771
FXUS63 KLSX 232316
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
616 PM CDT Sun Jun 23 2024

.KEY MESSAGES...

- Heat, and more noticeably, humidity will begin to build back
  into the region on Monday. Heat index values around 105F are
  forecast in parts of central Missouri Monday afternoon, with the
  dangerous heat likely expanding eastward on Tuesday.

- A cold front is expected to move through late Tuesday night.
  This front will provide a good chance for widespread showers and
  thunderstorms as well as a break from the heat (highs closer to normal).
  Some of the storms may be strong to severe Tuesday night,
  particularly in parts of northeast Missouri and west-central
  Illinois.

- Heat and humidity could return heading into next weekend, but at
  this early point in time, the magnitude does not look as strong
  as early this week. In addition, there is a fair amount of
  uncertainty as to how hot daytime temperatures will be due to
  multiple chances for showers and thunderstorms.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 139 PM CDT Sun Jun 23 2024

A clear, calm night is expected across the area as a weak ridge of
surface high pressure slowly slides eastward across the Mississippi
Valley. These conditions (combined with persistent dry low-level
advection during the day today) are very favorable for radiational
cooling. Have therefore stayed close to the 25th percentile of
available model guidance for low temperatures tonight, ranging from
the mid to upper 60s. The exception is in/around the urban heat
island of metropolitan St. Louis, which should only drop back into
the low 70s.

The most intense heat so far this summer is on track to advect into
portions of the area on Monday from the southwest. Increasing low-
level warm air advection along with a clear sky and building
mid/upper level heights all point to a jump in surface temperatures.
In addition, humidity will be on the increase along/behind a subtle
surface trough axis. Dewpoints are expected to pool near this
boundary as it heads eastward through the day. While there is more
confidence in high temperatures tomorrow (90s - with mid to
potentially upper 90s in parts of central Missouri), the dewpoint
forecast is less certain. A few factors that give me pause in
widespread low 70s dewpoints include surface winds increasing out of
the south (downsloping off of the Ozark Plateau), ongoing dryness
this month (dry soils), and MOS dewpoints (which tend to do verify
pretty well) being a bit cooler. Because of these concerns, did
lower dewpoints ever so slightly across parts of central Missouri.
This did not change peak heat index values much though given similar
high temperatures. Peak heat index values of around 105F are
forecast in parts of central Missouri where a heat advisory remains
in effect.

Warmer low temperatures are forecast Monday night due to increasing
southerly flow. Lows in the 70s are expected areawide, generally
about 7-10 degrees above normal for the date.


Gosselin

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 139 PM CDT Sun Jun 23 2024

(Tuesday - Tuesday Night)

Heat and humidity remains the focus on Tuesday, but unlike Monday,
there is an additional complicating factor. There remains a signal,
albeit weak, for a cluster of thunderstorms Tuesday morning exiting
the mid-Missouri Valley. The EPS remains the most bullish with
keeping convection going into at least northwestern sections of the
area by late Tuesday morning, with around 30% of its members showing
a wetting rain (>=0.10") ending at 1800 UTC. However, probabilities
from the GEFS (<10%) and the CMCE (10-20%) are quite a bit lower.
Any cluster(s) of thunderstorms though could provide enough debris
clouds to hamper solar insolation a bit and cool down temperatures
so we will have to keep an eye on this possibility. In the absence
of any direct or indirect effects however of this convection, the
heat (mid to upper 90s) should spread north and east compared to
Monday. Somewhat similar to Monday, there is more confidence in high
temperatures compared to dewpoints. There is concern that dewpoints
forecast by most model guidance is too high Tuesday afternoon,
particularly in parts of central, east central, and southeast
Missouri. With a wind direction out of the southwest, these winds
will be able to downslope off of the Ozark Plateau lowering
dewpoints. The ongoing dryness of June and lower values on the
MET/MAV MOS also cast doubt on widespread low 70s dewpoints. That
being said, it will still be hot and humid. Even lowering dewpoints
1-2F still yields peak heat index values of ~105-107F over most of
the area (exceptions being northeast Missouri and west central and
south central Illinois). A heat advisory likely will be needed, but
given this is still 48 hours away and there is at least a low
probability of convection yielding lower high temperatures, elected
to hold off on any heat headlines for Tuesday.

