168
FXUS63 KTOP 211705
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1205 PM CDT Fri Jul 21 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 232 AM CDT Fri Jul 21 2017

07Z water vapor imagery shows an upper level ridge centered over the
OK/AR state line. Meanwhile the mean westerlies were seen across the
northern tier of the country. At the surface, a rather weak trough
of low pressure was noted from the southern high plains through the
northern high plains.

The heat remains the main concern for today and tonight. Models prog
the center of the upper ridge to remain near southeast KS today even
as it begins to break down. Additionally models are in good
agreement that the thermal ridge will move a little further east
into the forecast area. So think highs will be the same or a degree
or two hotter than yesterday and have most areas in the 100 to 105
degree range. As for dewpoints, models tend to mix them out a little
more across central and western portions of the forecast area as
southwesterly low level flow increases and the boundary layer mixes
above 800MB. Still have dewpoints forecast to range from the lower
60s across north central KS to the lower 70s in the far eastern
counties. This still results in heat indices between 105 and 115
through the afternoon. And there doesn`t appear to be any relief
from the heat overnight as south and southwest winds persist ahead
of an approaching surface trough. Lows tonight are expected to be in
the upper 70s and around 80. So will leave the excessive heat
warning as is. With not real forcing expected to impact the forecast
area, precip chances look to be pretty slim through tonight.

.LONG TERM...(Saturday through Thursday)
Issued at 232 AM CDT Fri Jul 21 2017

Saturday, The upper level ridge centered across KS will slowly shift
southward as an upper level trough digs southeast across the
northern and central plains. A surface cold front across NE will
move southward into the northern counties of the CWA during the
afternoon hours. The ARW-WRF model shows the front pushing across
much of the CWA by 21Z. However, the 00Z GFS has the front just
south of the NE border by 21Z SAT. The NAM model show the front
north of I-70 by 21Z SAT. Most of the area will see heat indicies in
the 105 to 110 degree range during the afternoon hours. An isolated
thunderstorm may develop along the front but with the northern
periphery of the H5 ridge across much of KS, the synoptic scale
subsidence may prevent storms from developing along the front during
the afternoon hours. However, if a pulse storm does develop along
the front during the afternoon and evening hours, it may produce
strong wind gusts. The MLCAPE will be in the 1500 - 2500 J/KG
range but the 0-6 KM effective shear will be less than 20 KTS.

If the models prog the front to push to I-70
during the early afternoon hours, then the next shift may want to
expire the excessive heat warning for the northern counties late
Friday evening.

Saturday night through Sunday night, The best chance for showers and
thunderstorms may occur Saturday night into Sunday morning across
the CWA. An upper level trough across the northern high plains will
amplify as if digs east-southeast across the northern plains into
the midwest. The stronger ascent ahead of the H5 trough combined
with isentropic lift north of the surface front pushing south of the
CWA may provide enough ascent for at least scattered showers and
thunderstorms across the CWA. As the front shifts southward across
southern KS, the better zone of isentropic lift north of the
boundary will shift southward across the southern counties of the
CWA Sunday morning. The showers and thunderstorms may linger across
the southeast counties through Sunday afternoon. The GFS model is a
bit slower with the front and keeps the chance for showers and
thunderstorms across the southern counties of the CWA through Sunday
night. Highs on Sunday will be cooler with lower 90s expected
across the CWA.

Monday through Through Tuesday, both the GFS and ECMWF show the
upper ridge amplifying across the southern high plains. This will
keep eastern KS in northwesterly flow at the mid and upper level of
the atmosphere. low-level winds will become southerly and deeper
moisture will advect northward across the plains. The ECMWF
forecasts a more amplified upper trough moving southeast across
eastern KS Monday afternoon into Tuesday morning which may provide
for more widespread showers and thunderstorms. The GFS is not as
amplified and shows the more amplified portion of the upper trough
shifting southeast across southwest KS into eastern OK, which would
keep the more widespread showers and thunderstorms south of the CWA.
The best chance for showers and thunderstorms wil be across the
southern counties if the GFS solution were to verify. Highs wil
continue to be in the upper 80s to lower 90s.

Wednesday through Thursday, the upper level ridge across the
southern high plains may build east across the southern and central
plains. At this time it does not look as amplified thus, high
temperatures may only warm into the mid 90s with heat indices
increasing to around 100 degrees.

Thursday night through Friday, there could be another chance for
thunderstorms as an upper level trough digs southeast out of central
Canada into the eastern US. This will cause a surface front to move
southward across the CWA which may provide a chance for showers and
thunderstorms. Highs on Friday may cool back into the lower 90s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1158 AM CDT Fri Jul 21 2017

VFR conditions are expected through the period. LLWS is a bit more
marginal tonight with surface winds staying near 10 knots
overnight, but have opted to add it in beginning between 06-08Z
for all sites. LLWS will end near 12Z for all sites as mixing
increases.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Excessive Heat Warning until 8 PM CDT Saturday for KSZ008>012-
020>024-026-034>040-054>056-058-059.

&&

$$

SHORT TERM...Wolters
LONG TERM...Gargan
AVIATION...Heller