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FXUS63 KLSX 180905

Area Forecast Discussion
National Weather Service Saint Louis MO
405 AM CDT Wed Sep 18 2019

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 404 AM CDT Wed Sep 18 2019

Early this morning, an upper level RIDGE dominated the Mid-
Mississippi Valley, while Tropical Depression Imelda was now inland 
near Houston, Texas.  At the surface, a large area of high pressure 
continued to be centered over the Saint Lawrence Valley with its 
influence extending all the way westward into our region.  The 
extreme western periphery of this high pressure area continued to be 
represented by a "backdoor" frontal boundary draped over much of 
eastern MO, just west of the Mississippi River.  A few thin high 
cirrus clouds were passing through, with mainly clear skies. 
Temperatures are once again much above normal for this time of year, 
with readings in most locales in the upper 60s to lower 70s. 
Dewpoints were only a degree or two lower. 

The upper RIDGE is expected to maintain its strength and remain 
centered near western KY through Thursday afternoon.  At the 
surface, the "backdoor" frontal boundary is expected to remain 
nearly stationary today, oriented NW-SE across eastern MO, and will 
result in a similar setup from Tuesday afternoon:  MLCAPEs between 
1500-2000 J/kg, very weak 0-6km bulk shear of less than 20kts, no 
CINH, and weak moisture convergence near the frontal boundary.  Will 
maintain the slight chance PoPs for eastern and central MO for this 
afternoon.  There is even some signs of this frontal boundary 
maintaining enough integrity to do a repeat on Thursday afternoon. 
This should be the only chances for rain during this period. 

The remnants of "Imelda" are expected to drift north into the 
Arklatex region by the end of this period, but this position is 
expected to have no impact on our region...yet.

MOS continues to exhibit a cool bias with respect to max temps in 
this current pattern and have forecasted maxes a few degrees above 
MOS values, resulting in upper 80s to lower 90s today (very close to 
persistence), and similar values again on Thursday.  Min 
temperatures tonight will likewise reflect persistence, ranging from 
the mid 60s to around 70. 


.LONG TERM...  (Thursday Night through Tuesday)
Issued at 404 AM CDT Wed Sep 18 2019

Will begin to see a gradual cooling trend with high temperatures 
beginning on Friday and increasing pops as the upper level ridge 
shifts southeast of the region.  The remnants of Tropical Depression 
Imelda will lift northeastward into our area Friday afternoon and 
night. Low level moisture convergence will be increasing over east 
central and southeast MO on the nose of a southwesterly low level 
jet Friday night.  A moist southwesterly flow will become 
established this weekend with low-mid level warm air advection and 
increasing upper level divergence ahead of weak southwest flow 
shortwaves.  The highest pops and QPF will likely occur from 
Saturday night through Sunday evening as the ECMWF model has a 
deepening upper level trough approaching the region with an 
associated cold front dropping southeastward through our forecast 
area Sunday night into Monday morning.  Although the operational GFS 
model is weaker with the upper level trough and associated cold 
front, it still depicts relatively high QPF across our area.  With 
high precipitable water values over the region there will be the 
potential for heavy rainfall possibly resulting in flooding, 
although flash flood guidance values will be initially quite high 
due to the lack of rainfall the past week over our area. Still 
uncertainty this far out where the axis of heaviest rainfall will 
set up and the highest rainfall amounts expected.  Cooler high 
temperatures can be expected this weekend due to the cloud cover and 
the precipitation with highs close to seasonal normals by Sunday. 
There is a lot of forecast uncertainty Monday and Tuesday due to 
significant model differences.  The ECMWF model keeps it dry as a 
surface ridge moves eastward through our area after fropa.  The 
operational GFS keeps low QPF over parts of our region along and 
north of a weak warm front. For now will lower the pops by Monday, 
but keep slight chance to low chance pops going for most of our area
through Tuesday.



.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1042 PM CDT Tue Sep 17 2019

Mainly dry and VFR conditions are expected through the period.
The only exception will be late tonight into early Wednesday when
patchy fog will reduce visibilities to in the MVFR/possibly IFR 
range, mainly in river valleys. Winds are expected to remain light
through the period.


Dry and VFR conditions are expected through the period with light




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