Area Forecast Discussion
Issued by NWS Topeka, KS

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293
FXUS63 KTOP 202350
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
550 PM CST Sat Jan 20 2018

...Update to aviation forecast discussion...

.SHORT TERM...(This evening through Sunday)
Issued at 334 PM CST Sat Jan 20 2018

An interesting forecast for tonight through tomorrow as an upper
level system currently located over central Great Basin moves
towards the central Plains. An inverted trough axis has set up
across the area as a lee cyclone deepens across southeastern
Colorado. Stratus lingering from the morning may break up slightly,
but overall, clouds will linger overnight into tomorrow. Isentropic
ascent increases across the area overnight which may support some
drizzle, especially northern Kansas near the Nebraska border where
lift appears the strongest. There is also the potential for some
patchy fog formation overnight if clouds can break up enough with
soundings depicting saturation along with light winds. Continuing
into tomorrow, there is a small chance for some drizzle through the
day. However, this forecast leans towards a drier GFS solution, as
the NAM has continued to have model inconsistencies with multiple
parameters. This includes a tendency for the NAM`s depiction of the
boundary layer to be too cool and poorly mixed, while over-producing
precipitation today whereas dry conditions are observed. With that
said, there are still slight chance PoPs included through the day on
Sunday in the event some ascent is seen and drizzle occurs.
Temperatures will be generally be above freezing across the entire
CWA tomorrow, with the exception of very far north-central Kansas
where freezing drizzle may occur at the onset of precipitation
before surface temperatures rise.

On the topic of temperatures, an evolving warm sector associated
with the aforementioned deepening cyclone will extend northward into
the area. The region should see one more day of warmer temperatures
on Sunday -- ranging from the mid 40s north of the CWA-bisecting
trough (frontogenetic boundary) to upper 50s and perhaps low 60s
south of this feature. By the afternoon, the GFS/ECMWF depict
substantive moisture advection within the warm sector, supporting
dewpoints into the lower 50s extending into east central Kansas by
mid-afternoon. This will facilitate MUCAPE on the order of 200-400
J/kg within a northward-arching corridor preceding a weak-baroclinic
zone trailing south of 998mb-1000mb low pressure advancing towards
northeastern Kansas. Centered around peak heating, isolated to
perhaps widely scattered showers or even a thunderstorm may develop
along the weak-baroclinic zone as the leading edge of the dry slot
enters the area -- particularly in proximity to the surface low
where low-level convergence will be maximize. The current thinking
is that there may be enough pre-existing vertical vorticity near
the boundary -- especially near the surface low -- along with all
the other previously mentioned parameters, for a conditional risk of
a brief convective-scale circulation wrapping up within the most
robust convective elements. This will be highly dependent upon the
sustenance of any updraft structures -- which will likely be quite
shallow in nature, perhaps with little to no lightning. Spatially,
the most likely locations for this to occur would be from
Pottawatomie and Jackson Counties extending south to Coffey and
Anderson Counties. Keep in mind, the potential for this occurrence
is very low, but it does warrant a mention. Regardless, the
conditionality of such potential and overall very weak buoyancy
fields precludes direct mention of any severe-weather hazards at
this time.  As the evening continues, the dry slot continues into
the area before the deformation zone advances into the CWA.

.LONG TERM...(Sunday night through Saturday)
Issued at 334 PM CST Sat Jan 20 2018

A surface cold front will be clearing the region during the
overnight hours Sunday night, trailing south/southwestward from deep
surface low pressure tracking across the lower Missouri Valley
region vicinity. A tight surface pressure gradient will be
established across the region behind the departing surface low, with
strengthening west-northwesterly winds veering to north-
northwesterly into the daytime on Monday. These winds will
facilitate ample cold advection within the boundary layer, resulting
in Monday`s high temperatures being substantially colder than those
on Sunday -- i.e., Monday`s highs in the middle/upper 30s.

Meanwhile, forcing for ascent within the expanding/coiling
deformation zone of the low/mid-level cyclone will support
precipitation spreading across the region from Sunday night into
Monday. Cooling throughout the troposphere -- aided by strong ascent
preceding the deep midlevel cyclone -- will support sufficiently
cold/sub-freezing temperatures aloft for a mix of precipitation
types. This includes a combination of snow and rain, with the most
pronounced cold advection and related prospects for snow west of a
line from Seneca to Emporia. Across these areas, total snow
accumulations of one-half to one inch may occur -- with locally
higher accumulations possible north of Interstate 70. Moderately
steep midlevel lapse rates accompanying the cyclone`s attendant
midlevel cold pool will combine with moisture coiling into its
southwestern quadrant to support a slight convective enhancement to
precipitation character through the day. This may couple with the
strong winds for locally reduced visibilities in areas of snow.

Precipitation will exit the region late Monday into Monday night as
the deep cyclone becomes increasingly displaced away from the
region, while winds will be diminishing. Dry conditions are expected
on Tuesday and Wednesday with generally near-normal temperatures. By
late week, medium-range model guidance indicates a cyclonic
perturbation potentially progressing from the western states toward
the central CONUS. Warm advection preceding this feature, aided by
lee troughing over the High Plains, would facilitate a warming trend
and perhaps some chance for rain by Friday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 550 PM CST Sat Jan 20 2018

Again tonight main concern is timing and onset of stratus. MVFR
deck has moved to the north of the terminals, but backed surface
winds under inversion and high low-level RH likely will lead to
return of limiting deck over next several hour. Have again stayed
furthest from NAM-based guidance with their wet/cool near-surface
bias, but still anticipate IFR cigs to develop in the 06-10Z
window with gradual improving conditions after 14Z as the deep
cyclone approaches from the west-southwest and low-level winds
veer.

&&

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

$$

SHORT TERM...Heller
LONG TERM...Cohen
AVIATION...65



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