Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 191739

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1139 AM CST Thu Jan 19 2017

.SHORT TERM...(Today and tonight)
Issued at 440 AM CST Thu Jan 19 2017

The snow melt of yesterday has made for a rather moist boundary layer
out there this morning and that moist boundary layer is only being
aided by southerly winds. We saw one area of dense fog form across
central MN/western WI to the north of a weak warm front overnight.
There have been widespread visibilities of a quarter mile or less,
hence the dense fog advisory across central MN into NW WI. We went
with a 9am end time for this as the HRRR, which picked up on this
fog pretty good, shows it continuing its slow northward movement
this morning, reaching northern MN and mostly clearing the MPX CWA
by mid morning. Of course as we lose this area of fog, there is
another area of dense fog streaming north across IA right now. We
will be adding a dense fog advisory to areas south of the Cities
this morning for this fog coming out of IA. Will end this advisory
at 15z as well for simplicity, but this region may need to be
extended out in time as it looks to have a bit more staying power.

This fog also signals the arrival of the gloomy weather we have been
harping on the past few days. Even with the clouds though,
temepratures this morning are already in the lower 30s for most
locations and the continued WAA on south winds will send highs today
to similar levels to what we saw yesterday. We will just be
replacing beautiful sunny skies with a gloomy low stratus, fog, and
haze. Main change to the forecast today was to increase sky cover
and remove any drizzle mention, as moisture profiles are not deep
enough to support drizzle/precip generation until tonight.

As for tonight, it will be ugly for the aviation end of the
building. We will likely see more dense fog, ceilings likely under
500 feet, and to add to the fun, we`ll through in drizzle and rain
as well. The precip will come up out of eastern IA and will
primarily impact areas east of I-35, as this is where moisture is
deepest and clouds will likely be mixed phase. West of I-35, mid
levels look to stay dry and clouds will consist of only liquid
droplets, with drizzle generation looking possible all the way to
the SD border. Pops in there now are based on a blend of the CAMs
and are keyed in more on where the rain as opposed the drizzle will
be. The south wind and clouds will keep us mild again and did warm
lows a couple of degrees, with lows currently progged to be in the
30-34 range. Beside being a good 25 degrees above normal, it looks
like it will be warm enough to keep any potential freezing
rain/drizzle impacts to a minimum.

.LONG TERM...(Friday through Wednesday)
Issued at 440 AM CST Thu Jan 19 2017

By Friday the long wave pattern will be transitioning from a
split flow pattern to a more traditional dominate Polar Jet
across the southern CONUS. A west coast trough will slowly slide
east across the Plains and Upper Mississippi River Valley FRI-SAT.
There are two smaller waves embedded in the larger pattern - the
first coming through on Friday and the second quick on its heels
Saturday. These aren`t particularly deep lows, but given the
fairly high atmospheric moisture content over the next few days
with PWATS between 0.45"-0.60" - these two waves could produce
0.25" of precipitation for portions of the area. The forecast
soundings are warm, and we`d be surprised if there was anything
but light rain or drizzle for a vast majority of the area. It`s
possible a little sleet/snow mix in across central/western MN late
Saturday, but most of the area will have a 2000-4000ft freezing
level through Sunday. That would certainly be liquid. The ground
is still frozen, so there could still be slick spots around at
night with temperatures hovering near or just above freezing. The
first two weeks of the month left us well below normal for
temperatures locally, but the past two days alone have nearly
reversed the departure to above normal.

A deep trough moves across the central US during the middle of
next week. The GFS/ECMWF both show the potential for several
inches of snow with a mature cyclone moving through the region. We
will continue to monitor the track of this potential snow maker.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1140 AM CST Thu Jan 19 2017

Areas of fog/low clouds and low visibility that expanded northward
across Iowa and into southern Minnesota this morning had started
to erode on the northern edge as of 1130 am. Typically dense
fog/low clouds would have nearly dissipated if we had a higher sun
angle. However, this is winter and the low clouds (although
shallow) moved northward in different areas and not affecting the
major airports in MPX forecast area this morning. However, this
should not be the case this afternoon as both shallow moisture and
deeper moisture across Iowa will eventually spread northward
across the state during the afternoon. Therefore, it was mainly a
timing issue not so much as if IFR conditions would develop. Once
IFR cigs/vsby develop, these conditions will continue through the
taf period with some fluctuation in the cigs/vsby from the
afternoon through the overnight hours. Drizzle/Light rain,
possibly mixed with freezing drizzle/rain in west central
Wisconsin overnight as a storm system moves northward across Iowa.
Kept cigs/vsby simple after this evening as changes will likely
take place.


Complicated aviation forecast this afternoon as the northern edge
of the IFR/LIFR cigs/vsby are nearly at the airport grounds.
However, there could be a few hours this afternoon where cigs
could scattered out so I introduced a tempo period from 18-21z
for this scenario. Otherwise, IFR cigs will develop this evening
and continue through the forecast period with high confidence.
Lower confidence on the amount of drizzle/light rain developing
overnight, but it should be all liquid as temperatures should hold
above freezing.


FRI...IFR/MVFR. -DZ/-RA in morning. Winds E 5-10 kts.
SAT...IFR/MVFR. Chc -RA. Winds E 5-10 kts.
SUN...IFR/MVFR. Winds bcmg W 5 kts.




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