Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 292335 AAA
AFDTOP

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE TOPEKA KS
635 PM CDT Mon Sep 29 2014

...Update to aviation forecast discussion...

.SHORT TERM...(This Evening through Tuesday)
Issued at 303 PM CDT MON SEP 29 2014

On the large scale, the current weather pattern is  characterized by
surface high pressure over eastern KS and MO with a deepening
surface low in southeast Colorado. The surface low has been
developing in response to the approach of a long wave trough over
the Rockies and the ejection of a strong embedded short wave trough
from the four corners region. This initial strong short wave will
track into western South Dakota by Tuesday morning with the base of
the trough energy drifting across northern KS through the day on
Tuesday. Thunderstorms will develop in eastern CO and western KS
late today and progress east overnight. These may approach the
highway 81 corridor as early as 3 AM but will likely hold off until
closer to sunrise. A gradually veering low level jet will continue
to feed increasing theta-e air into the zone of ascent aloft and
should result in the axis of scattered thunderstorms persisting
through the day on Tuesday, although seemingly likely to show a
decreasing trend with eastward extent through the day. Instability
will generally be limited in association with these storms and
severe weather is unlikely through at least early afternoon. By mid
afternoon however, expect destabilization to occur in the wake of
the initial thunderstorms near and ahead of a low level convergence
zone in north central KS. This convergence will be enhanced during
the evening as a new jet max moves out of the Rockies and surface
low pressure again intensifies in southeast Colorado...backing winds
locally. Another round of storms will likely develop in response to
this in central and north central KS. 0-6 km wind shear will be
sufficient for organized storm structure, including supercells, but
the main question will be just how much instability can develop
before sunset. Most indications suggest in the neighborhoods of
around 1000 J/kg of SBCAPE but the mid-level lapse rates are not
particularly impressive. With all of this in mind, there seems to be
a period roughly from 4 PM to 9 PM during which semi-discrete storms
with embedded supercell structure are possible. LCL`s are elevated
and low level shear not particularly impressive so the main hazards
with any strong storms through the evening would be locally damaging
winds and perhaps severe hail (more likely if mid-level storm
rotation develops). The LLJ intensifies by late evening into the
overnight with a focus for thunderstorm development remaining in the
local area especially north of I-70. Moisture content will increase
and there is some potential for a bit of storm training so a
localized flash flood threat could develop by Wednesday morning.
Modest elevated CAPE in combination with decent elevated shear
profiles may allow a marginally severe hail threat to persist
through Tuesday night.

Through sunrise Wednesday, east central KS, mainly near/south of
I-35, is the most likely to be unimpacted by thunderstorms, and most
likely to have temperatures in the low to mid 80s for highs on
Tuesday.

.LONG TERM...(Tuesday Night through Monday)
Issued at 303 PM CDT MON SEP 29 2014

The next mid level shortwave and associated jet streak will begin to
lift into the plains Wednesday morning. At the surface a low
pressure forms across southwest KS and OK panhandle with a
quasi-stationary front which extends through north central KS.
Ongoing showers and thunderstorms will likely persist through the
morning hours on Wednesday, and possibly the afternoon. During the
day as the system ejects the Rockies high theta-E air surges north
into northeast KS south of the boundary. The main concern will be
can the warm sector recover from the ongoing activity. The 12z NAM
depicts decent boundary layer mixing, while mid to upper 60 dew
points wrap northward ahead of the sfc low covering the entire area
by the afternoon. Also, steep mid level lapse rates overspread the
region from the west allowing for an increase in MU cape. The NAM is
most aggressive with this instability, which could be as high as
2000 j/kg by late morning. As the same time deep layer shear greater
than 40 kts moves over the region. Therefore any storms forming
along the front in north central KS could be strong to severe by
late morning. Also, elevated storms across the warm sector could be
possible with a low level jet nosing into northeast KS as depicted
by the NAM, and given the shear these storms could be strong as
well. The other main issue with this forecast is that the models
show several minor shortwaves, which may or may not contribute to
upward motion and or subsidence at any time on Wednesday.

During the afternoon and evening hours the sfc low and associated
dry punch lifts into central KS with deep layer shear increasing to
as high as 55 kts. The sfc low should become a focus for development
of strong to severe storms late Wed. With the sfc low approaching
central KS low level winds in eastern KS begin to back to the
southeast. This causes the hodograph to exhibit strong clockwise
curvature contributing to high SRH especially around sunset. If
boundary layer mixing is efficient than LCLs are forecast to be
just over 1 km across most of the area. It is this time frame that
there may be a potential for a tornado or two. The most favorable
area should be northeast of the sfc low, which would cover our
western and southwest forecast area through the evening hours. If
the nocturnal inversion were to develop quickly than the tornado
and wind potential would diminish shortly after sunset. Another risk
to be aware of will be flooding. This system will pull plenty of
moisture northward with PWATs ranging from 1.5 to 1.8 inches. Storms
producing locally heavy rain and training over the same areas
could poss a threat.

There is slightly better agreement among the models with regards to
the timing of the main trough axis and associated energy. Although
the ECMWF continues to be the fastest solution, which starts pushing
the cold front through the area overnight Wednesday. The NAM and GFS
slowly push the front eastward leaving a potential for strong to
severe storms in east central KS Thurs afternoon. Will leave out the
details on how this system further evolves during this timeframe
given the uncertainty. Otherwise post frontal showers and some
thunderstorms are possible as the main trough axis passes
overhead during the day Thursday. Behind the system cooler and
drier air filters into the region making for a pleasant weekend
with temps closer to normal.

&&

.AVIATION...(For the 00Z TAFs through 00Z Tuesday Evening)
Issued at 616 PM CDT MON SEP 29 2014

Will continue with a dry and VFR forecast through 14z at all terminal
sites with showers then possible in the KMHK vcnty beyond 15Z as
the ejecting high plains trough skirts the area. For now...
appears precip chances in the KTOP/KFOE terminal vcnties thru
00Z/01 look too low to mention in the TAFS at this point. Either
way...vfr cigs should be aoa 5 kft all sites through the 00z/01.
Only potential vsby concern would be if the extent of mixing and
high clouds through sunrise are less than expected which could allow
a brief window for fog to form...but again...confidence on this
scenario remains too low to include in the TAFS at this point.

&&

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

$$

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






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