689
FXUS63 KMQT 211951
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
351 PM EDT Tue May 21 2024

.KEY MESSAGES...

- First round of showers and thunderstorms are already ongoing
  and will lift north over the west half of the UP potentially
  bringing small hail and stronger winds.

- Second round of thunderstorms moves from west to east this
  evening into tonight, bringing mainly some heavy rain showers
  and winds; there is a slight chance for all severe weather
  threats along this line.

- Windy conditions develop behind the system on Wednesday.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 319 PM EDT Tue May 21 2024

This afternoon brings the first wave of showers and possibly some
isolated thunderstorms to the UP with a second and more noteworthy
round expected this evening into tonight. A slight risk from the SPC
over the south-central seems to fit well for the going forecast with
potential for all severe weather threats accompanying the second
line of thunderstorms.

Latest WV imagery and RAP analysis has the first shortwave currently
over MN and northwest WI which is lighting up the radar data over
the western UP. This will continue to expand north and east over the
west half of the UP as the shortwave and mid level warm front lift
north, aided by isentropic ascent. With current RAP analysis showing
SBCAPE around 100 j/kg with mid level lapse rates below 6.5 C/km and
the 5/21 12z HREF mean SBCAPE holding around this through the
afternoon as the boundary lifts north, some isolated thunderstorms
are possible to bring some small hail. Severe weather is expected to
hold off until the second round.

The more vigorous shortwave is currently situated over east NE with
the associated sfc low over west IA. This shortwave trough will take
on an increasingly negative tilt as it lifts north-northeast through
the Upper MS Valley this evening and then across northern MN and
into Ontario later tonight. The associated sfc low will rapidly
deepen today aided by strong upper divergence from a coupled upper
jet structure (right entrance region of a 130 kt jet max over
northern Ontario and the left exit region of a 110 kt jet max over
the Southern/Central Plains). The most recent HREF mean has the low
deepening to around 984 mb near the MN/Canadian international border
by 12Z Wed as the center tracks just west of Lake Superior later
tonight. With this more northwest sfc low trend in the last 24 hours
has increased the severe weather potential. CAMs are in fairly good
agreement on this second round developing a QLCS over the western UP
this evening, quickly tracking east into tonight. With bulk shear
around 45-50kt, MUCAPE up to 1000 j/kg, and mid level lapse rates
around 7C/km, some severe thunderstorms are possible. With strong
LLJ around 60-70 kts, this should contain the severe weather threats
to winds. That said, mid level lapse rates could support hail up to
1 inch and low LCLs around 1kft with some low level helicity does
leave a low chance (~2% chance along WI state line) for a quick
tornado spin up. The limiting factor for severe weather is the
timing of the storms, as we progress into the evening and lose
daytime heating, storms are expected to weaken. Another factor in
limiting severe potential for the eastern UP is onshore flow off
Lake Michigan.

Most locations will see between half an inch to inch of rainfall
from this system with the highest amounts west. Models show shower
coverage diminishing significantly late tonight as we get into the
dry slot behind the front along with q-vector divergence/subsidence
as the system continues to lift north. Lows tonight will stay mild
in the 50s, cooler in the west.

&&

.LONG TERM /WEDNESDAY THROUGH TUESDAY/...
Issued at 350 PM EDT Tue May 21 2024

The extended forecast is dominated by two low pressure troughs
during the period, one west of Thunder Bay and north of the
Arrowhead of Minnesota by Wednesday morning, and another over the
Seattle area Wednesday morning making its way across the Upper
Midwest Friday afternoon through Friday night. While the first low
will be northwest of Lake Superior Wednesday, strong winds from
behind the first low will be the main story for the long term,
creating windy conditions across the area  (especially the west
where a High Wind Watch and Wind Advisories were issued).

As an impressively deep 986mb low continues lifting northwest from
just west of Thunder Bay towards James Bay Wednesday, a strong low-
level jet from between 5 and 30 thousand ft will move across Upper
Michigan, allowing for windy conditions to develop over the U.P.,
particularly the west where the strongest winds of the LLJ reside.
While 70 mph is the highest possibility (<10% chance), winds up to
50 to 60 mph are expected across the western U.P. along the
lakeshores and the Keweenaw. This will cause unsecured items to the
lofted away and for driving to become hazardous in spots. Therefore,
a High Wind Watch and a Wind Advisory have been issued for Ontonagon
and Gogebic counties Wednesday. While I`m confident that Ontonagon
will see wind gusts up to 45 mph, I`m only around 60% confident that
Ironwood and northward will see gusts up to that high. Likewise, I`m
only 70% confident that northern Houghton and Keweenaw counties will
see wind gusts up to around 60 mph Wednesday too. The main concern
is how well we will mix the higher winds down to the surface across
the area Wednesday; with low-level stratus looking to hang on until
the afternoon hours, we have have a more difficult time doing so.
Nevertheless, the friction that the land will cause along the
lakeshores (as well as the increase in elevation) will help to
increase the wind speeds too. As we move into Wednesday night, we
will lose the diurnal mixing from the sunlight, and thus more stable
air in the boundary layer will prevent the higher winds aloft from
mixing down. While we will likely (80+% chance) see breezy
conditions continue over the Keweenaw Wednesday night, expect
calmer winds across the rest of the U.P. by midnight.

