Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 251732

1232 PM CDT Fri Jul 25 2014

...Update to aviation forecast discussion...

.SHORT TERM...(Today and Tonight)
Issued at 340 AM CDT FRI JUL 25 2014

Water vapor imagery shows a mid-level low progressing eastward
across southwestern Canada, which has helped to flatten the
mid-level ridge over the Central and Southern Plains. A few weak
embedded shortwaves were noted along the northeast edge of the ridge
axis over far eastern South Dakota and Nebraska, which resulted in
scattered shower and thunderstorm activity early this morning across
eastern Nebraska and into much of Iowa. This precipitation was
tracking southeast along the ridge axis and progressing toward far
northeast Kansas. However, the regional radar was showing this
activity weakening as it approached the area. Much of the
precipitation should remain north and east of the area, but have
slight chance PoPs in for far northeast Kansas this morning in case
of few of these showers/isolated thunderstorms clip the area. These
passing shortwaves contributed to high clouds building southward
into the area early this morning. This cloud cover combined with
breezy southerly winds from a 40-50kt low-level jet resulted in
temperatures this morning remaining fairly steady in the 70s.

As the mid-level low over Canada progresses toward the Northern
Plains, it will help to push the area of low pressure currently
situated over the High Plains further southeastward into the Central
Plains today and tonight. With surface high pressure anchored over
the eastern U.S., winds were out of the south this morning and will
shift to the southeast through the day. As the area of low pressure
gets pushed into the region today, expect a tightening pressure
gradient with southwesterly winds gusting upwards of 25-30mph
through much of the day. While model soundings show some of these
high clouds sticking around through the day, expect fairly deep
mixing to occur by this afternoon with highs likely reaching into
the mid 90s to low 100s. Dewpoints this morning should be high in
the mid 60s to low 70s, however models show the drier air over the
Texas/Oklahoma panhandles and western Kansas advecting into the area
as winds shift to the southwest. Even with somewhat cooler dewpoints
by this afternoon, temperatures should be warm enough to still cause
heat indices to reach into the 103 to 108 degree range, so a Heat
Advisory remains in effect for the entire area.

Models show the weak cold front associated with the approaching low
pressure moving into north central Kansas by mid afternoon and stalling
out over northern Kansas through tonight. A few models are showing
the potential for a few scattered showers and thunderstorms to
develop along this boundary, however there will be a modest amount
of CIN that will need to be overcome and models show a very small
window of time (roughly between 21z-01z) in which this cap weakens
some to potentially produce some isolated precipitation before
quickly building back in through the remainder of the evening and
overnight hours. Model soundings also show that if any storms were
to develop they would be very high-based and it may be a struggle to
lift parcels high enough to tap into the available instability. So
while it`s more likely that the cap will be too strong for any
precipitation to develop, have added some very low slight-chance
PoPs in for far northern Kansas for early this evening.
Southwesterly winds will persist overnight with high clouds
remaining in place, so expect mild temperatures with lows only
dropping into the mid/upper 70s.

.LONG TERM...(Saturday through Thursday)
Issued at 340 AM CDT FRI JUL 25 2014

Temperatures will remain quite hot through Saturday as the same hot
airmass from Friday will still be in place. Morning lows will be
warmer than Friday morning and expect a quicker warm-up. A weak
front/surface trough will be nearly stationary across the area, but
this is another situation for which the post-frontal airmass is
still quite hot and dewpoints are likely to be substantially higher
immediately north of the front. South of the front, in the very hot
air, low level winds will have a westerly component but are not
expected to be particularly strong so it will be interesting to see
the extent of dry air mixing down to the surface by afternoon. The
current heat index forecast of 103-107 has a bit of uncertainty
owing mainly to the potential for fairly substantial dewpoint
fluctuations across the forecast area. Given the uncertainty and
current borderline conditions, will not issue a heat advisory at
this time but it remains a strong possibility for the advisory to be
continued into Saturday.

By late Saturday, expect a very unstable airmass near and
immediately north of the surface front/trough. The upper jet will be
encroaching on the area from the north with increasing wind shear as
well. Current indications are that the instability will be capped,
and parcels will need to be lifted through a significant mixed layer
depth and then also through the inhibited layer...possibly resulting
in a lack of storms similar to this past Tuesday. However, there is
also a strong short wave trough set to cross Minnesota Saturday
evening, and the glancing influence of this trough could be enough
to spark a few storms. At this point, do not feel comfortable going
any higher than slight chances. If a storm develops, it stands a
fair chance to produce locally damaging wind gusts.

As the short wave pushes through Minnesota and into the Great Lakes,
a substantial push of colder air will surge through the region. This
will bring a return of pleasant and cooler-than-normal temperatures
especially for Monday through Thursday when highs are likely to be
in the low to mid 80s and lows hovering around the 60 degree mark.
Some model guidance indicates a few periods of QPF, but this seems
largely unlikely given the dry airmass, lack of instability, and
negligible large scale forcing. Have kept slight chances Tuesday
night into Wednesday, and again on Thursday for a pair of weak
northern disturbances moving quickly through the flow, but the
period looks mainly dry.


.AVIATION...(For the 18Z TAFs through 18Z Saturday Afternoon)
Issued at 1232 PM CDT FRI JUL 25 2014

VFR conditions expected. SSW winds will be the rule, with some
gusts through the daytime hours. Isolated very high-based
convection is possible mainly in the later half of this forecast
but impacts on flight categories should be nil.


HEAT ADVISORY until 7 PM CDT this evening FOR KSZ008>012-020>024-



SHORT TERM...Hennecke
LONG TERM...Barjenbruch
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