Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 191150

Area Forecast Discussion
National Weather Service Topeka KS
550 AM CST Mon Feb 19 2018

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 408 AM CST Mon Feb 19 2018

A strong cold front will bring a wide variety of weather to NE
Kansas today and tonight, with the chance for thunderstorms in
east central Kansas and freezing rain tonight.

The morning surface analysis showed increasing baroclinicity along a
cold front stretching from Salina to Falls City as the attendant low
pressure center approached from the SW. Surface temperatures ahead
of the boundary ranged from the mid 50s to low 60s, but rapidly
dropped into the low 30s behind the front (Concordia`s temp dropped
from 52 to 35 in one hour). Light drizzle has been reported at a few
sites in the warm sector, but was rather limited in its overall
coverage. Aloft, water vapor imagery denoted a digging upper
tropospheric trough along the west coast of the U.S. and broad SW
flow and height falls ahead of this wave. Short and medium range
solutions are in good agreement in the overall progression of the
surface cold front through the day and tonight. However, the devil
is in the details with this forecast with respect to both convective
trends and freezing rain potential.

We start first with the overall thunder potential. The evening
sounding from TOP revealed a +10 C H850 inversion aloft associated
with a downslope air mass working through the SW mid-level flow. A
few showers and thunderstorms formed ahead of this warm nose in
central Missouri tonight, but have rapidly shifted away from the
region. The overall threat for showers and storms is very limited
given the strength of the cap, but have maintained a slight chance
of showers and storms in the SE CWA this morning given the strength
of the LLJ and some hints in the RAP/NAM forecast soundings of
diminished inhibition at times during the morning. Recent convective
development just to the south of the CWA in Wilson County would
justify this notion of slight chance POPs. Low stratus looks to
linger in the warm sector all day based on RAP/HRRR/NAM soundings,
limiting daytime insolation and the threat for surface-based
convection. Therefore, the best chance for thunderstorms will be
behind the front where upwards of 500-800 J/kg of MUCAPE may be
realized. Increasing isentropic lift over the warm nose keeps the
threat for thunderstorms continuing in the far SE CWA after
midnight, but with increasing CAA, the threat should be rapidly
waning at the end of the period.

As the aforementioned 295-300K isentropic upglide/moisture advection
interacts with the 950-850 mb front after 00Z tonight, additional
precip is expected to develop rearward of the surface boundary where
surface temps/wet bulbs will be in the upper 20s and H850 temps
still push +7 to +10 C. The strength of this moisture advection will
be key to how much precip is generated over the cold air, and
despite the event being within 24 hours of occurrence, models are
struggling with how much moisture will interact with the frontal
surface. The EC is the most aggressive in FZRA production, while the
GFS remains nearly dry. The NAM, which up until recently has been
similar to the EC, has started to trend slightly lower on QPF
amounts, but still keeps much of the p-type as FZRA. For now, have
not strayed too far from the going forecast for QPF and ice amounts,
which brings one to two tenths of ice to the central and NE CWA.
Since confidence is increasing that at least some amount of
impactful ice accretion will occur, have issued a winter weather
advisory for areas that will mostly likely see the highest ice
amounts. FZRA is possible to the south and east of the current
advisory outline, so expect additional areal adjustments before the
onset of the event.

.LONG TERM...(Tuesday through Sunday)
Issued at 408 AM CST Mon Feb 19 2018

For Tuesday the main questions is how quickly cold air moves
southeast and will there still be precip ongoing when temps fall
below freezing. The NAM continues to be the coldest solution while
the GFS remains the warmest. Have tried to stay in the middle of
these two ideas which is similar to the ECMWF and Canadian
solutions. In general this keeps east central KS above freezing for
much of the morning with a relatively narrow axis of freezing rain
on the back side of the expected precip shield. Confidence in this
idea is slowly increasing as models have been somewhat consistent.
Additionally the SREF tends to be a little warmer with surface temps
than the operational NAM. So think the potential for a significant
icing as suggest by the NAM soundings could be overdone. Models show
the moisture surge and frontogenesis moving southeast of the
forecast area rather progressively Tuesday afternoon and by the time
freezing temps reach east central KS think the precip should be
pretty much over. There continues to some indication for convective
instability along the frontal boundary Tuesday morning, so have a
mention of thunder until the front moves southeast of the forecast
area. For the most part there should not be much of a diurnal trend
in temps on Tuesday with persistent cold air advection and think
temps will generally be falling through the day.

For Tuesday night and much of Wednesday, dry weather is anticipated
with surface ridging and dry air over the region. However by
Wednesday evening models indicate a subtle wave moving within the
southwest flow aloft that will cause some warm air advection up and
over the shallow surface ridge. This warm air advection and
isentropic lift is generating some light precip from the models.
Initially forecast soundings show the saturation high enough for ice
within the clouds and snow the more likely precip type. But as the
night progresses, forecast soundings lose the ice in the cloud so
another freezing rain or freezing drizzle event is possible
Wednesday night into Thursday morning.

The synoptic pattern of a longwave trough to the west and the upper
jet overhead is expected to persist through the end of the work week
and into the weekend. This is expected to keep active weather over
the area until perhaps Sunday when a better defined shortwave lifts
out from the southwest and the longwave trough to the west breaks
down. Because of this there are chances for precip from Thursday
through Saturday night. Confidence in precip chances for Thursday
and Friday is not high since models have not had the best run to run
consistency and the QPF is generally derived from mesoscale forcing.
However there does appear to be some consensus with the shortwave on
Saturday and cyclogenesis developing with the wave. For now the GFS
and ECMWF track the surface low to the south of the forecast area
and keep the warm sector just south. With reasonable moisture able
to advect north, the pattern would support the potential for
convection within the warm sector. So will keep an eye on how later
model runs trend with this system. Temperatures are expected to
gradually moderate through the end of the work week and into the
weekend. Although a big warm up does not look to be in the offering
as the pattern favors high pressure at the surface remaining over
the central plains. Because of this, precip type could be tricky
dependent time of day whether it is rain or snow.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 548 AM CST Mon Feb 19 2018

MVFR to IFR ceilings persist for much of the morning, possibly
clearing this afternoon before returning tonight. LLWS continues
at TOP/FOE through 14Z today and then possibly redevelops
overnight tonight. Scattered showers and possibly a thunderstorm
will impact areas from TOP on east today, then spread back north
and west tonight. Freezing rain also is expected north of a line
from MHK to STJ between 00 and 12Z tonight. A strong cold front
moves through this morning, with winds veering to the northwest
to north at 10 to 20 kts after its passage.


Winter Weather Advisory from 6 PM this evening to noon CST
Tuesday for KSZ009>012-021>024.



LONG TERM...Wolters
AVIATION...Skow is the U.S. government's official web portal to all federal, state and local government web resources and services.