001
FXUS63 KMQT 211802
AFDMQT

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

.KEY MESSAGES...

- Strong storm system will bring rain and thunderstorms late
  today into tonight.

- Windy conditions develop behind the system on Wednesday.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 523 AM EDT Tue May 21 2024

Some patchy fog has developed this morning under weak high
pressure. Expect this fog to lift out later this morning.

Latest water vapor imagery has shortwave energy rounding the base of
the western mid-level trough through the Southern Rockies early this
morning. This shortwave trough will take on an increasingly negative
tilt as it lifts nne through the Plains and Upper MS Valley today
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 deepening
low is reflected nicely by models with healthy 12hr 500mb height
falls of 150-170m fcst ahead of system over the Arrowhead of MN and
northern Ontario by Wed morning. At the sfc, models indicate the
associated low pres will deepen to around 983 mb near the
MN/Canadian international border by 12Z Wed as the center tracks
just west of Lake Superior later tonight.

What weather/impacts can we expect from this impressive, and perhaps
historically deep, late spring storm system? Increasing isentropic
ascent, 850 mb theta-e advection and q-vector convergence on the
leading edge of the system will bring scattered showers and a few
thunderstorms into west half portions of the U.P. this afternoon.
Precipitable water increasing to around 1.5 inches or near the
climatological daily max for the day at ~200pct of normal along with
destabilization and steepening lapse rates out ahead of the
approaching frontal boundary and at the nose of an impressive 60-70
kt LLJ will set the stage for widespread showers and thunderstorms
by evening as the best mid-upper level forcing coincide with the
arrival of the frontal boundary. Elevated CAPE values around 1000
j/kg as depicted by the fcst models may even result in a few strong
to marginally severe storms given the very strong shear profile
noted on fcst soundings. However that said, believe SPC`s slight
risk of SVR issued for the southern UP may be a bit overdone
considering best forcing along the front won`t occur until after
sunset, so expect convection to be most likely elevated as depicted
by model soundings and the main threat would be marginally SVR hail.
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.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 611 AM EDT Tue May 21 2024

The long term forecast will mainly be driven by a pair of
troughs, one over the MN/Canada line by Wednesday morning and
another over the Seattle vicinity that will arrive in the Upper
Great Lakes by Friday and Saturday. Despite the surface low
associated with the first trough already being northwest of Lake
Superior by Wednesday morning, the impacts from this low will
be the main storyline of the long term forecast as strong winds
are expected.

By Wednesday morning, the surface low pressure will be over
northwestern Ontario at around 984 mb per the 00Z GEFS mean. CAMs
show a dry slot over the UP for much of the day Thursday with the
HREF hourly probabilities of measurable precipitation never
exceeding 40 percent and mainly being under 20 percent. While rain
will not be a major factor in the weather forecast, the winds will
take center stage. With 850mb winds on the south side of the low
pressure up to 50 kt, improving mixing throughout the day will bring
some of those winds to the surface, with the 12Z Euro ensembles
suggesting 30-50% chances of afternoon wind gusts in excess of 50
mph over the west half and near-certainty that gusts will exceed 35
mph across the UP. This is highly unusual for this time of year,
shown by EFI values for wind gusts in excess of 0.8 and NAEFS SLP
north of Lake Superior being below the last 20 years of climatology
for this time of year. Headlines will need serious consideration
following Tuesday`s active weather.

Thursday, as the low pressure lifts towards James Bay, the pressure
gradient relaxes and winds return to more normal speeds. A slight
(15-20%) chance of precipitation lingers, with daytime heating and a
weak remnant boundary supporting some isolated light showers, but
otherwise a seasonal day is in store with NBM highs around the mid
60s across the central UP, with mid 50s to the north and low 70s to
the south. While the weather is mostly benign over the Upper Great
Lakes Thursday, troughing moving over the Rockies will support lee
cyclogenesis, with the 00Z GEFS showing a mid-990s low over the
KS/CO border by 00Z Friday. This low will eject northeast Friday and
Saturday, but there`s considerable spread in the track and timing of
the storm. If the forcing lines up well with peak heating Friday,
some thunderstorms are possible, with a few GEFS members showing 500-
1000 J/kg of SBCAPE, but uncertainty is high. Ensembles go to chaos
beyond Saturday, but the pattern going into next week looks to favor
ridging over the west and troughing over the east, which should help
keep temperatures closer to normal relative to the warmer conditions
of recent, along with continued periodic rainmakers.

&&

.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...
None.
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...Voss
LONG TERM...GS
AVIATION...Jablonski
MARINE...GS