774
FXUS63 KMQT 161921
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
321 PM EDT Mon Sep 16 2024

.KEY MESSAGES...

- Isolated to scattered showers and non-severe thunderstorms today
over western Upper MI eastward to Marquette County.

- Extended stretch of well above normal temperatures continues
through at least Saturday with highs mainly in the upper 70s and 80s.

- Next credible chances of precipitation arrive Friday into the
weekend.

&&

.SHORT TERM /THROUGH THIS AFTERNOON/...
Issued at 322 AM EDT Mon Sep 16 2024

Water vapor imagery and RAP analysis show mid-level troffing along
the W Coast and a broad mid-level high encompassing the Great Lakes
to New England. A shortwave moving over Hudson Bay is aiding a 25-
30kt low-level jet from northern MN ne to James Bay. This also
coincides with 850mb theta-e ridging and 1000+j/kg of CAPE for
parcels lifted from around 850mb. As a result, radar imagery has
shown development of elevated isold shra/tsra across ne MN in the
last hr or so. Closer to home, it`s an incredibly warm night for mid
Sept across western to n central Upper MI in the s wind downslope
areas. Temps are in the lwr 70s F as of 07z, and that`s higher than
normal high temps at this time of year. Big Bay is the warm spot at
76F. Across the s central and eastern fcst area, largely decoupled
winds and nothing more than sct thin ci clouds have allowed temps to
fall mostly into the mid 50s to lwr 60s F range. Patchy fog has
formed in that area as well.

Developing shra/tsra in ne MN will likely further develop/push out
across western Lake Superior and may sneak into western Upper MI
this morning before low-level jet weakens. Models also indicate a
subtle 850mb wind shift that will further aid the convection. For
now, kept pops only in the 15-25pct range, but will be monitoring
radar trends over the next several hrs to determine if any increase
is needed based on how coverage of shra/tsra changes. This aftn,
attention turns to sfc-based convection potential. Models show quite
a range of potential MLCAPE this aftn with highest values over
western to n central Upper MI, ranging from around 700j/kg at the
low end to 2500j/kg from the NAM at the high end. Consensus seems to
run in the 1000 to ~1500j/kg range. With instability maximizing over
nw Upper MI eastward to Marquette County, which is in the same area
where lake breeze is likely to develop to provide a focusing
mechanism, will paint that area for aftn shra/tsra with pops in the
20-50pct range. With deep layer shear under 15kt, svr storms are not
expected. The lack of shear will lead to generally short-lived
convection with new development occurring along outflows of the
initial cells. With precipitable water upwards of around 175pct of
normal, very brief hvy rainfall will be possible. Otherwise, expect
another unseasonably warm day today with highs ranging thru the 80s
F with some locations reaching the upper 80s. Southerly winds will
hold temps to the 70s F lakeside of Lake MI.

&&

.LONG TERM /TUESDAY THROUGH MONDAY/...
Issued at 319 PM EDT Mon Sep 16 2024

Through the work week, the pattern aloft remains very blocky with a
closed low over the US Southeast through Thursday night. The rex-
block ridge that`s been residing north of the closed low lately will
be replaced by another ridge that will be causing 500mb height
anomalies over northern Ontario to climb to near +30 dam. By Friday,
the low wavenumber blocked pattern will shift to a high wavenumber
active pattern as the closed low shifts northeast from under the
second ridge, resulting in troughs over the Eastern Seaboard, the
Northern Plains, and over the Desert Southwest with ridges in
between each trough. The surface weather will reflect this pattern
change as the previous dry conditions and anomalous warmth will be
replaced by more widespread chances of rain into next weekend along
with more seasonable temperatures.

Tuesday looks to be a dry day as the most influential surface
feature remains a 1025mb high centered over New York during the
morning. Global deterministic models show a shortwave passing
through northern Ontario through the day and two of the members of
the HREF (NSSL WRF and FV3 WRF) show precipitation over Isle Royale
and western Lake Superior in response to surface troughing draped
south of the shortwave. Therefore, this forecast will add some
chance (15-26%) PoPs for those areas overnight. With HREF mean
MUCAPE of up to 750 J/kg, an isolated thunderstorm cannot be ruled
out, but severe weather is not expected as shear values of below 20
kt will not allow for updrafts to be organized in a meaningful
fashion.

Despite the erosion of the first ridge aloft over the Great Lakes,
it will be immediately replaced by another ridge moving over from
the Great Plains, resulting in Wednesday and Thursday being another
pair of dry days. Minimum RH values during those days do remain
above critical thresholds, but with wide swaths of the UP going
a full week without rain and nearly all of the UP having less
than a quarter inch of precip over the last week, drying fuels
may be a concern to some fire partners. Thankfully, under the
ridging aloft and with a lack of nearby low pressure centers,
winds remain light, alleviating some of those fire weather
concerns. A few record highs and record warm lows may be under
threat as the NAEFS highlights the near-surface temperatures
above the 90th percentile of its climatology. Lighter winds and
mainly clear skies will provide good setups for radiation fog
each overnight, especially in the south- central, where the Euro
visibility meteograms highlight 20% chances for dense fog.
Leaving just patchy fog in the grids for now until confidence
increases.

As the blocking pattern shifts quickly into a quite active pattern,
troughing will finally arrive from the west late Thursday into
Friday, allowing for higher PoPs to arrive with widespread wetting
rain being likely by the end of the weekend. Model spread is still
high in the details (which is understandable given the increased
number of features to track over the CONUS for next weekend), but
the LREF highlights the western half of the UP as at least
33-66% likely to reach a half inch of rain by the end of the
weekend with the east half with similar probabilities to reach a
quarter inch. NBM high temperatures will also cool behind the
trough with highs closer to normal for this time of year in the
upper 60s and lower 70s.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 158 PM EDT Mon Sep 16 2024

VFR conditions will continue to hold thru this fcst period at
IWD/CMX/SAW. However, there will be isold to sct shra/tsra
across western Upper MI this afternoon. CMX has the best chance
of seeing showers this afternoon so have included a VCSH there,
but confidence is too low to include any mention of showers in
IWD and SAW. Mostly southerly winds are expected this afternoon
though winds at CMX will shift e-se with lake breeze passage.
Some patchy is possible tonight but this is expected to stay
south of the terminals.

&&

.MARINE...
Issued at 319 PM EDT Mon Sep 16 2024

Light winds of 20 knots or less continue over Lake Superior this
week as high pressure ridging over the eastern U.S. influences our
weather pattern for the next several days. Nevertheless, some
showers and thunderstorms are possible tonight and again Tuesday
night as shortwave lows ride along the ridging overtop the Upper
Great Lakes. Chances for 20 kt gusts increase Friday into the
weekend as troughing finally arrives from the west, though details
on the exact track of the surface features are uncertain and will
have a large influence on the details of gusty conditions.

&&

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

&&

$$

SHORT TERM...Rolfson
LONG TERM...GS
AVIATION...Voss
MARINE...GS