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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

FXUS63 KTOP 200444

Area Forecast Discussion
National Weather Service Topeka KS
1144 PM CDT Thu Sep 19 2019

Issued at 331 PM CDT Thu Sep 19 2019

20Z water vapor imagery showed the remnants of Imelda remaining 
along the TX and LA gulf coasts, trapped within a broader upper 
level ridge over the southern U.S. Further west an elongated upper
trough was observed from NV to southern Alberta. At the surface, 
a weak outflow boundary had all but lost any definition across the
forecast area. South of the boundary, an unstable airmass existed
with scattered showers and thunderstorms developing in the heat 
of the day. Cooler temps north of the boundary had limited 
destabilization such that there was barely a CU field. Surface obs
from around the region showed good low level moisture remained 
over the central and southern plains.


...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Friday)
Issued at 331 PM CDT Thu Sep 19 2019

For the rest of this afternoon, models prog little in the way of 
deep layer shear. 0-6km bulk shear values were only around 15KT. 
With a lack of synoptic scale forcing over the central plains, the
showers and thunderstorms should dissipate with the lose of 
daytime heating. This should allow for dry weather to prevail 
through the night tonight. Lows tonight are expected to be similar
to the previous couple days with no change in airmass. So 
readings should fall into the upper 60s to around 70. For Friday, 
height falls are progged to spread east into the plains. With the 
potential for some weak perturbations within the southwest flow 
aloft and good moisture in place, the chances for precip will be 
on the increase. Since forcing appears to be rather disorganized 
in the model solutions and the upper trough axis remaining to the 
west of the forecast area, precip looks to be isolated to widely 
scattered. So only have POPs in the 20 to 30 percent range. The 
weak forcing is progged to begin affecting the forecast area by 
Friday afternoon so think the morning hours will remain dry with 
some chance for showers and thunderstorms for the afternoon and 
evening. Increased cloud cover on Friday is expected to limit 
insolation enough to keep highs in the mid 80s. If clouds fail to 
move in, forecast highs could be several degrees to cool. 

.LONG TERM...(Friday night through Thursday)
Issued at 331 PM CDT Thu Sep 19 2019

Friday night through Sunday morning looks to be stormy as the 
upper trough moves across the central plains and a cold front 
pushes southeast. Models show the better dynamics and the front 
impacting the forecast area mainly Saturday night and early Sunday
morning. So the highest POPs have remnant in this time frame. 
Forecast soundings continues to show a moderately unstable airmass
developing as the wind profile improves through the day. The NAM 
forecast soundings continue to suggest SRH values of around 200 
m2/s2 by Saturday afternoon as CAPE values range from 2000 to 3000
J/kg. So there remains a risk for severe storms. The main 
uncertainty is whether clouds and precip early in the day will be 
widespread enough to hinder destabilization. So the severe risk 
appears to be conditional on how warm we get. Given the improving 
wind profiles progged through the day, any discrete storms could 
have some potential for a brief tornado. Although forecast 
hodographs imply early on in the day there could be more storm 
mergers than isolated storms. The bigger concern may end up being 
flash flooding Saturday night and into Sunday morning. Models 
continue to show anomalously high moisture across eastern KS with 
precipital waters up to 2 inches. With a warm moist airmass, 
rainfall rates could be high enough to cause flash flooding 
overnight in spite of the flash flood guidance between 2 and 3 
inches. Considered issuing a flash flood watch, but there remains 
some variability in the placement of the heaviest rainfall. Have 
opted to hold off on a watch for now in hopes the models converge 
on a common location for the heavy rain.

By Sunday, all of the operational models are showing the front 
passing south of the forecast area through the morning. So have 
trended highs on Sunday cooler by about 5 degrees and also show POPs 
decreasing through the day a little sooner. By Sunday night, the 
front is forecast to be into central OK and AR so have removed the 
POPs from Sunday night. 

Uncertainty in the forecast increases by mid weak as the models 
handle a potential trough differently. The GFS keeps the northern 
and southern streams phased with a progressive and open wave 
impacting the central plains mainly on Tuesday. However the ECMWF 
cuts off the energy and digs it into southern AZ, bringing it back 
into the plains on Thursday. With the Canadian in between these two 
solutions, there is not a lot of confidence in one prog over another 
and have remained fairly close with the model consensus for the 
forecast Tuesday through Thursday. Unfortunately this spreads a 
small chance for precip through the whole period when it probably 
will not rain each of the days. Temps next week look to be more 
seasonal as generally northwest flow aloft slows any potential 
return flow. 


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1141 PM CDT Thu Sep 19 2019

For the 06z TAFs, southerly winds will become gusty around 20kts 
late Friday morning through the afternoon hours. Scattered 
showers and storms may develop near the TAF sites by the afternoon
hours and diminish by early evening. Expect cigs to drop to MVFR 
with these storms and remain MVFR (and possibly borderline IFR) 
through the end of the TAF period. 




SHORT TERM...Wolters
LONG TERM...Wolters