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Area Forecast Discussion (AFD)

Issued by Topeka, KS (TOP)

                            
000
FXUS63 KTOP 212336
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
636 PM CDT Sat Sep 21 2019

.SYNOPSIS...
Issued at 334 PM CDT Sat Sep 21 2019

20Z water vapor imagery showed an upper low lifting northeast 
through the northern plains into southern Canada with energy left 
behind over the four corners region. Southwesterly flow aloft 
continued across the central plains with a subtropical moisture 
plum seen streaming northwest from the gulf of California. A weak 
perturbation within the flow had aided some scattered storms to 
move across east central KS early in the afternoon. At the 
surface, low pressure has been gradually strengthening over 
southeast CO with a trough axis/frontal boundary extending north 
through north central KS and into northeast NEB. Dewpoints were 
generally in the lower 70s. 

&&

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Sunday)
Issued at 334 PM CDT Sat Sep 21 2019

For tonight and Sunday, precip chances will increase as the 
energy over the four corners eventually lifts into the plains. 
Convection is expected to begin along the surface trough axis 
where decent insolation and convergence should eventually overcome
the convective inhibition. Convective allowing models (CAMs) have
been consistent in storms developing between 5 and 7 PM across 
north central KS. Instability should be favorable for severe 
storms with improving deep layer shear as the 500MB jet 
strengthens. Forecast hodographs continue to show favorable low 
level winds initially for the possibility of a tornado if a storm 
can remain discrete. However the forecast soundings eventually 
show a mid level weakness in the winds causing an inflection in 
the hodograph implying storm mergers will become more likely. 
Additionally the linear lift from the boundary may favor updraft 
interactions as models show development on the boundary. So there 
may be a brief window for a tornado as storms initiate and they 
remain discrete. But the more likely hazards will be large hail 
and damaging winds. Cloud cover and precip has limited 
destabilization further east across east central and parts of 
northeast KS. So it is unclear how far east the severe weather 
threat will exist. Since the synoptic scale dynamics and frontal 
boundary don't really move through eastern KS until Sunday 
morning, storms may tend to weaken late in the evening 
transitioning to a heavy rain threat through Sunday morning. The 
operational models are in good agreement with the upper wave 
lifting across northeast KS through Sunday afternoon with the 
front pushing into southern MO. So precip chances should be on the
decrease through the afternoon with rain having moved out of the 
area by 00Z Monday. Models have again shown lower QPF amounts for 
the event. Think this may be due to the initial storms developing 
further north, but also the more widespread precip being delayed 
until Sunday morning. The 15Z SREF ensemble plums also show the 
envelope of possible outcomes diminishing with a clustering of 
rain amounts generally less than 2 inches for storm totals. Have 
continued the flash flood watch since there remains anomalously 
high moisture in the airmass. Also if storms start training, 
rainfall rates could quickly overcome the relatively high flash 
flood guidance values. 

.LONG TERM...(Sunday night through Saturday)
Issued at 334 PM CDT Sat Sep 21 2019

Dry and cool weather is expected on Monday with lows falling into
the lower and mid 50s. There is some potential for fog 
development Monday morning. Clouds and rain on Sunday are likely 
to keep high RH values in the boundary layer. Then as clouds clear
out Sunday night and winds become light, radiational fog looks to
become more likely. Have inserted a mention of fog into the 
forecast. The one element that casts some doubt is the dry air 
advection anticipated behind the front. But this may not be as big
a factor if the boundary layer decouples early in the night. Good
insolation is expected to help highs warm up into the upper 70s 
and around 80 for Monday afternoon. 

There continues to be a couple opportunities for precip for Tuesday 
through Friday. The first is a weak shortwave that propagates 
through the plains as the pattern splits and a closed upper low digs 
into the southwestern U.S. The second is when this closed low gets 
reabsorbed with the mean flow and lifts northeast through the plains 
on Friday. Models have not had the best consistency with these 
features so POPs are not terribly high. Temps look to remain 
seasonal with models absent of any strong temp advection. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 633 PM CDT Sat Sep 21 2019

For the 00z TAFs, the best chance for storms over the TAF sites
will be late this evening through the overnight hours, with off-
and-on showers and storms possible Sunday morning into the
afternoon hours. MVFR cigs will be likely during that time before
scattering out Sunday afternoon behind the exiting front. The
frontal passage will result in southerly winds shifting to the
northwest.  

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday afternoon for KSZ009>012-
021>024-026-034>040-054>056-058-059.

&&

$$

SYNOPSIS...Wolters
SHORT TERM...Wolters
LONG TERM...Wolters
AVIATION...Hennecke