Severe Thunderstorm Warning 17, NWS Marquette, MI

Area Forecast Discussion
National Weather Service Marquette MI
329 PM EDT Mon Jul 6 2020

.SHORT TERM...(This evening through Tuesday)
Issued at 225 PM EDT MON JUL 6 2020

On the large scale, the pattern continues to be defined by broad
ridging over much of the eastern half of the US and slight
amplification to the ridge over Ontario and Quebec. A stronger upper-
level disturbance exists well to the north of us over northern
Manitoba west of Hudson Bay with a weak, ill-defined cold front
stretched south and southwestward from it through northern
Minnesota. After a weak warm front moved through early this morning,
humidity is up from yesterday with dew points back in the mid 60s
and temps in the mid 80s to lower 90s yet again. This heat and
humidity has allowed 1000-2000 J/kg MLCAPE to build across the UP
with no cap which means that weak forcing with the front to the
west, plus lake breezes and remnant outflow boundaries are all
combining to lead to widespread weak thunderstorms. These storms are
expected to continue behaving the way they have for the past few
hours with a lot of pulsing up and down but no sustained strong or
severe storms since there continues to be little to no shear for
these storms to work with. Expect that these storms will continue to
fill in across the central UP along new outflow boundaries and the
lake breezes.

Still not sure exactly what happens this evening. Mesoanalysis shows
a mid-level short wave near the ND/SD/MN triple point which will
lift ENE into the UP by around 00z. This appears to be the feature
that the NAM and other NMM-core models have keyed in on as the
forcing for a larger and more significant bowing segment to develop
and move into the western UP this evening. Mesoanalysis also shows a
slight increase in effective shear to around 25-35 kts out ahead of
this wave. The big question, however, will be how worked over the
atmosphere is by that point by all of these earlier day storms that
were not handled well at all by the 12z run of the CAMs. Have to
think that the NAM, etc. are holding on to too much instability for
too long and that this possible secant round of storms this evening
won't materialize. Trended that way with POPs in keeping the highest
values towards early evening, but then lingered lighter values
through the evening just in case that does in fact materialize. Am
very curious to see what the 18z NAM/NAM nest and CMC-Reg/CMC-hi-res
show once they (hopefully) resolve the ongoing convection better. If
they still show a strong storm signal at that point even after these
storms today, may need to increase POPs and sky cover later on into
the evening.

Late tonight, the cold front will clear the area and become
stationary oriented east-west through Wisconsin. Initially there
will probably be lingering convective debris with low-level
scattered clouds through midnight or early morning, but eventually
the models show skies clearing towards dawn. The remnant moisture
and scattered low clouds should keep overnight lows warm, in the mid
to upper 60s. Behind the weak front, GEFS plumes show 850 mb temps
cooling by about 1-2 C compared to today, and thus highs should be
slightly cooler, although still in the mid 80s. Dew points should be
just a few ticks lower as well. With lighter low-level winds behind
the front, the Lake Superior lake breeze should make it farther
inland tomorrow which will also help keep things cooler along the
lakeshore where highs may not get out of the 70s. Depending on
exactly where the front settles, there could be some scattered
showers and storms again tomorrow afternoon over the south-central
and along the Bay of Green Bay. The environment tomorrow looks to
feature less heating and less moisture up here on the north side of
the boundary, and thus MLCAPE only around 500-1000 J/kg, but
stronger mid-level flow leading to 0-6 km bulk shear vectors of
around 30-40 kts which could make for better storm organization
should any form.

.LONG TERM...(Tuesday night through Monday)
Issued at 328 PM EDT MON JUL 6 2020

Ridging will move back into the upper Great Lakes into Wed. A 500 mb
trough will move into the northern plains Wed night and then into
the upper Great Lakes on Thu. Will still be unsettled for this
forecast period and did not make too many changes to the going
forecast.

In the extended, the GFS and ECMWF show a 500 mb ridge in New
England, another over the Rockies and a trough across the upper
Great Lakes 12z Fri. Thu trough remains over the area into 12z Sat
and then drops into the lower Great Lakes 12z Sun. Upper ridging
then moves into the upper Great Lakes 12z Mon. Temperatures will
remain at or above normal for this forecast period.
&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 143 PM EDT MON JUL 6 2020

Main story today will be scattered to numerous thunderstorms. Most
have already moved through IWD but additional storms could develop
next few hours so held on to a VCTS. Storms are just moving into CMX
now. Most will stay south but they could get close enough for a
brief vis reduction. Most storm coverage will be at SAW, beginning
in about 2-3 hours and lasting through the evening with occasional
MVFR vis possible.

Once storms clear tonight, expecting low-level scattered clouds in
their wake to linger through the evening, given that satellite shows
a lot of that ongoing in Minnesota right now. They should finally
clear late tonight. The W wind at CMX may push some marine fog
onshore but models for the most part don't show it so only went as
low as a 6SM BR for now.
&&

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 328 PM EDT MON JUL 6 2020

Light winds less than 20 kts are expected to continue throughout the
forecast period. Areas of fog are likely to persist, especially over
the east. Will still have to watch for dense fog at times with the
moist humid airmass over the colder water.
&&

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

SHORT TERM...RJC
LONG TERM...07
AVIATION...RJC
MARINE...07

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