301 
FXUS63 KLSX 300913
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
413 AM CDT Fri Jun 30 2023

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 405 AM CDT Fri Jun 30 2023

Key Messages:

1. The potential for dangerous heat continues today, and a large 
portion of the area is likely to see heat indexes between 105 and 
110 degrees. This is most likely across central and southeast 
Missouri. 

2. Another round of thunderstorms is possible this afternoon, and 
these storms may be strong or severe. The most likely hazards would 
be damaging straight line winds, with a more limited potential for 
large hail. Please see the discussion below for more detailed 
timing/location/confidence information. 

3. More thunderstorms are likely Saturday afternoon, again with the 
potential for strong or severe storms. Timing and coverage 
uncertainties remain high, but there is an increasing potential for 
damaging straight-line winds, along with at least some potential for 
large hail. 

All is quiet early this morning as we await another potentially busy 
day, featuring dangerous heat and possibly strong or severe 
thunderstorms later this afternoon. As of 4 AM, a weak surface front 
remains draped across northern Missouri and west-central Illinois, 
although surface wind fields are a bit washed out. Still, there is a 
clear E/W instability gradient along this boundary, and this is 
projected to remain largely stationary through the morning and into 
the afternoon, positioned roughly along the northeastern periphery 
of an upper level ridge. Farther upstream, a developing MCS is 
ongoing across southeast Nebraska, and accelerating eastward. 
Similar to yesterday, these thunderstorms are likely to play a 
major part in how things evolve today. 

While this activity will make its way east and some semblance of
it will likely eventually reach our area, current trends suggest 
that this may occur slightly later in the day than originally 
anticipated. Assuming this trend holds, this would result in a few
changes to today's thunderstorm and heat potential. First, storms
are increasingly likely to remain out of our area until around 11
am. This could allow for increased local heating in the 
afternoon, along with maximized instability along and south of the
surface boundary. Meanwhile, there is low certainty regarding 
just how this upstream convection will evolve. So while the timing
window for strong/severe storms appears to be shifting to the 
afternoon (noon to 7pm) compared to the previous forecast, there 
is some uncertainty regarding whether this will feature this  
MCS, or less widespread convection developing along its outflow
and/or the stationary front. In either case, significant 
instability appears likely (70% chance of <2000J/kg of CAPE per 
the HREF), with plenty of wind shear to support thunderstorm 
organization (35 to 45kt 0-6km shear) as well. Steep low level 
lapse rates and high LCLs, along with a slight preference for 
linear storm modes, point to damaging wind as the more likely 
hazard. However, large hail will also be possible should we see 
local development along the boundary that maintains some discrete 
structure. Meanwhile, this is most likely to occur in the vicinity
of the instability gradient beginning in northeast Missouri and 
curling southeast roughly along and east of the Mississippi 
river...including potentially the St. Louis metro area.

As for the potential for dangerous heat...this remains very likely 
across central and southeast Missouri, where heat indexes of 105 to 
110 can be expected. Farther north this is less certain due to the 
potential for thunderstorms and cooling outflow (much like 
yesterday), although a delayed timing of storms would lead to a
larger area of dangerous heat.

By this evening, this first wave of thunderstorms is likely to 
slide eastward, but redeveloping convection appears likely across 
northern Missouri and west-central Illinois along the nose of a 
resurgent low level jet. While mid-level winds will be increasing
as a mid-level trough approaches from the west, instability is 
likely to be marginal at best, and this should limit the potential
strength of any overnight activity. Still, coverage may be such 
that a decent portion of northern Missouri and west central 
Illinois observes a modest soaking rainfall overnight tonight. 

By Saturday morning, the previously mentioned upper trough and 
moderately strong belt of mid-level winds (35-45kt) will spread 
into the area. Meanwhile, much of the region will remain squarely 
in the warm sector, leaving the door open for significant 
afternoon instability to develop within a moderately sheared 
environment. However, after potentially multiple rounds of 
convection and possibly overnight convection, it remains to be 
seen how much of this potential instability will be realized 
Saturday afternoon. On the other hand, HREF probabilities of 2000 
J/kg of CAPE will once again be around 60%, with a reasonable 
potential (20 to 40%) for values of 3000 J/kg or more across parts
of the area, particularly eastern MO and southwest IL. As such, 
the potential for strong or severe storms remains very real 
Saturday afternoon, again with the most likely hazard being 
damaging straight line wind, with a more limited potential for 
large hail. While confidence is incrementing upwards regarding 
Saturday's thunderstorm potential, many details remain unresolved.
Later in the evening and overnight Saturday, a cold front will 
sink into the area, potentially providing a focus for additional 
showers and thunderstorms. 

