Area Forecast Discussion
Issued by NWS St. Louis, MO

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
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

000
FXUS63 KLSX 250818
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
318 AM CDT Wed May 25 2022

Key Messages:

1) There is a potential for a few severe thunderstorms this
afternoon, including a conditional tornado threat, mainly along
and east of the Mississippi River.

2) More thunderstorms are expected tomorrow afternoon, and while a
few may approach severe levels, an expected lack of organized
severe convection has precluded us from messaging a D2 severe
threat.

3) Confidence remains high that above normal temperatures will
build into the region beginning this weekend and lasting into next
week.

.SHORT TERM... (Through Late This Evening) Issued at 303 AM CDT
Wed May 25 2022

The upper trough that`s been centered over the western US for the
last several days is continuing to amplify and slide eastward. The
trough has now become cutoff over western KS and is reflected at the
surface by a low centered over eastern KS. This places the Mid-
Mississippi Valley in deep southerly flow. The resulting warm and
moist advection, coupled with intermittent support for ascent
from passing embedded shortwaves, has resulted in periods of
showers overnight.

The above cyclone will continue to drift eastward today, keeping the
return flow going until a dry slot punches into the region in the
mid levels this afternoon. Ahead of this feature, continued warm and
moist advection will keep precipitation chances in the forecast this
morning and into the early afternoon, though without a focused
forcing mechanism, it`s difficult to say exactly when or where
these showers will develop. Speaking of forcing mechanisms, there
is an increasing signal in the large-scale guidance for an
embedded shortwave to push into the region this afternoon, likely
in tandem with the dry slot. I expect this to result in a more
focused round of afternoon showers and embedded storms, spreading
northeast across the region. This solution is reflected in much of
the guidance, though uncertainty remains with respect to the
details of the convection.

Potential storm strength and storm hazards are a primary source
of uncertainty, and hinges largely on how much the atmosphere is
able to destabilize this afternoon. The persistent return flow
will be a double-edged sword, pumping mid-to-upper 60s dewpoints
into the region but also keeping the skies mostly cloudy into the
afternoon. Despite the expected clouds, the 00z HREF continues to
push a plume of 1,000-1,500 J/kg of surface based instability into
areas along and east of the Mississippi this afternoon. Should
that instability be realized, low level wind shear, bolstered by
slightly backed surface winds, would be sufficient to support
short-lived tornadoes (roughly 25 kts of 0-1 km shear and up to 30
kts of 0-3 km shear). Deep layer shear looks to be a bit less
impressive (about 35 kts 0-6 km shear), and given poor lapse
rates, there appears to be only a marginal threat for damaging
winds or large hail. Given this marginal wind/hail threat and a
conditional tornado threat, we`ll continue to message the SPC D1
marginal risk, with an emphasis on areas along and east of the
Mississippi.

BSH

.LONG TERM... (Late Tonight through Tuesday) Issued at 303 AM CDT
Wed May 25 2022

The cyclone will continue to drift east across the region through
Thursday night, finally nosing up the Ohio River Valley on Friday.
Persistent cyclonic flow aloft coupled with ample low level moisture
will result in additional intermittent showers from tonight into
Friday. There should be plenty of dry time during this period,
though unfortunately it`s difficult to determine exactly when
these additional showers will occur. It does appear that cooling
temperatures aloft will steepen lapse rates enough to support
renewed thunderstorm chances, mainly tomorrow afternoon. With
instability limited to less than 1,000 J/kg, poor lapse rates, and
deep layer shear topping out around 30kts, I doubt we`ll see much
organized severe convection. Therefore we`ve chosen to forgo
messaging the D2 severe threat.

Continued cloud coverage and increasing cold air advection will
keep Thursday`s temperatures on the cooler side, making for a bit
of a dreary spring day. The moisture associated with the low will
finally depart the region on Friday, bringing an end to the
clouds and precipitation chances by the afternoon. Nevertheless,
winds will remain out of the north, keeping temperatures below
normal.

The weather in the later half of the forecast period will be
dominated by increasingly strong and broad ridging over the
eastern CONUS, as well as troughing over the western CONUS by
early next week. This will cause low level flow to turn southerly
on Saturday before intensifying and becoming more southwesterly
from Sunday into Tuesday. The deep south/southwesterly flow will
continue to advect warm temperatures into the region, with
NAEFS/ECMWF mean 850mb temperatures approaching the 90th
climatological percentile by late Saturday, and topping the 99th
percentile by Monday. Anomalously warm temperatures at 850mb,
coupled with rising heights/mostly sunny skies will support above
normal temperatures at the surface. The latest NBM forecast pushes
highs into the mid/upper 80s by Sunday and to near 90 on Monday
and Tuesday, with narrow spread in it`s members. The narrow spread
and strong signal for anomalous warmth at 850mb gives me high
confidence that we`ll achieve these above normal temperatures as
we move into next week.

BSH

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1033 PM CDT Tue May 24 2022

Scattered showers continue to track south to north through
sections of central Missouri with thunderstorms being relatively
isolated for the time being. The eastern edge is making ever-so-
slow progress toward the east with surface observation showing
MVFR cloud bases mixing in with IFR bases just after 03z. Metro
terminals have been spared so far, but expect further eastward
progress over the next 2-3 hours with shra becoming likely.

Activity over Arkansas will continue to lift northward with a
little better potential for embedded thunderstorms as a warm
front lift north into the region. MVFR and IFR ceilings will
become commonplace until the front clears to the north, at which
point ceilings will gradually lift to MVFR/VFR levels during the
afternoon. While thunderstorm potential exists, there remains
quite a bit of uncertainty in overall coverage with latest
guidance hanging onto to IFR/MVFR ceilings slightly longer.
Nonetheless, the best potential for convection will be from 18z
through 01z or so. Any direct impact could bring heavy downpours,
visibility restrictions, gusty winds and the potential for hail,
should the full potential be realized.

SPECIFICS FOR KSTL:

Scattered showers and isolated thunderstorms remain west of the
terminal just prior to the 04z hour. Lightning has trended
downward since the last update with much of this type of activity
confined to Arkansas. There has been a bit of a gap between the
initial wave and what is expected to lift north with the warm
front late tonight into early Wednesday. This will introduce the
potential for scattered thunderstorms and MVFR/IFR ceilings.
Visibility restriction within showers have largely been 4-6SM with
brief 2SM observation in heavier rainfall.

Expect at least a few hours of IFR Wednesday morning until the
front pushes north. Behind the boundary, ceiling are expected to
gradually lift to MVFR and eventually VFR levels. While there is
an anticipated lull in the precipitation, isolate showers will
remain possible with better potential during the mid to late
after, if skies break and allows the atmosphere to destabilize.
Due to trend in holding onto cloud cover a little longer, this
remains in question. Thunderstorm potential is best after 19z up
through at least 00z. Shra was favored over vcts thereafter given
the limited confidence.

Maples

&&

.PRELIMINARY POINT TEMPS/POPS...
Saint Louis     79  62  71  56 /  80  70  90  60
Quincy          76  60  68  55 /  80  50  60  50
Columbia        75  57  66  52 /  90  50  70  40
Jefferson City  75  57  66  53 /  90  50  80  40
Salem           80  65  74  56 /  60  70  80  70
Farmington      76  60  70  53 /  90  70  80  40

&&

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

$$

WFO LSX


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.