Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 241145

Area Forecast Discussion
National Weather Service Marquette MI
745 AM EDT Thu May 24 2018

.SHORT TERM...(Today and tonight)
Issued at 459 AM EDT THU MAY 24 2018

Quiet again this morning. Had dense fog earlier near Lk Michigan but
that has now dissippated. Main focus is on shra/t-storms that are
now into the Duluth MN area. This convection is focusing in area
of stronger H85 warm air advection on nose of moisture transport
and along edge of EML/elevated MUCAPE spreading in from the
northern Plains. Based on radar loop, also looks like there is a
smaller convective shortwave carrying this convection along.
Various short range models, including the HRRR and RAP are
handling the shra and t- storms storms well and indicate that they
will hold together as the shortwave rolls along. Expect
shra/t-storms into western cwa by late morning and over the
Keweenaw and north central by mid aftn. Initially the convection
may diminish but as it meets up with building instability with
MLCAPES near 1000j/kg over cntrl Upper Michigan, think scattered
t-storms will regenerate this aftn. Given the MLCAPES forecasted,
it seems feasible that an isold storm this aftn could be strong
with stronger shear (up to 30 kts) present as the shortwave moves
through. Convergence along lake breezes will help this possibility
as will steeper low-level lapse rates as aftn temps top out in
the 80s again. Wet bulb zero heights only of 9-10 kft AGL are also
favorable for larger hail.

Once these shra/t-storms run their course by early evening, should
be a general lull. However, given persistent moisture transport
aimed into the area from the southwest and elevated MUCAPE up to
1000j/kg, think a slight chance of a pop-up shra or t-storm will
stay in play this evening. Main focus for more shra/t-storms arrives
overnight as stronger low-level jet and moisture transport aims into
western Upper Michigan and then shifts toward central Upper Michigan
by daybreak Fri. May be another shortwave to help this along but it
is unclear as it will develop out of the convection that occurs this
aftn over the central Plains. Prefer the non-GFS idea with how this
will evolve as the GFS seems way too muted on its degree of forecast
elevated instability. All other models and the SREF indicate this
cluster of shra/t-storms will be ingesting MUCAPES around 1000j/kg.
Effective shear is weak, under 20 kts, so attm do not expect any
strong storms late tonight though could be heavy rain and small hail
with extent of elevated instability. Warm night in store all of
Upper Michigan (most areas staying in the mid 50s to lower 60s) with
the clouds/shra/t-storms and steady south wind ahead of low and sfc
trough sliding across northern Plains.

.LONG TERM...(Friday through Wednesday)
Issued at 456 AM EDT THU MAY 24 2018

No meaningful changes in medium range guidance is noted from 24hrs
ago. Mean western Canada ridge/eastern Canada trof pattern that has
dominated for some time will be progressing downstream and breaking
down over the next several days as mean troffing tends to become
established from the Gulf of AK into western Canada/Pacific NW.
Southern stream trof that was over the western CONUS has been kicked
ne into the northern Rockies by a sharp trof approaching the W
Coast. Remnants of this trof now over the northern Rockies and the
associated shortwaves will shift eastward along the U.S./Canada
border region over the next few days, bringing an unsettled period
of weather to Upper MI thru at least the first half of the Memorial
Day weekend. With large portions of west and central Upper MI only
receiving less than 25pct of normal rainfall over the last 30 days,
widespread, decent rainfall is needed. While that probably won`t
happen, there is at least the potential for some areas to receive
much needed rainfall as sct showers and thunderstorms roam Upper MI
today thru Sun. With shortwaves slipping e of the area Sun, dry
weather should return for Memorial Day. Back to the w, the trof
currently approaching the W Coast will split with the southern
portion establishing a new western CONUS trof/mid-level low this
weekend. With the aforementioned troffing setting up off the Gulf of
AK into western N America next week, this western CONUS trof/mid-
level low will eventually get forced ne/weaken like its predecessor,
providing Upper MI with another opportunity of shra/tsra at some
point during the last half of next week. As for temps, with the
departure of the eastern Canada trof, the mechanism for sending any
strong surges of cooler air into the Upper Lakes will be gone.
Overall, the upcoming 7-10 day period looks solidly above normal,
though the typical cooler by the lake will occur frequently. Many
days will see more summerlike dwpts readings as well.

