660
FXUS63 KDLH 032118 AAA
AFDDLH

Area Forecast Discussion...UPDATED
National Weather Service Duluth MN
418 PM CDT Sun Aug 3 2025

.KEY MESSAGES...

- A slow pattern change continues tonight. Rain chances will
  start in central and north-central Minnesota and spread
  eastward over the next few days.

- Off and on rain and thunderstorm chances through this week.
  Wednesday and Wednesday night bring the best chance for
  widespread showers and storms.

- The potential for heat-realted illness will increase Thursday
  and Friday.

&&

.DISCUSSION...
Issued at 356 PM CDT Sun Aug 3 2025

Summary:

Precipitation chances return to the Northland this week along
with southerly winds. Wednesday and Wednesday night are shaping
up to bring the best chance of widespread rain and a chance of
thunderstorms as an area of low pressure propagates eastward
across the Canadian Prairies and northwest Ontario. That system
will draw a warm front northeast across the Northland followed a
few hours later by a cool front from the west. Rain chances
will start this afternoon in central and north-central Minnesota
and slowly expand eastward over the next two days. Despite the
rain chances, the southerly winds ushering in higher dewpoints
will raise a potential for heat related illness Thursday and
Friday.

Tonight through Monday night:

A stubborn blocking pattern remained in place across the CONUS
this afternoon. A large westward-drifting blocking high was over
the Desert Southwest this afternoon. The southern branch of a
split subtropical jet stretched from California to the Eastern
Seaboard. The northern branch was more wavy and arced north over
British Columbia into far northern Canada before curving south
over northern Ontario and turning east again over Quebec.
Between these branches was a high amplitude, short wavelength
ridge over the Canadian Prairies and the northern Plains. This
pattern will change very little over the next few days.

Meanwhile the main weather feature of concern was a remnant
multi-day old MCV over west-central Minnesota, anchoring a 700
mb trough over the eastern Dakotas and western Minnesota.
Cyclonic vorticity, warm air advection, and diurnal heating
contributed to a notable increase in the coverage of rain and
showers over western and central Minnesota during the past 2
hours. Look for this trend to continue for the rest of the
afternoon into early evening. The compact nature of the remnant
MCV will mean the best rain chances will be very near the center
of the vortex. Dry air in the low and mid-levels to the east of
this feature will make rain showers hard to come by over east-
central Minnesota, the Arrowhead, and northwest Wisconsin. Rain,
showers, and perhaps a few thunderstorms are forecast from
central Minnesota north into north-central Minnesota through
this evening. A diurnal trend in precipitation coverage and
intensity with that feature was observed over the past few days.
Think that trend will continue tonight with coverage and
intensity decreasing later tonight.

Low potential instability with MUCAPE values around 200 J/kg
will not entirely preclude a few lightning strikes this
afternoon and evening. The exception will be near the Canadian
Border where the latest RAP mesoanalysis finds about 500 J/kg of
MLCAPE and over 1000 J/kg of MUCAPE. Deep layer shear over that
area is nearly non-existant, around 10 knots. RAP, GFS, and NAM
lifted indices are negative over northern Minnesota this
afternoon and evening. Combine that with significant localized
upward vertical motion near the MCV and there should be
sufficient ascent for charge separation. I think "thunderstorms"
is going to be a stretch. It`s more likely we will see
occasional lightning strikes in the stratiform rain.

Rain chances will increase again on Monday. Forecast models are
handling the MCV poorly and struggled for the past 36 hours at
least. Therefore confidence in the location of that feature for
Monday is lower than average. As a result we broadened the rain
chances and lowered the PoP magnitude. Current expectations
carry the MCV slowly north-northeastward over the next 24 hours.
That would place it over northern St. Louis County for Monday.
Look for rain coverage to increase again Monday afternoon in
response to diurnal heating. LIs will trend negative again
Monday afternoon and I would not be surprised to have a few more
lightning strikes.

