Severe Thunderstorm Warning 76, NWS Duluth, MN

Area Forecast Discussion
National Weather Service Duluth MN
654 PM CDT Fri Jul 17 2020

.SHORT TERM...(This evening through Saturday night)
Issued at 313 PM CDT Fri Jul 17 2020

We have an active weather pattern for the next couple days,
including a significant severe weather threat overnight tonight
and again Saturday afternoon and evening.

The rest of today should be relatively quiet with high
temperatures in the mid-upper 80s with plenty of sunshine. A few
storms are possible over northwest WI late this afternoon as a
pool of deep instability is sitting over a large portion of the
area, but somewhat weak shear a very weak cap and not much of a
trigger. If storms can get going, they may be strong, but their
isolated nature and the weak dynamics should keep them from
growing upscale. They also may be elevated if the cap is stronger
than it currently looks. Of more interest is the convection that
has already developed over northwestern North Dakota. This
convection looks like the beginning of tonight's event, with
additional development expected in the next few hours out there.
We have been watching tonight for a few days, most of the models
in good agreement, depicting an MCS developing out there in
western ND, spreading east through the afternoon and evening
before blowing into the CWA by late evening, and then on though
the CWA overnight. Models are depicting a pool of moderate to
extreme instability from ND and SD into central MN, along with
deep layer shear of 30knots across much of this same area. I
expect the convection forming now to ride down the northern side
of the instability pool, which should take it right across the
central/southern CWA overnight tonight. The track and timing are
still somewhat uncertain, with some models running it more across
northern MN, and others farther south, some faster, some slower.
We may not be able to determine this until this evening once we
have a better idea of where the instability pool/warm front will
lie, and how strong the low level jet will be. Given the timing of
initiation, the event may be earlier than the models are
depicting. This event is looking somewhat like the July 20-21 2016
event, and we are including that in our messaging as many people
will remember the large swath of damage we had from that. We are
also in a Moderate risk from SPC for tonight. Once the line blows
through, things will quiet down for the remainder of Saturday
morning, only to redevelop the instability pool and deep layer
shear, ahead of a potent shortwave that is expected to move
through the forecast area during the afternoon and evening hours.
Much of this event will depend upon how tonight's event
progresses, and do not want to get hung up on too many details at
this point in time. Timing is going to be significant with it, as
I think the convection is going to develop right over the CWA, and
could get going by mid afternoon, and ride east from there.
However, it may hold off until later depending on the
destabilization trends, and we will just have to wait and see for
now. Given the deep layer shear and instability, it is no surprise
we are in a Slight risk for Saturday for mainly the southeast
half of the CWA.

Have kept the remainder of the forecast on the hot and muggy side,
with steamy dewpoints and otherwise normal summer conditions for
the Northland. Highs Saturday should get into the mid 80s to near
90 once again, with heat index values getting into the 90-100
range for a brief period Saturday afternoon.

.LONG TERM...(Sunday through Friday)
Issued at 313 PM CDT Fri Jul 17 2020

Sunday is expected to be cooler and less humid following the passage
of a cold front. There is a chance for showers and thunderstorms in
northern areas of northeast Minnesota Sunday evening into the
overnight hours as an upper level shortwave moves across western
Ontario. Rain chances remain in the forecast through much of next
week, as a series of progressive shortwave troughs move across the
region. The best chance for widespread showers and thunderstorms
appear to be Tuesday afternoon, as a shortwave moves directly over
the Northland. Temperatures in the first half of next week are
expected to remain comfortable, with highs in the upper 70s to low
80s with dewpoints remaining in the 50s. Temperatures and dewpoints
may rise in the latter half of the week, as a ridge builds over the
region.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 654 PM CDT Fri Jul 17 2020

VFR conditions are currently in place, but a complex of
thunderstorms will move through the terminals for this evening and
into the early morning hours. This complex will bring the
potential for IFR conditions along with wind gusts in excess of 60
knots, especially at BRD, HYR and DLH. INL will see less of an
impact, but could still see visibility reductions in heavy
rainfall. The most likely timing for the worst conditions at BRD,
HIB and DLH have been treated with TEMPO groups, but further
refinement will be need as storms approach. A break is expected by
the mid to late early morning hours before chances for storms
return for the afternoon hours. MVFR ceilings may linger at times
through sunrise in the wake of the storms. Outside of the storm
activity, generally light winds are expected.

&&

.MARINE...
Issued at 313 PM CDT Fri Jul 17 2020

Initial quiet conditions for western Lake Superior this afternoon
will turn active as a line of thunderstorms moves across the lake
late this evening or overnight tonight. These storms are likely to
be strong to severe, producing wind gusts to 50 knots, hail, and
frequent lightning. Ahead of the line of storms winds will
gradually turn to the southeast or even east, and increase into
the 10 to 15 knot range. After the storms move through, winds will
become southwest for Saturday in the 5 to 15 knot range. More
storms are expected, especially from late afternoon into the
evening hours. Some of these storms may be severe as well, and
produce strong winds, hail, and frequent lightning. Winds will
veer to west to northwest for Saturday night and Sunday and be
from 8 to 15 knots for most areas.

&&

.PRELIMINARY POINT TEMPS/POPS...
DLH  65  86  62  81 /  90  50  60  10
INL  65  84  59  77 /  80  20  20  40
BRD  68  87  61  79 / 100  50  50   0
HYR  69  89  63  82 /  90  60  90   0
ASX  67  89  65  84 /  90  60  70   0

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
LS...None.
&&

SHORT TERM...LE
LONG TERM...Unruh/LE
AVIATION...BJH
MARINE...LE

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