Duluth (DLH) — the National Weather Service forecast in plain English · Plaincast

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There is a slight risk (level 2 out of 5) for scattered strong to severe thunderstorms late this afternoon and evening.

Cooler fall weather moves in Saturday and into the foreseeable future.

Widespread rain and thunderstorms are likely Monday and Monday night. Severe weather is not expected. Localized minor flooding can't be ruled out.

Update

Quiet weather for the rest of tonight and through much of the day today, as southwesterly winds increase ahead of an approaching storm system. Winds will be breezy with gusts of 25-30 mph by this afternoon. Although clouds will be increasing, highs will still reach the upper 70s to lower 80s, with mid 70s along portions of the North Shore, under strong warm advection.

Severe storm potential for this Afternoon-Evening:

The airmass will become increasingly unstable through the afternoon, as moisture and warm air advects northward ahead of the approaching surface low and cold front. The parameter space remains unchanged from what was highlighted in the previous discussion below. What we will need to continue to watch is the strength of the cap around 750mb, as that will prevent development through much of the afternoon. As surface temperatures hit the low 80s, at the time where shear values are increasing, isolated cells could develop. These storms would pose the largest tornado threat, along with large hail. Storm mode will transition to more linear into the early evening, with the primary threat becoming damaging winds. This line will push eastward through the evening hours, with scattered showers hanging around portions of the Arrowhead and northwest WI for a few hours past midnight.

Discussion

Quiet weather abounds this afternoon with weak ridging aloft. As a surface high departs and moves into the central Great Lakes tonight, we will remain under an upper level ridge, which will be strengthened by a developing low-level jet tonight under warm southwest flow. Wind speeds will increase overnight, and fog is not expected to be a concern tonight.

On Friday, warm air and moisture advection will continue with a trough and developing surface low moving our way from the west. A trailing cold front associated with a low up in Canada will combine with the warm air and moisture advection to trigger showers and thunderstorms. The trend for storm initiation continues to be later, favoring storm development around 6 PM or so. Plenty ingredients will be in place for some severe storms, including: a cold front, up to ~2 kJ/kg MUCAPE (initially surface-based), favorable 30-40 knot 0-6 km shear (with some low- level streamwise vorticity ingestion possible which could result in a low-end tornado threat), and overall plenty of moisture with precipitable water inching up into the 1-1.75" range. The timing of storm initiation will be crucial, as a delay may result in a very narrow window for surface-based convection in the early evening. Large hail and damaging winds are still looking to be the main threats, especially if storms become largely elevated by starting later into the afternoon/evening. If they can get going early enough, a low-end tornado threat exists. The rain/storms should be progressive enough to minimize flooding risks, but with plenty of moisture available, any brief training of storms could result in some very localized and short-duration minor flooding, especially in developed areas. Storms are expected to move out with the cold front a bit after midnight.

The gates will open to more fall-like temperatures following the cold front into Saturday with cool northwest flow developing. Highs are likely only to reach the 60s in Minnesota and low 70s in northwest Wisconsin. We will remain in this cool air mass through the weekend with otherwise pretty quiet weather. Saturday and Sunday nights are looking rather chilly with high pressure slowly passing through at the surface. At this point, probabilities for temperatures cold enough for frost (~36 degrees) are about 10-15% for the usual cold spots. So, can't completely rule it out, but the chances are very low at this time. With that said, lows in the upper 30s are looking likely for the usual cold spots and low 40s for most other places.

Going into Monday, it's looking like we'll get a nice surge of moisture and warm air advection with Gulf of America origins. This should lead to more of a widespread light to moderate rain with some thunderstorms as well (though instability isn't currently looking overly impressive for anything strong or severe). There is some potential for locally heavy rainfall just north of the surface warm front, and there's some uncertainty where that might set up. Wisconsin is looking to be the main target right now, but whether means northwest Wisconsin or central Wisconsin is still up in the air. There could be some localized minor flooding issues with this, and part of that will depend on how much rain falls with Friday evening's storms (as of now, it looks like northeast Minnesota has the best chances, which is good news as that would spread the moisture somewhat evenly). Overall, this should be good news for ongoing drought with a good half inch of rain minimum for most places and >1" of rain very possible especially in northwest Wisconsin.

Another reinforcing cold front will pass through going into Tuesday. Some fall-style wraparound showers are possible Tuesday, then high pressure is likely to bring quiet but cool weather Wednesday and Thursday.