Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 030234

934 PM CDT Tue Jun 2 2015

Issued at 929 PM CDT Tue Jun 2 2015

Two area of concern overnight tonight:

1). As alluded to by day shift...stubborn stratus deck will
continue to be a focus overnight and into possibly early tomorrow.
Stratus has continued to shrink/erode on both the western and
eastern flanks so far this evening. Not sure whether or not this
will continue however as we head deeper into the night. Would
expect some migration to the northwest though starting around 0600
UTC possibly back toward portions of metro St. Louis as 925-hPa
winds veer to more of a southeasterly direction.

2). Latest HRRR/RAP guidance is suggesting development of a
cluster of thunderstorms late tonight across NW MO. Seems
plausible as LLJ strengthens overnight collocated with subtle
shortwave transversing the area evident on WV imagery. Did not
make large changes this early on but did increase sky cover late
tonight/early tomorrow for the NW third of the CWFA as well as
drag the sch PoPs a bit further to the SE tomorrow. Latest NAM
model also supports this scenario.



.SHORT TERM:  (Through Late Tonight)
Issued at 300 PM CDT Tue Jun 2 2015

The pesky stratus/stratocu remains the biggest issue tonight. In
the areas which have clouds that are more cumuliform in nature
such as St. Louis north and east, the clouds should dissipate this
evening with loss of heating. However for areas further south and
west, the clouds will be slower to dissolve, if at all completely.
Considering trends the last few days I have been rather pessimistic
and it would not surprise me at all if there are pockets/bands of
stratus aligned with the low level southeasterly flow at daybreak
on Wednesday.


.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 252 PM CDT Tue Jun 2 2015

The expansive lower troposheric anticyclone that has help maintained
the low clouds the last 3-4 days will continue to weaken and depart
east with winds veering to southeast-southerly on Wed. I suspect
we will have more diurnal cu again on Wednesday, probably not as
extensive as today, but enough for me to trim the high temps back
a few degrees from the previous forecast. While I am not overly
excited about the prospect, there is a slight chance of seeing
some showers or thunderstorms in parts of north-central/northeast
MO along the tail end of a weak impulse aloft and where there is
growing low-level WAA and moisture transport on the eastern
periphery of the return flow. The areal extent of the shower and
thunderstorm threat will increase across the northeast third of
the CWA Wednesday night due to improving low-level WAA/moisture
and potential for maintenance of any showers and storms moving
southeastward out of Iowa.

Overall the pattern will become warmer and much more active
Thursday into early next week with the possibility of multiple
organized thunderstorm episodes/MCSs and a nearly continuous
thunderstorm threat depending on your location. As has been
alluded to the last several days, while ridging aloft will be
building through the Plains and attempting to build into parts of
the MS Valley, short-waves will be topping the ridge crest and
moving east and southeast impacting the area. The most prominent
threat appears to be late Thursday into Friday when several of
these impulses will impact the region and an east-west front will
drop southward interacting with an increasingly warm, moist and
unstable air mass. There may be a lull on Saturday depending on
the exact location of the front. It is expected to pass through
the CWA but then waver and return northward Saturday night with
yet another front pushing into the area Sunday/Sunday night as
upper troffing becomes established across the northeast quarter of
the CONUS.



.AVIATION:  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 640 PM CDT Tue Jun 2 2015

None of todays/this afternoons guidance seems to have a firm grasp
on low level moisture that has become trapped on the southeast
perhiphery of the surface ridge, with a broad area of generally 3-4kft
cigs draped from se IA, across eastern MO, into s IL and the lower
Ohio Valley. Western edge of this cloud deck is showing a nice
ewd progression, while very ragged eastern edge near the
Mississippi River from KUIN to STL metro is holding quasi-stationary.
Based on above trends along with latest low level moisture
profiles from RUC, believe cloud deck will gradually erode over
eastern MO during the evening. If clouds do persist much past 04z,
will likely be chasing cloud trends through satellite depiction of
cloud deck, and its certainly not out of the question that bases
could lower into MVFR range if cloud deck persists into the
overnight and predawn hours. Have also added some early morning
fog at KCOU as this area has remained cloudy all day.

Specifics for KSTL: Based on above reasoning (and with above
caveats), SC deck around 4kft is expected to erode this evening,
with mostly clear skies overnight. Should see some CU form with
bases 2-3kft on Wednesday morning as inversion breaks,
with these bases increasing to aoa 3kft in the 16-18z time frame
as mixing heights increase across the area.





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