067
FXUS63 KMQT 192049
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
449 PM EDT Thu Sep 19 2024

.KEY MESSAGES...

- Extended stretch of well above normal temperatures continues through
  at least Saturday with highs mainly in the upper 70s and 80s.

- A cold front brings scattered showers and a few thunderstorms
  tonight into Friday. Some stronger storms are possible in
  western Upper Michigan this evening.

- Multiple rounds of precipitation possible through the first
  half of next week. Thunderstorms that do form are not
  expected to be severe.

- Temperatures around normal are expected to return for next week.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 446 PM EDT Thu Sep 19 2024

Water vapor imagery and RAP analysis show troffing over the western
U.S. with one well-defined mid-level low over southern Manitoba and
a second just off of the central CA coast. Some elevated showers and
a few storms have advanced into north central WI and western Upper
Mi this afternoon ahead of the low over Manitoba. The mid-level
ridge axis which was stationed over Upper Mi most of the week is
getting pushed slightly se this afternoon although the attendant
very dry air mass is still keeping the central and eastern U.P
shower-free this afternoon. Afternoon temps thus far have reached
into the mid 70s and lower 80s, coolest along the Lake Mi shore.
Higher dew points in the 60s have resulted in a more unstable
airmass south into north central WI where a cluster of elevated
storms have formed at the nose of a theta-e ridge.

Plenty of uncertainty regarding U.P weather for late this afternoon
into tonight as the mid-level low over Manitoba lifts north and
sends a mid-level trough and associated sfc cold front toward Upper
Mi later tonight. High resolution models suggesting stronger
convection and developing supercells now forming over southern MN in
environment of SBCAPE values 1500-2000 j/kg will pose a severe risk
into southern MN and western WI into early evening. These storms
over southern MN and western WI are then expected to transition to a
linear MCS tonight which moves east through WI. High effective shear
of 40 kts or higher and CAPE values of 1000-2000 j/kg will favor the
eastward movement of the MCS through WI tonight. Big question will
be how this MCS affects moisture for supporting convection over the
U.P tonight. A few of the CAMs suggest isolated to scattered showers
and t-storms this evening over the west could potentially become
more numerous in coverage into south central Upper Mi overnight if
northern extent of MCS moving through WI clips this portion of the
cwa. This will be something for the evening shift to keep an eye on.
Increasing effective shear to 30 kts could support some stronger
storms into the western U.P. this evening but severe threat will
likely be limited by mostly elevated CAPE of 500-1000 j/kg. Look for
min temps to stay around 60F tonight with increasing clouds and
chance for showers. Patchy fog will be possible again tonight,
especially central and east under moist southerly upslope flow.

&&

.LONG TERM /FRIDAY THROUGH THURSDAY/...
Issued at 400 PM EDT Thu Sep 19 2024

Active weather pattern to begin the long term forecast as a trio of
shortwaves will pass in the vicinity of the Upper Great Lakes Friday
through Tuesday. This will help keep more seasonal temperatures
(highs in the 60s) and periodic precipitation chances in the
forecast into the middle of next week. While there is enough spread
in the ensembles in the back half of the week to keep slight-chance
(~20%) PoPs in the forecast, ensembles have trended more dry over
the past 24 hours of runs and anomalous ridging looks to reside over
Manitoba and northern Ontario by the end of next week. Should this
ridging persist, the pattern could dry out to end September, which
aligns with the CPC 8-14 day and 3-4 week precip outlooks which put
the UP in a below normal precip pattern.

By Friday morning, the cold front associated with a 992mb occluded
low over Manitoba will be draped over central-to-eastern Upper
Michigan. This front will continue to drift eastward, though with
the parent low only slowly drifting northeast itself, the front will
take its time to push through. SBCAPE looks to be around 500-1000
J/kg along the front, so some thunderstorms are expected, but HREF
reflectivity paintball plots show that the coverage of thunderstorms
capable of 40+ dBZ will be sparse through the day Friday, so no
severe weather is expected. While not everyone will be getting a
soaking, the eastern 2/3 of the UP looks at least 50% likely to get
a tenth of an inch or more of precipitation tomorrow with the 75th
percentile of the HREF showing pockets of a half inch or more of
rain. CAMs show some afternoon redevelopment of showers in the
central UP, but otherwise, clearing skies behind the front should
allow for highs to climb up to the mid-upper 70s in the west half,
though highs will be closer to 70 in the east. Showers should exit
the UP by midnight, and overnight lows fall to the upper 40s to
upper 50s as a result.