Tuesday through Wednesday night:

The southwest US blocking high will remain in place heading
into midweek while the ridge over the Canadian Prairies drifts
east into Ontario and Quebec. The southern branch of the
subtropical jet will strengthen and become quasi-zonal over the
central Plains by Tuesday night. The remnant MCV may loiter over
the Arrowhead on Tuesday. Isentropic lift will remain over the
region to the south and west of the MCV which will keep a chance
of showers and storms in the picture.

A coupled jet will lead to surface low pressure development
over the northern Plains Tuesday night. A warm front will
develop and lift northeastward across northern Minnesota on
Wednesday. Meanwhile a cool front will sweep in from the west by
evening. Southerly Theta-e advection and isentropic lift in the
wake of the warm front will provide another chance of showers
and a few storms. Forcing for ascent will increase as the cool
front moves in. Shower and storm chances increase during the
afternoon and evening. There should be sufficient instability
over northern Minnesota in the warm sector. GFS progs indicate
1000-2000 J/kg of MUCAPE by late afternoon. Deep layer shear
will be a little more favorable for organized convection, 25 to
35 knots. A few storms may be strong to severe. There may also
be a heavy rainfall risk with southerly warm air advection
running parallel to the cool front.

Thursday through Saturday:

Warm air advection continues Thursday and Friday. Temperatures
will tick upward slightly. Dewpoints should respond more
strongly. That will set the stage for possible heat-related
illness from central Minnesota east into northwest Wisconsin
both Thursday and Friday. Heat Advisories may eventually be
needed.

The unsettled weather pattern will persist for the latter
portion of the week into the weekend. The southern branch of the
subtropical jet will remain strong with several impulses likely
advancing eastward through the flow into the Upper Midwest. A
shortwave trough will develop over Alberta and Saskatchewan and
propagate eastward through the Canadian Prairies Saturday night.
This will keep the Northland in a favorable area for several
rounds of showers and storms Thursday through the weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1239 PM CDT Sun Aug 3 2025

Slow-moving upper-level low pressure system centered over SE ND
this afternoon will continue to meander slowly north-
northeastward this afternoon. An area of light rain over west-
central and central Minnesota is forecast to move northward and
expand eastward this afternoon with scattered showers expected
to develop east of the main area of persistent rain. A few
lightning strikes are possible this afternoon despite limited
potential instability. The greatest chance of a storm is near
INL. Forecast models are struggling with the location and
affects of the upper-level low, so confidence in the
precipitation portion of the forecast is low. Smoke will loiter
around the terminals this afternoon and evening. Visibility
forecasts due to smoke are medium confidence and will vary
between 6SM and 4SM. Cloud cover at HIB overnight should limit
the fog potential there, while partial clearing over HYR may
permit IFR fog to develop overnight.

&&

.MARINE /FOR NEAR SHORE WATERS OF WESTERN LAKE SUPERIOR/...
Issued at 356 PM CDT Sun Aug 3 2025

High pressure slowly departs to the east tonight and Monday.
Winds will vary in direction depending on location tonight.
Winds along the South Shore to the Twin Ports will back easterly
to northeasterly while winds near the North Shore, northeast of
the southwest arm, will remain southwesterly. There a few gusts
to 20 knots are possible between Grand Portage and Isle Royale
this afternoon and evening. Winds are forecast to back easterly
and northeasterly on Monday as the surface high drifts farther
from the lake and weak low pressure develops over the northern
Plains. Wind and waves may become hazardous for smaller vessels
in the southwest arm of Lake Superior during the afternoon and
evening hours. The trend for easterly and northeasterly winds
will persist Tuesday and Wednesday. At the moment wind speeds
are expected to be 5 to 12 knots. Later forecasts may need to
increase wind speeds and gusts in the southwest arm if
confidence increases. Additionally, there may be areas of smoke
across the lake at times over the next several days.

For the open water discussion, refer to the NWS Marquette Area
Forecast Discussion at weather.gov/mqt.

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
MN...None.
WI...None.
MARINE...None.

&&

$$

DISCUSSION...Huyck
AVIATION...Huyck
MARINE...Huyck