898
FXUS63 KMQT 181909
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
309 PM EDT Sat May 18 2024

.KEY MESSAGES...

-Breezy south winds and very warm temps in the 70s and 80s today
 ahead of a cold front that will bring scattered
 afternoon/evening thunderstorms. The strongest storms may
 contain small hail.

-Borderline elevated fire weather expected this weekend with breezy
 conditions today then very dry with light winds on Sunday.

-More rain chances early to middle of next week as a series of
 disturbances moves across the area in a developing warm and
 moist southerly flow.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 309 PM EDT Sat May 18 2024

Forecast is mostly on track early this afternoon except for a few
pesky rain showers that continue to percolate across the Copper
Country on the cool side of a ~40 kt low level jet. There was some
thunder/lightning with this activity before it moved into our area,
but has just been showers since then. Looking around local weather
stations indicates morning rainfall has been difficult to measure,
probably due to the deep inverted-v signature evaporating most of
the droplets. Otherwise temperatures have warmed into the 70s and
80s in line with the morning forecast with the warmest spots being
L`Anse, Harvey, and Covington that all observed 84F around 2PM.
Relative humidity values decreased to 28% at Baraga Plains, Golden
Lake, and Michigamme sites with everywhere else staying at or above
30% at this time. The eastern edge of lower RHs appears to be a
subtle Lake MI lake breeze with a subtle wind shift from SSW to SSE
and dew points a few degrees warmer.

The main forecast challenge going through this evening continues to
be thunderstorm chances as a cold front sweeps east across Upper
Michigan. Many different observation platforms reveal a cold front
extending south from the MN arrowhead toward La Crosse, WI. A lake
breeze boundary over the Bayfield Peninsula is also apparent on DLH
radar with SPC mesoanalysis of surface vorticity and divergence
highlighting these features well. The latest 18z mesoanalysis also
shows SBCAPE around 1500 J/kg along the MN/WI state line beneath 0-6
km bulk shear of 30-40 kts, which is conducive to marginally severe
thunderstorms. Large hail and damaging winds are the most likely
hazards and deep inverted-vs with DCAPE of similar magnitude as
MUCAPE suggests damaging winds may be more prevalent. Several recent
HRRR/NAM nest runs have shown gusts at or near severe thresholds
with the most common location being near and northeast of the
Porcupine Mountains.

Satellite/radar imagery shows orphan anvils over northern WI
indicating updrafts are trying to get going, but have so far failed
to persistently break a mid level capping inversion. However, HREF
guidance indicates rapid initiation over northern WI around 20Z/4PM
eastern with coverage increasing as it moves into Gogebic County by
22Z/6 PM eastern then well into the western UP by 00z/8 PM eastern.
Thunderstorm activity is expected to be weakening as it moves into
the central UP due to the loss of surface heating. However, the
threat doesn`t end until the cold front moves through and the
frontal boundary apparent on DLHs area is about 5 hours away
according to the time of arrival tool. This is a fair amount slower
than CAMs implying storms may be outflow-dominant, surging ahead of
the boundary. As far as rain totals ensemble guidance favors a quick
0.10-0.20in of rain for the most part, but there is a chance (30-
40%) for heavier amounts up to a half inch in any heavier downpours.
Skies quickly clear from west to east behind the front while winds
veering westerly before becoming light and variable under surface
ridging.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 325 AM EDT Sat May 18 2024

After a continued warm and very dry day on Sunday, the focus then
shifts to a couple of widespread rain events for early/mid next week
before a transition to a period of below normal temperatures by late
week.

Beginning Sunday, surface ridging and its associated much drier air
mass will take hold in the wake of tonight`s frontal passage.
Abundant sunshine and light winds will result in deep mixing into
extremely dry mid-levels during the day. For example, the 18z NAM
fcst soundings valid at 18z Sunday over several of the interior west
half sites show -30C or drier dew points at nearly 800 mb or
approximately 5 kft AGL. Fortunately, surface dew points won`t mix
that low, but by incorporating the local mixed dew point tool I did
cut back on Td values resulting in min RHs closer to 20% across the
interior portions of the cwa. The good news is light winds generally
10 mph or less should ease fire wx concerns. Lake breeze
activity should limit fire wx concerns along the shorelines due
to cooler temps in the 60s and higher RHs. Expect max temps
reaching near 80F over much of the interior west half and mid to
upper 70s over the interior east half.

Models advertise a mid-level trough amplifying over the western
CONUS early next week. Periodic shortwaves in a warm/moist sw flow
developing ahead of the western CONUS trough will bring more rounds
of showers into the area early to mid next week. The first of these
shortwaves moves through late Sun night into Monday bringing the
initial batch of showers and isolated t-storms for early next
week. Models show this system tapping into Gulf moisture with
PWATs increasing to around 1.5 inches by midday Monday. This
increase in moisture along with increasing isentropic ascent
should lead to widespread wetting rain across much of the cwa.
Ensemble probabilities show much of the area (except maybe the
tip of the Keweenaw) with a 40-60% chance of seeing 0.5 inch of
rain from this system with the greatest chances south central.
After a period of drier weather Monday night into Tuesday
morning, the next shortwave moves through Tuesday afternoon into
Wednesday spinning up a 985-990 mb sfc low which tracks over
the far western portion of the fcst area late Tue night into
Wed. With PWAT values again increasing to nearly 1.5 inches
along with increasing isentropic ascent, PVA and upper
diffluence fcst with the shortwave, expect widespread rainfall
across the area. EPS probabilities again show a 40-60% chance
for getting half an inch of rainfall from this system with the
greatest chances southeast and west.

Despite the midweek system lifting north and east of the area,
models indicate wraparound moisture behind it could support
lingering isolated to scattered showers for late week (Thu-Fri) as
temperatures trend cooler than normal.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 138 PM EDT Sat May 18 2024

VFR cigs/vsby prevail thru this fcst period except for a brief 2-4
hour window when a cold front brings shower and thunderstorm chances
as it sweeps east across Upper Michigan. Uncertainty in thunderstorm
timing was addressed with a 4-6 hour VCTS group and a 2 hour TEMPO
group highlighting the most likely timing of thunderstorms.
Confidence in thunderstorms is lowest at SAW, but strong
thunderstorms capable of large hail and damaging winds are possible
at IWD/CMX this evening. Otherwise, gusty southerly winds up to 30-
35 mph today veer westerly and diminish to less than 10 mph behind
the cold front. Cloud cover also rapidly diminishes behind the front
with skies becoming clear by Sunday morning.

&&

.MARINE...
Issued at 325 AM EDT Sat May 18 2024

Stable conditions will lead to light winds of generally 20 knots or
less prevailing over the lake into early next week, except for some
20-25 kt southeast gusts today over the east half ahead of an
approaching cold front. Thunderstorms are possible today into
tonight as the cold front marches across the lake. These storms may
contain small hail, gusty and erratic winds, and frequent lightning.
More thunder chances return late Sun night into Monday and then
again late Tuesday into Tuesday night as a series of disturbances
track northeast across the Upper Great Lakes region. Models and
ensembles indicate winds will ramp up Tue night into Wed as a
stronger low pressure center tracks across the western and northern
portions of the lake. Ensemble probabilities show a 40-60 pct chance
of gale force gusts to 35 knots late Tue night into Wed across
western and northern sections of the lake.

&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
None.
Lake Superior...
None.
Lake Michigan...
None.

&&

$$

UPDATE...
SHORT TERM...EK
LONG TERM...Voss
AVIATION...EK
MARINE...Voss