Special Weather Statement, NWS Duluth, MN

Area Forecast Discussion
National Weather Service Duluth MN
1205 AM CDT Wed Jul 5 2017

.UPDATE...
Issued at 914 PM CDT Tue Jul 4 2017

Thunderstorms are percolating along and just ahead of a cold front
that extends from northern Lake of the Woods to southeast North
Dakota. These storms are losing some of their intensity as the
atmosphere is stabilizing. Capping inversion is building and
MLCAPE has decreased over the last two hours over the forecast
area. There has been virtually no shear, but lapse rates are
steep. Have made adjustments to timing and pops and favor the
Canada storms a bit. Other tweaks were also made.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 407 PM CDT Tue Jul 4 2017

Several rounds of strong to severe thunderstorms are expected
during the short term and heavy rainfall is possible Wednesday
afternoon.

Late this afternoon a weak area of low pressure was located over
southeast Manitoba with a warm front stretching south across
western Minnesota and a cool front to the southwest across North
Dakota and southern Montana. Ahead of the warm front, moist
southerly winds and abundant sunshine have resulted in
temperatures from the middle 60s along the shore near Grand
Marais, to the upper 80s in western Koochiching County, with most
of the Northland in the 80s. Dewpoints from central to north-
central Minnesota were in the upper 60s to low 70s. MLCAPE of
2000-3000 J/kg was found from the Brainerd Lakes northward toward
International Falls. A few isolated storms have developed near
Brainerd in the past hour ahead of a compact vort max indicated on
GOES-16 Channels 8 and 9 WV imagery. Think a few isolated strong
to severe storms will continue with that disturbance as it moves
eastward across our southern zones. Main concern for tonight will
be associated with the cool front. Storms are expected to fire
this afternoon over North Dakota and southern Manitoba and move
into our northwestern counties this evening. Moderate to strong
instability will continue into tonight and deep layer shear is
forecast to increase as the nocturnal low- level jet develops.
There will be a risk of heavy rainfall, large hail, and damaging
winds with the strongest storms tonight.

The area of storms will continue southeastward reaching northwest
Wisconsin Wednesday morning. A general weakening trend is
expected late overnight through early Wednesday morning. The
outflow boundary will act as a quasi-stationary boundary during
the morning and may provide focus for scattered showers/storms
during the morning from the Leech Lake vicinity to the Hayward
Lakes. Training thunderstorms are possible, which may lead to
excessive rainfall and localized flooding. Provided the convective
debris from the overnight storms can clear, a moderately to very
unstable airmass will develop. Deterministic models feature
anywhere from 1500 to 6000 J/kg of MUCAPE by mid- afternoon. While
the GFS solution seems astronomical, the consensus of the
remaining guidance converges around 2000-3000 J/kg once again with
0-6 km bulk shear of 20 to 30 kts. Strong to severe thunderstorms
will be possible once again. Moisture transport into the frontal
zone becomes nearly parallel with the boundary by late afternoon,
leading to increased precipitation rates and raising the potential
for localized flooding.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 407 PM CDT Tue Jul 4 2017

The cold front that sagged into the area Wednesday will continue
to slide south and then stall over the area Wednesday night,
Thursday and Thursday night. This creates two opportunities for
severe storms for parts of the forecast area. Wednesday evening a
weak shortwave moving through the northwest flow interacts with
the weak surface boundary to generate thunderstorms along it.
There is enough instability and shear that develops on the south
side of the boundary to generate strong thunderstorms over mainly
our Minnesota counties. Much is going to depend on the convection
tonight and Wednesday morning, and how far south that can push as
it decays Wednesday morning. There should be a lull in the
convection late Wednesday night and early Thursday. For later in
the day though, we have another strong shortwave that moves
through the flow across the lingering front. Due to the dewpoints
that will have been pooling along the boundary for a solid 24
hours instability will get pretty large over the southern half of
the forecast area, and as the shortwave moves into the area shear
will also increase. Storms should initiate in our area and then
move to the southeast, so our window of threat is relatively
short. Both nights we will have some pretty significant PW values,
so we may also be looking at locally heavy rain. For now have
kept to mainly chance POPs for both of these periods, and we will
have to refine later. Temperatures on the south side of the front
will be quite warm, with 80s on Thursday for most of the area, but
once the front slides through Thursday night it will become
cooler. Highs on Friday will only be in the 70s.

