000
FXUS63 KLSX 131938
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
238 PM CDT Sat Apr 13 2024

.KEY MESSAGES...

- Unseasonably warm temperatures (widespread 80s) will continue
  into early next week. Highs on Sunday and Monday may threaten
  daily records.

- Strong to severe thunderstorms are possible areawide Tuesday, mainly
  during the late afternoon/early evening hours. However, there
  remains uncertainty in details such as potential hazards, more
  favored areas for severe weather, and exact timing.

- Ahead of the possible severe weather, strong and gusty southerly
  winds are forecast. Sustained winds of 20-30 mph are forecast
  with gusts of 35-45 mph.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 223 PM CDT Sat Apr 13 2024


Unseasonably mild conditions are forecast overnight tonight with
increasing southerly winds. Lows are only expected to drop back
into the upper 50s to low 60s. In some locations this will be
about 20 degrees warmer than this morning!

A weak cold front will slowly approach from the northwest on Sunday,
reaching into parts of northeast Missouri and west-central Illinois
by early afternoon. Surface winds ahead of this feature will veer
more to the west/southwest. These winds are very favorable for
enhancing the warmup in parts of central, east central, and
southeast Missouri as these winds downslope off of the Ozark
Plateau. In addition, there should be plentiful sunshine and the
timing of the 850-hPa thermal ridge is favorable. Speaking of the
850-hPa thermal ridge, values of +14 to +18C would be above the 95th
percentile of climatology. This is where widespread mid to upper 80s
are forecast, and wouldn`t be shocked to see a location or two hit
90 degrees especially in/around metropolitan St. Louis. No chances
of showers or thunderstorms are anticipated with this weak front due
to rising mid/upper level heights (subsidence aloft), weak surface
convergence, and a strong cap in place.

Most areas likely will be slightly cooler for overnight lows Sunday
night due to much lighter winds and a clear sky. This cooling will
be most pronounced north of the frontal boundary in parts of
northeast Missouri and west-central Illinois as well as in valleys
due to the favorable conditions for radiational cooling.


Gosselin

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 223 PM CDT Sat Apr 13 2024

(Monday - Monday Night)

The stalled front on Monday is expected to lift back gradually north
as a warm front. Some guidance is suggesting some convective
initiation along this feature, but uncertainty is high. While
capping is much weaker Monday afternoon compared to Sunday
afternoon, the mid/upper level ridge axis approaches the Mississippi
Valley. It is not too common to see convective initiation directly
beneath a mid/upper level ridge which is building into the region.
If any thunderstorms do develop, model guidance does have moderate
instability (SBCAPE values > 1500 J/kg). Deep-layer shear is (not
surprisingly) pretty weak though given the synoptic setup, but 25
knots or so may be enough for some loosely organized
multicellular clusters. Marginally severe hail and damaging winds
would be the threats in this scenario though the most likely
scenario is for little/no storms to even form.

Precipitation chances are less clear on Monday night. Most
deterministic guidance stalls out the front near the northern CWA
border with broad low-level moisture convergence overhead. There
area also hints of a weak midlevel shortwave trough moving out of
the south-central Plains, but some pieces of guidance do not have
this feature at all. Given the uncertainty, focused higher PoPs (40-
60+%) near the frontal zone and where mid/upper level ascent is
most likely to be highest (areas further west...i.e, northeast
Missouri).

High temperatures likely will be slightly cooler across the area due
to a combination of an increase in cloud cover and surface winds
having more of an easterly component. However, the warm start to the
day (50s to low 60s) still will lead to very warm conditions
areawide by afternoon. High temperatures are forecast to range from
the low to mid 80s or 15+ degrees above normal for the date.


