773
FXUS63 KMQT 151105
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
705 AM EDT Thu Aug 15 2024

.KEY MESSAGES...

- Wet period with showers and some thunderstorms developing today,
  continuing through Friday, then diminishing on Saturday. Locally
  heavy downpours possible, especially on Friday.

- Mostly dry weather and seasonable temperatures Sunday through
  midweek.

&&

.UPDATE...
Issued at 652 AM EDT Thu Aug 15 2024

Some areas of fog, dense in some spots, have developed over
Upper Michigan early this morning. Expect these areas to
dissipate in the next few hours after the sun rises. Otherwise,
I`ve continued to slow the progression of the rainfall into the
area today following CAMs guidance; this has delayed rainfall by
an extra hour or two in comparison to the forecast earlier this
morning.

&&

.SHORT TERM /THROUGH THIS AFTERNOON/...
Issued at 220 AM EDT Thu Aug 15 2024

A deepening low pressure over the Northern Plains approaches Upper
Michigan today and will bring rainfall over at least the western
half before sunset this evening. Cloud cover increases from the west
today as the low approaches. This cloud cover will help to keep
temperatures cooler over the U.P. today, mainly over the west and
central, in addition to the rainfall; expect highs to range from the
upper 60s in the interior west to around 80 in the east. With strong
equivalent temperature advection moving in this afternoon and PWATs
around 1.75 inches, we could see some heavy rain showers from time
to time late this morning over the far west through this afternoon
across the western half. These showers will be a much welcome
reprieve from the abnormally dry conditions that we`ve been seeing
over much of the western half of the U.P. the past couple of weeks;
we can only hope that this will improve soil moisture conditions
across the area. Some limited CAPE in the atmosphere could produce a
few rumbles of thunder near the state border late this afternoon.

&&

.LONG TERM /TONIGHT THROUGH WEDNESDAY/...
Issued at 427 AM EDT Thu Aug 15 2024

Medium range model guidance remains in good agreement on the large
scale flow evolution over N America thru the weekend. Mid-level
troffing currently over the Northern Plains will progress over the
Great Lakes region Fri/Sat with a weakly closed off mid-level low
forming. Upstream, modest mid-level ridging into s central Canada
Fri/Sat will build northward with positive mid-level height
anomalies maximizing to the w and nw of Hudson Bay by Tue. This will
force the trof moving thru the Great Lakes region to gain amplitude
over eastern N America early next week. Model disagreement continues
on the amplitude/structure of the mid-level ridging thru central N
America early next week, including depiction of shortwaves rounding
the ridge or cutting thru it. Nonetheless, some degree of nw flow
will be favored into the Upper Lakes during the first half of the
week before flow deamplifies. For Upper MI, this evolution of the
flow with troffing developing thru the Great Lakes region will bring
shra to the area tonight thru Sat. While all day rain is not
expected, potential is high for all of Upper MI to see at least
light to mdt rainfall thru Sat. Ensemble guidance suggests the
probability of at least 1 inch of rainfall by Sat evening ranges
from around 10 pct over the eastern fcst area to around 60 pct from
Ironwood to Menominee. The rainfall is needed as the pattern has
turned drier over the last few weeks. A large portion of Upper MI is
currently running at 25-70pct of normal rainfall over the last 30
days with the largest area under 50pct over the w half of Upper MI.
After a return to dry weather on Sun, the uncertainty on progression
of shortwaves around the upstream ridge or even thru it leads to low
confidence on the pcpn fcst. Summertime NW flow can be notorious for
producing diurnal convection with even the subtlest of shortwaves,
so would not be surprised to see a few isold shra/tsra Mon/Tue.
There have been some indications for a more defined wave arriving in
the midweek period. So, pcpn chc should increase at that time.

