Area Forecast Discussion
000
FXUS63 KLSX 232340
AFDLSX
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
640 PM CDT Thu Mar 23 2023
.SHORT TERM... (Through Late Friday Night)
Issued at 304 PM CDT Thu Mar 23 2023
The primary concerns stem from the frontal boundary that is draped
across the region this afternoon. The boundary will become
increasingly active this evening into tonight with a two-pronged
threat. The first will be the chance for isolated severe weather,
while the second and likely greater threat will be flooding.
Surface observations show the front currently extending from just
south of Vandalia, IL to Rolla, MO. This is marked by 15-20
degree drop in temperatures over approximately 20-25 miles
distance. South of the front, temperatures have overachieved
dramatically in spite of the mostly cloudy skies. Upper 70s extend
across southeast MO into southwest IL with an isolated 80 degree
reading not out of the question. The warm sector includes
dewpoints in the low-60s, which drop into the 40s north of the
front.
There is initially some concern for marginally severe thunderstorms
over far southern sections of the forecast area. The greatest threat
is likely over southwest MO with the threat trending low further
east along the front. Grand ensemble means show 500-1000 J/kg
residing along the front with individual deterministic guidance
reaching upward of 1700 J/kg via the NAM. While this suggest
sufficient enough instability to result in thunderstorm
development, along with mid-level lapse rates of 7-8C, the better
lift initially remains west as the surface low lifts out of the
Plains. Better support does not look to arrive until later in the
evening, when surface instability begins to wane. I think the main
threat is once again hail, but concerns are fairly limited to
isolated development in the onset of the activity.
Between 22z and 00z this evening all CAMs indicate the front
becoming much more active as the surface low begins to ride the
front west to east. Forecast guidance has been fairly consistent
with very little north/south wiggle in the placement and magnitude
of the activity. This first wave tracks along the I-44 corridor and
roughly along and south of I-70 in Illinois through about 12z
Friday. There is high confidence that amounts of 1-2 inches will
fall within this first wave with locally higher amounts possible.
While rainfall is not expected to end entirely, there will be a
brief lull Friday morning into the early afternoon. The parent low
lifts northeast ahead of the upper trough by Friday afternoon,
resulting in another impressive wave of rainfall Friday afternoon
through Friday night with another 1-2 inches. The front shifts
slightly south with heavier rain allocated in the vicinity of the
boundary. However, the broader lift that track through with the
system will provide light to moderate rain further north than the
first round. There is likely to be a narrow area across southeast MO
and southwest IL where overlap of heavy rain may occur between the
two rounds. Localized amounts could exceed 5 inches in these
locations, most likely bordering the southern fringes of the CWA.
The current flood watch will continue with no need for adjustment at
this time. River flooding is likely within the watch area.
Fortunately, rainfall occurs over a roughly 36-48 hour period.
Maples
.LONG TERM... (Saturday through Next Thursday)
Issued at 304 PM CDT Thu Mar 23 2023
A negatively tilted upper trough will dig southeast from the
northern Plains into the mid-Mississippi Valley Saturday morning.
Surface low pressure deepens ahead of the upper trough as it tracks
into the Great Lakes. Deterministic guidance maintains saturation in
the 850-700MB layer before lifting northeast around 18z Saturday.
Light rain is likely to linger into the morning north of I-70,
especially over sections of northeast Missouri and west-central
Illinois. Further south, a dry slot looks to wrap around the
strengthening low, ending rainfall a bit earlier.
The system finally pulls northeast late Saturday with rainfall
ending across the entire forecast area. At that time, a long wave
upper trough will begin to influence, if not encompass, a large
majority of the Lower 48 states over the later half of the weekend
with surface ridging extending from the northern Plains into the
Ohio Valley. Though the surface high is not particularly strong, it
should be sufficient enough to keep conditions dry through the day
Sunday. Also, while the pattern might normally relate to colder than
normal temperatures, zonal flow is largely maintained over the
region with limited CAA behind the system. Temperatures will only be
around 5 degree below climatological norms with highs in the 50s
over much of the area and a few location in southeast MO and
southwest IL reaching the low-60s.
Ensemble spread (NBM) begins to increase slightly Monday into
Tuesday as long range deterministic guidance varies on the handling
of a couple of weak shortwaves embedded in the mean westerly upper
flow. Though the NBM introduces slight chance (15 percent) PoPs
Saturday night over central MO and higher (40-50 percent) chances
across the area Sunday night into early Monday, confidence is fairly
low given the lack of organization and the track of the system being
north of the CWA. Any precipitation will rely on the available
moisture running along a weak warm air advection wing extending
southeast of the low.
By midweek, ensemble spread increases rapidly with zonal flow
favored over the central sections of the U.S. The dominant feature
remains well west in the form of a broad upper trough with a close
low parked over CA. Upper ridging attempts to work west to east with
surface ridging building into the Midwest. However, spread between
individual guidance lends too many questions to have much confidence
beyond Tuesday.
Maples
&&
.AVIATION... (For the 00z TAFs through 00z Friday Evening)
Issued at 640 PM CDT Thu Mar 23 2023
IFR conditions continue across the area, although ceilings have
improved to MVFR at Quincy. A cold front has sagged south into
southern Missouri, and we are expecting an area of showers and
thunderstorms along and behind the front this evening with periods
of LIFR possible. Best chance of thunder at a TAF location is in
the St Louis metro between now and 6Z tonight. We should see a
lull in the showers tomorrow morning before another more
widespread round begins tomorrow afternoon. It`s likely that
ceilings stay IFR through the day tomorrow.
Kimble
&&
.HYDROLOGY...
Issued at 328 AM CDT Thu Mar 23 2023
Specifics on the synoptic set up of the anticipated heavy rain can
be found above in the "Short Term" section. Two rounds of rainfall
are expected, with the first and most substantial occurring this
evening into the overnight. The heaviest rainfall tonight will
occur generally along the I-44 corridor in Missouri to the I-70
corridor in Illinois affecting the Meramec and Kaskasia River
basins. Confidence is high that a swath of around 2 inches of rain
will fall with this, though uncertainty still remains in the
exact location of the heaviest rainfall and amounts which will be
impacted by the location of individual thunderstorms. The next
round of rainfall arrives Friday afternoon, with the heaviest rain
from this occurring further south, affecting the Black River as
well as the tributaries of the Meramec. The increasing confidence
in the location of the heaviest rainfall with respect to the
basins has prompted the issuance of several river flood warnings
along the Black, Kaskaskia, Meramec, Bourbeuse, and Big Rivers
where points are expected to crest above minor and moderate flood
stage with crests expected over the weekend and into the
beginning of the work week. Other locations could also rise to
flood stage, although confidence is lower outside of these basins.
Kimble
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through late Friday night for Crawford MO-Franklin
MO-Gasconade MO-Iron MO-Jefferson MO-Madison MO-Osage MO-
Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis
City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-
Washington MO.
IL...Flood Watch through late Friday night for Bond IL-Clinton IL-
Fayette IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint
Clair IL-Washington IL.
&&
$$
WFO LSX
NWS LSX Office Area Forecast Discussion
