/Issued 5:51 PM CDT, Monday, July 24, 2017/
Here's the setup for Tuesday, and where I think the best chance for some severe thunderstorms will be especially from late Tuesday afternoon into Tuesday night.
20z HRRR model
18z NAM temperature forecast valid at 4 PM Tuesday, July 25, 2017.
Map below shows the average number of thunderstorm days per year -- Most of Minnesota and Wisconsin typically get between 30-40 thunderstorm days a year with 40 or more thunderstorm days a year on average in far southern Minnesota and southwestern Wisconsin.
Number of thunderstorm days so far this year
Minneapolis, MN: 25
Duluth, MN: 23
Marquette, MI: 19
International Falls, MN: 12
Low clouds and fog burned off this morning which gave way to mostly sunny to sunny skies over the Northland this afternoon with visible satellite imagery at 3 PM showing some patchy fair weather cumulus clouds in extreme northeast Minnesota while some high level clouds push slowly E/SE out of western Minnesota. Highs today were mainly in the 70s with Bigfork and International Falls close to 80 degrees (3 PM) It was cooler near Lake Superior with highs today in the 60s.
Tonight: Clear to partly cloudy skies this evening giving way to partly to mostly cloudy skies later on. A few showers and thunderstorms spreading into mainly western/southern sections of the Northland overnight. Brief heavy downpours and lightning will be the main hazards from this activity. Elevated instability will be on the increase later tonight over the southwest portion of the area with CAPE building to a few hundred up to 1000 j/kg, this combined with a warm and moist advection pattern and a 30-40 knot SW low level jet should help to develop some convection in parts of the Northland mainly after midnight.
Tuesday: Turning more humid as southerly winds help transport a muggier airmass into the Northland. Dew points this afternoon ranged from the mid 40s in far northeast Minnesota to around 60 degrees over the Brainerd Lakes -- Dew points by Tuesday afternoon will range from around 60 to 70 degrees over all of the Northland. PWATS will also climb from current values late this afternoon of 0.70"-0.80" to between 1.50" to ~2.00" Tuesday afternoon -- Bottom line is that there will be abundant moisture in place for a thunderstorm to tap into which could result in very heavy rainfall rates and localized flooding Tuesday and or Tuesday night.
The risk of severe weather is more uncertain here in the Northland. Will have the necessary lift in place as a cold frontal boundary approaches from the west, will also have very steep mid level lapse rates advecting into the area with values of 7.0-8.0 c/km. Winds in the mid and upper levels of the atmosphere are also favorable for severe weather with 80-100 knot upper level winds just off to our NW with mid level winds around 30-45 knots covering the Northland. SW low level jet of 30-40 knots should also persist ahead of the cold front on Tuesday. Shear is forecast to be in the 30-40 knot range...So all those ingredients are favorable for severe weather. The unknowns at this time is the sky condition, and just how much instability can build up, up here in our area as well as the strength of the cap and whether or not that will break. Note: The convection allowing models have been hinting at most of the storms from late Tuesday afternoon into Tuesday night remaining across southern, central and eastern Minnesota (Possibly clipping the far southern portions of the Northland) with just spotty weaker storms possible over most of our local area. Note: There could also be a round of scattered showers or thunderstorms that affect parts of the Northland Tuesday morning into early afternoon associated with whatever develops to our west/southwest later tonight, those storms shouldn't be severe though.
Wednesday: Cold front exits the Northland to the south/east during the morning hours. Showers or thunderstorms are possible mainly in the morning over east central Minnesota and northern Wisconsin. Turning less humid from north to south with dew points dropping into the 50s to lower 60s by Wednesday afternoon. Highs Wednesday in the 70s to lower 80s.
Forecast for Duluth and Superior
.Tonight... Mostly clear this evening then partly to mostly cloudy overnight. A few showers possible toward daybreak. Low 55 to 60. Wind east to southeast at 10 to 15 mph.
.Tuesday... Turning more humid. Partly to mostly cloudy. Scattered showers or thunderstorms possible. High 71 to 76. Wind south to southeast at 10 to 20 mph. Dew point around 65 degrees during the afternoon.
.Tuesday night... Humid. Mostly cloudy. Scattered showers or thunderstorms possible. Lows in the 60s.
.Wednesday... Turning less humid in the afternoon. Partly to mostly cloudy. High 75 to 80.
Normal temperatures for Tuesday
Sunrise Tuesday: 5:41 AM CDT
Sunset Tuesday: 8:49 PM CDT