576
FXUS63 KMQT 182257
AFDMQT

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

.KEY MESSAGES...

- Increasing threat for severe thunderstorms capable of
  damaging winds and large hail across western Upper Michigan
  late this afternoon into the evening hours.

- Borderline elevated fire weather conditions on Sunday with
  very dry conditions, but winds on the lighter side.

- Frequent periods of showers Monday through late week, but
  mainly focused on Monday and also Tuesday afternoon and
  Tuesday night.

- Above normal temperatures early in the week falling to below
  normal late week.

&&

.SHORT TERM /THROUGH EARLY THIS EVENING/...
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 /TONIGHT THROUGH SATURDAY/...
Issued at 551 PM EDT Sat May 18 2024

An active pattern will continue across the U.S. during this week.
Troffing currently over western Canada into the northern U.S.
Rockies will amplify over the next few days, leading to western
Canada/western U.S. troffing this week. A series of shortwaves
dropping into this trof from the N Pacific will then eject e and ne
across the U.S., supporting the active pattern. For Upper MI, the
result will be several opportunities of rainfall this week. The
first is just beginning and continues into tonight. The second
follows late Sun night thru Mon evening, the third Tue aftn thru
Wed, possibly lingering into Thu, associated with the strongest low
pres of the week, and finally the fourth at some point Fri thru the
weekend. Models are in overall good agreement thru midweek. Timing
of the last system is most uncertain, but that`s expected given the
farther time range out in the model runs. So, expectation is for
overall above normal pcpn across Upper MI over the next 7 days. This
will work to ease still lingering drought over portions of western
Upper MI where, per the 5/16 U.S. Drought Monitor, severe drought
was still indicated, centered around the Porcupine Mtns, and also in
eastern Upper MI where moderate drought was still indicated from
eastern Delta to western Mackinac counties. As for temps, above
normal readings will be the rule thru Mon, warmest on Sun. Temps
will then trend downward to blo normal Thu/Fri, due to the flow
around the deep low that passes across the area midweek. A return
toward normal expected over the weekend.

Beginning tonight, a mostly narrow band of convection associated
with cold front still currently w of Upper MI will continue
progressing eastward with a tendency to diminish with time once it
reaches central Upper MI due to loss of daytime heating and
resulting waning instability. Risk of damaging winds/large hail
exists across roughly the w half of Upper MI owing to 500-1000j/kg
of MLCAPE over the next few hrs and deep layer shear of 30-40kt.
Last of the diminishing shra/tsra will exit the eastern fcst area 06-
09z. Skies will quickly clear from w to e after fropa. Expect lows
generally in the upper 40s to lwr 50s F across the area.

On Sun, sfc high pres ridging will move to the Upper Great Lakes
along with and an associated very dry air mass. Full sun thru the
morning will give way to some mid/high clouds arriving from the w in
the aftn. Strong insolation will result in deep mixing into
extremely dry mid-levels. NAM shows late morning dwpts at
800mb/~5kft AGL ranging from -20 to -40C, though this is the low end
of the model guidance. Moisture aloft does increase during the aftn,
but mixing potential still supports sfc dwpts falling thru the 30s
F. Some locations will see upper 20s F. The low dwpt potential is
evident thru local mixed dwpt tool and simply in raw model guidance,
HRRR most dramatically (it often captures mix down drying quite
well). With high temps in the mid/upper 70s F across the interior,
the low dwpts will drive RH down to around 20pct across much of
Upper MI Sun aftn. The good news is winds will be on the lighter
side, though a little higher than previously expected. Sustained
winds will be around 10 mph with gusts to around 15 mph. Winds will
be strongest across the Keweenaw with gusts to around 25mph there.
Will need to monitor for lightning strike started fires, especially
if rainfall from storms is more limited tonight. Lake breeze
development will limit fire wx concerns close to the Great Lakes
shorelines due to cooling conditions (temps falling into the 60s F),
and higher dwpts, resulting in higher RH.

Out of the amplifying mid-level trof over the western CONUS, next
shortwave will lift to the Upper Great Lakes on Mon, accompanied by
a surge of precipitable water up to ~190pct of normal. This increase
in moisture along with increasing isentropic ascent will lead to
shra/isold tsra and a widespread wetting rain across much of the
fcst area. EPS ensemble probabilities show much of the area, except
the Keweenaw, with a 30-50% chance of seeing 0.5 inch of rain from
this system. Shra will arrive late Sun night and end most areas Mon
evening. After a period of drier weather late Mon night thru Tue
morning, a notably stronger shortwave ejecting from the western trof
will reach the Upper Mississippi Valley Tue night. With this wave
potentially closing off a mid-level low, it may be slower to exit
across Upper MI into Ontario Wed/Thu. The GFS has been and continues
to be on the faster edge of the model solutions. This fcst leans
toward the slower consensus. Associated deepening sfc to 985-990mb
range will be in the vcnty of western Lake Superior Wed morning.
While the low track passing to the w of Upper MI will support the
widest coverage of heaviest rainfall to the w of Upper MI, mdt to
locally hvy rainfall will still be possible across the fcst area
during the period of strong isentropic ascent/strong 850mb theta-e
advection to the e of the low Tue night. Precipitable water
increases to ~200pct of normal as well. Expect widespread rainfall
across the area. EPS probabilities show a 30-70% chance for at least
a half inch of rainfall from this system with the highest
probabilities se and w. Shra coverage will diminish significantly
for Wed, some areas may slip in dry weather for much or all of the
day, especially e half. As the low lifts across northern Ontario,
isold/sct shra may linger under cyclonic flow on Thu.

Timing shra potential Fri/Sat is uncertain. For now, of the 2 days,
Fri has the better shot at being a dry day.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 657 PM EDT Sat May 18 2024

VFR conditions prevail thru this fcst period. A brief 2 hour window
through 02z at SAW and CMX could see some thunderstorms when a cold
front sweeps east across Upper Michigan.  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.

&&

$$

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