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

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397
FXUS63 KEAX 182124
AFDEAX

AREA FORECAST DISCUSSION
National Weather Service Kansas City/Pleasant Hill MO
324 PM CST Mon Dec 18 2017

.Discussion...
Issued at 320 PM CST MON DEC 18 2017

The upper level split flow with zonal flow along the northern US and
a shortwave trough over the SW US will start to break down and shift
into a more meridional pattern mid week.  The shortwave trough over
the SW US will slide south of our region keeping our region
relatively cut off from any air mass changes and temperatures
consistently in the mid to upper 50s through Thursday.  This
incoming shortwave trough over the NW US will initiate cyclogenesis
over the central plains.  This low and associated cold frontal
boundary will be the beginning of a strong arctic outbreak that will
dive into our region this weekend.  Currently the guidance has most
of this cold frontal boundary dry with exception of northern
Missouri where some isentropic ascent will increase the chances of
rain turning to snow as temperatures drop behind the frontal
boundary.

Another potential hazard that may occur with the passage of this
cold front will be freezing drizzle. As this strong cold front
undercuts the warmer air it will form a thick stratus deck that may
have enough lift to produce drizzle as no moisture is in place above
-10C to produce ice crystals. Temperatures will quickly fall below
 freezing behind the front which may increase an icing hazard as
 the temps continue to drop, especially over bridges. Currently
 freezing drizzle is not in the forecast with this being four days
 out and mainly based on forecast soundings, but it may be added
 in the coming days if this trend continues to show this
 possibility Friday morning into the afternoon. Temperatures will
 drop almost 25F degrees from Thursday to Friday with highs only
 reaching the mid 30s. This cold air will continue to funnel in
 with the high pressure as lows Saturday morning drop into the
 teens along northern Missouri and low 20s in central Missouri.
 There are some slight PoPs in the forecast from Friday night into
 Saturday with some snow accumulation associated with a weather
 system passing through southern Missouri that is only seen
 utilizing the Canadian model. This snow forecast has very low
 confidence as the Canadian is the only model with a developed
 surface low moving through southern Missouri. This forecast has
 been consistent with multiple runs so it was not completely taken
 out of the forecast, but was significantly reduced in PoPs and
 QPF. This was widely collaborated between the surrounding offices
 to make this change in our forecast grids.

A secondary blast of arctic air will plunge down into our area
Saturday night into Sunday which will not only bring unseasonably
cold temperatures (teens to single digit lows Sun/Mon) but also a
chance for snow again on Christmas Eve. Model Guidance has varied
greatly the last few days on where the frontogenetic boundary will
develop over the central plains. Currently the GFS has this boundary
well south of our area and much drier in the lower levels, but the
ECMWF and Canadian have consistently put this frontogenesis region
between the MO/IA border and southern Missouri.  Almost all the
PoPs, QPF, and snow totals for Christmas Eve all come from a blend
between these two models with the lack of anything on GFS keeping
the PoPs and forecast confidence low. It is really going to come
down to which model handles the long wave trough and jet streak
placement as this feature seems to be the driving factor for where
the frontogenesis will occur on the right entrance region of the jet
streak.  The ECMWF and Canadian show this feature further north over
northern Missouri and the GFS has it over central Missouri. The
trend of this feature as we move closer to the event may provide the
best guidance on increased or decreased chances of snowfall.


&&

.Aviation...(For the 18Z TAFS through 18Z Tuesday Afternoon)
Issued at 1133 AM CST MON DEC 18 2017

VFR conditions with a 10-15kt southwest wind will persist through
the afternoon and into the evening. Overnight winds are expected
to weaken near sunrise accompanied by potential for fog along the
Missouri River valley and even south of I-70. If areas don`t cool
down enough or winds stay higher this fog potential will not
develop. Since there is still some uncertainty to where the fog
bank will develop the TAFs only have MVFR visibility at that time.
If it starts to look like areas will cool enough tonight then IFR
may be added during that 12-16Z timeframe. This fog is expected to
burn off rather quickly leaving VFR for the remainder of the TAF
period.


&&

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

$$

Discussion...Barham
Aviation...Barham



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