271
FXUS63 KMQT 210925
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
525 AM EDT Tue May 21 2024

.KEY MESSAGES...

- Strong storm system will bring rain and thunderstorms late
  Tuesday into Tuesday night.

- 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 /TUESDAY THROUGH MONDAY/...
Issued at 545 PM EDT Mon May 20 2024

Persistent troughing in the vicinity of Alberta, Canada sets the
stage for a deepening surface low tracking northeast from western
Kansas to near Duluth, MN by early Wednesday morning. Warm air
advection ahead of this system may support a few strong to severe
storms late on Tuesday, especially closer to Lake MI. An impressive
pressure gradient combined with steep low-level lapse rates south of
the low pressure supports potential for a windy Wednesday. Cooler
and quieter weather is expected for Thursday into Friday with the
next weather maker moving in Friday night or Saturday.

Beginning Tue, an energetic and negatively tilted shortwave lifts
northeast across the Plains resulting in a sub-990 mb low over MN
Tuesday evening which is at or near the minimum of CFSR climatology.
This unseasonably deep low pressure is forced 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). As a result,
12hr 500mb height falls of 150-180m predicted by the models over the
Upper MS Valley/Upper Great Lakes region late Tue night/early Wed.
This impressive cyclone results in two waves of weather hazards 1)
potential for strong to severe thunderstorms along/ahead of the cold
front Tuesday evening and overnight then 2) potential for a windy
Wednesday as the system departs the area.

Regarding the thunderstorm potential, SPC continues to outlook our
entire area in a marginal/slight risk for severe thunderstorms. HREF
guidance shows a cluster of storms developing near the IA/NE state
line Tuesday afternoon that rapidly tracks northeast across the
Upper MS River Valley. HREF means indicate a narrow corridor of
MUCAPE increasing to 500-1000 J/kg immediately ahead of the cold
front with bulk shear increasing to near 50 knots. In general, I`d
like to see more CAPE to better balance the strong environmental
shear, especially considering the unfavorable diurnal timing around
or after sunset. Additionally, E-SE flow ahead of the front is not
favorable for severe storms due to the stabilizing maritime
influence from the still cold Great Lakes implying exclusively
elevated storms. Despite all the above, there`s a chance for a few
strong to potentially severe thunderstorms that may pose a threat
for large hail or damaging winds especially closer to Lake MI where
instability is greater. Moderate to locally heavy rainfall is also
possible across Upper MI during the period of strong isentropic
ascent/strong 850mb theta-e advection to the e of the low track Tue
night. Precipitable water also increases to around 1.5 inches or
near the climatological daily max for the day at ~200pct of normal.
The biggest factor working against heavy rainfall is the rapid
system motion with ensemble means suggesting only localized 24-hour
amounts greater than 1 inch. Shower coverage diminishes
significantly for Wed as we get into the dry slot and q-vector
divergence/subsidence behind the system as it lifts north. Some
areas may see dry weather for much of or all of the day, especially
e half.

The biggest impacts from this system may end up being strong winds.
ECMWF EFI/Shift of Tails continues to increase into Wed with Upper
MI mostly around 0.9 now, suggesting an unseasonably strong wind
event based on last 20 years of model climate. The EPS probabilities
essentially shows a 80-100% chance that the entire fcst area will
see wind gusts exceeding 40 mph Wed afternoon into Wed evening with
a 10-30 pct chance of western U.P. higher terrain areas seeing high
wind warning criteria gusts of 50 mph or greater. While winds for
this fcst were increased from what NBM indicates, they are not as
high as the median from the EPS (40-55 mph) and will likely need to
be bumped up more. There is still time to reevaluate for higher
winds in later fcsts as wind direction will be important for
identifying the windiest spots, but at this time the strongest winds
are expected across the far western UP. Continued to mention the
potential for higher wind gusts on Wed in the HWO product.

As the low lifts across northern Ontario toward James Bay on Thu,
isolated showers will remain possible under lingering cool, cyclonic
flow. Thu should be the coolest day of the week with highs in the
lower 50s to the lower 60s F warmest s central. Another shortwave
trough lifting through the Plains into central Canada late week will
send a sfc low pressure trough/frontal boundary into Upper Mi late
Friday/Fri night which could linger over the area into Sunday
bringing yet more opportunities for showers.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 142 AM EDT Tue May 21 2024

KIWD will stay IFR overnight into Tue morning. KIWD will see MVFR
conditions return by Tue afternoon and LLWS sets in by the end of
the period. KCMX will stay mostly MVFR before improving to VFR by
late Tue morning and when rain arrives conditions will lower to
IFR with LLWS by Tue evening. KSAW will lower to IFR overnight
in fog/stratus before improving to MVFR late Tue morning and
then VFR in the afternoon. Rain returns to SAW Tue evening
along with LLWS as MVFR conditions settle back in.

&&

.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 from 5 PM EDT /4 PM CDT/ this afternoon to 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...EK
AVIATION...07
MARINE...GS