Attention for Tuesday night turns to the increasing chances of
showers and thunderstorms along/ahead of a cold front. The timing of
this front has been consistent, passing through late Tuesday night
into Wednesday morning. This front should have more to work with
compared to the one which passed through last night. Namely,
stronger surface convergence along the boundary, better mid/upper
level support with the shortwave moving through, and increasing low-
level moisture convergence Tuesday night. PoPs have been raised
about an additional 10% compared to the previous forecast, with
chances of showers and thunderstorms centered on late Tuesday night
in the 60-80% range. Probabilities on the LREF for a more
substantial rain (>=0.25") are also trending upwards. Chances are
now 40-70% between Tuesday night and Wednesday morning of at least a
quarter inch and 25-50% for 0.50" of rain. Of course, with
convection, there likely will swaths with substantially more/less
rainfall, but the good news is that the strength of the forcing
mechanisms suggest that convection should be widespread. There will
also be a threat for organized strong/severe convection. While deep-
layer shear is likely to be weak (~25 knots), better forcing for
ascent and high instability/CAPE (LREF probabilities of 3000+ J/kg
60-80%) suggest that multicellular clusters should be able to
amalgamate cold pools that would be prone to produce strong-severe
wind damage. This threat is most likely in northern sections of the
forecast area through the early overnight hours. Some type of
weakening trend is likely late Tuesday night further south as
instability gradually wanes.


(Wednesday - Thursday Night)

All signs continue to point toward a break in the heat (and
eventually humidity) for the middle of the week behind the cold
front. Cooler temperatures are forecast to arrive first on Wednesday
(mid to upper 80s), though cooler readings are possible if the
front is faster or convection maintains itself through the morning
hours. The high humidity levels however are likely to maintain
themselves across the area, with a very gradual drop of dewpoints
behind the cold front (similar to last night/this morning`s
fropa).

Thursday`s highs may be slightly cooler, but more importantly, be
accompanied by much more comfortable humidity levels. Ensemble
guidance has trended noticeably cooler with the air mass coming in
behind the cold front, with 850-hPa temperature anomalies some
1-2+C cooler than yesterday`s guidance. This has led to a decrease
in the high temperature forecast on both Wednesday and Thursday,
with additional cooling possible if these trends continue.


(Friday - Next Sunday)

Forecast confidence decreases late week into the following weekend,
namely due to uncertainty with the position and strength of a
mid/upper level ridge centered across the southern CONUS. A slight
majority of guidance is weak enough and just far enough south to put
our area on the northern periphery/"ring of fire" region where
multiple shortwaves moving through could initiate one or multiple
rounds of convection. While low temperatures will assuredly be above
to well-above normal, there is a lot of uncertainty with high
temperatures during this time period. Interquartile ranges on the NBM
are generally in the 6-10F range, but the cold tails/10th
percentiles are several degrees cooler. For now, leaned toward the
more conservative scenario (temperatures not getting impacted by
thunderstorms) with highs getting well back into the 90s Friday
and Saturday, but those temperatures could come down several
degrees if confidence in the overall pattern and the potential for
thunderstorms increases.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 612 PM CDT Sun Jun 23 2024

High pressure will support mostly clear skies and light/variable
winds overnight. As the surface high moves east Monday, surface
flow will turn out of the southeast and increase from mid-morning
onward. VFR is expected throughout the period.

Maples

&&

.CLIMATE...
Issued at 247 AM CDT Sun Jun 23 2024

With hot temperatures forecast, here are some of the daily high
temperature records:

            MON JUN 24     TUE JUN 25
ST LOUIS    102 in 1988    102 in 1954
COLUMBIA    103 in 1988    102 in 1988
QUINCY       99 in 1988    102 in 1931


&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon to 8 PM CDT Monday for Boone MO-Cole MO-
     Moniteau MO.

IL...None.
&&

$$

WFO LSX