543
FXUS63 KMQT 200719
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
319 AM EDT Mon May 20 2024

.KEY MESSAGES...

- A round of showers passes through the UP today. While brief
  heavy downpours and occasional lightning are possible
  (15-30%), severe weather is not expected.

- 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 318 AM EDT Mon May 20 2024

GOES-16 Nighttime Microphysics shows multiple cloud layers
overspreading the UP this early morning, with a lower water cloud
deck around 8kft per METARs and higher cirrus pouring in downstream
of a complex of storms over the Upper Mississippi River Valley. RAP
analysis shows this as a 500mb shortwave currently over Iowa and
tracking northeast. Indeed, the forecast is for this shortwave and
associated showers to arrive in the UP throughout the morning. The
aforementioned cloud cover will prevent much destabilization, though
the 00Z HREF 90th percentile of SBCAPE over much of the west half
does show up to 600 J/kg, so a few thunderstorms are not out of the
question, though severe weather is not expected. PWAT values
climbing to 1.25+ inches (90+ percentile per NAEFS climatology)
means ample moisture is present and some showers may have briefly
intense downpours, though widespread impacts are unlikely as HREF
50th percentile QPF by Tuesday morning is only 0.1-0.4 inches.

As the initial wave of showers departs around sunset tonight,
attention will need to be paid upstream tonight. The available 00Z
CAMs are split as to the presence of a decaying MCS over northern WI
passing through the western UP overnight into Tuesday morning. The
HRRR and HRW ARW are the strongest proponents of that MCS solution,
but the remainder of the CAMs hold off on further noteworthy
precipitation until later Tuesday. Should the MCS hold together (an
already unlikely solution), the western UP could see some isolated
strong winds from the decaying MCS, but more likely (80%) is that
even if the MCS holds, it will be moving into a weaker convective
environment and damaging weather will not maintain into the UP.

&&

.LONG TERM /MONDAY THROUGH SUNDAY/...
Issued at 350 PM EDT Sun May 19 2024

Southwesterly midlevel flow between deepening troughing over the
northern/central Rockies and ridging extending form the Deep South
through the Northeast will result in the preferred storm track being
located over the local area through midweek. This will lead to
periods of rainfall through at least Wednesday. Shortwave ridging
favors a drier period to end the week Thu/Fri, with precip chances
reentering the picture to start the holiday weekend Saturday. Aside
from a brief cool-down Thursday, temperatures look to average near
to slightly above normal for the upcoming week into the start of
next weekend.

Initial shortwave of concern looks to touch off convection over the
Central Plains today, and may itself be convectively enhanced as it
ejects northeastward to the upper Great Lakes by Monday. Low-level
jet strengthening to 30-40 kt at 850 mb will enhance moisture
transport into the region, with PWATs increasing to 1.25-1.50
inches, or +1 to +2 stdev. High confidence in an area of rainfall
continuing over the western UP by the start of the period (12Z/7 am
CDT Mon), continuing into the central UP by mid-morning, and likely
reaching the eastern UP by early afternoon. Although a couple rumbles
of thunder will be possible, the corridor of better instability
should remain well to the south. Confidence is lower on what happens
during the afternoon and evening hours. It`s reasonable to expect
some subsidence on the back of this initial wave to dry things out
for the afternoon, especially west/central. However, some CAMs,
namely the 12Z.19 NAM3 and HRW-ARW have a trailing convectively-
generated wave bringing in another round of rainfall for the
afternoon central and east. Reduced PoPs a bit here for the
afternoon/evening as I feel that this scenario is not the most
likely. Forecast soundings suggest some lower clouds may linger
through the day which would limit surface-based instability to
redevelop; however, should the convectively generated wave
materialize, some elevated convection would be possible. Monday
night should be quiet and partly to mostly cloudy in the wake of
Monday`s rain.

Tuesday morning will start out quiet, then attention turns to the
highest potential impact portion of the period, late Tuesday into
Wednesday. The culprit is a negatively tilting, vigorous shortwave
trough ejecting out of the central Plains into northern MN/Lake
Superior. This will foster an unseasonably strong cyclone. The 00Z
NAEFS mean has this low bottoming out at 996 mb near the Keweenaw at
12Z Wed, which is below the 1 percentile relative to climatology.
However, the 12Z EPS/GEFS mean have this low at 992 mb, and several
members (and the 12Z ECMWF) have this low down in the mid/lower
980s, which is highly anomalous for this time of year. There is
still considerable uncertainty on the track, with generally a
stronger storm favoring a westward and slightly slower track.
Regardless, this storm will feature incredibly strong dynamics and
moisture transport, with PWATs likely above 1.50 inches Tuesday
night. Showalter values go negative Tuesday night as steeper
midlevel lapse rates move in. With at least modest instability
coupled with very strong wind shear, there is potential for large
hail along with heavy downpours, as SPC has included portions of the
central and southern UP in a Slight Risk (level 2/5). Thinking it`s
unlikely that we get into the warm sector with model soundings
showing convection being elevated, especially as the strongest
forcing is likely to arrive after sunset, so wind threat is more
uncertain at this time. Heaviest rainfall looks to favor the western
UP, with EPS probabilities of 2"+ at 20-30%. However, potential for
a more widespread flooding event should be mitigated due to ongoing
drought conditions in the region.

A westerly track of the system (e.g., 12Z ECMWF) would open the door
for synoptic-scale wind concerns on Wednesday in the favorable
location beneath the midlevel dry slot. If low-level moisture scours
out, strong mixing could occur and tap into the strong low-level
wind field. ECMWF EFI values exceeding 0.8 signify potential for
unusually strong winds. EPS mean wind gusts are widespread 40 kt
here. Something to monitor in the coming days, and will wait until
storm track becomes more certain to message this threat more
strongly.

Cold advection on the back side of this system will keep things on
the cool and dry side for Thursday, with a modest warming trend
Friday and the next system possibly (40-60% chance) bringing rain
back Friday night into Saturday.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 124 AM EDT Mon May 20 2024

VFR cigs/vsby prevail into Mon morning at all terminals. Conditions
deteriorate Monday morning as a wave of rain lifts north across
Upper Michigan. Model guidance brings in MVFR conditions first at
IWD Monday morning and Monday afternoon at CMX and SAW as this area
of rain moves through. Could be some IFR briefly at SAW late in the
afternoon and at CMX Mon evening. IWD will improve back to VFR by
Mon evening.

&&

.MARINE...
Issued at 350 PM EDT Sun May 19 2024

With the exception of some locally higher gusts of 20-30 kt along
the tip of the Keweenaw and in the vicinity of Isle Royale through
early this evening, winds should remain around 20kt or less through
Tuesday morning. Main period of interest is Tuesday afternoon
through Wednesday as an unseasonably strong low pressure system
tracks from the central Plains across Lake Superior and into
southern Ontario. There are still important model differences that
will dictate peak wind speed and direction, but right now the
highest probabilities for northeasterly gales are Tuesday night over
the western and north-central part of the lake, with 30-60 kt
probabilities here. Winds remain elevated as they shift to a
westerly component on Wednesday with 25-30 kt expected. If the low
track ends up being further west than anticipated, Tuesday night
northeasterly gales would be less likely, but Wednesday
southwesterly gales would be more likely, possibly lingering into
Wednesday night.

&&

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

&&

$$

SHORT TERM...GS
LONG TERM...Thompson
AVIATION...07
MARINE...Thompson