SHORT TERM /THROUGH THIS AFTERNOON/
Issued at 333 AM EDT Wed Sep 18 2024

Water vapor imagery and RAP analysis show mid-level troffing over the western U.S. and downstream ridging from the Upper Mississippi Valley to New England. A vigorous shortwave is over northern Ontario, and it is producing a cluster of tsra about 180 miles n of Isle Royale. Recently, a few shra have developed w and sw of Duluth on the eastern edge of the low-level jet and within a local 850mb theta-e ridge. Closer to home, for the 3rd day in a row, it's an incredibly warm early morning for mid Sept in the downslope areas near Lake Superior from western to n central Upper MI. Stirring s winds are holding temps up into the upper 60s and lwr 70s as of 07z, and for the third night in a row, Big Bay is the warm spot. Two nights ago the temp at Big Bay was 76 at 07z, last night it was 75, and currently, it's 74F. Just as in previous nights, temps fall off significantly to the e and s. Across s central and eastern Upper MI, temps are as low as the mid 40s F at traditional interior cold spots as a drier column has further enhanced radiational cooling on this night, but temps ranging thru the 50s F are generally the rule. With the cool conditions, patchy, shallow radiation fog has once again developed in that area. The patchy fog, locally dense, will burn off an hr or so after sunrise.

Another unseasonably warm day is on the way under sunny skies.
Models show mid morning 850mb temps today about 0.5C lower over the eastern fcst area and maybe about 1C higher over the w. So, overall, today's highs will be about the same as yesterday, maybe a degree lower e and a couple of degrees F warmer w. In general, expect low to mid 80s e and mid to upper 80s F w. As in recent days, southerly winds will hold temps to the 70s F near Lake MI. The low-level air mass today is a little drier than yesterday as noted by 850mb dwpt depressions, and this will result in sfc dwpts mixing out a bit lwr than yesterday as well. Much of the fcst area, except areas immediately downstream of the southerly flow off of Lake MI, will see dwpts fall thru the 50s with low end in the upper 40s F. This will drive RH down thru the 30s with low end slipping into the 25- 30pct range at some of the typically lower RH locations.
Fortunately, wind gusts will generally be no higher than the 10- 15mph range this aftn, providing some limiting of fire wx concerns.
Still, the ongoing lengthening dry spell is becoming an ever greater concern for wildfire potential. Over the last 2 weeks, pcpn has been been less than 50 pct of normal across all but nw Upper MI, and large areas are at less than 25 pct of normal. Many locations have not seen any rain for at least a week.

LONG TERM /TONIGHT THROUGH TUESDAY/
Issued at 325 AM EDT Wed Sep 18 2024

We get one more mostly dry, late summer-like day on Thursday before the high pressure ridging over us gives way to a cool front moving in from the west late Thursday through Friday. Afterwards, expect temperatures to decrease closer to normal, with a low pressure over the Canadian Prairies lifting towards Hudson Bay and another low lifting from the Desert Southwest/Southern Rockies into the Upper Midwest possibly bringing back rainfall over us as well next week.

Another mostly clear night is in store tonight as the ridging persists but slowly weakens over us with time. As temperatures cool during the overnight hours, we could see some patchy fog develop in the south central and near Lake Michigan late as the temperatures reach the dewpoint. Overall, thinking low temperatures tonight will get into the 50s, with a few spots in the interior central possibly getting into the upper 40s via the help of the radiational cooling.
Moving into Thursday, expect another very warm day and mostly sunny skies. While we won't be as warm as the past few days, we can still expect high temperatures to get into the low to possibly mid 80s in the interior areas. Fire weather concerns should be lessened Thursday too, as min RHs aren't looking to get below the upper 30 percents. In addition, rainfall looks to finally make its way into our area come Thursday as troughing from the Plains begins to push into our far west. As this happens, we could see some isolated showers and maybe a stray storm or two during the morning hours near Ironwood, in addition to partly cloudy skies over the rest of the western U.P. throughout a significant portion of the day. While CAMs are generally showing the showers and storms dying out and avoiding the Upper Peninsula by the early afternoon hours, there looks to be another line of convection that develops over Minnesota late in the day that makes its way into the U.P. after sunset. This line of convection looks to follow a cool front moving from west to east across the area late Thursday into Friday as the parent low lifts from the Canadian Prairies towards western Hudson Bay. While not much rainfall is expected with this cool front, we could see some heavier downpours in the stronger showers and storms that produce some locally soaking rainfall over the U.P. as guidance shows PWATs around 1.50 inches along the frontal boundary. While the European ensemble shows us that we do have enough instability to get thunderstorms and showers going, with bulk shear remaining fairly modest (25 knots or less) across the area, no severe weather is expected at this time along the cool front. With the thicker cloud cover overhead, we could see some record high minimum temperature records broken Thursday night (and possibly Friday night too across the eastern U.P.). The cool front looks to leave our area by Friday night as weak ridging fills in behind it.

Model guidance begins to significantly diverge past this Saturday as each medium range suite has different strengths and locations for two vertically-stacked lows over the CONUS that could impact our weather. The first of which is one over the Canadian Prairies Saturday that lifts towards Hudson Bay for the latter half of the weekend. This low looks to bring another cold front over us Saturday night into Sunday. This front may bring us some additional showers and thunderstorms across our area during that time period, but with the lifting mechanisms weakening as it moves over us, some guidance looks to keep us dry but cloudy through the latter half of the weekend. However, once this front passes, expect to see temperatures closer to normal with highs only getting into the lower 70s post cold front on Sunday to the mid to maybe (?) upper 60s come Monday.
A second vertically-stacked low looks to lift from the Desert Southwest and the Southern Rockies towards the Upper Midwest early next week. Depending on how the cold front of the first low phases with the developing warm front of the second low, as well as where the second low tracks, we may or may not receive rainfall during the first half of next week; while the GFS and Canadian suites are generous with the rainfall over us early into mid next week, the Euro is suggesting that we will remain dry through the remainder of the period. Hopefully, the former two will occur, as we really need some meaningful rainfall across our area.

AVIATION /12Z TAFS THROUGH 12Z THURSDAY/
Issued at 716 AM EDT Wed Sep 18 2024

Under a warm, dry air mass, VFR will continue thru this fcst period at IWD/CMX/SAW.

MARINE
Issued at 325 AM EDT Wed Sep 18 2024

Light winds of generally 20 knots or less continue today into Thursday before southeast to southerly winds pick up to 20 to 25 knots over the north central lake late Thursday ahead of an approaching cool front from the west. As the front weakens while traveling eastward across Lake Superior Thursday night into Friday, expect the winds to die down to 20 knots or less again by Friday morning. The light winds continue through Saturday as weak ridging moves through the Upper Great Lakes, before possibly another cold front from the west increases winds to greater than 20 knots Saturday night/Sunday.

As for thunderstorm chances over Lake Superior the next several days, outside of a stray thunderstorm or two near Isle Royale early this morning, expect the lake to remain storm-free until the cool front reaches the far western lake near Duluth Thursday morning.
Shower and storm chances continue over the far western lake Thursday before a line of convection moves from west to east across the lake along the cool front late Thursday through Friday. There is a marginal risk (2+% chance) for some strong, erratic winds and damaging hail over the far western lake Thursday. As the showers and storms move eastward into a less favorable environment, the severe weather threat diminishes. The last of the showers and storms looks to leave eastern lake Superior by Friday night.

MQT WATCHES/WARNINGS/ADVISORIES
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.