Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 062310

610 PM CDT Wed May 6 2015

.SHORT TERM: (Through Friday)
Issued at 355 PM CDT Wed May 6 2015

A slow-moving shortwave will lift from NEB into MN tonight and
tomorrow. Most of the forcing with this system will remain focused
to the west and to the north of the LSX CWA, however the eastern
edge of an area of large-scale ascent will brush the western part
of the LSX CWA towards morning. This may be sufficient for
isolated SHRA/TSRA after 09z. The tail end of the LLJ may also
support isolated to scattered convection across the western CWA,
especially after 12z when it starts to veer and becomes more
focused over MO.

Although the exact precipitation picture is more nebulous between
12-18z on Thursday, the overall trend through Thursday night
should be for scattered convection to gradually increase in
coverage and gradually spread eastward with time. This is
especially true on Thursday night into Friday morning when
moisture convergence increases across the area due to a favorably
positioned southwesterly LLJ. Models also show a couple of weak
shortwaves which could provide additional lift during this time.


.LONG TERM: (Friday Night through Next Wednesday) Issued at 355 PM
CDT Wed May 6 2015

Meanwhile, a large low pressure system will be dropping southward
along the west coast before briefly closing off over CA and then
migrating slowly eastward through the desert southwest. This
feature then lifts northeastward into the plains and induces lee
cyclogenesis near the OK/TX panhandles on Saturday. A trailing
cold front will have nosed into northern MO and stalled on Friday
night when the flow aloft becomes parallel to the boundary, but
the developing surface low will start to lift the boundary back
northward on Saturday as a warm front. There is still some
question regarding how far south the boundary will progress before
it starts to lift back to the north, and this could affect both
high temperatures and precipitation chances for parts of the CWA.

Much like the low pressure system which was moving slowly
northeastward late this afternoon, the aforementioned low pressure
system is also forecast to move slowly northeastward with time
over the weekend, which will keep periodic SHRA/TSRA chances in
the forecast through the weekend and into early next week. The
cold front associated with this system finally pushes through the
area on Sunday night and Monday. High pressure builds in behind
the front on Monday night, and precipitation chances diminish

Recent models runs are in general agreement that 0-6 km shear
remains fairly weak across the LSX CWA, at least until Sunday/Sunday
night into Monday when models show it increasing to around 30-50
kts. This suggests that although a few strong storms are possible
over the next few days, widespread severe weather is unlikely. Later
shifts will need to keep an eye on the evolution of the
Sunday/Monday system and the potential for severe weather given
the forecast values of shear and instability ahead of the
approaching cold front.

Temperatures will remain unusually warm through the weekend
followed by cooler temperatures early next week behind the cold



.AVIATION:  (For the 00z TAFs through 00z Thursday Evening)
Issued at 550 PM CDT Wed May 6 2015

There may be isolated showers/thunderstorms near COU and UIN
early this evening, but latest radar trends do not indicate any
convection will impact these taf sites at this time. The diurnal
cumulus clouds will dissipate this evening with just some mid-high
level clouds remaining. Diurnal cumulus clouds will develop again
late Thursday morning and afternoon with scattered showers and
storms expected as an upper level disturbance moves through the
area with a moist and unstable atmosphere. Will include VCTS for
the taf sites for Thursday afternoon into the evening. S-sely
surface winds for tonight, then gusty sly winds on Thursday.

Specifics for KSTL: Just some mid-high level clouds tonight, then
diurnal cumulus clouds will develop late Thursday morning and
afternoon with isolated to scattered showers/storms for late
Wednesday afternoon and evening. Gusty s-sely surface wind will
weaken this evening, then become strong and gusty again from a
sly direction on Thursday.





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