263
FXUS63 KMQT 211920
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
320 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 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...
  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...GS
AVIATION...Jablonski
MARINE...GS