Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Topeka Kansas
1134 PM CST Fri Dec 13 2019
..update to aviation forecast discussion...
Short term...(this evening through saturday)
issued at 350 PM CST Fri Dec 13 2019
A quick hitting band of rain/snow pushes across central Kansas
this evening, with colder air filtering southward tonight.
A compact mid to upper level vorticity lobe over eastern Nebraska
--well depicted on water vapor imagery--will continue to track
southeastward through the course of the evening. A region of mid-
tropospheric frontogenetical forcing and deformation has led to
the development of a broad, but weak, band of precipitation along
the southern flank of the pv lobe. This area of light rain/snow
likewise shifts southeastward this evening and gradually decays
before midnight over east-central Kansas. A modest sub- 700mb dry
wedge, coupled with the transient nature of the lift, means that
little in the way of impacts are expected from this event.
The passing of this upper level wave Ushers a cold front
southward this evening, with high temperatures on Saturday a solid
15 to 20 degrees colder than today as h850 temps fall to -5 c. An
upstream low amplitude upper tropospheric ridge axis will keep
the weather quiet on Saturday before the next system knocks on our
doorstep for later in the weekend.
Long term...(saturday night through friday)
issued at 350 PM CST Fri Dec 13 2019
The main focus of the forecast period continues to be the wintry
system expected Sunday into Monday morning. There is still a
greater than normal amount of uncertainty in both timing and
amounts, with guidance varying with regards to the speed and
placement of key features.
By Sunday morning, a large positively-tilted upper trough will be
located over the southwest, with a surface low developing in the
Lee of The Rockies and broad isentropic ascent increasing over the
plains. This will lead to a band of light to moderate snow
developing during the morning over north-central Kansas, slowly
moving to the east through the afternoon and evening. The axis of
heaviest snow currently looks to set up along or just north of
I-70, though again there is some variation amongst the guidance
as to its exact location. With temperatures throughout the
vertical profile below freezing, snow is expected to be the
primary p-type, especially in areas of steadier precipitation.
There is the potential for some freezing drizzle south and east of
the Turnpike during times of lighter precipitation, as a dry slot
aloft may lead to a loss of cloud ice necessary for snow
formation. At this time, any freezing drizzle/rain accumulations
are expected to remain around or less than a tenth of an inch.
Sunday night into Monday morning, the initial band of snow will
continue to move off to the east. At this time, a large spread
exists with regard to the speed of the surface low, with some
guidance depicting the low over Ohio while others have it over
Arkansas. A quicker and weaker low would lead to a faster shutoff
of any precipitation, but a slower low could lead to a band of
snow lingering into late Monday morning on the northwest side of
the low. With regards to amounts, we are currently taking a middle
ground approach given the large spread in the guidance solutions,
but with room to adjust either way in future updates. For now,
highest amounts of 2-4" most likely along and north of I-70, with
accumulating snow still likely elsewhere. Despite the uncertainty
in exact amounts, focus should remain on the likely impacts to
travel as a result of the accumulating winter weather.
After the storm, the rest of the period looks to quiet down as a
ridge pushes east onto the plains. Temperatures should start out
cold, with highs in the 30s through Tuesday. While potential snow
cover may by a reducing factor on temperatures as the ridge
continues to build over the plains, highs will continue to
moderate into the mid-40s by Thursday.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1134 PM CST Fri Dec 13 2019
A deck of MVFR clouds was working its way to near the Kansas border
from the north at this writing and still looks to move over the
terminals in the 08z-12z window. Can't completely rule out IFR
levels in slightly greater moisture locally. VFR conditions will
be the rule over the last 2/3rds of this forecast.