Area Forecast Discussion
Issued by NWS St. Louis, MO

Current Version |  Previous Version |  Text Only |  Print | Product List |  Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
FXUS63 KLSX 141121

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
621 AM CDT Sun Jul 14 2024

Issued at 613 AM CDT Sun Jul 14 2024

Showers and a few weak thunderstorms have developed across
northern Missouri along an elongated area of mid-level forcing and
a strong low-level jet. Further north, a decaying MCS is diving
south along the Mississippi River in east-central Iowa and
northwest Illinois. While this convective activity is forecast to
wane over the next few hours, the remnant cloud cover will
continue to advect into northeast Missouri and west-central
Illinois. These clouds may slow warming and restrain our high
temperatures today depending on how long they persist. The
forecast precipitation probabilities were updated to reflect the
ongoing activity.




- Dangerous heat and humidity will grip the Mid-Mississippi
  Valley through Monday, with heat index values up to 105 degrees
  in spots today and widespread 105-110 degrees tomorrow. A Heat
  Advisory remains in effect areawide through 8 PM Monday.

- While the triple-digit heat index values may linger into
  Tuesday, there is growing confidence that more widespread cloud
  cover and rain surrounding a cold front will bring relief

- Multiple rounds of thunderstorms along the front are forecast
  late Tuesday through Thursday, some of which may lead to
  flooding. Our short-lived heat decidedly ends with the front`s
  arrival, dropping highs into the upper 70s to mid-80s from
  Wednesday onward.


.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 306 AM CDT Sun Jul 14 2024

Strong upper-level ridging centered over the Four Corners region
persists, and despite ongoing convective activities eroding its
eastern periphery across the Upper Mississippi Valley, it is
slowly expanding east. As it does, and as a strong longwave trough
in eastern Canada and several shortwave impulses begin to impinge
on the ridge in south-central Canada, an area of weak, broad
baroclinicity will develop in the Plains and strengthen the low-
level pressure gradient. Strengthening low-level southwest winds
will draw a 22-24 degrees Celsius 850mb thermal ridge into
northern Missouri and west-central Illinois.

While under normal circumstances, these temperatures and the overall
pattern would support heat index values of 105 degrees, convective
debris from the several convective complexes will threaten some
degree of cloud cover that will mute temperatures a degree or two
today there. That said, humidity will be abundant (dew points of 70-
75 degrees) which will bring heat index values to near 105 this
afternoon. Further south along the I-44 corridor, removed from the
thermal ridge aloft and cloud cover, deeper mixing may limit
humidity during the afternoon but temperatures may be able to
compensate. All of this is to say today`s heat index values will
straddle 105 degrees for different reasons depending on where one
is. The exceptions are the metropolitan areas (St. Louis and
Columbia/Jefferson City) that will have the added urban heat island
effect. Dangerous heat index values of 105+ degrees will be observed
later this afternoon.

After a largely-calm night under upper-level subsidence with the
ridge expanding further east and a low-level jet far removed to our
north, the most dangerous heat and humidity will be on tap for
Monday. By this point, convective activity will shunt north and east
a bit more, removing any threat of cloud cover or cold pools
limiting temperatures. Winds become a bit more south-southwesterly
and stronger capping aloft with an even-warmer 850mb thermal ridge
will limit afternoon mixing. With highs forecast in the mid/upper
90s (perhaps triple digits in St. Louis and central Missouri), there
is little doubt dangerous heat will be observed areawide on
Monday. Some parts of the region may see heat index values of near
110 degrees, particularly north of I-70 where evapotranspiration
from maturing crops may locally increase humidity. Those with
preexisting medical conditions, young/elderly, or those who work
outdoors will need to take great care on Monday during the day
when heat and humidity will peak.



