711
FXUS63 KMQT 151845
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
245 PM EDT Thu Aug 15 2024

.KEY MESSAGES...

- Low pressure system to bring soaking rain (up to 1-1.5
  inches), gusty winds (up to 35 mph) and some thunder (30-40%
  chance) to the UP by Friday morning.

- Scattered to numerous showers and some thunderstorms with
  locally heavy downpours through Friday, then showers
  diminishing on Saturday.

- Mostly dry weather and seasonable temperatures Sun through
  midweek.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 243 PM EDT Thu Aug 15 2024

GOES-16 Visible Imagery shows a swirling low pressure in northwest
MN with a band of precipitation extending southeast to IN. KMQT
radar reflectivity shows this band of precipitation over the state
line from Menominee to Iron Mountain and extending northwest to
Ontonagon at 17Z. No thunder so far with this first band of rain
showers, which is not entirely surprising given the meager
instability (12Z HREF mean SBCAPE of <100 J/kg). Models continue to
trend slower and slower with the progression of showers as well as
the arrival of better instability, with HREF mean MUCAPE not
exceeding 500 J/kg until at least 10Z Friday now. Despite poor
instability and shear, abundant PWATs of 1.5-1.75 inches will allow
for some heavier showers, with mean accumulated precip totals of 0.5
inches to 1 inch by Friday morning, and the 75th percentile of the
HREF suggests some areas could reasonably realize up to 1.5 inches
of precip. No flooding is expected given the antecedent dry
conditions (much of the western UP at 50-90% of normal precip the
past 30 days). Efficient mixing even overnight will allow for some
gusty conditions, with KSAW already gusting to 27 mph and the HREF
mean gusts suggesting 20-50% chances of 35 mph gusts within the
initial band of showers. Once the first band is through, a dry slot
is expected to reduce the coverage of showers from definite to more
scattered/isolated early Friday morning.

&&

.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 /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 124 PM EDT Thu Aug 15 2024

As a low pressure system continues to push east, flight categories
will begin to deteriorate with IWD already at MVFR. All 3 sites will
reach IFR as heavier rainfall moves in at IWD this afternoon and
CMX/SAW tonight, with around 25% chances of occasional LIFR at all
sites overnight. TSRA will not be present at the initial onset of
showers but will be more likely overnight, up to 33% at SAW/CMX and
up to 45% at IWD. Gusty SE and S winds will gradually become W and
NW by tomorrow as the front passes over the UP.

&&

.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...
SHORT TERM...GS
LONG TERM...Rolfson
AVIATION...GS
MARINE...Rolfson