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Area forecast discussion
National Weather Service Topeka Kansas
334 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 Kansas early in the afternoon. At the
surface, low pressure has been gradually strengthening over
southeast Colorado with a trough axis/frontal boundary extending north
through north central Kansas and into northeast Nebraska. Dewpoints were
generally in the lower 70s.

&&

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 Kansas. 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 Kansas. 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 Kansas 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 Kansas 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 quantitative precipitation forecast 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 relative humidity 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 18z tafs through 18z Sunday afternoon)
issued at 1221 PM CDT Sat Sep 21 2019

MVFR ceilings are likely to persist through the afternoon at top and
foe as low level moisture continues to advect northeast. Mhk is
expected to see some improvement in the ceilings in the next several
hours. The low clouds are expected to eventually lift by this
evening as ts development focuses along the frontal boundary
across north central Kansas. Ts are most likely to impact the
terminals overnight and Sunday morning. Timing ts will be the main
challenge with the forecast.



&&

Top watches/warnings/advisories...
Flash Flood Watch from 7 PM CDT this evening through Sunday
afternoon for ksz009>012-021>024-026-034>040-054>056-058-059.

&&

$$

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