Area Forecast Discussion
Issued by NWS Topeka, KS

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FXUS63 KTOP 091731
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
1131 AM CST Mon Jan 9 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 325 AM CST Mon Jan 9 2017

Overnight lows tonight are several degrees warmer than in nights
past as southerly winds and warm advection return over the area.
Surface pressure gradient tightens up once again today and brings
sustained winds 15 to 25 mph across the forecast area, with southern
counties on the higher end.  Not seeing the potential for the higher
gusts as lapse rates in the low levels are modest, but still
anticipate 20-30mph with slightly higher in the far southeast.  The
winds are slow to diminish in the overnight hours as weak upper wave
approaches toward morning hours. After daytime highs in the 40s,
this should aid in keeping overnight lows up from the middle 30s
north to the lower 40s in the far southeast.  Enough isentropic lift
after midnight to bring lower clouds back in and with drizzle
chances increasing in the east toward morning, with temperatures
above freezing.

.LONG TERM...(Tuesday through Sunday)
Issued at 325 AM CST Mon Jan 9 2017

On Tuesday a shortwave trough tracks over the northern and central
plains. Ahead of this system low level moisture will be in place,
and depending on the depth of saturation and strength of the lift
there could be drizzle during the morning. Surface temperatures
should be generally be above freezing with the exception of far
northern KS. Although through out the day temperatures will
continue to warm into the 40s and 50s despite a weak cold front
moving through the area. On Wednesday an arctic high pressure will
build in from the northwest. Ahead of this front highs will reach
the mid 50s to lower 60s. As that front moves through the area
drizzle will again be possible, but mainly over far eastern KS.

The main concern for this forecast period will be the potential for
a winter storm across the central US late week into the weekend. On
Friday a reinforcing high pressure slides through the northern
plains and upper Midwest, which will be driven by a strong shortwave
that tracks over southern Canada. At the same time a shortwave
trough will dig over the southwest US. This along with ridging
over the southeast US will cause an increase in the southerly
flow over the central US and set the stage for an overrunning
event. With this flow pattern plenty of warm moisture air will
also surge northward up over the arctic air mass. The increase in
lift will cause precipitation to develop sometime during the day
Friday. At this time surface temperatures will be below freezing
while temperatures aloft remain above freezing. The models do
disagree on how warm these temperatures aloft will be, which will
dictate precipitation type. A few degrees aloft will mean the
difference between freezing rain, sleet, and snow. The depth of
the surface cold air will also play a role in the chances of
whether sleet or liquid reaches the surface. As of now that
precipitation type gradient will set up somewhere in the region.
As it does a prolonged period of precipitation will occur
especially Friday night through Saturday. There are also hints of
a lead shortwave ejecting out of the southwest trough, which will
enhance the large scale lift.

This pattern closely resembles those of storms capable of producing
significant icing. A majority of the models agree on the synoptic
features, and CIPS analog guidance is showing similar outcomes to
this pattern. Therefore have decided to introduce a winter mix to
the forecast given the thermal profile similarities from several
models. There is still plenty of time and there will likely be
deviations in the mesoscale features, which will have huge
implications on precipitation types and locations.

Towards Sunday into Monday the southwest trough begins to lift
northeastward while interacting with shortwaves dropping south
out of Canada. As these waves interact over the central US one
last round of precipitation is possible. This could be in the form
of snow and or a winter mix. The models are still struggling with
this and it is much too far out to focus on those details.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1130 AM CST Mon Jan 9 2017

VFR conditions are expected through the period.  Have opted to
include LLWS in after 02Z at TOP/FOE where winds above the low level
inversion are strongest.  Models have backed off on lower
visibilities early morning due to winds still up to 10kts overnight.

&&

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

$$

SHORT TERM...67
LONG TERM...Sanders
AVIATION...Heller



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