Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232347

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
647 PM CDT Thu Mar 23 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 337 PM CDT Thu Mar 23 2017

A tranquil night appears to be on tap across the area. In response
to surface pressure and low level height falls centered in the
Plains, the warm front currently stretching through central MO into
southeast MO will gradually lift nortward and a stout 60+ kt south-
southwesterly LLJ will develop. This will result in the onset of
improved moisture transport through the CWA and much milder low
temps. The LLJ may also bring some stratus be parts of southeast MO
and southern IL during the predawn hours, however most of the night
is expected to be largely void of clouds.

The southwest upper low/trof will move from the Rockies into the
Plains on Friday, with a broad warm sector through the Mid MS
Valley. Height falls, and large scale ascent attendant with a lead
shortwave ahead of the upper low will contribute to the development
of a north-south band of showers and some thunderstorms in the
afternoon across western MO. The lack of steep mid level lapse rates
will result in only weak instability across eastern MO and western
IL, resulting in lesser thunderstorm chances and coverage as the
band pushes through central MO in the afternoon. Temperatures look
rather mild with highs in the 70s owing to morning sun and a good
advective component.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 337 PM CDT Thu Mar 23 2017

First closed low to lift northeast through forecast area Friday
night through Saturday evening. As prefrontal trof moves into
region, will see ongoing line of convection sliding east through
area. 12z model runs showing some timing and strength differences
between the NAM and GFS/ECMWF, so will lean more towards the
slower/stronger GFS/ECMWF solution with a band of strong low level
convergence moving out ahead of the cold front. So best chances
of showers and storms will be Friday night. With the upper low
moving over the area, there is still the potential for some modest
instability to develop near the low, and ahead of the cold front.
As mentioned in previous discussion, will need to monitor the
possibility for low-topped supercells capable of producing all
hazards near the path of the upper low with more of a hail threat
along the cold front during the day on Saturday. Latest SPC Days 2
and 3 outlooks have a marginal risk over parts of the area Friday
night through Saturday.

Then as surface low exits region, will see wrap around showers and a
few storms develop Saturday afternoon and evening.

Should see a break on Sunday as weak surface ridge builds in, before
next closed low system lifts out of southwestern US into region late
Sunday night through Monday night. Best instability with this system
will be along and south of I-70.

Upper level and surface ridges build in by Tuesday with dry wx
through mid week. Some showers are possible over central MO during
the day on Wednesday as next system approaches region. Otherwise,
better chances of showers Wednesday night through Thursday as parade
of closed lows moving through region continues.

As for temperatures, will see lows in the 40s and 50s with daytime
highs in the 60s.



.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 647 PM CDT Thu Mar 23 2017

VFR conditions will prevail for the TAF sites for much of the
valid period. LLWS will be the main issue overnight with a 45kt
low level jet building overhead by 03z with a base at about
1600ft AGL. Some potential for low stratus clouds to form towards
daybreak Friday morning but best potential will be initially well
to the SE of STL metro but is expected to expand and lift somewhat
during the day on Friday but for now have forecast as low-end VFR
when it does impact STL metro sites. The main band of TSRA will
develop over western MO by midday Friday and propagate eastward,
reaching COU by mid-late afternoon and STL metro and UIN by early
evening Friday.





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