Area Forecast Discussion
Issued by NWS Kansas City/Pleasant Hill, MO

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FXUS63 KEAX 171831
AFDEAX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE KANSAS CITY/PLEASANT HILL MO
1231 PM CST Wed Dec 17 2014

.DISCUSSION...
Issued at 434 AM CST WED DEC 17 2014

Chilly conditions continue this morning as low temperatures remain
in the middle 20s to upper teen. Area clouds are somewhat spotty,
and should gradually wane in coverage as we proceed through the day.
The focus then becomes centered on a fairly low-impact winter
weather system that is poised to move into the area tonight bringing
some light snow and freezing drizzle.

Water vapor imagery around 10z indicates the trough in question over
Las Vegas, moving eastward toward the Rocky Mountain region. In
response to the approaching mid level wave a lee side surface trough
is forming over the High Plains. This low will help to shunt the
surface ridge, which is responsible for the chilly surface
temperatures to ever so slowly creep eastward. Its impacts will be
felt through the day though as it only makes enough eastward
progress to cause the surface winds to become easterly, as opposed
to southerly, as would be the case if it moved far enough east. This
will keep the chilly surface temperatures in place through the day.
Expect high temps today to reside in the lower to middle 30s. As the
mid level trough approaches this afternoon and this evening a
decent..but not incredibly strong isentropic lifting of warm/moist
air over the low level cold air will take place. Condensation
pressure deficits by tonight approach 0 mb along the 275K to 285K
surfaces, indicating deep saturation and good lift. Mid level ascent
will be realized by the quickly moving mid level trough. All told,
good, deep ascent, on the order of around 10 to 15 -ubars/sec will
be realized through the saturated dendritic growth layer after
sunset.

As far as onset times go, it`s a bit tricky to determine exactly
when the snow will begin to fall. Forecast soundings over the
southern part of the CWA (south of I-70) indicate some snow
formation as early as 00z this evening. But NAM soundings -
corroborated by upstream observed soundings - indicate a VERY dry
lowest 6000 feet of the atmosphere.  While it`s possible that some
of that high altitude snow may overcome that dry air and make it to
the surface it`s unlikely that it will be able to accumulate much.
By midnight however, enough sublimation and lift will have occurred
to adequately saturate the column and produce accumulating snow. The
best window for lift and saturation will be in the 03z to 12z time
frame tonight. Expect enough snow production to at least cause the
ground to be covered, and perhaps produce up to 3 inches.

The mesoscale forecast for this event is also tricky. 00z NAM
indicated several enhanced bullseyes of precipitation concurrent
with a completely frozen thermal profile, indicating pockets of
heavy snow. The 06z NAM has since backed off a little bit on those
pockets of enhanced precip formation. These pockets of enhanced
convection almost always signal some enhanced low/mid level
frontogenesis, but in this case forecast cross sections only yields
marginal frontogenesis. However there is some indication that
Saturated Theta E contours will decrease with height in some
locations, yielding some fairly decent equivalent potential
vorticity values approaching -1. This would indicate that there will
be a somewhat moderately unstable atmosphere through the overnight
hours, and this could yield the pockets of convective snow showers.
Areas where these convective showers form could see snowfall values
exceeding the general 1-3 inch snow range.

As you go farther to the NE the more dry air is in place for the
moisture to overcome, so snowfall totals are about 1/2 of what they
are expected to be along and south of I-70. Issued the Winter
Weather Advisory over areas most likely to exceed the 1" snowfall
total.

Once the system departs early Thursday morning there will remain
some shallow saturation, which could yield some freezing drizzle.
Expect that hazard to persist until temperatures warm up above
freezing, likely by mid to late Thursday morning.

The next system, late this week into the weekend should remain too
far to the south to really affect much of the forecast area. So
expect dry conditions to remain in place through the weekend. The
next trough moves through early next week and will likely bring some
rain, as thermal profiles at this point look to remain mostly above
zero, aside from some overnight hours where temperatures go below
freezing. By the time we approach the Christmas Holiday there are
beginning to show up some signals that a cold blast will be on its
way for Christmas week. Precip chances look somewhat meager, but
that could change as we approach the middle part of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
Issued at 1230 PM CST WED DEC 17 2014

MVFR has generally eroded westward and out of the KC area, so VFR
should be the rule for the next several hours. A large area of light
to perhaps moderate snow will overspread the area overnight with
anywhere from 1 to 3 inches of accumulation. Most of this should fall
through 12Z or so, with light snow or perhaps some light freezing
drizzle lingering into the late morning.

&&

.EAX WATCHES/WARNINGS/ADVISORIES...
KS...WINTER WEATHER ADVISORY from 6 PM this evening to 9 AM CST
     Thursday FOR KSZ025-057-060-103>105.

     WINTER WEATHER ADVISORY from Midnight tonight to Noon CST
     Thursday FOR KSZ102.

MO...WINTER WEATHER ADVISORY from 6 PM this evening to 9 AM CST
     Thursday FOR MOZ028>031-037>039-043>046-053-054.

     WINTER WEATHER ADVISORY from Midnight tonight to Noon CST
     Thursday FOR MOZ011>014-020>024-032-033-040.

&&

$$

DISCUSSION...Leighton
AVIATION...Hawblitzel





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