661 
FXUS63 KEAX 121746
AFDEAX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE KANSAS CITY/PLEASANT HILL MO
1145 AM CST SAT JAN 12 2013

...UPDATED AVIATION DISCUSSION...

.DISCUSSION...

Short Range (Saturday through Monday)...

The main forecast concern centers on when/where the expected snow
will fall. Earlier today the SREF began to shift the snow band
further northwest. The 00z NAM and to a lesser extent GFS also
latched onto this trend. A strong cold front is currently sliding
southeast through the CWA sending temperatures below freezing. The
degree of cold air advection brought in by a stiff northwest wind
prevent temperatures from rising more than a few degrees. It is
conceivable that some areas will hold steady for the better part of
the day. 

A well defined positively tilted upper trough extends from the Upper
MS Valley through the Desert Southwest. Attention is focused on the
base of the upper trough where a shortwave and speed max will rotate
rapidly northeast and into KS and OK by late afternoon. A southwest-
northeast band of frontogenesis sloped between 850-650mb is expected
to form early this afternoon from OK into east central KS and be
enhanced by an impressive 300mb jet streak. A strong region of upward
vertical motion co-located within a region of very moist air and
negative equivalent potential vorticity (epv) will lead to rapid top-
down saturation. This set-up favors banded precipitation which is
characterized by narrow bands of enhanced snow rates. The models have
trended to the northwest with the heaviest snowfall and have keyed in
on a well-defined baroclinic zone centered around 700mb. What is
keeping me from going with higher snow amounts is the best lift and
negative equivalent potential vorticity are forecast to occur outside
of the dendritic snow growth area. Should the models change their
tune we will need to adjust snow amounts upwards.

Have opted to issue a winter weather advisory for a relatively large
area with the idea that later shifts may need to better define the
region of heaviest snowfall as the bands of heavier snow form and/or
the best lift and negative epv fall within the dendritic snow growth
region. Felt it was best to go this route than have to play catch up
later in the afternoon and issue advisories with short or non-
existent lead times.

This should be a rather quick snow event and by midnight the bulk of
the snow should have moved out of the CWA. Cold high pressure will
settle over the CWA Sunday and Sunday night. This combined with a
fresh snow cover will require going a bit colder than the current
forecast so have lowered temperatures a bit. 

MJ

Medium Range (Tuesday through Friday)...

Latter half of the forecast continues to be relatively quite. As 
Tuesday arrives a large trough will be shifting across the nation as 
a blocking ridge across the eastern Pacific holds strong well to our 
west. Resulting adjustment in the flow across the nation will evolve 
from the highly amplified pattern, currently in place, to a more 
broadly zonal flow by the end of the work week. With a dry surface 
high still oozing down through the Plains States, settling along the 
Gulf Coast, don't expect any precipitation as moisture will be long 
gone from our region. Otherwise, only issue to be watched looks to 
be temperatures. As the mean trough slides past the Plains the 
prevailing westerlies will shift from south of the forecast area to 
north during the tail end of the work week. This will allow for 
moderation of temperatures through the work week, though with a 
nominal northwest flow still nosing into the Northern Plains 
thoughts are that temperatures wont climb too high, staying within a 
category or two of seasonal normals.

Cutter

&&

.AVIATION...

For the 18Z TAFs, recent model guidance still suggests the bulk of 
this evening's winter weather event will likely skate just east of 
the terminals with MKC seeings the greatest impacts of some 
accumulating snow. A signficant layer of dry air both above and 
below a narrow corridor of strong elevated frontogensis is the main 
prohibiting factor. 

VFR conditions will continue through 00Z with gradually lowering mid 
cloud. Ceilings should steadily fall thereafter. 

Basically have kept STJ dry, MCI on the fringe with MVFR to perhaps 
brief IFR conditions this evening, and MKC in the steadier snow this 
evenint with IFR conditions likely. Conditions should rapidly 
improve after around 07Z. 

Winds will remain northwest to north-northwest at around 10 knots. 

Bookbinder

&&

.EAX WATCHES/WARNINGS/ADVISORIES...
MO...WINTER WEATHER ADVISORY from 3 PM this afternoon to Midnight CST 
     tonight FOR MOZ021-028-029-037-043-053.

     WINTER WEATHER ADVISORY from 6 PM this evening to 3 AM CST 
     Sunday FOR MOZ016-017-022>025-030>033-038>040-044>046-054.

KS...WINTER WEATHER ADVISORY from 3 PM this afternoon to Midnight CST 
     tonight FOR KSZ057-060-103>105.

&&

$$

WFO EAX