Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 270447 AAA
AFDTOP

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE TOPEKA KS
1147 PM CDT Fri Sep 26 2014

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Saturday)
Issued at 230 PM CDT FRI SEP 26 2014

Mid/upper low continues to spin over southwest IA this afternoon and
overnight. The rest of today will remain mostly clear with
temperatures in the low to mid 80s. There are indications that the
mid/upper low could generate isolated to scattered showers with
possible thunder after midnight. Soundings show moisture around the
850 mb layer with marginal lapse rates aloft allowing for some
elevated instability. None of the high res models are highlighting
any of this activity with the exception of north central MO. The NAM
and GFS show some isentropic lift basically beginning to increase
over far eastern KS so overall confidence is low at this point. Like
this morning this minimal lift should limit the coverage which will
be confined to far northeast Kansas after midnight until a few hours
after sunrise. Elsewhere will be partly to mostly clear with light
winds and low temperatures in the upper 50s to near 60. Tomorrow
after the slight rain chances end in far northeast KS expect partly
to mostly sunny skies as mid level dry air works into the region.
High temperatures similar to today should be in the low to mid 80s.

.LONG TERM...(Saturday Night through Friday)
Issued at 230 PM CDT FRI SEP 26 2014

The long term forecast is fairly active with several chances for
thunderstorms. The first will be for mainly isolated activity on
Sunday morning and perhaps lingering into the afternoon. This will
be associated with a weak vorticity max wrapping around the west
side of the closed upper low. Cooler mid level temps and a bit of
vertical motion associated with this vort max will provide an
opportunity for isolated to scattered convective shower
development with thunder possible. This should be focused in
northern KS but could extend south of I-70 during the morning. For
the most part though, Sunday through Tuesday morning will be dry
for the majority of the forecast area with highs in the lower to
middle 80s and lows around 60.

By late Tuesday, a large trough will be situated over the Rockies
with a strong lead short wave ejecting from Colorado to the NNE.
Deep surface low pressure will move almost due North from far
northeast Colorado and this northward trajectory will cause the
frontal boundary to have only limited eastward progression through
Tuesday night. Even with only a weak front, the upper trough axis
itself will likely provide enough forcing to support scattered
thunderstorms into north central KS by late Tuesday afternoon and
perhaps into northeast KS by Tuesday night. Instability is not
expected to be particularly impressive but SBCAPE around 1000 J/kg
and 0-6 km shear of 30 to 40 kts suggests storm organization is
possible late Tuesday and could see a few strong storms.

The front stalls and re-organizes from southwest into north central
Kansas on Wednesday. Could see some residual showers/thunderstorms
in eastern KS through early afternoon tied to the LLJ and previous
day`s activity, but expect more widespread thunderstorm
development along the front later on Wednesday with the approach
of another upper level short wave trough and the continued
approach of the long wave trough axis. Expect moderate instability
of 1000-2000 J/kg in/near the frontal zone with deep layer wind
shear again in the 30-45 kt range. This would again support storm
organization with some uncertainty in how widespread the activity
will be playing a role in storm intensity. Wednesday may provide
the best chance of the week for severe storms locally.

By Thursday, this slow moving system begins to push toward the
east, although there is substantial model uncertainty in the speed
of the surface front. The ECMWF holds the front in central KS
until Thursday afternoon while yet another strong short wave
rounds the base of the long wave trough. Meanwhile, the GFS blows
the surface front through the forecast area by Thursday morning.
Forecaster preference at this time is a hedge toward the ECMWF
solution which would provide another afternoon of favorable
parameters for strong to severe storms. The main question
throughout this convective 3-day event is how much destabilization
can occur with potential for ample cloud cover and convective
interference. Will need to monitor the period closely though as
some forecast elements do suggest severe weather for one or more
days.

Much cooler weather is expected behind the front with lows falling
into the 40s and highs around 70. Have kept slight chance POPs for
Friday for the potential of a secondary vort max rotating around
the west side of the long wave trough axis and providing scattered
showers.

&&

.AVIATION...(For the 06Z TAFs through 06Z Saturday Night)
Issued at 1134 PM CDT FRI SEP 26 2014

Although a very small chance for convection will linger over
northeast Kansas through 12z...confidence remains too low to
mention in the TOP/FOE TAF sites at this time as the better
chances remain north of the terminals. Will therefore maintain a
dry and VFR fcst at all terminal sites with light southeast winds
becoming southerly at 10 to 12 kts aft 15z/27.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Sanders
LONG TERM...Barjenbruch
AVIATION...63






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