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000
FXUS63 KTOP 222338
AFDTOP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TOPEKA KS
638 PM CDT Tue Jul 22 2014

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Wednesday)
Issued at 326 PM CDT TUE JUL 22 2014

There are two key features being carefully watched this afternoon.
The first is a cold front extending at 3 PM from near Belleville to
just north of Falls City NE, with slow but steady southward
progression. The second is a weak short wave trough that is tracking
into eastern NE & KS. The trough axis seems to extend from just west
of Omaha to near Concordia judging by subsidence and dissipation of
mid cloud deck steadily progressing southeast. Another somewhat
interesting feature is a pre-frontal trough extending from near St
Joseph to Manhattan to Great Bend with weak low level convergence
seemingly maximized in central KS where some cumulus has recently
bubbled up.

Ahead of the main cold front, temperatures will remain very hot well
into the evening. Dewpoints have mixed out into the mid 60s to lower
70s (west to east gradient) which has kept heat indices in the 103
to 111 range so far with a bit of an increase still possible.
Immediately behind the front is a band of moisture pooling with
dewpoints back into the middle 70s. So, while the temperatures
decrease a bit with the front, the heat index remains nearly the
same or even increases.

Model guidance remains split on the topic, but isolated to scattered
storms are still expected to develop along the surface front...with
a smaller chance of development near the pre-frontal trough. The
favored timing will be immediately in advance of the short wave
trough axis. While the trough is very weak, the weak subsidence in
the wake of the trough seems that it will be too much for the weak
low level forcing along the front to overcome. Its a delicate
convective balancing act between multiple factors this afternoon but
still think a few storms will develop. Wind shear is weak,
instability is moderate to strong, and the low level lapse rates are
very steep over a significant depth with a max theta-e difference of
30 to 40. This environment favors relatively short lived individual
storms with very strong cold pools and potentially strong
downbursts. There is a very small potential for hail due to pockets
of very strong instability but damaging winds will be the main
hazard and they should be fairly localized to the storm downdrafts.
At this point, a bit unsure how strong a cell will have to be to
support severe winds, but it could be a situation where fairly weak
reflectivity could support 60 mph downbursts.

There is some indication that another weak short wave trough will
move out of NW Nebraska into north central KS, and could set off
additional storms during the early morning hours. Expect this
activity to be scattered and non-severe if it develops. The cold
front will continue to push south overnight into Wednesday, with a
cooler and drier airmass gradually filtering in. Highs on Wednesday
will be a good 10 degrees cooler with a light northeast breeze and
lower dewpoints.

.LONG TERM...(Wednesday Night through Tuesday)
Issued at 326 PM CDT TUE JUL 22 2014

By Wednesday evening northeast KS will be situated down stream of
the upper level ridge axis as several shortwaves are forecast to
ride along the northern periphery. The first wave drops down on
Thursday and early Friday morning bringing a slight chance for
precip to northeast KS through weak isentropic lift. The ECMWF
has been consistent in taking a slightly stronger wave through the
Midwest, and keeping this lift northeast of the area. Temperatures
for Thursday appear to be closer to normal with highs in the mid
80s to mid 90s. On Friday a boundary sags south through the area
although we remain mostly dry until it hangs up somewhere across
eastern KS. The GFS is suggesting another weak wave ejects from
the central Rockies and bisects the front Friday night causing
slight chances in precip across the area. Ahead the front temps
heat up again Friday afternoon reaching the mid 90s to near 100
especially in central KS. Towards the end of the weekend the deep
upper level low is still forecast to dig into the Great Lakes
region. A secondary front and additional mid level support arrives
on Sunday, although there are still timing differences among the
models. Half of the GFS ensembles agree with the ECMWF and the slower
solution, therefore have chances for both Saturday night and
during the day Sunday to account for these differences. High
pressure builds in behind the front on Monday and ushers in
another unseasonably cool air mass for a good portion of early
next week. The longevity of this air all depends on how long the
upper low spins over the Great Lakes keeping the upper ridge
suppressed over the western US.

&&

.AVIATION...(For the 00Z TAFs through 00Z Wednesday Evening)
Issued at 635 PM CDT TUE JUL 22 2014

An isolated storm may develop this evening along or north of the
surface cold front. I am not confident enough to insert VCTS in
TAFS. Otherwise, expect a wind shift at the terminals to the
north-northeast this evening and VFR conditions.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
HEAT ADVISORY until 9 PM CDT this evening FOR KSZ008>012-020>024-
026-034>040-054>056-058-059.

&&

$$

SHORT TERM...Barjenbruch
LONG TERM...Sanders
AVIATION...Gargan