Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 172331

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
531 PM CST Tue Jan 17 2017

Issued at 530 PM CST Tue Jan 17 2017

Updated to include 00Z aviation discussion below.


.SHORT TERM...(This evening through Wednesday)
Issued at 330 PM CST Tue Jan 17 2017

A shortwave aloft has moved east of the area bringing gradual
clearing to a remnant stratus deck from west to east. Expecting
skies to clear out by midnight across MN counties, and by sunrise
for W WI. Any significant diurnal cooling tonight will be offset by
light southerly winds and subsequent warm advection, a continued
theme throughout the week. Lows tonight will range from the middle-
teens across west-central MN to the low 20s across W WI and S MN. As
is typical in warm advection regimes in winter, some patchy fog is
expected overnight, mainly in low-lying areas where winds are calmer
and temperatures locally cooler.

A sunglasses advisory is in place for Wednesday as sunny skies and
continued warm advection allow temperatures to rise near 40 across
the area, about as nice a day you can draw up for mid-January. It
will be short-lived unfortunately as clouds build in again Wednesday
night as the next shortwave approaches from the south, leading to a
warm but gloomy rest of the week.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 330 PM CST Tue Jan 17 2017

The main forecast challenge is the amount of cloud cover
expected/anticipated due to a mild air mass riding over the cold
snow pack across the Upper Midwest. Usually in January to achieve
mild temperatures (at or above freezing) for the Upper Midwest,
you need a zonal flow aloft, and a predominately west/southwest
flow near the surface. Current snow pack across the northern
United States has a very large area of deep snow from Montana,
eastward across the Upper Midwest. Once temperatures rise above
the freezing mark, and melting starts, the atmosphere in the
boundary layer will become saturated. Depending upon the amount of
melting and boundary layer mixing, stratus/low clouds or fog
forms. This will probably be the case as winds become more
west/southwest along the leading edge of the very mild air mass
moving across the Canadian Prairies/Northern Plains this
afternoon. As clouds and fog form, temperatures will likely only
rise a few degrees from morning lows/afternoon highs. Kept
temperatures in the extended period in the 30s/lower 40s which is
still 15 to 25 degrees above normal for the third week of January.

The second forecast challenge is precipitation chances
Friday/Friday evening, and again early next week. The mean upper
level pattern/flow has a deep trough of low pressure across the
desert southwest, with a large ridge across the deep southeast.
This has allowed for record highs today in the Ohio Valley and
southeast corner of the U.S. A very powerful Pacific system will
affect the west coast for the next few days. This powerful system
will aid in ejecting the upper low in the desert southwest to the
northeast across the plains. However, as an upper ridge builds
across the Great Lakes, confluent flow (blocking ridge) should
weaken this system as it moves north/northeast toward the Upper
Midwest on Friday. However, enough moisture in the boundary layer
should allow for drizzle or freezing drizzle depending upon
surface temperatures as temperatures aloft are quite mild. There
is some colder air associated with the upper low moving out into
the plains Friday, but the milder air mass during the onset of
this system will likely keep the predominate weather type liquid
vs. snow/sleet.

Eventually as the upper ridge begins to break down across the
east, and a long wave trough becomes more established in the
center part of the country, cooler air will eventually filter
across the region. Although there is no significant arctic air
intrusion expected for the remainder of January.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 530 PM CST Tue Jan 17 2017

Main aviation concerns look to be in the first 6-9 hours of the
forecast before we get somewhat better advection and drier
boundary layer air into the area. The IFR ceilings currently over
the southeast half or so of the area are slowly working east, but
there is some disagreement in the guidance as to how quickly
they`ll exit the area, with the HRRR being more pessimistic than
LAMP and consensus guidance. For now trended a bit closer the
HRRR, but thinking amendments will be needed as things unfold.
Also have concerns about some patchy fog with lingering low level
moisture and light winds, with that appearing most likely at KSTC
and KRNH which will see the best overlap of those conditions and
lack of lower ceilings.

KMSP...Primary uncertainty is with respect to the departure of the
current low ceilings. Went on the pessimistic side of things for
now, but perhaps by the 03Z amendment time will be able to refine
things some.

Wednesday overnight...MVFR possible. Southwest wind 5 to 10 kt.
Thursday...MVFR expected, IFR possible. Southeast wind 5 to 15
Thursday night...MVFR expected, IFR possible. East wind 5 to 10
Friday...IFR expected. East to southeast wind 5 to 15 kt.
Friday night...IFR expected. Southeast wind around 10 kt.




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