Area Forecast Discussion
Issued by NWS Duluth, MN

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FXUS63 KDLH 220556 AAB

Area Forecast Discussion...UPDATED
National Weather Service Duluth MN
1256 AM CDT Sat Jul 22 2017

Issued at 1256 AM CDT Sat Jul 22 2017

Update for 06Z Aviation Discussion below.

UPDATE Issued at 715 PM CDT Fri Jul 21 2017

Severe thunderstorms were occurring this evening from just north
of Duluth west through the Grand Rapids area into the Pine River
area. Instability has been steadily increasing across the
Northland and the storms have shown little signs of weakening as
they move toward a less unstable airmass to the east. The
afternoon KINL sounding showed dry air in the lower to mid levels
which will enhance the wind threat. Per coordination with SPC,
we`ve added Aitkin, Carlton, and Saint Louis Counties to the
watch. The latest HRRR shows the storms continuing for the next
few hours but gradually decreasing in intensity as they move
further east.


.SHORT TERM...(This evening through Saturday)
Issued at 336 PM CDT Fri Jul 21 2017

Storms have formed over northwest Minnesota this afternoon that
we have been watching very carefully. A Severe Thunderstorm Watch
has been issued, and I will leave it to the reader to refer to the
specifics of that watch in the WCN. A plume of 2000 J/kg plus
most unstable CAPE extends up the Red River valley, overlapping
much of western Minnesota and eastern North Dakota and South
Dakota. In addition, this area has decent shear, a weak shortwave
aloft and is underneath the right entrance region of the upper
level jet. We are looking at getting a round of severe storms for
the next few hours, mainly affecting the far western areas of the
CWA along the high CAPE gradient. After that have poor confidence
in additional storms, though several models bring a second round
of convection through the southwestern half of the forecast area
after midnight tonight.

Attention then turns to Saturday, when the tight upper low/vort
max currently over the Saskatchewan/Manitoba border drops
southeast cross the forecast area during the afternoon and early
evening. Expect that depending on the overnight convection we will
have an additional round of showers and thunderstorms move across
the area during the afternoon and early evening. Depending on the
amount of CAPE we can develop severe weather is a real threat,
but we will have to watch what happens overnight tonight to
determine the daytime threat for tomorrow.

.LONG TERM...(Saturday night through Friday)
Issued at 336 PM CDT Fri Jul 21 2017

The extended forecast period features a couple of chances of
showers and thunderstorms over the Northland, with temperatures
forecast to remain around seasonal averages.

Saturday night begins with continuing chances of showers and
storms from Saturday afternoon as a compact mid-level shortwave
dives across our north and eastern areas of the CWA Saturday
night. There should be some good forcing associated with this
shortwave as there is a large lobe of positive vorticity
advection, along with higher 850-300 mb omega, per the Thaler QG
analysis. The Storm Prediction Center currently has the entire
Northland in a Slight Risk of severe weather for the Day 2
convective outlook. The magnitude of instability from 00Z Sunday
between the synoptic models differs somewhat, with values ranging
between 300 up to 2000 J/kg between the GFS/NAM/ECMWF/CMC models.
The expectation is that any strong to severe convection on-going
at 00Z Sunday will dissipate as the sun sets, decreasing
instability rapidly. There could be some damaging winds and large
hail possible, if some strong convection continues into this
period. As the shortwave departs, there may be some lingering
showers Sunday morning as the region remains under the cyclonic
flow of the mid-level wave. Non-zero MUCAPE could provide enough
instability for a rumble of thunder, but no severe weather
expected at this time.

By Sunday afternoon, drier conditions will return to the
Northland, as surface high pressure builds. Skies look to clear
out overnight Sunday into Monday morning, so there could be some
patchy fog, especially over areas that receive rainfall Sunday.
Winds look to be light as well, especially over the Minnesota
Arrowhead and along Lake Superior. Dry conditions will continue
through the day Monday before another area of low pressure
develops over northern Manitoba and northwest Ontario Canada,
bringing a cold front boundary through the Northland. The
GFS/ECMWF/CMC models are showing some decent agreement with the
areal coverage and timing of precipitation, so confidence is
fairly high this precipitation will move through Tuesday morning
and afternoon.

Temperatures will be around seasonal averages, with highs in the
70s across the Northland.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1256 AM CDT Sat Jul 22 2017

Shower/storm coverage has been on the decline over the past
couple of hours and that will continue over eastern areas with an
increase again from west to east later tonight into Saturday. An
upper level shortwave will continue to move east-southeast toward
the Manitoba/northwest Ontario/Minnesota border by 18Z Saturday.
The shortwave will continue to western Lake Superior Saturday
night. A frontal boundary will also be moving through the
Northland and strong to severe thunderstorms will be possible
Saturday afternoon into the evening over portions of the
Northland. We highlighted time periods where we have the most
confidence some storms will occur but timing will continue to be
refined through the period.

Fog was already forming as of late evening and we expect that to
continue. IFR/MVFR conditions will expand tonight then improve
through the day Saturday.


DLH  59  73  57  70 /  70  40  40  20
INL  60  77  56  74 /  30  60  40  20
BRD  65  83  60  75 /  70  30  10  10
HYR  63  80  58  71 /  60  40  30  20
ASX  58  74  56  67 /  70  30  30  30




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