A Severe Thunderstorm Watch has been issued for northern Minnesota and western Lake Superior until 1 AM Saturday morning (yellow area on map)
Some cities in the Northland that are included in this watch are…International Falls, Bigfork, Grand Rapids, Cass Lake, Walker, Longville, Crane Lake, Orr, Ely, Hibbing, Duluth, Isabella, Two Harbors, Silver Bay, Lutsen and Grand Marais.
Primary hazards in the watch area include
- Scattered large hail and isolated very large hail events to 2 inches in diameter possible
- Scattered damaging wind gusts to 70 mph possible
A severe thunderstorm watch means conditions are favorable for severe thunderstorms in and close to the watch area. Persons in these areas should be on the lookout for threatening weather conditions and listen for later statements and possible warnings.
Note: No, a severe thunderstorm watch doesn’t mean every single location in the watch will get hit by a severe thunderstorm or receive damage, all a watch means is that conditions are favorable for severe weather development. Usually only a small percentage of a watch area will see severe weather/damage.
A cold front (blue line on map) will move across the Northland through Saturday morning. As this front moves east it will interact with a warm, humid and unstable air mass, while lift provided by the front will help produce scattered convection in the Northland tonight.
A broken line of strong to occasionally severe thunderstorms continue to move east-southeast across northern Minnesota as of 6 PM Friday. This line of storms should hold together as it continues to track east-southeast this evening, while it eventually reaches the Arrowhead and North Shore. Note: Additional storms could also develop and or move into other portions of the Northland later this evening/overnight.
A look at the 18z NAM 3km model shows scattered clusters/lines of showers and thunderstorms impacting much of the Northland tonight, with this activity moving out of the area early Saturday morning.
Torrential downpours and lightning will be threats with any thunderstorm tonight.
On average we’re looking at anywhere from 0.10 to 0.50″ of rain in northeast Minnesota and northwest Wisconsin tonight, but as is often the case with thunderstorms, some locations could receive a lot more rain, possibly >1″ totals, while other areas receive a lot less rain.
Certainly have a juicy air mass in place for storm development this evening with dew points on the tropical side (mid 60s to low 70s) A much drier air mass arrives by Saturday afternoon and sticks around into Sunday.
Quite a bit of instability (CAPE) covering much of the Northland late this afternoon, with Mixed Layer CAPE values ranging from ~1000 to 2500 J/kg, while elevated and surface based cape were approaching 3000 J/kg over parts of northern Minnesota late this afternoon.
We do have a change in wind direction with height as SW winds at 850mb veer to W to WNW at 500mb, but speed shear is on the marginal side, with low and mid level winds generally around 20 to 35 knots, but this will likely be enough given the other parameters in place to sustain thunderstorm activity for much of the night. Whether or not severe weather occurs through the night remains to be seen, however.
With the wind shear in place, this does open up the possibility for a few rotating/supercell type thunderstorms with a threat for large hail and damaging winds this evening.
A blow up of strong to severe thunderstorms mid to late this afternoon across far northern Minnesota as seen on Goes-16 sandwich imagery from Friday, July 23, 2021.
Fire Weather Concerns return to the Northland this weekend as a very dry air mass moves overhead, with afternoon relative humidity values Saturday and Sunday of only 15-25% over much of the area, this combined with gusty winds to 20 mph could produce critical fire weather conditions in parts of the area this weekend given the drought we’re in, but if we see widespread/significant rains tonight, that may ease the fire weather issues just a bit this weekend.
18z NAM 3km model relative humidity forecast valid from Noon Saturday to 7 PM Sunday (dry air shows up in brown colors)
Highs Saturday are forecast to mainly be in the 80s in northeast Minnesota and northwest Wisconsin, and similar high temps are likely on Sunday.