SHORT TERM /THROUGH TONIGHT/
Issued at 322 AM EDT Sat Jul 27 2024

Unseasonably warm and muggy conditions settle in today...

Axis of prominent 588-590 dam 500 mb ridge centered over the Upper Great Lakes early this morning will gradually get displaced eastward later today into tonight as a shortwave trough moves through northern Manitoba and northern Ontario. There will remain enough influence from this ridge and anticyclonic flow from its associated sfc ridge to keep conditions dry across Upper Mi through tonight.
WAA South-southwest flow on the backside of the sfc-850 mb ridge will maintain 850 temps of 16-18C across the fcst area, warmest west. Model guidance has been pretty consistent forecasting max temps today in the mid to upper 80s across much of the west half with even lower 90s readings for some downsloping locations along Lake Superior. Moderation off Lake Mi in the southerly flow will keep conditions cooler over the east half, generally from the lower 80s inland to the mid to upper 70s right along the Lake Mi shoreline. These very warm temperatures combined with dew points rising into the 60s will yield muggy and more uncomfortable conditions today. Some lower stratocu clouds which have formed early this morning over nw and northern wi as noted on IR satellite imagery could sneak into interior west portions of Upper Mi along the WI border this morning but should lift out later this morning with diurnal mixing.

Continued mixing of the lower levels in a southerly wind flow along with higher dew points in the upper 50s to mid 60s persisting over the area will yield min temps tonight mostly in the 60s, except for the potential of lower 70s readings for west half downsloping locations along Lake Superior.

LONG TERM /SUNDAY THROUGH FRIDAY/
Issued at 502 AM EDT Sat Jul 27 2024

Ridging aloft will be kicked to Quebec by Sunday morning by a pair of troughs, one approaching the western end of the Hudson Bay and another over Missouri. The more southern trough will approach the Upper Great Lakes basin by Monday which will result in rain showers, but the trough will be weakening as it approaches, and as such the effects will not be particularly hazardous. A more zonal pattern aloft then settles in through the week, leading to periodic rain chances throughout the week, with the highest chances (up to 40%)
being with the approach of a stronger trough on Thursday. Warmer than average temperatures will prevail, but areas of high temperatures exceeding 90 are only likely (50+%) Sunday and Wednesday.

For Sunday, despite the center of the surface high pressure shifting over Appalachia, a lack of any other forcing and mild 500mb ridging will keep the UP dry. Efficient radiational cooling and downsloping southerly flow will allow for locations near Lake Superior shorelines to reach the mid 80s to near 90 degrees. CAMs do show some showers bubbling up over northeast Wisconsin and approaching the MI/WI state line, so the 12Z HRRR may need to be watched in case those showers trend further north. However, the higher chances of precipitation late Sunday will be from weak surface troughing passing through the MN Arrowhead Sunday night which the NBM paints a ~15% chance that showers may brush the western shores of Lake Superior with PoPs near 30 over Isle Royale.

Better chances of rain will arrive when the main trough aloft arrives over the UP Monday morning. With the amplitude of the trough becoming less and less as the trough approaches, showers will have less 500mb vorticity to work with for lift. Only mild 850mb warm advection is expected to provide supporting lift and the resultant surface low looks to only be around 1005mb per the GEFS. With the LREF showing mean surface CAPE of around 500 J/kg, thunderstorms are possible (~40%), but with a bunch of weak forcing and poor shear (mean 0-500mb shear below 20 kt), nothing stronger than a garden variety thunderstorm is forecast.
This will also result in less than a 10% chance of daily precip rates exceeding 1 inch, so the risk of flooding is negligible.

Beyond Monday, 500mb flow becomes more zonal, which will lead to a number of weak impulses being possible that could initiate showers, but ensemble spread is high on location and timing of such showers, though without the presence of any extremes in QPF anywhere in the ensembles, impacts from these showers should be very low. The next particularly notable trough will arrive from the west on Thursday, bringing another low pressure in the 1000-1010mb range, though ensembles do have some spread as to the latitude of this surface feature. PWATs of near 1.5 inches are approaching or exceeding the 90th percentile of the NAEFS climatology, so some heavier rain showers are within the realm of possibility but are challenging to predict at this time frame. Another impact of this approaching trough will be that on Wednesday, the warm advection from the southerly flow will lead to the warmest temperatures of the forecast, with the NBM forecasting widespread highs around 90 degrees. Ensemble spread increases drastically following the passage of this trough, but continued warmer than normal weather is expected as the calendar rolls into August.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/
Issued at 732 AM EDT Sat Jul 27 2024

VFR conditions are expected to prevail through the TAF period. There is around a 15% chance of seeing MVFR cigs developing at IWD 12-15Z. Southerly winds will gust to around 20 kt at IWD/SAW during much of the daytime hours today. Some LLWS will redevelop later tonight at IWD and CMX.

MARINE
Issued at 502 AM EDT Sat Jul 27 2024

Generally southerly winds of 20-25 kt will frequent the lake through Sunday evening before high pressure to the east weakens and the pressure gradient slackens in response. Generally weak pressure systems over the Great Lakes this upcoming week will keep wind gusts below 20 knots through the forecast period. This evening, wave heights over the east-central portions of the lake will swell to 4-6 feet before falling below 4 feet by mid-morning Sunday. Waves near the US/Canada maritime border will peak again to near 4 feet Sunday evening before sub-3 foot waves characterize the lake Monday and beyond.

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