Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 171736

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1136 AM CST Sun Dec 17 2017

.SHORT TERM...(Today and tonight)
Issued at 330 AM CST Sun Dec 17 2017

We find ourselves this morning within a zone of mid level
convergence between a positively trough stretching from northwest
Wyoming into Manitoba and an h5 low moving northeast across KS. This
has high pressure over us and limited mixing, meaning we have a
combination of fog and low stratus out there. Decided to go with a
dense fog advisory through the morning out in western MN where
dewpoints have remained in the mid 20s and visibilities have been
down in the 1 to 4 mile range much of the night, with further
visibility degradation expected. In addition, the stratus expanding
west out of southeast MN is starting to put out 1/4sm visibilities
down by Rochester, so we may need to expand this east across the
rest of south central MN. With virtually no wind expected today, we
will have no mechanical mixing to help improve these visibilities
and ceilings, so went with the relatively late end time for the
advisory of noon. Unfortunately, we aren`t going to get rid of this
low level moisture until we get some mechanical mixing going and
that won`t happen until tonight as southwest low level winds
develop, so outside of western MN we get to spend another daylight
period enjoying gray skies.

The one late addition to the forecast is the chance for snow this
afternoon in western WI. It is not much, but the HopWRFs and HRRR
have been showing a narrow band of light snow developing in western
WI. Looking at the RAP, this looks to be in response to a band of
fgen that develops in the h85-h7 layer within the right entrance
region of a jet streak up across Lake Superior. Main concern for
getting precip comes from the 17.00 KMPX sounding, which has a very
pronounced dry layer of air between roughly h85 and h5, but enough
of the rapid updating hi-res models have this snow developing to
include some snow chances in the forecast. Beside the precip
chances, we also added a little mention for a half inch of snow
based on a consensus forecast of the short term models.

.LONG TERM...(Monday through Saturday)
Issued at 330 AM CST Sun Dec 17 2017

If you`re an aficionado of snow and cold then you may be in luck
later this week.

First and foremost, there is still a large model spread after
Tuesday, which means it will likely be at least another day until
we can begin to hone in on the temporal and spatial details of
this system. What we can say at this point is there is a fair
amount of certainty with the large scale pattern, with the general
regime pointing toward a Colorado Low winter storm archetype. On
average, this type of pattern is conducive to an 8 inch snowfall
somewhere in our forecast area. The 17.00z ECMWF just happened to
pin point that on the Twin Cities this go-round. However, we`ve
seen that wobble around from model run to model run over the last
several days due to the difficulties models have resolving the
phasing of northern/southern stream jets, so it`s certainly too
early to make a call. The 00z ECMWF also slowed the system down
and lingered snow into Friday. Again, still too early to jump on
that, but a trend to watch.

What we are a bit more confident about is intrusion of Arctic air that`s
looking to arrive by Christmas. In fact, some of the Christmas
Day highs that guidance indicates would make it the coldest
Christmas in over 20 years (when we had a high of -9F in 1996).
In fact, the CMC (-1F), ECMWF (-4F), and MOS (+5F) would all point
toward that.

In the meantime, the week starts off on a benign note, with mild
highs in the 30s and 40s on Monday and Tuesday. Snow may work
south into the area as early as Wednesday and Wednesday night with
the northern stream, with the main show expected to arrive on
Thursday. Stay tuned...


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1140 AM CST Sun Dec 17 2017

Continued the same trend with the worst conditions in south
central/east central Minnesota, and into portions of central
Minnesota, and west central Wisconsin. The best conditions will
occur in west central/southwest Minnesota, especially once
south/southwest winds develop later today, tonight. Confidence
remains high that the IFR, MVFR cigs will hold through the next 6
hours for KMKT, KRNH, KEAU, KAXN, KMSP. Elsewhere, a period of
IFR or MVFR cigs are still possible, but confidence is much lower.
After 00z, and once winds shift to the southwest/south,
conditions will improve especially in western/southwestern
Minnesota. Conditions in west central Wisconsin may not become VFR
until Monday afternoon or later.


IFR conditions, with a slow trend toward low end MVFR cigs will
continue through the afternoon with high confidence. Lower
confidence that cigs will lower once again tonight as winds become
more southwest. By morning, MVFR cigs will lift above 1.7k, and
VFR by the afternoon. Confidence still remains low for VFR cigs by
Monday afternoon.


Mon evening...VFR. Wind SW 10-15G25kts.
Tue...VFR. Wind NW 20G30kts.
Wed...VFR. Chc IFR/-SN. Wind E 5-15kts.


MN...Dense Fog Advisory until noon CST today for MNZ084-085-093.



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