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FXUS63 KLSX 162338

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
638 PM CDT Wed Oct 16 2019

.SHORT TERM...  (Through Late Thursday Night)
Issued at 139 PM CDT Wed Oct 16 2019

Cool, dry northwest upper level flow will persist through Thursday 
as high pressure continues to build into the region. NW winds ahead 
of the surface high will continue to advect the clouds currently 
across the Midwest out of the area, and the mid-Mississippi Valley 
should be mostly clear by midnight as the high becomes centered over 
southern MO. The combination of mostly clear skies and calm winds 
should allow overnight temperatures to fall off quite a bit, with 
morning lows falling into the mid to upper 30s across the area. The 
high will shift to our east during the day tomorrow, swinging 
surface winds around to the south. These weak southerly winds and 
mostly clear skies will help temperatures moderate a bit, though 
with the cool start to the morning, temperatures will still 
struggle to get much above 60. An upper level ridge will build 
into the region on Thursday night. High clouds riding atop this 
ridge appear likely into Friday morning, and should help keep 
overnight temperatures a bit warmer than what we're expecting 


.LONG TERM...  (Friday through Next Wednesday)
Issued at 139 PM CDT Wed Oct 16 2019

A broad trough will begin establishing itself across western North 
America by the end of the work week, setting the stage for a period 
of active weather across the Midwest. On Friday, the first of a 
series of shortwaves will push into the Great Plains before sweeping 
through the Great Lake on Saturday. In response to this wave, a 
surface low will move across the northern Plains on Friday and 
Saturday. Southerly winds will pick up ahead of the low, finally 
boosting temperatures to near normal levels on Friday. The warm 
up will be short lived however, as the low is expected to drag a 
cold front through the mid-Mississippi Valley on Saturday. Moisture
return ahead of this front is progged to be rather weak, raising 
questions about precip likelihood along the front. While guidance 
shows good consensus with precip reaching NW MO on Saturday 
morning, I'm less certain that precip will sweep across the entire
area. Therefore have introduced likely PoPs in NE MO on Saturday 
morning and stuck with just chance wording for the rest of the 
area. Given poor moisture return ahead of the front, there looks 
to be little to no instability, so I'm not expected thunder at any
time on Saturday.

We'll briefly will dry out behind the front on Saturday night and
Sunday. Meanwhile, a more robust trough will dig into the Plains 
before deepening into a cutoff low and curling into SW Ontario by
Tuesday morning. Models are in remarkable consensus with the 
timing and placement of this feature, though the operational GFS 
does show the low getting quite a bit deeper than the majority of 
deterministic or probabilistic guidance. Regardless, a strong 
surface low is likely to track across the northern tier of the 
central US on Sunday and Monday. The first local impact of this 
stout low will be brisk southerly winds on Sunday, boosting temps 
back into the low 70s while ushering surface dew points in the 
60s across portions of the region. The cold front trailing behind 
the surface low is then progged to move through the region on 
Sunday night or Monday, likely bringing showers and thunderstorms.
While showers and storms are likely, guidance differences in 
fropa timing have resulted in an extended period of chance PoPs.

The very strong dynamics forecast with this system are worth 
noting and are reflected by 0-6km shear in excess of 50 kts and 
0-1km shear in excess of 30 kts as the front sweeps across the 
region. With these strong shear values, there is some concern for 
severe storms despite the limited instability forecast to be in 
the vicinity of the region. CIPS severe probability guidance 
depicts this threat with a 15% probability contour extending from 
the lower Mississippi Valley into southern Lake Michigan. However,
with this event still 4-5 days away, fropa timing differences, 
and the potential for the front to sweep through when instability 
is at its nadir, I'm not yet confident enough in severe storms to 
mention the threat beyond this discussion. That said, this will be
an event worth watching as we move into and through the weekend.

Deep NW flow will return to the region behind the early week
cyclone. This cool, dry flow should help keep things dry, while
pushing temperatures back towards or perhaps below seasonal 
normals by midweek.



.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 557 PM CDT Wed Oct 16 2019

Existing stratocumulus clouds are expected to clear more slowly 
than previously forecast given the time of year and the time of 
day. At this time, cloud bases are expected to remain VFR (except 
at KCOU where lower cloud bases were noted on recent METARs), but 
if the clouds do not clear out as anticipated then nocturnal 
cooling could lead to MVFR ceilings at the other terminals after 
03z. The overall trend will be for clouds to dissipate and move 
away from the terminals as high pressure builds in to the region. 
Northwesterly winds will diminish as the high pressure center 
moves across the region before turning southeasterly to southerly 
around its back side. 




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