294
FXUS63 KLSX 182333
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
633 PM CDT Sat May 18 2024

.KEY MESSAGES...

- Seasonably-warm temperatures persist through Tuesday, with
  potential for some to see 90 degrees Sunday and Tuesday.

- Other than the low (15-20%) threat for thunderstorms over parts
  of the region tomorrow, rain chances stay at bay until at least
  Monday.

- An active pattern with multiple thunderstorm chances is a near-
  certainty for next week. The ingredients for severe
  thunderstorms are most likely to converge over our region on
  Tuesday, but another threat Wednesday will depend on the speed
  of a cold front. &&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 219 PM CDT Sat May 18 2024

Clear skies gave way to diurnal cumulus development earlier this
afternoon amidst weak southerly surface winds and warm temperatures.
A cold front is slowly approaching the region from the northwest
attending a surface low tracking across south-central Canada.
Directly along it, brief and meager convection from earlier this
morning is no longer evident. A ridge aloft is gradually impinging
on the region from the southwest and working with the weak surface
convergence to inhibit much convection. As the front approaches
and clips northeast Missouri and west-central Illinois, there is a
very slim (less than 15%) chance for a thunderstorm early this
evening. However, with synoptic-scale subsidence in place,
anything that manages to develop would quickly collapse.

The front itself won`t make much progress into the region
overnight, mostly due to the surface low associated with it being
so far north. Ahead of it, the weak winds and generally- clear
skies will allow for another round of efficient radiational
cooling. Despite mixing today, low-level moisture will still be in
relative abundance from recent rain and dewpoints near the 75th
climatological percentile. This will lead to another threat for
fog overnight across most of the region (save for areas along and
behind the front). Patchy dense fog will threaten river valleys
from roughly midnight to about 8AM Sunday, by which point surface
heating will cause the fog to dissipate. The weak boundary
lingers across northern Missouri and west-central Illinois during
the day Sunday, but temperatures on both sides of it jump into the
mid/upper 80s. A few spots, St. Louis and Columbia in particular,
will make a run at 90 degrees thanks in part to the impact of the urban
heat island.

As instability builds through the morning and early afternoon, with
SBCAPE values forecast to exceed 2000 J/kg, the front has potential
to serve as a focus for isolated thunderstorms. That said, mid-level
subsidence will still be strong and work with meager shear to keep
any convection in check. If a storm can manage to deepen as a weak
shortwave approaches, it would threaten a marginally-severe hail
threat in northern Missouri. This is an outside potential at this
time, so we are not messaging a severe thunderstorm threat. The
thunderstorm chances diminish with sunset and loss of surface-
based instability. Meanwhile, flow aloft begins to amplify and
southerly surface winds strengthen. Temperatures overnight Sunday
will stay warm for mid-May, and further promote seasonably-warm
temperatures into Monday.

MRB


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 219 PM CDT Sat May 18 2024

Clear skies gave way to diurnal cumulus development earlier this
afternoon amidst weak southerly surface winds and warm temperatures.
A cold front is slowly approaching the region from the northwest
attending a surface low tracking across south-central Canada.
Directly along it, brief and meager convection from earlier this
morning is no longer evident. A ridge aloft is gradually impinging
on the region from the southwest and working with the weak surface
convergence to inhibit much convection. As the front approaches
and clips northeast Missouri and west-central Illinois, there is a
very slim (less than 15%) chance for a thunderstorm early this
evening. However, with synoptic-scale subsidence in place,
anything that manages to develop would quickly collapse.

The front itself won`t make much progress into the region
overnight, mostly due to the surface low associated with it being
so far north. Ahead of it, the weak winds and generally- clear
skies will allow for another round of efficient radiational
cooling. Despite mixing today, low-level moisture will still be in
relative abundance from recent rain and dewpoints near the 75th
climatological percentile. This will lead to another threat for
fog overnight across most of the region (save for areas along and
behind the front). Patchy dense fog will threaten river valleys
from roughly midnight to about 8AM Sunday, by which point surface
heating will cause the fog to dissipate. The weak boundary
lingers across northern Missouri and west-central Illinois during
the day Sunday, but temperatures on both sides of it jump into the
mid/upper 80s. A few spots, St. Louis and Columbia in particular,
will make a run at 90 degrees thanks in part to the impact of the urban
heat island.

