Area Forecast Discussion
Issued by NWS St. Louis, MO

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244
FXUS63 KLSX 190220
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
920 PM CDT Thu May 18 2017

.UPDATE...
Issued at 918 PM CDT Thu May 18 2017

We continue to track convective evolution out to the west this
evening. A broken line of thunderstorms exists over eastern
KS/eastern OK from earlier convection across the central and
southern Plains. A few supercells also have developed ahead of
this broken line, mainly north of a surface warm front near the
Kansas City metro area.

Convective evolution remains a bit uncertain overnight, as many of
the CAMs had suggested an organized cold pool would have developed
by now. While there have certainly been some bowing structures at
times, it appears convection remains fairly disorganized at this
time. Nonetheless, an increase in a nocturnal low-level jet is
likely underway and should lead to continued thunderstorm
development/sustenance over portions of eastern KS and into
western MO. While forecast soundings suggest increasing nocturnal,
surfaced-based CIN with time over central and eastern MO, think
if storms are able to develop a cold pool we may still see some
strong/locally damaging wind gusts along with some locally severe
hail as the convection moves into central/eastern MO. Have updated
the forecast to reflect these latest trends.

KD

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 333 PM CDT Thu May 18 2017

Main concerns through tomorrow will be the potential for severe
thunderstorms.

Cold front has become quasi-stationary from near Kansas City to
Quincy to Chicago.  To the south, there have been a few showers and
thunderstorms that have developed across southeast Missouri.  There
is still potential for thunderstorms to develop farther north near I-
70 late this afternoon into the early evening hours according to the
latest runs of the CAMS.  This falls in line with the RAP which
shows some weak surface moisture convergence south of the front in
the 2000-3000 J/kg MLCAPEs.  There may be a few storms continue to
develop along the front into the early evening hours.

Focus for storms will increase later this evening and overnight
farther west where severe thunderstorms that are expected to develop
over the Central Plains will grow upscale into an MCS per the latest
runs of the HRRR and move east across the CWA between 08-14Z.  There
will be the possibility for a few strong to severe thunderstorms
during this time with some large hail and damaging winds.  This
complex will lift to the northeast by mid morning allowing for dry
time from mid morning until early afternoon.  If the atmosphere can
recover from the morning convection, then a few severe thunderstorms
will be possible in the afternoon given MLCAPES between 1500-2000
J/kg and deep layer shear values 40+kts along and south of the warm
front.  Main focus for severe storms in the afternoon would be along
the warm front.

Britt

.LONG TERM...  (Friday Night through Next Thursday)
Issued at 333 PM CDT Thu May 18 2017

(Friday Night - Saturday Night)

Best chance of convection early in the period will be along/north of
the slowly retreating warm front. Some disagreement on how
widespread this activity will be with CAMs suggesting isolated-
scattered activity while global models seem a bit more bullish.
Regardless, favorable environment will exist for strong to severe
thunderstorms capable of producing large hail and damaging winds,
with a tornado threat for any storms that can travel along the warm
front.

Another round of storms then looks to move in from western Missouri
overnight Friday night. Straight-line winds look like the primary
threat with convection expected to be more linear by the time it
reaches our CWA. Could Some signs the quasi-linear convective system
will weaken as it heads toward the Mississippi River toward daybreak
as low-level moisture convergence and instability slowly abates.
Will have to also monitor the potential for flash flooding with this
round of activity. Thankfully, the ground has gotten a good week to
dry out so flash flood guidance is fairly high. Concern would be for
any areas that get hit fairly hard tonight through Friday afternoon
and then get hit again Friday night. Will continue to monitor this
potential, but for now will hold off on a flash flood watch and see
how the next ~24 hours evolves.

Uncertainty increases on Saturday with respect to convective trends.
There will likely be some ongoing convection on Saturday morning as
alluded to above. How the morning convection evolves will be
important regarding afternoon destabilization and severe potential.
Does look like more of a marginal setup however with quite a bit
uncertainty with the amount of destabilization. Higher chance for
strong/severe storms at the moment look east of the Mississippi
River where higher instability will likely reside. Do expect at
least another round of showers and storms regardless of severity to
form along/ahead of the cold front which will sweep through the bi-
state area.


(Sunday - Next Thursday)

Well-below normal temperatures continue to increase in likelihood in
the extended with a deep upper-level trough carving out across the
eastern two-thirds of the CONUS. Highs generally will range from the
mid 60s to low 70s each day with lows as cool as the mid to upper
40s. Best chance of rain, with a few thunderstorms possible,
continues to look like the Monday night through Tuesday timeframe
coincident with a closed midlevel circulation moving
southeastward into the mid-upper Mississippi Valley and eventually
the Ohio Valley.


Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 636 PM CDT Thu May 18 2017

Main TAF concern will be convective trends overnight into Friday.
While a few storms continue to linger over eastern MO early this
evening, think we will see a general lull in thunderstorms from
this evening into the early overnight hours. However, a line of
showers/storms should approach COU/UIN after 07Z, spreading into
the St. Louis metro sites after 08-09Z. These storms will bring
lowered vsbys in heavy rain along with locally gusty winds.
Showers and a few thunderstorms will likely persist into the day
on Friday, but confidence is fairly low on timing/coverage.
Outside of any showers/storms on Friday, UIN will reside on the
cool side of a warm front which will allow for some MVFR cigs to
work in from the north.

SPECIFICS FOR KSTL:
Main TAF concern will be convection early Friday morning. Think
convection may begin to develop near the terminal as early as
7-8Z, but think a more concentrated line of showers/storms is
likely to move through between 10-14Z Friday morning. Any storm
will likely bring vsbys into the MVFR/IFR range. After the round
of convection Friday morning, confidence decreases in convective
coverage for Friday afternoon/evening. For now, will keep VCTS out
of the forecast for the afternoon until trends are better
ascertained.

KD

&&

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

$$

WFO LSX



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