Severe Thunderstorm Warning 13, NWS Duluth, MN

Area Forecast Discussion
National Weather Service Duluth MN
630 PM CDT Sun Jul 9 2023

.MESOSCALE UPDATE...
Issued at 630 PM CDT Sun Jul 9 2023

Strong thunderstorms were occurring this evening along the
International Border near Voyageurs National Park on east. These
were occurring ahead of a cold front in a favorable environment
with MUCAPE values around 1000 J/KG, effective shear of 40-45 KT,
and 0-1KM shear of 20-25 KT. Other showers and storms have
developed much further south in an area of low level convergence
in the Brainerd Lakes region. RAP forecast soundings at KINL were
inverted-V with DCAPE values of 800-900 J/KG. There will be a
threat for severe storms over the next few hours, mainly in far
north to northeast Minnesota with damaging wind the main threat.
There will also be a threat for large hail as we've observed some
three body scattering with a couple of the cells. The storms were
developing into a line so the hail threat will be secondary to the
wind. Although not likely, the 0-3KM shear along the
International Border supports a chance for a QLCS tornado.

&&

.DISCUSSION...(This evening through Saturday)
Issued at 330 PM CDT Sun Jul 9 2023

Summary: Thunderstorms and rain showers are expected to move into
the Minnesota Borderlands between 6-8 PM this evening, with storms
as far south as MN Hwy 210 and possibly limited coverage into
northwest Wisconsin later tonight. The strongest storms could
produce wind gusts of 40-50 mph, but an isolated severe wind gust
to 60 mph cannot be entirely ruled out for far northern Minnesota.
These overnight rain chances end last in the Arrowhead by very
early Monday morning. The next chance at rain and thunderstorms
sets up Monday late afternoon in inland northwest Wisconsin around
eastern Sawyer, Price and southern Iron Counties. The strongest
storms in this area could also be a period of isolated severe
thunderstorms producing large hail and strong wind gusts into late
Monday evening. Cooler temperatures in north-central Minnesota
Monday spread areawide by Tuesday.

Deep low pressure remains entrenched over far northern Manitoba
today. A low to mid-level shortwave trough is seen rotating
through this feature on afternoon infrared imagery. This has
already produced daytime showers and isolated thunderstorms in
northwest Ontario, per CASDR radar imagery. The trailing cold
front is expected to slowly move southward this evening and create
increasing precip chances first over the Minnesota Borderlands
between 6-8 PM today. Forecast model soundings show a period of
500 to 1000 J/kg of CAPE with a fairly dry environment aloft. That
dry air aloft and fairly dry mixed layer late this afternoon,
expected to last into this evening, may create gusty and erratic
winds as the main thunderstorm threat through 03-04Z tonight
mainly for northeast Minnesota. Most wind gusts are expected to be
from 40-50 mph, but an isolated severe thunderstorm gust to 60
mph cannot be entirely ruled out. The best forcing lining up with
available instability and very limited shear keeps the severe
threat mainly to the Iron Range, Arrowhead and Borderlands, but
scattered showers and thunderstorms may be observed as far south
as the MN Hwy 210 corridor and into lake shore counties of NW WI
through tonight. Showers chances decrease last in the Arrowhead by
early Monday morning. So the main message for this evening
continues of widely scattered strong thunderstorms in northeast
Minnesota through very early tonight, but an isolated severe
thunderstorms has a low probability of occurring too.

Lighter northwesterly flow behind the front allows cooler
temperatures to set up over north-central Minnesota Monday, while
much of far eastern Minnesota remains seasonable and still warm
into northwest Wisconsin.