As instability builds through the morning and early afternoon, with
SBCAPE values forecast to exceed 2000 J/kg, the front has potential
to serve as a focus for isolated thunderstorms. That said, mid-level
subsidence will still be strong and work with meager shear to keep
any convection in check. If a storm can manage to deepen as a weak
shortwave approaches, it would threaten a marginally-severe hail
threat in northern Missouri. This is an outside potential at this
time, so we are not messaging a severe thunderstorm threat. The
thunderstorm chances diminish with sunset and loss of surface-
based instability. Meanwhile, flow aloft begins to amplify and
southerly surface winds strengthen. Temperatures overnight Sunday
will stay warm for mid-May, and further promote seasonably-warm
temperatures into Monday.

MRB

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 219 PM CDT Sat May 18 2024

As we enter the next work week, southwest flow aloft will return to
the region. After a week-long respite from widespread active
weather in the region, this all-too-familiar setup will bring a
renewed threat for severe thunderstorms. On Monday, there are some
indications that a decaying convective complex from the central
Plains will develop an MCV that tracks across the northern
forecast area during the morning. If this occurs, and if it can
tap into instability during the late morning, additional
thunderstorms are possible as it passes through. Regardless of the
MCV, a warm front across the Missouri/Iowa border will be a focus
for scattered thunderstorms in the afternoon. South of the front,
temperatures will remain at least seasonably-warm, climbing to
the mid/upper 80s areawide. Depending on how unstable the air is
and whether upper-level support will exist for deep convection,
some marginal threat for severe weather would exist. We aren`t
currently messaging the threat due to the low confidence in its
occurrence and potential intensity.

The warm air advection becomes much more pronounced on Tuesday
amidst deep southwest flow over the Great Plains and ahead of a
surface low ejecting from the Rockies into the Upper Mississippi
Valley. As a result, warm air advection will intensify and send
850mb/925mb temperatures to the 90th climatological percentile or
higher. This will bolster temperatures into the upper 80s to low
90s, as we`ll have no issues mixing to those levels, but the
bigger story will be the threat for severe thunderstorms during
the afternoon and evening. A cold front and upper-level shortwave
(with some favorable, albeit weak, jet dynamics) approach the
region through the afternoon and early evening, with copious
instability available in the open warm sector. There are still
slight timing differences in the overall system speed that linger,
but they make little difference in the grand scheme of this
threat. Particular attention will be needed to the northern half
of the forecast area, where confidence is higher in the sufficient
shear, unstable profiles, and stronger upper-level ascent needed
for severe thunderstorms.

Like many of our events this year, convective initiation looks to
begin along a pre-frontal convergence zone during the afternoon.
Deep-layer shear orientation looks sufficiently perpendicular to
the initiating boundary to support discrete supercells at first,
growing upscale over time into a line as those shear vectors back
out of the south. While a tornado threat certainly will exist with
any discrete thunderstorms, model hodographs are not particularly
noteworthy to suggest it being the main concern. Strong low/mid-
level lapse rates and vigorous instability will lead to a very
real hail/wind threat initially. As the thunderstorms grow
upscale, a wind and QLCS tornado threat becomes predominant with
30- 45kts of 0-3km shear in place. The threat won`t completely end
until whatever complex that forms exits the forecast area, mainly
because the effective cold front will still be lagging back to
the northwest.

Again depending on the speed of the front, we may be dealing with
another round of strong to severe thunderstorms on Wednesday as
well, primarily along and south of the I-44 (in Missouri) and I-64
(in Illinois) corridor. At this stage, however, there are
appreciable differences among ensemble guidance regarding available
instability, front timing, and upper-level ascent. That said, what
happens on Tuesday in conjunction with how the synoptic system
evolves will play key roles in the severe weather threat Wednesday.
There looks to be a brief lull in precipitation chances late
Wednesday into Thursday behind the front, which will also cool our
temperatures closer to normal. Before too long, another wave
amplifies and approaches the forecast area and leads to more
thunderstorm chances Friday into the weekend.

MRB

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 625 PM CDT Sat May 18 2024

VFR flight conditions are expected to prevail at least through
this evening across the area. A cold front over northwest Missouri
will dip into northeast Missouri and west central Illinois by
early Sunday morning. The front will stall near a line stretching
from KVER to KPPQ before lifting back to the north during the
afternoon. Current thinking is that upper level ridging will
suppress any potential thunderstorms in our area along the
front...but I wouldn`t say there`s a zero chance for a shower or
thunderstorm in northeast Missouri and west central Illinois this
evening. Maybe 5 percent... Looking farther south tonight, another
round of fog looks likely across the eastern Ozarks and parts of
southwest Illinois. That being said, the signal for fog is not as
strong, so vsbys will likely not drop as far as last night,
staying mainly MVFR, except in river valleys and other low-lying
and fog prone areas. Any fog that develops should dissipate
quickly after sunrise on Sunday morning.

Carney

&&

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

$$

WFO LSX