312
FXUS63 KMQT 202231
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
631 PM EDT Mon May 20 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 148 PM EDT Mon May 20 2024

Light rain showers continue across the U.P. and end from west to
east this afternoon as a low pressure continues to travel from
southern Wisconsin/northern Illinois to northern Lower Michigan the
rest of today through tonight. With the SPC mesoanalysis page
showing pathetically low lapse rates and CAM soundings showing
almost no CAPE to work with, expect at most just a rumble or two of
thunder across the central and east the rest of today. With the
light rain taking its time to get into the far eastern U.P. this
afternoon, min RHs have dropped down to as low as 40% at the
Newberry airport. However, now that RHs are beginning to recover,
expect the upward trend to continue. Likewise, with light southerly
winds dominating today, the warmest temperatures thus far have been
near the Lake Superior shoreline, with some spots even getting into
the upper 60s. Expect the U.P. to be pretty rain-free by tonight as
very localized and weak sfc high pressure moves across the area. As
this occurs, we could see some areas of fog and low-level cloud
cover develop across the entire Upper Peninsula; with visibilities
dropping down to 1/4 mile or less in some areas, be careful if
driving late tonight and early Tuesday morning. With the areas of
cloud cover across the area tonight, low temperatures are expected
to get to around 50.

While there was a low chance earlier of a dying MCS moving into far
western Upper Michigan late tonight, the CAMs have trended away from
this solution entirely now; at most, we will see some light rain
showers move into the far west near Ironwood late tonight/early
Tuesday morning now (and even that is a fairly low (10%) chance).

&&

.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 /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 630 PM EDT Mon May 20 2024

KIWD will stay MVFR until early morning when IFR conditions set in
through Tue morning. KIWD will see MVFR conditions return by Tue
afternoon. KCMX will go from VFR to IFR later tonight and then back
to MVFR by Tue morning and then VFR for most of Tue afternoon. KSAW
will be MVFR this evening and then briefly VFR before fog and IFR
conditions move into the site later tonight and lasting into Tue
morning. KSAW will then have VFR conditions in the afternoon.

&&

.MARINE...
Issued at 545 PM EDT Mon May 20 2024

Weak pressure gradients will keep winds less than 20 knots across
Lake Superior through Tuesday morning. However, an unseasonably
strong low pressure system approaches Lake Superior Tuesday
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). Winds
may increase further Tuesday night into Wednesday when our local
probabilistic tool indicates a 30% chance of gales exceeding 40
knots over the western and north-central portions of the lake.
Confidence is lower that gales will occur across the eastern half of
Lake Superior, but a considerable pressure gradient combined with
uncertainty in the low pressure track indicates it`s too early to
drop the gale watch. Low level stability further reduces forecaster
confidence as a very warm air mass flows across the still cold lake
water implying strong gusts at elevated platforms. Despite the
strong stability depicted over the lake on model soundings, the
strong pressure gradient should be sufficient for gales near the sfc
of the lake especially at higher elevation observing platforms. In
short, decided to continue the Gale Watches for LSZ162 21Z Wed-00Z
Thu, for LSZ263-264 02Z Wed-06Z Thu and for LSZ265>267 06Z Wed-21Z
Wed. It is worth noting that some of the models that track a deep
low just w of Lake Superior into northern Ontario show a potential
for higher end westerly gales across the lake Wed into Wed night,
especially across the w half. Will be something to monitor. At any
rate, west-northwest winds should gradually diminish to 20 to 30
knots late Wed night into Thu and fall generally to 20 knots or less
late Thu night into Friday while gradually becoming easterly.

&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
  Gale Watch from Tuesday afternoon through Wednesday evening
     for LSZ162.

  Gale Watch from Tuesday evening through late Wednesday night
     for LSZ242>244-263-264.

  Gale Watch from late Tuesday night through Wednesday evening
     for LSZ245>251-265>267.

Lake Michigan...
None.

&&

$$

SHORT TERM...TAP
LONG TERM...EK
AVIATION...07
MARINE...EK