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

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242
FXUS63 KEAX 162339
AFDEAX

AREA FORECAST DISCUSSION
National Weather Service Kansas City/Pleasant Hill MO
539 PM CST Sat Dec 16 2017

.Discussion...
Issued at 217 PM CST SAT DEC 16 2017

A cut off upper level low and associated shortwave trough over
northern Mexico will finally eject out into the southern plains this
afternoon and become the main weather producer over the next 24
hours.  Strong southerly winds and warm air ahead of this system
will help temperatures climb into the upper 50s to lower 60s in the
southern CWA. Temperatures were increased a degree or two from the
operational guidance to account for this unseasonable warm air
advection occurring today. As this system moves to the NE it will
combine with enough mid level moisture and lift to develop an area
of rain showers. These showers will move into our southern CWA
overnight tonight and to the KC Metro by the morning timeframe.
Some showers along our southern CWA may have enough moisture and
lift to actually accumulate up to a quarter inch of precip but the
majority of the area will see between 0.05 and .15 inches. This
system will move quickly as it embeds into the zonal pattern over
the midwest with precipitation east of our CWA by the afternoon.

After this shortwave trough pushes through a very similar pattern
will set up again with the long wave trough shifting down towards
Mexico almost becoming cut off Mon into Tue before it weakens and
slides east Wed. This will keep our weather fairly consistent with
highs in the mid 50s and a weak southerly wind through Tuesday.
Wednesday the pattern shifts slightly as a weak backdoor cold front
will push through from the north shifting winds to the NE and
dropping temperatures down into the lower 50s.

The pattern will start to change after Wednesday as a large
amplitude blocking high will build into the eastern Pacific helping
surge colder air down through the plains. This surge of colder air
will start off with the passage of a shortwave trough and surface
cold front Thursday afternoon into the evening.  Temperatures will
start off in the upper 40s to 50s Thursday afternoon and plummet
into the teens over northern Missouri by Friday morning.  While most
of these cold fronts lately have been dry this system will tap into
Gulf moisture in place as it pushes through creating an overrunning
situation over our northern CWA as the front passes through.
Initially the precipitation type will be rain Thursday afternoon,
but will transition to frozen or even mixed type precipitation as
the strong CAA in the lower levels quickly builds into the region in
the lower levels. Currently the forecast indicates a rain to snow
transition occurring overnight Thursday as the colder temperatures
move in from the NW. Precipitation type will need to be watched
with this system as sounding profiles do not show temperatures
conducive to much ice crystal growth. The majority of the
moisture and lift is below -10C which could indicate more of a
freezing drizzle scenario than a snow scenario if enough lift is
available. This far out and with some uncertainty to the exact
moisture to temperature profile, as well as placement of the QPF
as some guidance has it only along Northern Missouri, the forecast
will show snow for now but expect possible changes if this trend
of supercooled droplets instead of ice crystals continues. The
pattern that sets up after this cold frontal passage may bring the
coldest air into the region that we have seen so far and it may
stick around through the end of the year if that blocking ridge
doesn`t budge.

&&

.Aviation...(For the 00Z TAFS through 00Z Sunday Evening)
Issued at 531 PM CST SAT DEC 16 2017

Although VFR conditions are currently prevailing, increasing
clouds and reduced ceilings overnight into tomorrow morning will
bring it down a category or two. Light rain is anticipated to
overspread the terminals by around dawn Sunday with the rain
looking to taper off midday. However, the lowered ceilings/low-
level stratus is expected to linger around through the end of this
TAF period. Winds through much of the period will primarily be out
of the south, becoming more WSW toward the latter half of
tomorrow.

&&

.EAX WATCHES/WARNINGS/ADVISORIES...
KS...NONE.
MO...NONE.
&&

$$

Discussion...Barham
Aviation...lg



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