Next weekend starts out dry and warm with the large surface ridge
over Iowa by early Saturday and northwest flow aloft. There is a
shortwave that dives through the upper level flow, but for now the
track appears to be far enough to the south of us to not bring
any convection into the forecast area on Friday night or Saturday.
By this time however, there are some pretty significant
differences showing up in the models. The upper level trough axis
is broader and less progressive in the ECMWF as compared to the
GFS. This means that the ECWMF keeps us cooler with chances of
showers and thunderstorms than the GFS does through early next
week. For now have just kept the consensus slight to low end
chance POPs through the early part of next week with near to below
normal temperatures.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1205 AM CDT Wed Jul 5 2017

A cold front is forecast to work its way past the terminals.
Showers and thunderstorms will occur along and ahead of the front
and gradually affect all terminals. Timing is in question as
models all offer different solutions. Confidence is below average.
VFR is expected through the forecast with MVFR in the vicinity of
thunderstorms.

&&

.PRELIMINARY POINT TEMPS/POPS...
DLH  59  82  60  82 /  30  40  20  30
INL  62  81  57  80 /  70  20  30  20
BRD  66  86  65  85 /  30  40  30  30
HYR  63  83  64  85 /  10  40  40  40
ASX  59  83  61  84 /  20  40  30  40

&&

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

UPDATE...GSF
SHORT TERM...Huyck
LONG TERM...LE
AVIATION...GSF

Hazardous Weather Outlook...UPDATED
National Weather Service Duluth MN
354 PM CDT Tue Jul 4 2017

MNZ010>012-018>021-025-026-033>038-WIZ001>004-006>009-051030-
Koochiching-North St. Louis-Northern Cook/Northern Lake-
North Itasca-Central St. Louis-Southern Lake/North Shore-
Southern Cook/North Shore-North Cass-South Itasca-South Cass-
Crow Wing-Northern Aitkin-South Aitkin-Carlton/South St. Louis-
Pine-Douglas-Bayfield-Ashland-Iron-Burnett-Washburn-Sawyer-Price-
354 PM CDT Tue Jul 4 2017

This Hazardous Weather Outlook is for portions of northeast
Minnesota and northwest Wisconsin.

.DAY ONE...This afternoon and tonight

There is a chance of thunderstorms this afternoon and this
evening from near Walker eastward through the Brainerd Lakes to
Pine City, Siren, and Spooner. Thunderstorms are also possible
north of the Iron Range to the International Border. A few of the
storms may be strong or severe with large hail the main threat.
Another round of thunderstorms will develop this afternoon over
North Dakota and southern Manitoba, and move southeastward across
most of the Northland tonight. Strong to severe thunderstorms are
expected with large hail, gusty winds, locally heavy rainfall, and
frequent cloud-to-ground lightning.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Thunderstorms are possible Wednesday afternoon into Wednesday
night across the Northland. A few storms may be strong to severe,
and produce heavy rainfall. The Storm Prediction Center has the
Northland under a Marginal Risk of severe thunderstorms Wednesday
and Wednesday night. The strongest storms may produce large hail,
damaging winds and torrential rainfall.

Severe thunderstorms are much more likely on Thursday and
Thursday evening, especially across the eastern third of Minnesota
and northwest Wisconsin. The Storm Prediction Center has
indicated a Slight Risk of severe thunderstorms across much of the
area, along with an Enhanced Risk across portions of northern
Wisconsin. Very large hail and damaging winds will be the main
threats.

There is a small chance of thunderstorms Saturday afternoon and
evening. Severe weather is not anticipated at this time.

.SPOTTER INFORMATION STATEMENT...

SKYWARN Spotters may be needed this afternoon and tonight.

LSZ121-140>148-051030-
Bayfield to Oak Point WI-Grand Portage to Grand Marais MN-
Grand Marais to Taconite Harbor MN-
Taconite Harbor to Silver Bay Harbor MN-
Silver Bay Harbor to Two Harbors MN-Two Harbors to Duluth MN-
Duluth MN to Port Wing WI-Port Wing to Sand Island WI-
Sand Island to Bayfield WI-Oak Point to Saxon Harbor WI-
354 PM CDT Tue Jul 4 2017

This Hazardous Weather Outlook is for portions of the Nearshore
waters of western Lake Superior.

.DAY ONE...This afternoon and Tonight

Thunderstorms are possible across western Lake Superior late
tonight.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Thunderstorms are possible Wednesday and Wednesday night. Some of
the storms may be strong or severe with large hail, gusty winds,
frequent cloud-to-water lightning, and heavy rain.

Thunderstorms are likely Thursday and Thursday evening, some of
which may be severe. Very large hail, damaging wind gusts,
frequent cloud-to-water lightning, and torrential rainfall are
possible.

There is a small chance of thunderstorms Saturday afternoon and
evening.

Huyck

More information may be obtained at the following web address:
http://www.weather.gov/duluth (all lower case)

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