parts of central and northeast Missouri between ~5-8PM. Storm
motions are likely to be near 50 knots. Hodographs continue to look
pretty straight at least initially due to mostly unidirectional
shear through the column so some splitting supercells are possible.
Large hail is the main threat with these storms, with a tornado or
two also possible. Veered winds and marginal low-level
shear/helicity suggest a low(er) tornado potential initially but LCL
values will likely be quite low.

Tornadic potential should increase during the evening hours as the
low-level jet intensifies and hodographs in the low-levels become
more elongated/curved. However, there is more uncertainty in
convective coverage by late evening into the early overnight hours.
The dryline becomes a bit less defined as it heads eastward,
instability will likely be a bit more muted, and the mid/upper level
forcing for ascent shifts rapidly into the Upper Midwest/Great
Lakes. The environment will still be very favorable for severe
weather given 500-1000 J/kg of CAPE and 50-60+ knots of deep-layer
shear. As alluded to above, the low-level shear/helicity also should
be on the increase so tornadic potential is also certainly there.
The threat becomes more and more conditional with time though,
particularly after 0600 UTC Wednesday.

Gradient winds will be another story on Tuesday, with sustained
southerly flow of 20-30 mph and gusts of 35-45 mph. Models have
backed off the intensity of the winds ever so slightly compared to
yesterday. Soundings show some 40+ knot winds at the top of the
mixed layer, but not as long lasting or as prevalent spatially.
Current thinking is that there may be some sporadic/isolated
advisory (45+ mph) level wind gusts across parts of the area, but
not confident in anything widespread at this time which would
necessitate a wind advisory headline.


(Wednesday - Next Sunday)

There may be a narrow window for redevelopment along the dryline
late Wednesday morning for a few strong-severe thunderstorms in
parts of southeast Missouri/southwest Illinois. Much better chances
would be to our southeast, but this is something to keep an eye on
in case the boundary`s progress to the east/southeast slows more
than currently forecast. Otherwise, Wednesday should be drier and a
bit cooler with the trailing cold front moving through the area by
afternoon. Highs are expected to be mostly in the 70s across the
area.

Forecast uncertainty begins to increase for late week as models
continue to struggle with the strength/track of anomalous troughing
near the US-Canadian border. Three of the four clusters of the 500-
hPa height pattern from the WPC favor quasi-zonal flow aloft. This
combined with a strong low-level baroclinic zone across the area
would seem to favor increasing chances of widespread rain,
potentially multiple rounds. However, timing any individual impulses
or their tracks this far out is a futile exercise. Best rain chances
right now are between Thursday and Friday night. Drier weather is
forecast for the weekend as the low-level baroclinic zone shifts
gradually southeastward. Below normal temperatures are also
expected, but guidance has muted the strength of the incoming shot
of cold air a bit. This is due to a propensity for more members of
the grand ensemble to keep the brunt of the colder air further to
the north. That being said, this is still 6-7 days out and if things
shift a bit colder, this would increase chances of a freeze in our
north once again.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 634 PM CDT Sun Apr 14 2024

VFR flight conditions are expected through much of the period. A
surface frontal boundary is slowly trekking southward from northeast
Missouri and west-central Illinois. This will allow winds to be
light and variable overnight then increase again Monday morning.
With the passage of the front winds will shift to be northeasterly
then southeasterly through the day on Monday.

Isolated to scattered showers/thunderstorms are possible at the end
of the TAF period, but confidence in specific timing and location of
activity is too low to mention in TAF at this time.

MMG/Byrd

&&

.CLIMATE...
Issued at 225 PM CDT Thu Apr 11 2024

DAILY HIGH TEMPERATURE RECORDS

          |=== 4/14 ==|=== 4/15 ==|
St. Louis | 92 (2006) | 89 (2002) |
Columbia  | 89 (2006) | 90 (1896) |
Quincy    | 86 (2006) | 88 (2002) |


&&

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

$$

WFO LSX