Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 201016

Area Forecast Discussion
National Weather Service Topeka KS
416 AM CST Sat Jan 20 2018

.SHORT TERM...(Today and tonight)
Issued at 411 AM CST Sat Jan 20 2018

The midlevel trough of interest is currently located over CA and is
forecasted to continue progressing eastward over the Four Corner
region today. Surface high pressure currently centered over the
southeastern US is far enough east to allow gulf moisture to spread
northward towards the area. This moisture in the form of stratus is
building into south central KS now and will continue push into east
central KS this morning. Wind speeds have kept the boundary layer
mixed enough to prevent any fog from developing, but the models are
struggling with the mixing and suggesting lower visibilities. An
inverted surface trough draped across north central KS has caused
the winds to become light and the remaining snow pack in this area
could cause lows to reach the lower 30s. If the higher dew points
can manage to reach this area then fog could develop especially
along the northern edge of the stratus. Any fog that does develop
in northern KS this morning should mix out later today as evident
by the model soundings. This evening low level isentropic ascent
increases as the surface winds begin to back in response to the
frontal boundary, which will bisect the area. This ascent evident
on the 285 K theta surface should support drizzle and or fog
especially with the wind speeds generally under 5 mph. Guidance is
trending warmer with the temperatures tonight so the chance for
freezing drizzle is now confined to far north central KS. Again
the remaining snow pack could contribute to sub-freezing
temperatures at these locations by Sunday morning.

.LONG TERM...(Sunday through Friday)
Issued at 411 AM CST Sat Jan 20 2018

On Sunday the deepening midlevel low pressure ejects out of the
Rockies and the associated surface low moves through southwestern
KS. Temperatures are forecasted to rise above freezing and the
isentropic ascent should migrate to NE. Expect that there should be
a dry period late Sunday morning and early afternoon especially
given the modest EML in place. As the midlevel low lifts
northeastward over KS the EML erodes and allows for convective
instability over portions of eastern KS. In far east central KS
the steep level lapse rates become co-located over the mid 50 dew
points resulted in MUCAPE upwards of 200 j/kg. Most of the models
suggest the initiation of showers and storms along the prefrontal
trough or synoptic dry line as it quickly races eastward. The
shallow nature of this convection makes it hard to know how much
effective shear will be available for any of these potential
storms to become strong. General rain shower activity is a more
likely scenario across the as the large scale ascent becomes
maximized. Although the coverage appears limited as the dry slot
passes almost directly overhead.

Attention then turns to the deformation zone on the backside of the
system that is forecasted to progress through portions of north
central KS Sunday night. The temperature profile suggests mostly
snow within this band and the lift supported by lower tropospheric
frontogenesis. Mesoscale banding driven by CSI will be possible with
negative EPV evident above this frontogenesis. Snow accumulations
are expected to be minor for now although there are signs of
multiple snow bands. Into Monday the frontogenesis weakens, but
tracks over eastern KS as well as several potential vorticity
maxes rotating around the parent midlevel low pressure. A lack of
strong cold air advection in the wake of this system allows
temperatures to hover around freezing as this precipitation moves
through. Northerly winds are expected to be gusty, but with
temperatures slightly below freezing not sure if blowing snow will
be an issue. There is the potential for moderate to heavy
snowfall rates if any of these bands materializes, which could
restrict visibility. With highs in the mid to upper 30s on Monday
some of the snow could change over to rain. If the lift is
significant enough dynamic cooling could keep it all snow. Most
of the precipitation comes to an end by Monday evening. The
extended remains dry with more seasonable temperatures expected
and another system tracks over the plains next weekend.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1138 PM CST Fri Jan 19 2018

Have leaned closer to the GFS guidance for this forecast, with
overall less confidence in IFR ceilings. NAM is suffering from too
much snow depth and is too cool in the boundary layer, helping
raise near-surface saturation. Low MVFR cigs seem more likely for
much of this forecast, with some concern for lowering to IFR as
surface winds back.




SHORT TERM...Sanders
LONG TERM...Sanders
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