Beginning tonight, mid-level trof reaching the Upper Mississippi
Valley tonight will only drift e on Fri. Models show a mid-level low
closing off within the trof, and the center of that low will be over
northern WI/western Upper MI Fri aftn. A good surge of isentropic
ascent and 850-700mb moisture transport ahead of the trof will
support a band of shra moving e and ne across Upper MI today into
this evening. After the initial band of shra, shra coverage will be
on the sct/nmrs side tonight. There is some instability for non sfc-
based parcels, so isold to sct tsra are expected. With precipitable
water around 150pct of normal and warm cloud depths near 12kft
tonight to increase warm rain processes, expect some locally hvy
downpours as well.

Mid-level low moving over the area on Fri will provide a good setup
for diurnally aided shra/tsra. Typically, a mid-level low overhead
or in the vcnty during summer raises concern for strong and svr
storm potential. Fortunately, in this case, there is not much of a
mid-level cold pool associated with the low to steepen lapse
rates/lower wetbulb zero heights/further increase instability, and
deep layer shear is very weak, aob 15kt, even less than 10kt over
much of the area, to prevent organized storms. So, svr storms not
expected. Building instability will lead to expansion of slow moving
shra/tsra in the aftn, and would expect greatest coverage probably
across central Upper MI due to weak lake breeze convergence. Sfc low
pres center may also end up in central Upper MI during the aftn,
aiding low-level convergence. Torrential downpours will be possible
with the slow moving convection, particularly if there are more
breaks in the cloud cover to further increase instability.

Sct/nmrs shra and some tsra will continue into Fri night, though
coverage and rainfall intensity will diminish with waning
instability. Trof/mid-level low will shift e of the area during Sat,
but will remain close enough to keep shra in the fcst, especially e.
Over the w, there may be a diurnal component to the shra, resulting
in some expansion of shra during the aftn, but overall, coverage
will be less than over the e and also less than what occurs on Fri.

Dry weather is expected on Sun as trof continues shifting farther to
the e of the area. As previously mentioned, Upper MI will remain
under some degree of NW flow early next week, and during summertime,
NW flow can result in isold convection with even the subtlest of
shortwaves. Probability is less than 20 pct for Mon/Tue. Last day or
so of medium range model runs have suggested that a more defined
wave may arrive in the midweek period, Wed-Thu, offering a better
potential of something more than isold shra/tsra.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 705 AM EDT Thu Aug 15 2024

Cigs and vis will deteriorate to MVFR/IFR across the TAF sites today
as a low pressure moves in from the Northern Plains. Rain showers,
sometimes heavy, look to continue through the rest of the forecast
period. While the chance is too low to include in the TAF, some
rumbles of thunder could be heard from time to time across the TAF
sites too late this afternoon through tonight. While winds at 2 kft
look to be gusting up to 30 to 40 knots, with mixing continuing
through the overnight hours, LLWS is not expected at this time; the
best chance of it occurring looks to be at KIWD, but it would only
last an hour or so, would be fairly marginal, and only have like a
30 percent chance of occurrence (if that). Breezy south to southeast
winds look to continue through the period. Heading into the
overnight period, we could see LIFR cigs drop over the TAF sites,
particularly KSAW as rain shower coverage lightens up and upslope
flow brings very low cigs to the TAF sites (KIWD has the lowest
chance of this happening given the southeasterly wind).

&&

.MARINE...
Issued at 427 AM EDT Thu Aug 15 2024

Low pres developing eastward, reaching northern WI/Upper MI on Fri,
will bring increasing easterly winds to Lake Superior. Expect winds
under 20kt to start the day today, then gusts up to around 25kt this
aftn/evening across the w half of Lake Superior and tonight into
early Fri morning across the e half of the lake. On Fri, pres
gradient will overall relax, so winds will settle back a bit across
the lake, especially across eastern Lake Superior. As the low
departs to the e, expect northerly winds gusting to about 20kt Fri
night/Sat, but these winds will diminish across the western lake
during Sat. High pres ridge will build se and s into the Upper Great
Lakes on Sun/Mon. Winds will be mostly under 20kt during this time
with the stronger winds on Sun across eastern Lake Superior ahead of
the approaching ridge.

&&

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

&&

$$

UPDATE...TAP
SHORT TERM...TAP
LONG TERM...Rolfson
AVIATION...TAP
MARINE...Rolfson