Localized ridging will allow for a mainly dry day on Saturday with
highs back to around 80. While this is unseasonably warm, the NBM
gives less than a 5% chance of breaking a record high at the MQT WFO
that day. The break in the clouds and precip will be brief however
as a ~1005mb low will quickly jet along the Manitoba/Ontario line,
bringing showers and non-severe thunderstorms to the UP for
overnight Saturday through Sunday along the low`s cold front. Mean
daily precip rates with this system range between 0.2 to 0.4 inches,
so nothing extreme, but welcome rainfall to some areas that are
running well below normal over the past week and month.

Beyond the weekend is a surprising amount of ensemble spread. 500mb
height plots show a trough tracking from Colorado on Sunday towards
the southern Great Lakes basin by late Tuesday but weakening as it
approaches. Chances that precip arrives from this system are highest
in the east (40-50%) than in the west (20-30%), but spread in
solutions keeps the details rather muddy, though it is important to
note that these PoPs are slightly lower than the previous NBM runs.
Moving into the second half of the work week, the 12Z GEFS shows
anomalous ridging building over the Northern Plains and Canadian
Prairie with 500mb height anomaly reaching +20 dam over Manitoba by
Thursday morning. Downstream high pressure of that ridge would tend
to have the UP rather dry moving into late September, and with the
potential for higher heights to persist (LREF chance of 500mb
heights 580+ dam around 40%), the CPC outlooks of less than normal
precipitation will be on-track.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 133 PM EDT Thu Sep 19 2024

VFR conditions should prevail thru the evening hours at
IWD/CMX/SAW. Southerly winds will prevail, gusting to around
20kt at IWD. Shra and some tsra will spread w to e across Upper
MI tonight. Models indicate the coverage of showers is still
uncertain, but expect isolated to scattered showers and t-storms
reaching IWD late evening and then CMX closer to 04-05Z and SAW
around 07Z with showers lasting approximately 2-3 hours before
moving east. There is a potential for MVFR during any periods of
heavier rainfall that may occur. At SAW, upslope southerly
winds and shra should result in MVFR developing overnight and
MVFR conditions could linger a few hours after sunrise before
dissipating. Other than the possible lingering MVFR stratus/fog
at SAW Fri morning, expect VFR conditions to prevail at the
terminals on Friday. &&

.MARINE...
Issued at 315 AM EDT Thu Sep 19 2024

Light winds of 20 knots or less today give way to southeast to
southerly winds of 20 to 25 knots over the central lake this evening
as a cool front slowly trudges from west to east today through
Friday. This front will bring some showers and storms across the
lake, with some storms possibly being strong to severe over the west
(the main threats being strong, erratic winds and large, damaging
hail). As the front moves eastwards with time, expect the severe
threat to diminish, particularly by around midnight tonight EDT.
While winds generally decrease to 20 knots or less across Lake
Superior by Friday morning, the showers and storms continue along
the front through Friday night. Weak ridging over the area keeps
winds light Saturday before a cold front passes through Saturday
night and Sunday from the west. Currently, we could see
northwesterly winds gust to 20 to 25 knots behind the cold front
over the western half of the lake late Saturday night and Sunday. In
addition, expect to see some showers and a few thunderstorms with
the front`s passage Saturday night through Sunday, although the
convection isn`t a guarantee as some models do keep the lake rain-
free (50 to 60% chance of occurrence). Another low lifting from the
Southern Rockies may lift into the Upper Great Lakes around the
Tuesday time period too and bring stronger winds and thunderstorms
back across the lake, but uncertainty on this occurring is higher
(around 30 to 40% chance of occurrence).

&&

.MARINE...
Issued at 400 PM EDT Thu Sep 19 2024

Ahead of a cold front passing over Lake Superior tonight, wind gusts
out of the south and southeast are gusting to 25 knots over central
Lake Superior and will persist through the evening. Wind gusts relax
behind the front and will be near or below 20 kt for the remainder
of the forecast, with the highest chances of exceeding 20 kt coming
near a weaker cold front Saturday night and then depending on the
track of a shortwave trough, another front on Monday. Waves will be
4-6 ft over central Lake Superior tonight, subsiding below 4 ft by
Friday afternoon.

&&

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

&&

$$

SHORT TERM...Voss
LONG TERM...GS
AVIATION...Voss
MARINE...GS