Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS63 KLSX 201142

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
642 AM CDT Sun Aug 20 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 356 AM CDT Sun Aug 20 2017

An ongoing MCS is currently located across the mid-Missouri Valley 
moving into northwestern Missouri. A northwest to southeast 
propagation is likely to continue per latest Corfidi vectors, and is 
also further supported by a similar orientation to the instability 
gradient and a veering low-level jet. Predawn CAMs are really 
struggling with what is currently going on upstream (moving the 
complex much more west to east), so they were not relied upon much 
at all for this forecast. Believe weakening low-level jet and 
associated moisture convergence should lead to weakening, and 
eventual dissipation of the MCS by mid/late morning. Have 
significantly upped PoPs and sky cover however for roughly the 
northwestern 1/2 of the CWA, introducing likelies as far southeast 
as a KJEF>>KPPQ line. Given that the MCS has maintained itself 
fairly well over the past couple of hours, may need to further 
increase PoPs as the morning hours draw closer.

Expectation is for the MCS to lay out an outflow boundary, which may 
serve as a focus for diurnal convection this afternoon into early 
this evening. Best guess as to where isolated storms may fire along 
this boundary is across south-central and southeastern Missouri 
based on the expected track of the MCS. 

Knocked high temperatures down just a degree or two over regions 
where increased sky cover will likely reside through at least 
midday. Depending on exactly how the complex evolves, may not be 
cool enough but hard to get too aggressive as clouds could easily 
still thin out by early afternoon allowing for a quick recovery.

A bit more uncertain forecast for tonight. All models pretty much 
have the synoptic warm front oriented west to east across central 
Iowa into southern Wisconsin, with convection firing along it 
overnight tonight. However, as alluded to above, no model is really 
handling what is going on currently well at all so hard to have much 
confidence in any solution for tonight. That being said, with the 
expectation of a warm front just north of the area, believe highest 
chances for showers and storms will be across the far northern CWA. 

Temperatures tonight with a partly to mostly cloudy sky and light 
southerly flow will be quite mild with lows in the 70s across much 
of the region.  


.LONG TERM...  (Monday through Saturday)
Issued at 356 AM CDT Sun Aug 20 2017

(Monday - ECLIPSE DAY)

There remains a greater than normal amount of uncertainty in the 
forecast. Adding to that discomfort is the rather poor performance
of the model guidance, especially the ECMWF, of the current 
southeastward moving MCS centered in far southeast Nebraska. When 
the models can't get the first 12-18 hours correct in this pattern
it doesn't give you a great deal of confidence in the forecast 
details at 36-48h. 

The overall synoptic situation that is forecast remains the same - a 
elongated upper ridge will be anchored from the southwest Atlantic 
into the lower MS Valley and southern Plains with the main band of 
westerlies across the northern tier of the CONUS where the main 
short wave track will reside. A baroclinic zone will be drapped from 
the central Plains across the upper MS Valley and this boundary, a 
pronounced southwesterly LLJ, and disturbance tracking along the 
southern edge of the westerlies should support an ongoing MCS at 
daybreak. Within the broader southwest low level wind maxima 
extending south of the LLJ core, low level moisture convergence and 
WAA focused from northeast KS across northern MO also suggest 
additional less organized elevated showers and thunderstorms could 
be ongoing within that corridor. Troublesome is the 00Z NAM suggests 
the MCS across IA could have sagged into northeast MO and west 
central IL with a outflow boundary on its southern flank. The NAM 
then wants to sink the active boundary and ongoing MCS south-
southeast through the morning and afternoon right into St. Louis. 

The flow aloft will be a bit more backed on Monday than currently 
with a west-southwest component at 500 mb and at 300 mb gradually 
backing from west-northwest in the morning to westerly by early 
afternoon. These changes result in less southeastward steering flow 
aloft for MCS motion which is a good thing, and do not support the 
00Z NAM solution. These changes to the flow however still allow for 
any convective debris cloud to the west to spread into the CWA. I 
still think the areas to watch for morning precipitation are across 
northeast MO/west central IL into central MO due to the previously 
mentioned low level WAA/MCON pattern. By afternoon I think any 
threat of precipitation will be diurnally driven thunderstorms which 
should be more isolated in nature given the lack of large scale 
support. The impacts of eclipse cooling on the diurnal cycle are 
unknown but I would tend to think it would delay the diurnal 
development in general. How much will temps fall off in the roughly 
3 hour period from partial eclipse onset to end? This will really be 
interesting to see. A recent modeling study on the effect of the 
eclipse on temperatures, showed temp reductions can peak at 9-12 
degF in clear sky areas, however cloud cover will reduce that amount.

My current thinking remains that the best odds for optimal viewing 
will likely be in far southeast MO and southern IL. Potential 
impacts from cloud cover are too iffy further west-northwest as the 
odds of greater cloud cover increase. In reality we really won't 
know the details until we move through the morning hours on Monday 
and are able to assess the real-time cloud trends.

After the eclipse cool down, temps should quickly rebound and we are 
still expecting a seasonably hot day with highs in the lower 90s.

(Monday Night - Tuesday Night)

All indications are that a wet period with showers and thunderstorms 
can be expected. An upper trof will dig out of south central Canada 
into the north-central U.S. on Monday night. This will help drive a 
cold front southward and into northwest MO by 12Z Tuesday. Height 
falls/large scale forcing with the digging trof, ample instability, 
and low level forcing via a southwesterly LLJ should result in 
extensive thunderstorms along and well ahead of the front in the 
prefrontal warm sector impacting most of the northern CWA thru
early Tuesday morning. The cold front will advance through the 
CWA Tuesday into Tuesday evening as the upper trof continues to 
deepen. There will likely be some decrease in coverage and 
intensity of storms on Tuesday morning and then an upswing in 
intensity across southeast MO and southern IL on Tuesday 

(Wednesday - Saturday)

A rather deep upper trof will evolve across eastern NOAM through the 
end of the week. This will maintain an impressive and expansive 
surface high/low level anticyclone whose center will move slowly 
to the east from WI through the Great Lakes with time. This high 
will dominate our weather through Saturday resulting in below 
average temperatures and pleasantly low humidity. 



.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 638 AM CDT Sun Aug 20 2017

Complex of showers and thunderstorms approaching from the WNW.
This will likely affect KCOU and KUIN this morning. Worst
conditions expected at KCOU with a TEMP for gusty NW winds and
brief IFR visibility with storms. Believe this area will weaken
and dissipate before reaching metro terminals. Diurnal convection
possible late this afternoon and early evening for KCOU and metro
terminals though probability too low to mention in TAFs. More
storms expected overnight tonight but most expected north of CWA
near a warm front. KUIN will be closest to this activity.


Expect ongoing showers and storms to gradually weaken this morning
and dissipate before reaching the terminal. Increase in midlevel
clouds expected and if outflow reaches the metro, winds may swing
around to the northwest briefly. Isolated showers and storms
possible this afternoon and early evening but probability too low
currently to mention in TAF. 




National Weather Service Glossary of Abbreviations