Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 242008

Area Forecast Discussion
National Weather Service Marquette MI
408 PM EDT Sat Jun 24 2017

.SHORT TERM...(This evening through Sunday)
Issued at 349 PM EDT SAT JUN 24 2017

Tonight: The shortwave/upper low that has been producing showers and
isolated thunderstorms this afternoon will continue to shift to the
southeast of the area through this evening. At the same time, a weak
surface trough will linger across the area. The combination of the
forcing/moisture associated with the departing low and the trough
will keep isolated to scattered showers across the area through
tonight. The better chance of scattered rain showers will be over
the east half as that area will be closer to the system forcing
associated with the shortwave/upper low as it departs. The west half
will likely see a bit of a break in the precipitation or at best
isolated rain showers as minimal daytime heating decreases. At this
point, the east half looks to gain about a tenth of an inch or less
of QPF tonight with the west half looking at a trace to 5 hundreths
of an inch of additional QPF tonight. Any instability will diminish
quickly after sunset, so the small thunder potential tonight will be
limited to mainly the evening hours south-central and east.

Sunday: The aforementioned surface trough will linger nearby
throughout much of the day, helping to focus showers and
thunderstorms across the Upper Peninsula. At the same time, yet
another shortwave will rotate through the broad troughing across the
Upper Great Lakes. This shortwave is expected to slide directly
overhead during the late afternoon into the evening hours. The
combination of the cooler air aloft associated with the shortwave
along with surface heating during the afternoon, will give steadily
increasing chances for showers and thunderstorms. MUCAPE values
around 800 - 1000 J/kg along with freezing hgts and wet bulb zero
hgts at or around 5kft to 6kft agl will give increased chances for
stronger storms to produce small hail. Similar to Saturday, very
little shear in the 10 to 15 knot range will support only
pulse/unorganized thunderstorm activity. No widespread severe
weather is expected with this, just mainly small hail. Some of the
storms that pulse may cause some heavy downpours, again, the best
timing for this would be the afternoon into the early evening hours
and mainly inland from the Great Lakes.

.LONG TERM...(Sunday night through Saturday)
Issued at 408 PM EDT SAT JUN 24 2017

Mid-level positive height anomalies currently over western N America
are supporting downstream troffing over eastern N America. Over the
next 7 to 10 days, the positive height anomalies out w will weaken
while expanding ne into n central Canada. This will result in
relaxation and realignment of the eastern N America trof from
eastern Canada into the Northern Plains. This large scale pattern
will support cool conditions/blo normal temps for the Upper Lakes on
most days. Coolest conditions will occur over the next 2 days as
negative 500mb height anomalies max out in the 150-200m range over
the Great Lakes region. Warming will occur into the mid week period
of the upcoming week as a rather strong shortwave cutting thru the
western ridge early in the week goes on to set up troffing from
eastern Canada toward the northern Rockies, backing flow downstream.
Temps will trend to normal to even above normal for a time. A return
to somewhat cooler conditions will then occur into next weekend as
the upstream trof reaches the Upper Lakes. As for pcpn, after a
showery period into Mon with mid-level trof over the Great Lakes,
dry weather returns for all of Upper MI on Tue as trof exits well to
the e and high pres ridge passes. Shra/tstms will return midweek
ahead of the approaching trof extending from eastern Canada toward
the northern Rockies. There should be additional shra/tstms late
week as the trof moves closer, arriving over the weekend.

Beginning Sun night/Mon, would expect shra/tstms that increase in
coverage Sun aftn to tend to diminish Sun night with the loss of
daytime heating. However, shortwave moving across the area suggests
otherwise. With decent deep layer forcing noted per q-vectors,
expect shra to persist thru the night, probably most nmrs across
central section of Upper MI with some increase in shra coverage into
the eastern fcst area as stronger forcing shifts eastward thru the
night. These more synoptically driven shra should then diminish
and/or end from nw to se on Mon as shortwave departs and heights
begin to rise. However, if clouds break enough to allow sufficient
heating/instability, lingering cold pool aloft will support some
additional shra development, perhaps an isold tstm, mainly across s
central Upper MI.

As mid-level trof shifts farther e, expect clearing skies Mon night,
leading to a pleasant day on Tue as sfc high pres ridge passes
across Upper MI. Under mostly sunny skies, temps will rise into the
70s across the area, locally cooler along the Great Lakes.

In developing wsw flow mid week, medium range model guidance remains
in good agreement in showing a shortwave approaching the Upper Lakes
on Wed. Although strengthening low-level jet ahead of this wave
across the Plains Tue night will largely remain focused w of Upper
MI thru the night, not out of the question that some shra could
reach the far w before 12z Wed. Shra/tstms will then increase from w
to e Wed/Wed evening as shortwave reaches the Upper Great Lakes.
Depending on cloud cover and affect on instability build-up, there
may be some risk of severe storms as deep layer shear should be on
the order of 30kt or so, supporting potential of organized storms.
Right now though, it appears instability may be lacking.

Cold front associated with shortwave should clear the fcst area
during Thu morning, leading to a drier day. However, there are some
suggestions that additional shortwave energy will be moving across
the area, so probably can`t guarantee that there won`t be some shra.

There is considerable uncertainty in the structure of the upstream
trof as it reaches the Upper Lakes over the weekend. The more
amplified GFS would bring another low pres wave into the Upper Lakes
for Sat, resulting in a wet day. While the CMC is similarly
amplified, it is farther se with its wave, for a drier solution. The
ECMWF is the least amplified solution, but the 12z has trended a bit
more amplified, increasing the potential of shra on Sat. At this
point, consensus of current and recent model runs suggests minimal
chc of shra on Fri and a greatest chc on Sat.

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 129 PM EDT SAT JUN 24 2017

Showers and possibly a thunderstorm or two are possible over mainly
over the KSAW TAF site this afternoon into this evening. Ceiling
will be in the MVFR to IFR range at each of the TAF sites this
afternoon as a system shifts by to the south. There may be some
minor improvement tonight before the next system approaches toward
Sunday afternoon. Visbiltiy may be restricted at times under heavier
rain showers; however visibility should generally be in the VFR
range. Winds will be fairly light under a weak pressure gradient
across the Upper Peninsula.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 349 PM EDT SAT JUN 24 2017

Winds of 10 to 15 knots are expected to continue through the first
half of next week as the pressure gradient decreases and high
pressure builds in for the first part of the work week. Towards the
middle of the week, winds will start to increase to around 20 to 25
knots as a system lifts across the Upper Great Lakes. Winds will
then decrease to around or less than 15 knots towards the end of the
week and next weekend as the pressure gradient relaxes again.

Upper Michigan...
Lake Superior...
Lake Michigan...


LONG TERM...Rolfson
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