Area Forecast Discussion
Issued by NWS Topeka, KS

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128
FXUS63 KTOP 171145
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
545 AM CST Sat Feb 17 2018

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 341 AM CST Sat Feb 17 2018

A band of mixed precipitation will affect areas along and
southeast of the Turnpike early this morning, but otherwise quiet
weather is expected for the rest of today and tonight.

At 09Z, a broad SW/NE-oriented band of precipitation extended across
Oklahoma and into SE Kansas and SW Missouri attendant with a mid-
level impulse and associated Fg region. Pinning down the evolution
of the northern boundary of this precip shield has proven slightly
challenging with the plethora of dry air present in the lower
troposphere as shown in the 00Z TOP sounding. Radar echoes have been
present across SE Kansas for the last several hours and it hasn`t
been until recently that surface observation sites have started
reporting light rain. Stronger echoes are starting to be seen on
KTWX from Butler to Woodson counties just south of the CWA, but with
ceilings of 8-12 kft currently being observed in the area, it is
likely that additional saturation of the column is needed across the
CWA to allow any wetting precip to reach the surface. Surface Td
values under the band have risen into the mid 20s, but remain in the
mid teens further north in Emporia and Ottawa.

The 05-07Z HRRR runs has been overly bullish in the coverage of
precip just south of the CWA and while it has accurately predicted
the redevelopment along the northern side of the precip shield, did
not feel confident in boosting POPs to the level the HRRR predicts
in the next few hours given its recent history and the ambient
environment. Did use these HRRR trends, however, to model the
outward extend of mentionable POPs in the short-term forecast (which
is close to the 17.00Z EC POP forecast. However, will continue to
monitor trends in the next few hours and adjust POPs as needed.

Precip type for this band is also somewhat finicky and rests on just
a couple of degree variations in surface temperatures/wet bulb
values. Forecast soundings are in agreement that the much of the
moisture in the column will be liquid, though there is a chance for
some ice introduction. Omega fields peg the bulk of the lift
occurring between 0 and -7 C, promoting predominantly liquid drop
development. Should the column saturate ahead of the surface theta-e
advection wing, there is a short-lived window for freezing rain
along the Turnpike between 10 and 14Z. However, even a one or two
degree rise in temps will result in p-type changing over to rain.
RWIS pavement temperature sensors generally range from 32 to 37
degrees, which means that any freezing of precipitation will likely
accrete on elevated surfaces, if at all.

The forcing quickly wanes between 12 and 14Z, bringing an end to the
precipitation threat. Westerly downslope flow behind the departing
surface trough will facilitate ample mixing and allow highs to reach
the mid to upper 50s. Little in the way of sensible weather is
forecast for tonight as a second surface high translates through the
region.

.LONG TERM...(Sunday through Friday)
Issued at 341 AM CST Sat Feb 17 2018

There is not a lot of difference in the models until Tuesday night
and Wednesday when there are some minor variations which impact
the placement of QPF.

Return flow is expected to bring warmer temps and higher moisture
to the area Sunday and Sunday night. Forecast soundings continue
to show the boundary layer moisture remaining capped for the most
part by an elevated mixed layer (EML). Meanwhile upper level energy
is progged to dig into the southwestern U.S setting up southwesterly
flow over the central plains. Eventually a cold front should
slowly move through the area Monday night and Tuesday. This is the
setup for an active start to the first half of the work week. With
not a lot of differences in the models, the forecast follows the
general consensus with chances for precip Sunday night into
Tuesday. The first concern may be the potential for elevated
convection Sunday night into Monday morning. Although the bulk of
the moisture return is capped, there remains some marginal
indications for some mid level saturation along with the general
set up of a low level jet with some of the stronger moisture
return so far this year. So have continued with a slight chance
for thunderstorms into Monday. Another concern with the forecast
is a general lack of saturation in the dendritic growth zone
throughout the duration of the event Monday and Tuesday. Thinking
there may be a lack of ice in the cloud as temps fall below
freezing Monday night, have included a chance for freezing rain.
For now precip amounts look to be rather light so ice
accumulations so not appear to pose a large impact, but it will be
something to keep an eye on.

The main difference in the solutions is the Canadian generating
precip over the area Tuesday night and Wednesday. The Canadian
looks to be slightly more amplified with a shortwave lifting out
of the southern Rockies. As a result the Canadian shows a brief
period of a warm conveyor belt setup Tuesday night as shallow
cold air moves southeast. However the GFS and ECMWF do not support
this and show ridging building in with much dryer air
overspreading the forecast area. Have tended to lean more towards
the GFS and ECMWF as even the Canadian progs the frontal boundary
to be well south of the forecast area by Tuesday night. Have left
some small chance POPs across far southeastern portions of the
forecast area Tuesday evening and some slight chance POPs
Wednesday morning to match up with neighboring offices. There
could be a very tight gradient between the deep moist axis along
the boundary and the dry air to the northwest.

After the surface ridge passes by Wednesday and Wednesday night,
the ECMWF and Canadian show another surface wave potentially
developing Thursday night and Friday with return flow
redeveloping. The GFS is not nearly as amplified with the synoptic
pattern late in the week and shows shear out southwesterly flow
and a dry forecast. So confidence in the forecast is a little
below average given the differences. Have included a chance for
rain and snow Thursday night and Friday and will watch how things
evolve with later model runs.

Very mild temps are expected Sunday and Monday before the front
brings cold air back into the state for Tuesday and Wednesday. A
moderation in temps for the last half of the work week should
allow temps to trend closer to normals Thursday and Friday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 545 AM CST Sat Feb 17 2018

Area of light rain/FZRA and accompanying MVFR/IFR ceilings east of
a line from EMP to TOP will quickly exit NE Kansas before mid-
morning today. Skies clear with current S/SE winds veering to the
W/NW and increasing to 10 to 20 kts this afternoon. Winds lessen
and back to the south this evening, but increase to 10 to 15 kts
overnight. Marginal LLWS may develop after 06Z tonight, but
confidence in its magnitude and timing was not high enough to
mention in the 12Z TAFs.

&&

.FIRE WEATHER...
Issued at 341 AM CST Sat Feb 17 2018

Winds veer to the W/NW with the departing boundary this morning and
drier air will work into central Kansas. RH values fall to between
20 and 25 percent with winds of 10 to 15 kts, gusting to 20-25 kts
from the Flint Hills on west. A very high fire danger will exist
from the Flint Hills and west this afternoon as a result of these
conditions.

Southerly winds will increase on Sunday with gusts expected to be
around 35 MPH. The winds will create an elevated fire danger, but
will also increase humidity levels such that min RH values are
expected to remain above 35 percent.

&&

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

$$

SHORT TERM...Skow
LONG TERM...Wolters
AVIATION...Skow
FIRE WEATHER...Skow/Wolters



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