Otherwise, with the potential for multiple rounds of thunderstorms 
in some areas over the next two days, some areas are likely to 
receive a beneficial soaking rain and modest relief from drought 
conditions...particularly across eastern MO and western IL. 

We will also need to keep an eye on the potential for another day of 
near-Advisory level heat across southeast Missouri and southwest 
Illinois, although current projections keep values just short of 
that threshold.

BRC

.LONG TERM...  (Saturday Night through Thursday)
Issued at 405 AM CDT Fri Jun 30 2023

By Sunday morning, much of the area will be well within a post-
frontal regime featuring breezy northwesterly winds. However, the 
slow eastward progressing of the upper trough may maintain the 
potential for some showers and possibly a few thunderstorms into 
Sunday afternoon, with eastern MO and western IL as the favored 
areas for lingering showers. Meanwhile, afternoon temperatures will 
be noticeably milder, both due to the passing cold front and 
lingering cloud cover, and may not climb above seasonal averages in 
many areas. 

Monday and Tuesday though, the upper low will be well east of the 
area, and as a result precipitation chances diminish considerably. 
While a few ensemble members hang on to a smattering of showers on 
either day, a clear drying trend is evident, and any additional 
precipitation early in the week is not likely to be meaningful. 
Temperatures are also likely to begin a gradual warming trend as 
upper level heights slowly build. 

Wednesday through the end of the work week, confidence continues to 
increase that an active pattern will resume, featuring a broad 
trough across the central U.S. and a baroclinic zone sinking into 
the area. This is also accompanied by a gradual increase in moisture 
content, with ensemble mean PWATs increasing to around 1 standard 
deviation above climatological means by Thursday. While day-to-day 
details remain sparse, this provides at least some optimism that 
more needed rainfall is on the way toward the middle and end of next 
week.

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1045 PM CDT Thu Jun 29 2023

The main challenge will be fine tuning the thunderstorm potential
early Friday morning. Isolated thunderstorms may develop along a
warm front that is hinged up near the Missouri/Iowa border as
early as 12z. The bigger concern is the potential for a complex 
of thunderstorms over the central Plains to once again track along
the border and turn southeast through mid-morning/early afternoon.
This could be some strong winds, hail and heavy rain. While the
potential is certainly there, VCTS was maintained with latest
trends once again showing a slight delay in timing. Otherwise, up
until then, patchy fog is possible and may need to be added via
tempo groups late tonight into the early morning for just a couple
of hours. 

Elsewhere, metro terminals could be skimmed by the southeast-
diving thunderstorm complex somewhere in the 17z-21z window. For
now, limited the potential to KSTL/KCPS with KSUS just far enough
west to avoid this first round. However, later in the forecast
period, additional scattered thunderstorms are expected to impact
the area, but coverage, timing and placement will all likely be
impacted by how the initial complex evolves through the morning. 

Outside of any thunderstorms, the main story will be the erratic
wind behavior with VFR condition prevailing under mid/high clouds.

Maples

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     98  76  94  73 /  40  40  70  70 
Quincy          91  71  87  68 /  40  60  80  80 
Columbia        99  74  91  69 /  20  40  60  60 
Jefferson City 102  75  93  69 /  20  30  60  60 
Salem           95  73  94  69 /  40  50  70  70 
Farmington     102  74  97  69 /  30  20  60  60 

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT this evening for Audrain MO-Boone 
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison 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-Warren MO-Washington MO.

IL...Heat Advisory until 7 PM CDT this evening for Bond IL-Calhoun IL-
     Clinton IL-Jersey IL-Madison IL-Marion IL-Monroe IL-
     Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX