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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

FXUS63 KTOP 181037

Area Forecast Discussion
National Weather Service Topeka KS
537 AM CDT Wed Sep 18 2019

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 313 AM CDT Wed Sep 18 2019

Very sticky to uncomfortable dewpoints remain in place for the 
foreseeable future -- especially in the near term -- with 
temperatures around 90 yielding heat index values in the mid to 
upper 90s.  

Overall, the upper flow pattern remains devoid of an significant 
trough bringing cooler and drier air into the region probably until 
next week when a more significant modified Canadian high pressure 
cell can build into the region.  Until then, the broken record of 
hot and humid conditions holds.  The near term forecast for today 
and Thursday doesn't really change much or offer much in the way 
of precipitation chances. Currently, the high amplitude ridge that
has been over the area is flattening and shifting east of the 
area. Tropical depression Imelda is slowly trying to lift north 
through southeastern portions of TX. Meanwhile, a compact upper 
shortwave is continuing to lift into the Dakotas with the best 
forcing remaining well north of the area. The main Pacific trough 
is still trying to make its way onshore over the Pacific 
northwest. This will leave a southwest flow regime over the 
central Rockies into portions of the central Plains. Lee troughing
remains in place along the front range of the Rockies into 
portions Nebraska. Thus the best convergence and isentropic ascent
along with the LLJ tonight also remains northwest of the area. 
Elevated storms remain well north of the area and are currently 
struggling to back build into extreme northeastern Nebraska as the
upper trough axis is lifting trough the region taking on a more 
negative tilt. 

With the orientation of the previously mentioned shortwave and the 
lack of a significant convergence associated with the LLJ over the 
area, most short term model guidance has continued to back off of 
developing precipitation into the area.  There may only be some 
isolated showers or a storm develop into the early morning hours. 
This similar pattern remains into Wednesday night and Thursday 
morning.  However, have kept slight chance PoPs mainly north of I-70 
due to a a bit better LLJ convergence.  But, moisture again remains 
at a premium aloft, so not too optimistic in any widespread shower 
or storm development even into Wednesday night and early Thursday 
time frame.  Highs again Thursday remain into the 90s with high near 
surface dewpoints with little relief from daytime mixing giving way 
to upper 90 heat index values again.  

.LONG TERM...(Thursday through Tuesday)
Issued at 313 AM CDT Wed Sep 18 2019

Into the weekend, a frontal system may begin to finally move into 
the area as the previously mentioned Pacific trough impacts the area 
and lead waves emanating from this trough lift into the Plains.  

Still low predictability in the overall evolution of the larger 
scale western trough into the Saturday and Sunday time frame, but it 
does look like a wet pattern will be in place for much of the 
upcoming weekend.  This could be further enhanced by the northward 
advection of higher theta-e air associated with tropical depression 
Imelda.  As more southwest flow component remains and upper flow 
increases over the region along the eastern flank of the deeper 
trough spanning through much of the western CONUS, can't rule out
some episodes of severe weather and heavy rainfall. The ECMWF 
suggests stronger upper flow than the GFS and to some extent the 
Canadian solution. This could lead to shear profiles more 
supportive of more organized severe weather. Will have to wait a 
bit longer to tell for sure. The other main hazard would be 
related to the heavy rainfall potential. With no antecedent 
rainfall, concerns overall may be mitigated. But, with PW amounts 
easily pushing the 90th percentiles for time of year, concerns 
would likely be elevated and dependent on additional moisture 
associated with previously mentioned tropical influences. Also, 
frontal positioning is going to be critical. Right now the frontal
zone is anything but clear as to where it may linger. The GFS 
focuses mostly over east central areas into Missouri. The ECMWF 
has the focus further north essentially along the KS/NE border. 
Again, this positioning will be critical. 

Have kept precipitation chances into the early portion of next week 
as the boundary struggles to push very far south of the region with 
return bring back isentropic ascent back over the boundary into the 
region.  Eventually, into the latter portions of the week, it 
appears most deterministic guidance is trying to suggest that a 
stronger and drier Canadian airmass pushes into the central Plains. 
 As a result the area could potentially see dewpoints dip into the 
40s!  Relief with this dry air has not been felt for a long time and 
would certainly be newsworthy.  


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 526 AM CDT Wed Sep 18 2019

VFR continues at KTOP/KFOE/KMHK with southerly winds increasing to
around 10 kts during the mid to late afternoon. Isolated TSRA
develops north and west of terminals this aftn, remaining far
enough from terminals to not mention in TAFs.