Area Forecast Discussion
Issued by NWS Duluth, MN

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FXUS63 KDLH 171607 AAC
AFDDLH

Area Forecast Discussion...UPDATED
National Weather Service Duluth MN
1107 AM CDT Sun Sep 17 2017

.UPDATE...
Issued at 1107 AM CDT Sun Sep 17 2017

The cold front attendant to the surface low located north of
Winnipeg is now well east of the Duluth CWA as of late this
morning. Large scale subsidence has spread across all of the
Northland, but there is still quite a bit of moisture trapped in a
rather stout post frontal inversion as evidenced by the 12z KINL
sounding. Clouds have been a bit more persistent and ceilings
slower to rise than previously anticipated, which is likely
keeping wind speeds a bit lower as well despite a rather tight
west/southwesterly gradient and 2-4 mb/3 hour pressure rises
across the northern part of Minnesota. We have made a few
adjustments to account for these observational trends, which
support a bit slower improvement than most short term guidance
seems to be insistent on.

For marine, west winds continue rather strong out over the open
waters of western Lake Superior, but strong winds/gusts have been
very slow to materialize in the nearshore zones, particularly
along the north shore. Despite a subtle uptick in winds with gusts
above 15 knots now at the UMD buoy 5 miles offshore of Duluth,
the wave heights have actually decreased by around a half a foot
during the same time window, likely due to the increasingly
offshore wind direction and very limited fetch. We have held on to
the small craft advisory, as conditions 3 to 5 miles offshore are
likely to be considerably worse than near the shore line and
harbor entrances, but we did trim the ending time to 4 PM, as the
pressure gradient will be rapidly weakening by that time with the
surface ridge building in, and the cessation of meaningful
pressure rises to augment the acceleration component. We also
lowered the rip current risk on the MN/WI Point beaches to low.
Offshore flow is not conductive to the accumulation of wave energy
in the surf zone (the forcing driver for rip currents), and all
available observational evidence from webcams on the beaches
support this, with marginal swell of about a half a foot at most
as of 11 AM - which is expected to continue for the afternoon.

UPDATE Issued at 730 AM CDT Sun Sep 17 2017

The cold front has departed and high pressure is building into the
region. Fog and rain has ended. Cloud cover is still expected to
dissipate through the morning. Updated to account for these
trends. The rest of the forecast is on track.

UPDATE Issued at 703 AM CDT Sun Sep 17 2017

Update for new 12Z Aviation Discussion below.

&&

.SHORT TERM...(Today through Monday)
Issued at 429 AM CDT Sun Sep 17 2017

A pair of surface low centers were near the forecast area at 07Z.
One in the southeast corner of Manitoba, the other over Lake
Superior. A trof connected the two and was located right along the
international border. A cold front extended through northwest
Wisconsin from the Lake Superior low. Clouds were lifting to the
North from southern Minnesota toward northwest Wisconsin.
Meanwhile, plenty of low clouds were found over the area with more
developing just to the West and were moving into the forecast
area. Even with this cloud cover, low level drier air was working
its way across the region as dewpoints were dropping from the 60s
near the cold front, to the 40s over the western edge. This was
thanks to high pressure that was building into the area with the
center currently in western South Dakota. This high will drift
over southern Minnesota by late afternoon. Will see a gradual
clearing trend through the day. Max temps will be quite a bit
cooler today with 50s north, and 60s south.

The high will waft over to north central Wisconsin late tonight.
This puts the forecast area into a return flow pattern overnight.
Moisture will be advecting into the region late tonight and clouds
will be on the increase as a result. Warm air advection will also
be occurring tonight and will help to keep minimum temps in the
40s.

Even through the high will be near eastern Lake Superior through
the day, an upper level short wave trof moves into the region in
the afternoon. The moisture profile is not very deep, until late
afternoon, but with cold air advection from the south moving into
the area, will see some showers develop by late morning over the
western and southern edges of the forecast area. The shower chance
increases over the southern portion of the forecast area in the
afternoon and northwest Wisconsin, while diminishing over the
north. Model differences in the potential for QPF and placement
were handled by using a blended approach. No thunder is expected
due to the lack of instability. Max temps will be around 60 over
the Arrowhead, with lower to middle 60s elsewhere.

.LONG TERM...(Monday night through Saturday)
Issued at 429 AM CDT Sun Sep 17 2017

Mid-level ridging expected to bring relatively quiet conditions
Monday night and Tuesday transitioning to a more active regime
Tuesday night through the end of the period.

A deep longwave trough is forecast over the Intermountain West
Monday evening with a mid-level ridge over the western Great Lakes
and the Canadian Prairies. As high pressure departs to the east
Monday night and Tuesday, the building ridge aloft should be
sufficient to keep a dry forecast for the Northland. South to
southeasterly winds are expected and wind speeds will increase
during the day as the pressure gradient tightens between the
departing ridge and developing low pressure over southern Alberta.
The southeasterly breezes will work to keep much of northeast
Minnesota a little cooler thanks to the influence of Lake
Superior. Elsewhere, temperatures will trend a little warmer with
highs from the low 60s in the Arrowhead to near 70 degrees in
central and north-central Minnesota, and much of northwest
Wisconsin.

A strong cold front will advance eastward into Minnesota Tuesday
night associated with a strong shortwave trough rotating through
the Northern Plains and into northwest Ontario. The combination of
falling heights and convergence along the front should be able to
set off numerous showers and thunderstorms. The best signal for
storms is over western Minnesota and will spread into the
Northland overnight and Wednesday morning. High pressure will
build quickly behind the front Wednesday afternoon and a
strengthening mid-level ridge over the eastern United States will
keep conditions relatively quiet until Thursday evening. Southerly
flow over the nation`s midsection will pump progressively warmer
air and additional moisture into the Northland late in the week.
Look for unseasonably warm temperatures in the middle 70s to low
80s by Friday, 10 to nearly 20 degrees above normal. Another round
of showers and thunderstorms is expected Friday through the
weekend as a quasi- stationary front sets up across the region.
Several shortwave troughs will ride along the boundary and rich
southerly flow in the boundary layer and mid-levels will support
successive rounds of rain and thunderstorms. By Sunday most of the
instability will be located south and east of the CWA with
showers expected.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 703 AM CDT Sun Sep 17 2017

Stubborn MVFR ceilings will linger over much of the area through
mid-morning at least before clouds break up with high pressure
building in. Strong west winds will continue as the pressure
gradient tightens over the Northland. Winds will gradually subside
late this afternoon or this evening as the ridge axis moves into
the area. A warm front will push into the region this evening
bringing a chance of rain showers into BRD and HIB late in the
forecast period. Opted to handle with VCSH for now due to
substantial amount of dry air between the cloud bearing layer and
the surface.


&&

.PRELIMINARY POINT TEMPS/POPS...
DLH  59  45  61  49 /   0   0  20  10
INL  55  38  64  46 /   0   0  10  10
BRD  60  45  65  50 /   0   0  20  10
HYR  62  43  63  49 /   0   0  40  20
ASX  63  42  64  48 /   0   0  30  10

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
LS...Small Craft Advisory until 4 PM CDT this afternoon for LSZ121-
     140>148.

&&

$$



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