Monday late afternoon looks to be the next chance for severe
weather in northwest Wisconsin as the trough aloft moves
southeastward tomorrow daytime. Inland northwest Wisconsin
counties including eastern Sawyer, all of Price and across
southern Iron Counties have the best chance at seeing isolated
severe thunderstorms in the late afternoon to mid-evening hours
Monday. Southern Price County could even see more like a
scattered severe thunderstorm regime. This is largely conditional
on just how far north the warm air is able to keep around in our
far southeast counties as the front moves southward tomorrow ...
right now the severe threat is there but this could easily push
just south of our forecast area over the next 12-18 hours. At this
point the 25-or-so knots of effective shear, CAPE to 1700 J/kg
and steeper lapse rates promote the potential of large hail around
1" in diameter as a primary severe threat for Monday late
afternoon into mid-evening. Some guidance has non-severe
thunderstorms back into the lakeshore counties of NW WI, but
confidence remains low on any development at this time so the
forecast remains limited to only isolated to scattered storm
mentions over just a few hour period tomorrow in those counties.

Temperatures were dropped for Monday night from the base national
blend guidance over northeast Minnesota as clearing skies, calm
winds and dryer sounding profiles are expected to promote
efficient cool-basin radiational cooling. Temperatures may have to
be further lowered over the next 24 hours from current
expectations of most locations north of US Hwy 2 in MN reaching a
few degrees either side of 40 F by sunrise Tuesday morning.

Fog was added to Price County in northwest Wisconsin for the
expected localized late-evening heavier rainfall tomorrow night
creating pockets of low-basin fog throughout Monday night after
the showers and storms taper off.

Those cooler temperatures set up areawide by Tuesday predominant
northwest flow. Surface winds should be fairly light.

The pattern of west to northwest flow persists into the middle to
the end of this week as the low pressure is stationary over
Manitoba. The next shortwave could move over our area Wednesday
evening and bring an overnight rain chances into Thursday. Similar
low PoPs remain in the forecast into next weekend.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 630 PM CDT Sun Jul 9 2023

A cold front from northwest Ontario into far northern North Dakota
will continue south tonight. There will be a chance for showers
and thunderstorms both ahead and behind the front with the
stronger storms ahead and especially affecting far
northern/northeast Minnesota through the late evening hours. A
dip to MVFR or IFR will occur with the thunderstorms and gusty
winds will be likely with the storms as well. Coverage of
showers/storms are expected to diminish overnight but some are
expected to hold together. We highlighted the most likely time in
the TAFs and will update as necessary. A brief dip to MVFR
ceilings will also occur right along the front but they should
only last a few hours.

Gusty southwest winds will gradually diminish this evening. A low
level jet will cause low level wind shear at KBRD/KHYR tonight.

The front may kick off more thunderstorms in southern parts of the
Northland including northwest Wisconsin on Monday. We left the
mention out of KHYR for now as confidence is too low to include.

&&

.MARINE...
Issued at 310 PM CDT Sun Jul 9 2023

Gusty southwest winds will persist over the head of the lake
through the mid-evening hours today as the strong winds shift east
towards the Apostle Islands and near Saxon Harbor. The earlier
Small Craft Advisories remain in effect for those conditions, with
a new set now in effect until early tonight covering the Apostles,
Chequamegon Bay and up till Saxon Harbor. The low level jet moves
out of the western waters after Midnight tonight, with a cold front
eventually moving southeastward. Scattered showers and thunderstorms
associated with the front may produce a lightning and gusty wind
hazard over the Arrowhead through much of tonight. Winds will
shift westerly Monday morning and midday behind the front. There
may be another period of limited thunderstorm coverage along the
South Shore late Monday afternoon and evening, but confidence is
not high on just how far north the storms will travel away from
inland northwest Wisconsin.

&&

.PRELIMINARY POINT TEMPS/POPS...
DLH  61  75  47  71 /  40  10  10   0
INL  53  69  41  70 /  60  10   0   0
BRD  63  77  47  72 /  20  20   0   0
HYR  62  81  45  73 /  20  40  30   0
ASX  63  76  46  71 /  40  30  20   0

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
LS...Small Craft Advisory until 1 AM CDT Monday for LSZ121-147-148-
     150.

     Small Craft Advisory until 7 PM CDT this evening for LSZ143>146.

&&

MESOSCALE UPDATE...Melde
DISCUSSION...NLY
AVIATION...Melde
MARINE...NLY

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