993 
FXUS63 KLSX 191947
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
247 PM CDT Tue Oct 19 2021

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 247 PM CDT Tue Oct 19 2021

Key message in the short-term period through tomorrow night:

A cold front will enter the area tomorrow afternoon, bringing a 
threat for scattered showers and an isolated thunderstorm or two. 
While gusty winds will be the predominant threat, severe storms 
appear unlikely.

Current surface analysis places a developing cyclone lee of the 
Rockies across the northern Plains, placing most of the Mississippi 
Valley under relatively weak southerly winds. Aloft, increased 
moisture return on the backside of a mid-level ridge is evident in 
WV imagery and the high clouds across eastern Missouri and western 
Illinois. Winds this evening will remain elevated enough to inhibit 
radiation fog from developing in any notable way.

Attention turns to the aforementioned cyclone and associated cold 
front, which will eject east today through tomorrow into the region 
along with its upper-level support. The low itself will shunt north 
as the upper-level system pivots into a negative tilt, and the 
associated cold front will advance east through tomorrow afternoon. 
Convergence along the front is meager at best according to all 
deterministic and short-range ensembles, and upper-level support is 
confined north of the CWA where more impressive mid-level lapse 
rates reside. 0-6km bulk shear values of 40kts across most of our 
region will only have upwards of 500-750 J/kg of MUCAPE to work with 
in aiding deep convective initiation, thanks to poor moisture return 
ahead of the front. The higher shear and "higher" CAPE are also not 
coincident for more than an hour or two in the early evening. Model 
soundings also depict a strong capping inversion that the cold 
front's limited convergence would have difficulty overcoming. 

All of this is to say that it appears that both the coverage and 
intensity of convection tomorrow is unimpressive. While gusty winds 
typical of any thunderstorm are certainly in the cards with the few 
storms that can more fully develop, severe storms are not likely. 
After sunset, any remaining instability will be elevated and likely 
won't have much forcing to maintain precipitation beyond tomorrow 
evening when the front clears the area.

MRB

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 247 PM CDT Tue Oct 19 2021

Key messages through the extended forecast period:

1) Dry weather and seasonable temperatures stay in place behind 
tomorrow's cold front through the end of the work week.

2) Rain chances ramp up late Saturday into Sunday, with most of the 
area expected to see showers and scattered thunderstorms. 
Temperatures will rise next week, but how much they will warm is 
uncertain.

Deterministic and ensemble guidance are in good agreement that upper-
level northwest flow will bolster seasonable temperatures and dry 
weather through the next few days. Though subtle shortwave ripples 
are depicted in the guidance, they will be moisture-starved and will 
likely not produce anything more than cloud cover.

Active weather returns to the region this weekend when longwave 
troughing and broad diffluent upper-level flow set up just lee of 
the Rockies. The resultant surface low that develops Saturday 
afternoon with an upper level shortwave will deepen and move east 
Saturday night. WPC Cluster Analysis depicts agreement among the 
EPS/GEFS/CMCE members on a swath of precipitation associated with 
stronger WAA and broad isentropic ascent oriented perpendicular to a 
lifting warm front on Sunday. However, subtle differences in the 
position of the low (and thus the warm front) place the QPF maximum 
in slightly different locations depending on the cluster. 

The ECMWF EFI, which measures the extremity of the EPS guidance 
relative to its model climate, shows values of 0.5-0.6 for QPF in 
central and east-central Missouri. These values are indicative of 
unusually-high QPF values appearing in the EPS guidance for this 
area. NAEFS guidance also depicts IVT values near the 90th 
climatological percentile on Sunday afternoon, which backs up the 
potential for heavy rain during this time period. While most of the 
area could stand to see some rain, attention will be given to the 
potential for localized flooding if waves of showers repeatedly 
impact the same locations. A brief respite from rain appears 
possible following this system, but guidance diverges regarding the 
next chances for rain. Though this forecast features high 
temperatures next week that are warm for late October, uncertainty 
exists over where another warm front sets up and how anomalous 
temperatures aloft will be. As better agreement on the pattern 
emerges in the guidance, these temperatures will be refined.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1129 AM CDT Tue Oct 19 2021

VFR conditions are expected at all terminals through this TAF 
period. The main concern over the TAF period will be an approaching 
cold front that brings the threat of scattered showers and isolated 
thunderstorms. It appears that the onset of any showers/storms that 
develop will fall just outside the valid TAF period (between 18z 
Wednesday and 00z Thursday), however thunderstorm activity will be 
quite limited and terminal impacts are not expected to be widespread 
or long-lived.

Otherwise, a threat for LLWS also exists at KCOU, KJEF, and KUIN 
overnight tonight. Winds aloft do not appear particularly strong, 
but with due southerly winds at the surface and west-southwest winds 
aloft, directional LLWS is possible. Right now, confidence in the 
LLWS criteria for TAF mention is low.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX