920
FXUS63 KMQT 201748
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
148 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 SUNDAY/...
Issued at 513 AM EDT Mon May 20 2024

An active pattern will continue across the Upper Great Lakes
in the coming week as an amplifying mid-upper level trough over
western North America allows for a series of shortwaves to eject ne
from this trough to support more rounds of rainfall for Upper
Mi.

Beginning Tue, After a period of dry weather late tonight into Tue
morning behind today`s shortwave and associated bout of rainfall,
another stronger shortwave is expected to eject ne from the western
CONUS trough and continue to deepen on Tue as it lifts through the
Plains and reaches the Upper Mississippi Valley Tue night. The
deepening of the system 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) is reflected in healthy 12hr
500mb height falls of 150m predicted by the models over the Upper MS
Valley/Upper Great Lakes region late Tue night/early Wed. At the
sfc, models indicate the associated low pres will deepen to between
985-990mb as it approaches and then lifts across western and
northern Lake Superior on Wed. While this low track passing to the w
of Upper MI will support the widest coverage of heaviest rainfall
over MN and nw WI, moderate to locally heavy rainfall will still be
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.
Expect widespread showers and scattered t-storms across the area as
mid-level lapse rates steepen across the area. 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 late Tuesday into the southern UP may
be a bit overdone considering we will be in an unfavorable e-se flow
ahead of the approaching low for much of the day and with the
strongest forcing from the system probably not arriving until after
sunset, expect convection to be most likely elevated. GEFS
probabilities show a 40-80pct chc of at least a half inch of
rainfall from this system with the highest probabilities w and se.
Models show shower coverage diminishing 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 range now, suggesting a climatologically strong
wind event for this time of year based on last 20 years of model
climate. The EPS probabilities basically 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
knots or greater. While winds for this fcst were increased from what
NBM indicates, they are not as high as the median from the EPS and
will likely need to be bumped up more. There will still be time to
reevaluate for higher winds in later fcsts. For now, will
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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 112 PM EDT Mon May 20 2024

KIWD continues to flirt between high-end IFR and low-end MVFR this
afternoon through this evening as rain showers have currently
stopped here. Meanwhile, low-level cloud cover behind the light rain
showers early this afternoon will drop the VFR cigs at KCMX and KSAW
to MVFR. Moving into the overnight hours, areas of FG and low-level
cloud cover look to develop over Upper Michigan. This FG/low-level
cloud cover has a good chance (60%) of dropping conditions down
close to airport mins across the terminals late tonight. Moving into
Tuesday morning, expect conditions to slowly improve to IFR and MVFR
across the TAF sites, with rainfall possibly moving back over KIWD
near the end of the period as an impressively strong low pressure
lifts towards far western Lake Superior. Beyond the TAF period,
expect LLWS across the TAF sites Tuesday night.

&&

.MARINE...
Issued at 513 AM EDT Mon May 20 2024

Weak pressure gradients will keep winds less than 20 knots across
Lake Superior into Tuesday morning. Deepening low pres will then
lift toward western Lake Superior, reaching the lake Wed morning.
Expect e to ne winds to ramp up late Tuesday as the low approaches.
NE winds will increase to 25-30kt over western Lake Superior by late
Tue afternoon, reaching gales of 35-40 knots by Tue evening. Winds
increase further Tue night. Probabilistic guidance indicates a 50-80
percent chance of reaching gale gusts later Tue night into Wed
across w and central Lake Superior. Although, model probabilities
were low (generally less than 20%) for gales over the east half of
the lake, I did bump up the gusts to 35 to 40 knots late Tue night
into Wed due to a 60 to nearly 70 kt southerly low-level jet moving
across the area at that time east of the low track. Gut feeling is
despite the strong stability depicted over the lake by model
soundings, believe that some of this wind will be realized near the
sfc of the lake over higher platform observing stations. In short,
decided to issue Gale Watches for LSZ162 00Z Wed-00Z Thu, for LSZ263-
264 04Z 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 evening through Wednesday evening for
     LSZ162.

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

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

Lake Michigan...
None.

&&

$$

SHORT TERM...TAP
LONG TERM...Voss
AVIATION...TAP
MARINE...Voss