(Tuesday - Tuesday Night)

Attention for Tuesday/Tuesday night remains on the potential for
strong to severe thunderstorms, most likely during the late
afternoon/early evening hours. However, plenty of uncertainty
remains with specifics, in large part due to the possibility of
antecedent convection at least limiting instability somewhat later
in the period. While the signal for morning convection fades pretty
quickly, there is a stronger signal for storms to develop in/near
the Arklatex area Tuesday morning. This is in an area of strong
low-level moisture convergence and strongly diffluent flow aloft
south/southeast of the closed low. This activity should spread
northeast, likely into parts of east central and southeast
Missouri as well as southwest Illinois by early-mid afternoon.
Some of these storms may be strong to severe given the strong
deep-layer shear (50- 60 knots), but thicker cloud cover
downstream of this convection and a temperature inversion aloft
help to limit instability. Probabilities for >500 J/kg of CAPE at
1800/2100 UTC in these areas are generally below 10 percent from
the GEFS.

The better threat for severe weather I think will be behind this
activity, beginning further northwest in parts of central and
northeast Missouri. The focus for initiation likely will be a pseudo
dryline/surface trough axis. This feature will move east/southeast,
but slow overnight in parts of southeast Missouri and southwest
Illinois. The convective mode at least initially likely will be
supercells given the very strong deep-layer shear, with the vector
oriented about 45 degrees off of the initiating boundary. Hodographs
are mainly straight, also suggesting the potential for splitting
supercells. There is uncertainty as to how quickly this mode may
turn into more line/bowing segments. There remains a strong
orthogonal component to the deep-layer shear into the early evening
hours, but often when the synoptic forcing is this strong
(particularly in northern sections of the area), the transition to a
more linear mode is pretty quick.

The amount of instability remains a question mark along/just ahead of
the dryline/surface trough axis, but there is likely to be at least
enough to produce some instances of severe weather. Probabilities
for at least 500 J/kg of SBCAPE on the GEFS/EPS are greater than
60%. Given the strong mid/upper level forcing for ascent and 60-70+
knots of deep-layer shear, it will not take much in the way of
instability. All hazards are on the table for late Tuesday/Tuesday
night, with large hail more favored earlier in the event (with
supercells) and damaging winds later in line/bowing segments.
Tornadoes will also be possible, with a sharp increase in winds
within the lowest 1 km and low LCL values.

Outside of thunderstorms, we are also expecting very strong
southerly sustained winds/gusts due to the strong pressure gradient
in place across the region. Sustained wind speeds of 20-30 mph
appear likely with gusts of 35-45 mph. Soundings show 40-45 knot
winds at the top of the mixed layer, with unidirectional winds which
suggests momentum transport to the surface. Southerly winds aren`t
climatologically a favored direction for wind advisories, but the
signal is there and the data supports it. The EPS Extreme Forecast
Index (EFI) has 0.7-0.8+ values Tuesday, which suggests a high
confidence in anomalous wind gusts. The shift of tails (SOT) is near
zero however, which indicates that the event likely will not be too
extreme.


(Wednesday - Next Saturday)

Dry and cooler weather is likely on Wednesday after the passage of
the trailing cold front.

Forecast uncertainty increases heading into late week. Models agree
that anomalous mid/upper level troughing will dig toward the US-
Canadian border, but differ on the strength and location. The
general theme should be for a gradual step down in temperatures
heading into next weekend, but exactly how cold is a question mark.
The EPS is stronger and further south with this feature, and
therefore, colder. Temperature anomalies at 850 hPa on the EPS are
in the -8 to -10C range while the GEFS is closer to -5 to -8C. We
may have frost/freeze concerns though, especially if the colder EPS
verifies. Even last night`s CIPS analog guidance from Saint Louis
University (which is based on the GEFS) showed a chance
(>40%) for a freeze in northern sections of the forecast area
next weekend.

Precipitation wise, the best chance for rain appears to be
Thursday/Thursday night as strong low/mid level frontogenesis moves
southeastward across the CWA. After that, drier and drier air should
advect into the region so precipitation chances look minimal
thereafter.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1209 PM CDT Sat Apr 13 2024

Dry, VFR conditions are forecast through the period. Southerly
winds are forecast to increase this afternoon, with gusts around
20 knots northwest of St. Louis. These gusts should diminish
around sunset, but pick back up overnight tonight as the pressure
gradient tightens. Winds will also veer more to the southwest and
eventually west/southwest by Sunday morning.

Gosselin

&&

.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