Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 120806
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
306 AM CDT Thu Oct 12 2017

.SHORT TERM...(Today and tonight)
Issued at 304 AM CDT Thu Oct 12 2017

Surface high pressure is currently centered over southern MO and is
forecasted to gradually retreat eastward today. With southwesterly
flow aloft a lee side trough has developed in the high plains. This
building pressure gradient is keeping the surface winds from fully
decoupling this morning. This may limit the potential for ground fog
with the exception of low lying areas in far eastern KS. The VWP
show 925 mb wind speeds are around 15 kt, which could explain the
lack at any fog at this point. There may be a brief window around
sunrise when ground fog develops. This fog could lift into a low
stratus deck just after sunrise before completely mixing out. A lot
of the short term guidance is still suggesting this fog potential,
but their coverage is already overdone in portions of MO. Some of
the soundings are showing that this residual low level moisture
could form into low stratus. A few model soundings suggest this and
show better saturation just above the surface. Current satellite
shows stratus back building westward, which could reach portions of
far eastern KS. Lows this morning are expected to drop into the mid
40s across most of the area. With the increased return flow and
mostly sunny skies today temperatures should reach the 70s with a
few locations around 80 in central KS. Higher dew points also try to
move northward across the plains today, but decided to keep them
slightly lower than model the model consensus since they tend to
overestimate initial advection within return flow.

.LONG TERM...(Friday through Wednesday)
Issued at 304 AM CDT Thu Oct 12 2017

Thursday night through Friday night, a broad upper level trough
across the western US will slowly shear into more a positive tilt
trough across the northern high plains and central Rockies. A
cold front across the upper Midwest, extending southwest into the
central high plains will shift southeast as a lead H5 trough
ejects northeast out across the northern plains on Friday.
Stronger low-level CAA across the northern plains will cause the
surface front to push southeast into the northern and western
counties of the CWA Friday. There may be enough residual moisture
for a few showers and isolated weak thunderstorms north of front
due to weak isentropic lift across northeast KS. The EML across
the warm sector should provide a strong enough CAP to prevent
surface base thunderstorms from developing along or ahead of the
surface front. I suppose if there was slightly more surface
convergence ahead of the front and the CAP did break there could
be a few strong surface based storms with half inch to 1 inch hail
and some gusty winds given an environment with sfc to 6 km
effective shear of 40 to 50 KTS and MLCAPES around 1000 j/KG. The
GFS is the only model that does develop QPF along the front across
northeast KS. As the front pushes southward through the night,
residual moisture transport will cause enough isentropic lift for
isolated to scattered showers. MUCAPES may increase to 50 to 100
J/KG, so I cannot rule out a weak elevated thunderstorm across the
CWA. The front will stall out across southeast KS around 6Z SAT,
then slowly move northward across the southeast counties of the
CWA during the early morning hours of Saturday.

850mb temperatures will warm to 15 to 18 deg C range and deeper
mixing will occur within the warm sector and should allow highs to
reach the mid 80s across much of east central KS. Northwest of
the front across north central and northeast KS, highs will be in
the upper 60s to lower 70s with winds switching to the northwest.
If the front shifts southeastward across the CWA a bit faster then
the mid 80 degree temperature readings for east central KS may be
too warm.

Saturday through Saturday night, The upper trough across the central
Rockies will continue to shear out as it moves east into the
central high plains. The northern portion of the trough will lift
east-Northeast into the upper midwest by 00Z SUN. The weak warm
front will gradually shift north-northwest across central and
northeast KS and become stationary by noon. The stronger
isentropic lift will shift northward across the northern counties
of the CWA and then into southern NE. There could be periods of
showers and isolated elevated thunderstorms through the day across
north central and northeast KS to the northwest of the stationary
front. Strong low-level CAA will develop across the northern and
central high plains through the day Saturday. This will cause a
much stronger front to rapidly push southeast across western and
central KS through the afternoon hours of Saturday. A surface and
850mb ridge axis will extend from the southeast US, west-southwest
across the TX gulf coast. This would keep the true transport of
gulf moisture well south of the plains across extreme southern TX.
We will see a modified return flow, though most models show 850mb
Td of 12 to 14 deg C. This may allow for moderate instability to
develop east of the surface front across east central KS. The 00Z
NAM forecasts MLCAPES of 2000-2500 J/KG to develop across east
central KS Saturday afternoon, these vales look a bit over done.
While the GFS MLCAPE forecast of 1200-1500 look more reasonable.
As the stronger surge of colder and drier air surges southeast
across north central KS, the once stationary front will begin to
push rapidly southeast across east central KS during the late
afternoon and early evening hours. A line of storms will develop
but the only hazard I can forecast is for gusty winds along the
squall line and possibly some isolated severe wind gusts as
updrafts may be strong enough to entrain the 50-55 KT
southwesterly 700mb winds towards the surface. However, every
updraft that develops along the front will probably be undercut by
the outflow. Therefore, I do not expect any large hail from any
of the updrafts that develop along the line. The low-level winds
veer more along the front even though the NAM and GFS forecast 0-1
SRH around 200 J/KG ahead of the surface front. The 0-3 KM shear
vectors looks more parallel to the front, so I have very low
confidence of any meso-vortices developing within the line of
storms. The front should push southeast of the CWA by 3Z, thus any
minimal damaging wind hazard should end once the leading edge of
the convective line pushes southeast of the CWA. Through the night
there will probably be post frontal showers and perhaps a few
isolated elevated thunderstorms as the positive H5 trough moves
east across the central plains.

Highs on saturday will be dependent on how far northwest the warm
front will move across the CWA and how quick the surging front
across northwest KS will reach the CWA during the afternoon hours.
At this time I have much of east central and portions of
northeast KS in the lower to mid 80s. Northwest of the surface
front highs will be in the lower 70s across north central KS.

Sunday through Thursday, the northwest flow at mid levels across
the plains will become zonal. The ECMWF keeps the CWA dry through
the period. The GFS has a minor H5 trough tracking east across the
central plains which may provide the CWA with a chance for showers
Wednesday night. Highs will only reach the upper 50s to lower 60s
behind the surface front on Sunday. As a lee trough develops
across the high plains southerly winds Monday through Thursday
should help to warm highs into the lower to mid 70s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1123 PM CDT Wed Oct 11 2017

Models have been backing off on fog development tonight, so the
VFR forecast will continue through the period. There still may be
some ground fog early this morning, but expect that to be limited
to river valleys and low lying areas. There is a chance for LLWS
across the area late Thursday night, but have left out of this TAF
issuance with it being at the very end of the forecast period.


&&

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

$$

SHORT TERM...Sanders
LONG TERM...Gargan
AVIATION...Heller



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