Assessing the model fcst of shortwave progression in addition to the
uncertain spin-up of waves by convection over the Plains will make
for a complicated and uncertain fcst with regard to timing better
opportunities of shra/tsra Fri/Sat. However, surprisingly, there is
reasonable agreement for a pair of shortwaves to affect Upper MI
Fri, one roughly in the morning hrs and the second late
aftn/evening. In a broad sense, model qpf guidance aligns with the
timing of these waves. 30-35kt low-level jet in response to the
first wave should support more organized convection spreading across
the area in the morning, weakening as it reaches the eastern fcst
area. Air mass/instability recovery behind the morning convection
will then occur for the next shortwave to take advantage of late
aftn/evening. Convection in the morning will be elevated, and models
show a wide range of mucape values for parcels lifted from an
elevated sfc. Best estimate is that 500-1000j/kg will be avbl. With
effective shear under 20kt, not anticipating any svr storms during
the morning. With precipitable water up around 1.5 inches, 250pct of
normal, locally hvy rainfall is certainly possible. Late
aftn/evening mlcapes are fcst into the 1500 to around 2000j/kg
range. With deep layer shear under 25kt, organized svr storms are
not expected with the late aftn/evening convection. However, given
the instability, can`t rule out pulse type isold svr storms. Delta
theta-e from the sfc to mid-levels is over 20K, supporting a risk of
strong winds while WBZ heights btwn 9 and 10.5kft over the w half
supports a risk of larger hail. Shra/tsra will wind down during Fri

On Sat, another shortwave, is fcst to approach. Weak pres gradient
over the area will support lake breeze development which will likely
become the focusing mechanism for convection. Thus, pops will be
highest in the interior on Sat. With mlcape increasing to 1000j/kg
or so and deep layer shear increasing to around 30kt, there is some
potential of organized storms and a svr risk, but potential is
still low unless instability is greater.

Shortwave trof will be shifting e of the area on Sun, and so there
should be a trend toward drier weather. Will maintain schc/low chc
pops in the morning which will then become confined to s central
Upper MI in the aftn where lingering moisture/lake breeze
convergence and a little instability may still support some shra.

Memorial Day should be dry, but there are some hints that there may
be sufficient instability in the aftn to generate isold shra along
lake breezes. Will retain a dry fcst for now.

Tue thru Thu, attention turns to the ejection of the trof/mid-level
low becoming established over the western CONUS this weekend. Ahead
of this feature, guidance has trended toward a less amplified/weaker
mid-level ridge moving across the western Great Lakes on Tue.
Whether this will allow shortwave energy ejecting from the trof to
bring some shra/tsra into the area as early as Tue remains to be
seen, but there are trends to support that potential. Wed/Thu look
more unsettled with shra/tsra possible as the remants of the western
CONUS trof lift ne across the area.

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 743 AM EDT THU MAY 24 2018

Main focus today will be shra/t-storms across the area as warm front
approaches. Already seeing cluster of t-storms at IWD and those
should persist through the morning. Expect some shra/t-storms to
reach CMX and SAW by midday. Maybe a brief IFR vsby during heaviest
rain, otherwise conditions should stay VFR during these
shra/t-storms. This evening should be quiet though some LLWS will
develop at IWD and SAW as the warm front lifts north of Upper
Michigan. After midight another round of shra/t-storms should
push over all the TAF sites. Will start out the forecast with
VCSH and later forecasts can refine.


.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 459 AM EDT THU MAY 24 2018

Winds less than 20 kts are expected into next week. Expect areas of
fog to develop late tonight into the weekend as some rainfall occurs
over the lake and a humid air mass arrives. May have dense fog at
times with best chance setting up Fri night and Sat as a trough
slides across the lake.

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


LONG TERM...Rolfson
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