Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 171207
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
607 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 12Z TAFS through 12Z Monday morning)
Issued at 607 AM CST Sun Dec 17 2017

We will have weak low level flow all day, which means this stratus
is going nowhere. We don`t expect much improvement in cigs until
we begin to establish southwest winds, but that doesn`t happen
until late this afternoon in western MN and late tonight in
western WI. Also, IR satellite shows a rapidly cooling band of
clouds across southwest MN and is associated with a band of snow
the HRRR develops this afternoon from MSP northeast to Ladysmith
and included a quick snow mention for this for MSP/RNH.

KMSP...Developing snow band to the west looks quite impressive on
satellite, so believe the HRRR and HopWRFs in the idea of some
light snow impacting MSP around noon. Low confidence on when the
clearing will occur this evening, but like the idea of the GFS
that it happens between 6z and 10z

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

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...Dense Fog Advisory until noon CST today for MNZ041-042-047>049-
     054>058-064-065-073-074-082>085-091>093.

&&

$$

SHORT TERM...MPG
LONG TERM...LS
AVIATION...MPG



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