851 
FXUS63 KTOP 042334
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
634 PM CDT Fri Sep 4 2020

...Update to aviation forecast discussion...

.UPDATE...
Issued at 630 PM CDT Fri Sep 4 2020

Added a slight chance of thunderstorms to the forecast for late 
tonight after 09Z through midday Saturday in conjunction with a 
wedge of weak theta-e advection working northward on the return 
southerly flow. RAP/HRRR/NAM forecast profiles all show the base 
of the EML saturating by 09-12Z along and southeast of the 
Turnpike with a resulting 1500-2000 J/kg of MUCAPE residing above 
this layer. The overall kinematic support for convective initation
is weak to modest, with an H850 jet of <25kts, but a subtle H500 
shortwave noted in water vapor imagery upstream over the Black 
Hills that is progged to amplify slightly as it drops 
southeastward and reach the forecast area around 12Z. 

The low to mid-level flow is weak and any cells that do develop 
will be poorly organized and likely present a low threat outside 
of lightning. The threat for convection should shift eastward 
after midday as the upper tropospheric ridge and H850-700 thermal 
ridge build from west to east over the forecast area. Also 
increased cloud cover for late tonight and through the day on 
Saturday east of Manhattan given the moistening BL and likelihood 
that convective temps will be reached late Saturday morning. This 
overall forecast is supported by several recent CAM/CPM runs and 
leveraged the SREF for PoPs and HREF for sky cover trends for the 
next 18 to 24 hours.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 359 PM CDT Fri Sep 4 2020

We continue to see northerly mid to upper-level flow over the 
forecast area as the trough in the Great Lakes region slowly moves 
east and ridging remains in the western third of the CONUS, putting 
us between the two features. Sfc high pressure has already started 
to slide eastward and this will be the trend through the evening 
into Saturday. This will allow for southerly flow in the low levels 
to advect moisture back into the area, bringing a return of heat and 
humidity for the holiday weekend. Have increased overnight lows as a 
result of the moisture advection with temps only falling to the low 
to mid 60s. Highs tomorrow are expected to reach the low to mid 90s 
as dew points rise through the 60s and into the low 70s for some 
parts of the area. This puts heat indices in the upper 90s to around 
100.

.LONG TERM...(Saturday night through Friday)
Issued at 359 PM CDT Fri Sep 4 2020

Saturday night into Sunday, lee troughing deepens and strengthens 
the pressure gradient across our area. This in addition to downslope 
flow and strengthening WAA will allow for temperatures to warm 
further, with Sunday expected to be the hottest day in the forecast 
period. Highs range from the low 90s in the east to upper 90s in 
north central KS , but with more mixing farther west, heat indices 
should still be in the upper 90s across most of the area. 

The pattern then changes drastically through the rest of the week, 
as an upper trough swings down from western Canada and ultimately 
becomes a cutoff low by mid-week. This brings the much-advertised 
cold front Monday into Tuesday. This morning's GFS/NAM had shown 
hints of the sfc boundary starting to pass through during the day on 
Monday, while the 00Z EC lagged behind with fropa occurring Monday 
night into early Tuesday. This is likely due to the EC keeping the 
upper trough a bit further north into Tuesday and ultimately 
tracking the upper low into the Four Corners region, which was much 
farther west than the GFS/NAM which tracked the low right over KS. 
Now, the 12Z EC run is more in line with the GFS/NAM both with the 
track of the upper low and in the timing of the front. This slightly 
earlier frontal passage results in highs on Monday in the 80s as dew 
points start to fall back to the 50s. CAA strengthens Monday night 
and especially into Tuesday, yielding highs in the low to mid 50s in 
north central KS to the 60s in east central KS. The much cooler air 
sticks around through the week as the upper low pivots around the 
state (although the track diverges after Thursday between the 
GFS/EC). This keeps Wednesday's highs only in the 50s before 
managing to get back to the 60s Thursday and Friday.

Regarding precip associated with the front, the greatest likelihood 
of showers and thunderstorms is on Tuesday behind the front, 
possibly lingering into Wednesday. Small shower chances continue 
into Thursday and Friday with wrap-around moisture from the upper 
low, although we'll need to continue monitoring its track for run-to-
run consistency before we can increase confidence on those.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 630 PM CDT Fri Sep 4 2020

Predominantly VFR conditions are forecast for the TAF period with
low VFR ceilings and an isolated thunderstorm possible along and 
southeast of an EMP to TOP line late tonight through Saturday 
morning. Light and variable winds tonight shift to the south-
southwest at 5 to 10 kts for the day on Saturday.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Skow
SHORT TERM...Picha
LONG TERM...Picha
AVIATION...Skow