955 
FXUS63 KLSX 312150
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
350 PM CST Wed Dec 31 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 350 PM CST Wed Dec 31 2014

West-southwest flow has developed across the area on the northeast 
periphery of the Arctic high which is now centered over northeast 
Oklahoma.  Southwest flow will continue tonight, with variable 
cloudiness.  Clouds over southwest Missouri are spreading east 
between 850-700mb at this time.  There isn't much in the way of 
model guidance that's handling the clouds right now, but indications 
are that the clouds will overspread much of our southern CWFA before 
midnight and then should thin out somewhat overnight as some drier 
air punches south at those levels in cyclonic flow aloft.  Between 
the southwest flow and the clouds, expect temperatures to be between 
5-10 degrees warmer tonight than last night.

Carney

.LONG TERM:  (Thursday through Next Wednesday)
Issued at 350 PM CST Wed Dec 31 2014

A stagnant pattern of a longwave RIDGE over western North America 
and a TROF over eastern North America is expected to persist...with 
some ebbs and flows...over the next seven days.  This pattern will 
be very favorable for regular intrusions of Arctic air.

(Thursday - Friday)

A well moderated airmass will be in place during this period, 
featuring seasonable temps with maxes in the 30s and mins mainly in 
the 20s. 

There is much better model agreement on how to handle the initial 
wave that kicks out from the SW CONUS storm, maintaining the 
position of the weak upper RIDGE to our southwest and thereby 
shunting the light pcpn from this to our south.  Have maintained 
slight chance PoPs for parts of SE MO and S IL...south of a 
Farmington MO to Chester IL line...Thursday night and Friday for 
what could be a light wintry mix, consisting of mainly FZRA and PL, 
but it will be tough to get anything measurable as it stands now. 

(Friday Night - Saturday Night)

The models have maintained their good consistency on track, while 
tightening up significantly on the timing as well.  The overall 
track has trended a tad more northerly as well, with a new surface 
low triple-point to track thru SE MO and S IL into northern Indiana 
by late Saturday evening. 

There will be two phases to this system, the first will be primarily 
driven by WAA with the weak upper RIDGE axis expected to pass thru 
to our north on Friday night, this should give the full go-ahead to 
the pcpn set to advance northward.  Reasonably good confidence 
exists on the northern edge of the pcpn reaching I-70 by daybreak 
Saturday with decent evidence of a more western surge pushing thru 
central MO and into northeast MO by this time as well.  Pcpn will 
then overspread the entire area for a period on Saturday morning 
with a dry slot then poking thru late morning and afternoon.

Pcpn-types with this first phase will be primarily liquid, with some 
frozen on the northern edge.  The location of the 32F isotherm will 
be critical to the forecast with FZRA being a likely possibility for 
most locations for a short time, and considering how the surfaces 
may be decently cold from this last Arctic airmass, another thing to 
consider is the possibility of FZRA for a longer period of time.

The second phase of the storm will begin to take shape for our 
region on Saturday evening with a weak deformation zone of pcpn that 
will swipe thru, mostly for areas to the north and west of STL 
metro.  For a period during the evening, the warm wedge of air aloft 
will be strong enough to maintain the predominant liquid nature of 
the pcpn, with some frozen perhaps holding on in the far north 
around UIN, but with passage of the upper system center and surface 
cold front, this warm wedge is then anticipated to collapse rapidly 
turning the pcpn-types into a R versus S question for late evening 
and overnight.  Snow will have the best chance of being seen where 
this cold air can reach first before the def zone pcpn exits, 
meaning a COU-UIN axis.

Putting it all together, it looks like minor ice accums from FZRA of 
less than a tenth of an inch are likely for a broad swath of the 
forecast area late Friday night and Saturday.  Snow accums, perhaps 
as high as 3", could be seen in parts of NE MO and west-central IL, 
but it is walking a fine line depending on how far north that warm 
air aloft gets. 

(Sunday - Next Wednesday) 

Another intrusion of Arctic air will follow the storm on late 
Saturday night and Sunday morning and persist into Monday.  A weak 
system will squirt thru Monday night and early Tuesday, warranting a 
low PoP for snow, with another Arctic invasion set for Tuesday.  
Below normal temps...if not well-below normal...look to be the rule 
here with any pcpn types, chances as meager as they are now, set to 
be snow.

TES

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1124 AM CST Wed Dec 31 2014

VFR flight conditions and west-southwest flow will prevail through
Thursday as high pressure remains in control of the region.

Specifics for KSTL:

VFR flight conditions and west-southwest flow will prevail through
Thursday at Lambert as high pressure remains in control of the
region.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX