Area Forecast Discussion
Issued by NWS Topeka, KS

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135
FXUS63 KTOP 271133
AFDTOP

Area Forecast Discussion
National Weather Service Topeka KS
533 AM CST Mon Feb 27 2017

...Update to aviation forecast discussion...

.SHORT TERM...(Today and tonight)
Issued at 340 AM CST Mon Feb 27 2017

Early this morning areas of dense fog formed across far east central
KS due to the light winds in place and residual moisture from
yesterday`s light precipitation.  Expect these areas of fog, with
one-quarter mile visibilities or less, to become more widespread
before sunrise and then dissipate by mid-morning as mixing
increases.  With temperatures in the 20s this morning, freezing fog
may cause some slick road conditions, primarily on bridges and
overpasses.

Southwest-to-northeast oriented mid-level flow will be fairly zonal
today.  However, models show an embedded shortwave developing over
Oklahoma and lifting northward into eastern Kansas this morning into
this afternoon.  Model soundings show increasing mid-level moisture
with this embedded wave, but low-level dry air will likely remain in
place.  As a result, the question becomes whether or not this mid-
level moisture can saturate low enough to produce some scattered
light precipitation across eastern Kansas by this afternoon.  Due to
the low confidence in this precipitation from the low-level dry air,
have only slight to low-end chance PoPs in the forecast.  Models
show the area of low pressure centered over Colorado shifting
further eastward into Kansas today, resulting in an increasing
pressure gradient and, thus, increasing southerly winds.  These
breezy southerly winds will support decent warm-air advection into
the region so have increased high temperatures a bit into the upper
50s to low 60s.  With these mild conditions in place, any
precipitation that develops will be in the form rain. Better
moisture and lift will be focused over western Missouri by early
this evening, however some of these scattered rain showers may clip
far eastern Kansas. With some weak elevated instability noted in
model soundings, cannot rule out the potential for some isolated
thunder early this evening.

By tonight, surface low pressure will track northeastward into
northwest Kansas and into northeast Nebraska by Tuesday morning.
Models show a decent pressure gradient remaining in place overnight
across east central Kansas to support ongoing warm-air advection, so
increased low temperatures for tonight across that region.  However,
across north central Kansas, the associated cold front will be
tracking into the area by Tuesday morning.  As a result, expect a
large temperature spread tonight with lows ranging from the lower
40s over north central Kansas to the middle 50s over east central
Kansas.

.LONG TERM...(Tuesday through Sunday)
Issued at 340 AM CST Mon Feb 27 2017

On Tuesday morning, surface low pressure will be moving northeast of
the area with cooler air building into far northern KS. A new
stronger surface low will be developing in NW Oklahoma as an upper
trough and associated jet streak cross the Rockies. This surface low
will track east along the KS/OK border, eventually pulling the cold
front south and through the area by Tuesday evening. As the front
shifts south, there is a chance for thunderstorms to develop in
eastern KS south of the turnpike. Chances for development this far
west are on the low end and will depend on how quickly the front
surges but if a storm develops in the forecast area it could become
strong before moving off to the east.

Tuesday evening also remains very interesting, meteorologically
speaking, for the potential of a relatively narrow band of snow.
Multiple forcing mechanisms are likely to align within the forecast
area, including a band of strong frontogenesis, pressure advection
on the 1.5 PVU surface with the incoming upper trough, and a strong
signal for a deep layer of convective instability above the
frontogenesis. NAM and GFS forecast soundings have actually
suggested that the convectively unstable layer could extend through
more than 10,000 feet vertically...although still less than 200 J/kg
of MUCAPE. Low levels look to be very dry, and overcoming this dry
layer will be a significant limiting factor, but the lift will be
more than sufficient to overcome this specifically where the
strongest frontogenesis band develops. The biggest question with the
greatest impact on sensible weather will be the thermal profiles
associated with the precip. Temperatures will be very warm at precip
onset with surface T in the upper 40s and a warm layer depth
greater than 5000 feet. However, the dry low-level air will have
wet-bulb temperatures very near the freezing mark with cold
advection expected through the evening as well. NAM is most
aggressive in the wet-bulb cooling of the profile but also seems the
most thermodynamically reasonable within the heaviest precipitation.
GFS forecasting soundings make good sense except for the lowest
layers which it keeps inexplicably warm with superadiabatic layers.
IF precip changes to snow within the heaviest parts of the band, the
snow fall rate would very likely out-pace the melting rate for a 1-3
hour period.

With all of those factors considered, believe that a band of
precipitation will develop with precip rates at times in the 0.10-
0.25" per hour range. This precipitation will initially be rain, but
believe that it will be heavy enough to effectively wet-bulb the
thermal profile toward a rain/snow mix and then a slushy snow. Also
believe that this will only occur within the heaviest precipitation
within the band, and perhaps at points north of that heaviest precip
area. Some accumulating snow seems likely over a relatively small
portion of the CWA, wherever those heaviest precip rates develop,
and should be mostly relegated to grassy areas but there is some
potential for a bit of slushy accumulation on roads too. At this
time, the area with the best chance would appear to be within about
40 miles N/S of a Minneapolis to Hiawatha line. Max accumulation
amount within the forecast area could easily range from only a Trace
(if temperatures stay too warm to change to snow until very late) to
a roughly 25% chance that some relatively small location in the area
will exceed 1.5 inches.

After Tuesday, a steady warm up is predicted through Sunday with
temperatures well into the 60s and even some 70s by the weekend.
Precipitation-free conditions are also expected through this time
frame. A new storm system is forecast to impact the Plains by the
Sunday/Monday time frame with colder temperatures after that time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 533 AM CST Mon Feb 27 2017

LIFR conditions at TOP/FOE are expected to improve to MVFR by 13Z,
with fog clearing by 16Z. VFR is expected through the afternoon,
with stratus likely to return after 00-02Z this evening. Have kept
conditions MVFR currently, although IFR ceilings will be possible.
Marginal LLWS is possible this evening as well with winds near
40kts at 20kt.

&&

.TOP WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 7 AM CST this morning for KSZ026-039-
040-054>056-058-059.

&&

$$

SHORT TERM...Hennecke
LONG TERM...Barjenbruch
AVIATION...Heller



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