212 
FXUS63 KMPX 112147
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
347 PM CST Sat Mar 11 2017

.SHORT TERM...(This evening through Sunday)
Issued at 345 PM CST Sat Mar 11 2017

Although confidence has increased enough to warrant a winter storm
warning along and southwest of the Minnesota River Valley, there
remains uncertainty on how much the drier air will play a role
during the onset of snow on Sunday. First, there is no question
that most of central and southern Minnesota, and west central
Wisconsin will see light snow before Monday afternoon. The main 
question is how fast does the very dry air mass overhead begin to 
saturate the low levels of the atmosphere and allow snow to fall
to the surface. Dew points this afternoon are slightly higher 
than yesterday, but humidity levels remain very dry in the 20-30% 
range in the boundary layer. Just as a previous winter storm that 
had a very sharp gradient of where the snow fell, to nearly 
nothing on the northwest side, this system has similar 
characteristics. However, the orientation of the sharp snow line 
is different. Another aspect to this storm is the relatively 
short period of strengthening Sunday before the main forcing moves
south/southeast of Minnesota by Sunday night. This forcing is 
associated with a strong thermal gradient oriented from west 
central to south central Minnesota Sunday morning, through Sunday 
afternoon. This is where the best frontogenesis, omega and the 
dendritic growth zone coincide. 

Initially the dry air mass needs to saturate which might be 
easier in southwest Minnesota due to the current system leaving 
behind some moisture in the boundary layer. However, further to 
the east/northeast the air mass may take a few hours to saturate 
leading to lower confidence on snowfall amounts. The dendritic 
zone does last longer further to the east across south 
central/east central Minnesota Sunday afternoon, but the forcing 
is weaker and may hold off on accumulating snowfall until the late
afternoon. Another aspect to this storm is the concern with 
snowfall ratios which initially will be 18/20-1 in eastern 
Minnesota, to 15/17-1 in southwest Minnesota. These numbers will 
rise during the day, so the combination of snowfall ratios and qpf
amounts will be critical on snowfall totals, especially Sunday as
the atmosphere saturates in the boundary layer. 

.LONG TERM...(Sunday night through Saturday)
Issued at 330 PM CST Sat Mar 11 2017

Longer term concerns remain placement of the heavier snow band and 
overall headline resolution.

Model trends indicate heavier snow will be ongoing over south 
central MN during the evening. Best vertical velocity in the 
dendritic growth zone remains over the southwest metro by late 
afternoon and shifts into south central MN mainly after 00z Mon. We 
do feel model qpf may be a bit high...especially on the NAM...which 
also seems to shift heavier snow band farther to the northeast. 
Also...adjusted the SLR values during the evening as well... with 
18t to 20:1 a bit high when compared with the COBB and other model 
trends...especially to the south.  A more 14-17:1 ratio looks be 
better here. Meanwhile...the 12z GFS holds onto its snow band 
placement...mainly to the southwest. Looking at the GEFS probability 
trends last few runs...they have shown pretty good continuity with 
the >6 inches snow accumulation in 24hrs...which places it along the 
Minnesota River Valley region. Still have time to monitor one last 
model run to finalize over all headlines. The bulk of the snow 
should occur before 12z Monday across the area...with some lingering 
light snow possible over the eastern cwa Monday morning. Wind 
concerns appear minor at this time with wind gusts around 20 to 25 
mph expected at the tail end of the accumulating snow.

We see cooler air to follow the system through Wednesday with a 
gradual warming trend...above normal into next weekend. The 
deterministic runs diverge some on how much warm air lifts northeast 
into the area...ahead of the next trough.  It looks warm enough for 
at least a rain/snow mix...if not all rain developing during the day 
Friday the surface cold front.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1135 AM CST Sat Mar 11 2017

Main concern is timing of IFR cigs/vsbys in western/southwestern
Minnesota as a winter storm begins to affect the region late
tonight, and into Sunday. Since these TAFs end at 18z, and the
worst conditions are after 18z Sunday, am leaning toward lowering
vsby/cigs at KRWF/KAXN between 14-16z. Elsewhere VFR conditions
will continue. Winds will start light from the north/northeast, 
and increase overnight/Sunday morning and veer to the east-southeast.
Western/Southwest Minnesota could receive up to 1-3 inches before
noon Sunday. 

KMSP...

As previously stated, timing of the IFR/MVFR cigs/vsbys is the
main aviation concern. Kept VFR conditions through 18z with 
MVFR/IFR conditions after 18z. After 21z, vsbys/cigs could drop to
LIFR in snow. Winds will be light from the north/northeast today,
tonight, and veer to the east/southeast Sunday and increase. 

/OUTLOOK FOR KMSP/

Mon...MVFR/IFR with -sn early, then VFR. Wind bcmg NE at 5-10 kts.
Tue...VFR. Wind N-NE 5 kts. 
Wed...VFR. Wind SW-S 5 kts. 

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Winter Storm Watch from Sunday afternoon through Monday 
     afternoon for MNZ059-060-068>070-076>078-083>085-092-093.

     Winter Storm Watch from Sunday morning through Monday morning 
     for MNZ047>049-055>058-065>067-075.

     Winter Storm Warning from 10 AM Sunday to 7 AM CDT Monday for 
     MNZ054-064-073-074-082-091.

&&

$$

SHORT TERM...JLT
LONG TERM...DWE
AVIATION...JLT