.LONG TERM...  (Monday Night through Saturday)
Issued at 306 AM CDT Sun Jul 14 2024

The longwave trough continues to deepen slowly late Monday into
Tuesday, forcing a west-east oriented cold front through the Upper
Mississippi Valley. As it approaches, some CAMs spark an MCS along
the boundary at the nose of the low-level Jet and propagate it
south and east. While most depictions of this feature miss us, a
few have the complex scraping northern Missouri and west-central
Illinois. The SPC Day 2 Convective Outlook highlights a very small
sliver of our northern forecast area in a Marginal Risk for
severe weather, which we are not currently messaging given the
rather low threat. The front continues its slow march south
Tuesday, and by most accounts will enter northern Missouri by
dawn. For this area, the heat and humidity will come to an end by
this point, though areas further south have a low (20-30% chance)
potential to see another day of near 105 degree heat index values.
That said, a combination of increased cloud cover, more westerly
surface winds, and increased rain chances along the front all work
against the potential for dangerous heat on Tuesday. With this
threat in serious doubt by then, the Heat Advisory was not
extended in time.

By late Tuesday evening, nearly all scenarios have the front
stalling somewhere around the I-70 corridor as a low-level jet
develops and works against its southward movement. The LLJ will
impinge the boundary and exist under modest jet-level divergence,
all of which will lend forcing and synoptic-scale lift for
additional thunderstorms overnight Tuesday. Concern is growing that
training convection along/in the vicinity of the boundary will lead
to heavy rain and localized flooding. Precipitable water values of
2.00 inches are not very alarming when compared to climatology,
but considering we are approaching the time of year when PWATs
peak, this is still a sufficiently-moist environment.
Deterministic guidance highlights warm cloud depths exceeding
13kft in the vicinity of the boundary and large-scale lift, and
Corfidi vectors (while weak) are parallel to the boundary
suggesting training. One factor working against this potential
hazard is the strength of the LLJ itself: most guidance does not
depict a very robust jet which may weaken low-level forcing
mechanisms. Nevertheless, amidst an otherwise-favorable
thermodynamic environment for heavy rain, the threat of localized
flooding will be monitored overnight Tuesday into Wednesday
morning. There is also a low threat for severe weather late
Tuesday evening into the overnight along the front, though any
stronger convection will be isolated in coverage and threaten
gusty winds. The SPC Day 3 Convective Outlook is not being
messaged at this time owing to uncertain shear strength and
frontal position.

As the LLJ weakens Wednesday morning, the front will be unimpeded to
continue drifting south with reinforcing shortwaves aloft aiding its
progress. While it`s not exactly clear how cool the region will be,
Wednesday will undoubtedly start a welcomed reprieve from the heat
for everyone in the forecast area. Highs Wednesday fall as far as 10
to 15 degrees from Tuesday`s values, though low-level humidity is
forecast to linger behind the effective front with high surface
pressure not fully-established in the region. Rain and thunderstorms
in the vicinity of the front will persist into Wednesday, though it
doesn`t look to rain all day. By Wednesday night into Thursday, the
front appears far enough south to preclude a heavy rain threat for
our forecast area. If the front slows, however, we could deal with a
similar situation as the night prior. Nearly all available guidance
clears the front by Thursday morning and establishes a strong
(around the 90th climatological percentile) surface high in the
region. As a result of this Canadian airmass, highs will approach
the 10th climatological percentile and stay that way into the
weekend. The high will also end our rain chances for the foreseeable



.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 613 AM CDT Sun Jul 14 2024

Two areas of convection exist in the region that may impact the
central Missouri terminals and KUIN through 16z, one in north-
central Missouri and another diving through eastern Iowa. Both of
these features are weakening and are forecast to continue doing
so, but did warrant addition of VCSH at KUIN at least. Most of the
activity approaching the terminals is devoid of thunder, but will
be monitored for deeper convective elements.

After activity dissipates later this morning, dry and VFR
conditions are forecast. Winds strengthen out of the southwest
today before weakening after sunset.



MO...Heat Advisory from 1 PM this afternoon to 8 PM CDT Monday for
     Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Knox MO-Lewis
     MO-Lincoln MO-Madison MO-Marion 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-Shelby MO-Warren MO-Washington

IL...Heat Advisory from 1 PM this afternoon to 8 PM CDT Monday for
     Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe
     IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-
     Washington IL.