Besides the winds Wednesday, there is a 20% chance that we will see
some light rain showers moving through the central and east
Wednesday before more light rainfall from moisture that`s wrapped
around the low returns and brings isolated showers and thunderstorms
back over the western half by the afternoon hours. As the low
continues to pull away Wednesday night, some of the CAMs hint at a
line of light rain showers moving over the western half late as a
secondary shortwave rotates around the low over us. However, the
shower activity is expected to die out Thursday morning as weak
ridging begins to build back into the area. With warm air advection
returning to the area Thursday, expect highs in the 60s near the
Great Lakes to the 70s in the interior areas. With mostly sunny
skies being over the area Thursday, we could see some strong
dewpoint depressions develop, possibly getting min RHs down into the
low 30 percents in the interior west. There is a chance that some
convection will pop up during the afternoon hours Thursday as a
localized sfc low moves through the U.P.. Whether this occurs or not
will depend if this sfc low materializes, and if so where it tracks.
As it stands right now, I think the NBM`s 30% handles these rain
chances fairly well.

Our second big low of the period looks to impact us around the
Friday through Friday night time period. While we are looking to be
initially dry Friday, especially over the east where min RHs could
dip into the lower 30 percents, rain showers and thunderstorms from
the approaching low look to move into the west by no later than the
afternoon. I lowered thunderstorm chances from likely to chance
wording over the interior west late Friday afternoon into early
Friday evening as guidance could change rapidly over the next 3 days
in regards to the track of the low (and thus thunder chances). That
being said, should everything come into place and the low track just
right, we could see some severe weather return to the western half
as MUCAPEs could get over 1000 J/kg and 0-6 km bulk shear be around
40 to 50 knots. As the cold front of the low moves over the U.P.
Friday night and Saturday morning, expect the rain chances to cease
behind it.

Dry weather returns for this weekend as ridging builds across our
area. For the beginning of next week, model guidance begins to
significantly diverge; we could see dry conditions and ridging
continue, or we could see a Colorado low lift through the Plains
into the Great Lakes and bring more rainfall to the area.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 202 PM EDT Tue May 21 2024

As a strengthening low pressure system currently over western IA
lifts north into northern MN tonight, two rounds of showers and
thunderstorms will move though the area with it. The first one is
already progressing its way over the west third of the UP bringing
SHRA to IWD. This will continue expanding north and west over the
west half of the UP into this evening bringing SHRA to CMX and some
VCSH to SAW. While this first round may bring some thunder,
confidence remained low so VCTS and TSRA were left out for the first
round of precip.

A line of stronger thunderstorms is expected to accompany a front
lifting through the area this evening into tonight. TSRA arrives
first at IWD around 0Z Wednesday, then the line progresses to CMX
and SAW around 2-3Z Wednesday. MVFR conditions are expected to
accompany these showers and storms tonight with IWD and SAW lowering
to IFR late. Gusts to 25-30 kts are expected at all sites tonight
into Wednesday. LLWS is also expected this evening and tonight as a
strong low level jet moves across the area.

While windy conditions continue on Wednesday behind the low pressure
system, expect vis and cigs to return to VFR through the day.

&&

.MARINE...
Issued at 353 AM EDT Tue May 21 2024

Early this morning, wind gusts across the lake are light. However,
an unseasonably strong low pressure system approaches Lake Superior
this afternoon and evening resulting in rapidly increasing northeast
winds across western portions of the lake. Northeast winds increase
above 20 knots midday Tuesday over the far western lake then
increase to 35-40 knot gales by Tue evening (60-90% chance). While
the likelihood of gales is high, there is uncertainty in whether
high-end gales can be achieved. Late May gales are rare due to the
stable air immediately above the lake, but the strength of the
surface pressure gradient close by the low pressure should support
at least gales to 40 knots today. While a brief respite from gales
is expected overnight in the west, the environment behind the low
will make it much easier to tap into the low-level jet Wednesday,
leading to high end gales expected (50+%) over the west half of Lake
Superior with a few storm-force gusts possible (~20%), but current
model guidance has storm force gusts isolated and brief, so most
marine products will reflect gales to 45 knots. As the low continues
to quickly depart, winds fall below gales late Wednesday and below
20 knots Thursday afternoon. Another low pressure will pass through
the Upper Great Lakes region Friday, but is likely (75+%) to be much
weaker and uncertainty exists regarding the track of the low, so the
current forecast reflects 20-25 knot gusts Friday afternoon with
gusts near 20 knots through Saturday.

Other marine hazards include chances (30-50%) of thunderstorms over
the lake this afternoon in the west and across all of Lake Superior
tonight, with a 5-15% chance of severe winds associated with the
thunderstorms along with small hail. With the first round of gales
this evening, significant wave heights will be highest in the far
west at near 12 ft, with the Wednesday gales forcing waves as high
as 15 feet in the north-central portions of the lake.

&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
  High Wind Watch from Wednesday morning through Wednesday
     evening for MIZ001-003.

  Wind Advisory from 9 AM EDT /8 AM CDT/ to 10 PM EDT /9 PM CDT/
     Wednesday for MIZ002-009.

Lake Superior...
  Gale Warning until 8 PM EDT /7 PM CDT/ Wednesday for LSZ162.

  Gale Warning from 7 AM EDT /6 AM CDT/ to 8 PM EDT /7 PM CDT/
     Wednesday for LSZ240-241.

  Gale Warning from 10 PM this evening to 11 PM EDT Wednesday
     for LSZ242>244-263-264.

  Gale Warning from 2 AM to 11 AM EDT Wednesday for LSZ245>251-
     265>267.

Lake Michigan...
None.

&&

$$

SHORT TERM...Jablonski
LONG TERM...TAP
AVIATION...Jablonski
MARINE...GS