Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 240547

Area Forecast Discussion
National Weather Service Marquette MI
147 AM EDT Sun Sep 24 2017

.SHORT TERM...(This evening through Sunday)
Issued at 239 PM EDT SAT SEP 23 2017

Upper air pattern consists of a 500 mb over the ern U.S. with a deep
500 mb trough over the western U.S. and Rockies. It is a tale of two
seasons with summer in the east and fall and winter in the western
U.S. for temperatures. The ridge remains over the upper Great Lakes
this forecast period with the area being in a warm and humid airmass
while the trough remains over the Rockies through 00z Mon. The
moisture and dynamics remain to the west and north of the area this
forecast period. Kept in some slight chance pops for this evening
across the far west and far north as about half the models have some
type of convection forming late this afternoon into this evening and
the other half are dry. Only went slight chance pops though.
Otherwise kept with persistence. Still looks very warm for Sunday
with southwest downslope flow helping to break a few records across
the area.

.LONG TERM...(Sunday night through Saturday)
Issued at 405 PM EDT SAT SEP 23 2017

Amplified pattern currently dominates with a deep trof over the
western CONUS and a strong ridge over eastern N America. 500mb
height anomalies of around 230m are centered over se NV and ssw of
James Bay. Resulting deep sw flow has led to the long advertised
unseasonable late season heat/humidity over the Upper Great Lakes.
In the coming days, the western trof will weaken while progressing
downstream, reaching the Great Lakes midweek. A couple of days ago,
medium range guidance was beginning to suggest that a ridge
developing into western N America mid and late week may trend more
amplified. That model trend has held up, and it will result in some
amplification/slowing of the trof once it reaches the Great Lakes.
This obviously has implications for temps here over the next 7 days.
Initial unseasonable warm/humid conditions will fall back to around
normal by mid week, then fall to blo normal late week under trof
amplification thru the Great Lakes. Farther ahead, looks like the
period of blo normal temps may be short-lived as the western ridge
progresses downstream next weekend into the first week of Oct. Temps
will probably return to at least a little above normal. As for pcpn,
potential of shra/tsra will increase from w to e Mon/Tue as cold
front shifts across the Upper Lakes in response to progression of
the western trof. Given days of deep sw flow ahead of the trof,
resulting in precipitable water of at least 1.5 inches (200-250pct
of normal) in the vcnty of the front, potential will be there for
mdt to hvy pcpn as front moves across the area. A trend to dry
weather should occur Wed behind the front, then expect showery
weather at times for Thu/Fri under mid level troffing.

Beginning Sun night, sfc low pres wave lifting from northern MN into
northern Ontario along nearly stationary frontal boundary should
carry focus of shra/tsra from northern MN into northern Ontario as
that area will be under and/or at the nose of the low-level jet.
However, will maintain a small chc of shra/tsra over far western
Upper MI late as the frontal boundary may drift e a bit once the sfc
low lifts into northern Ontario. It will be another unseasonably
warm/humid night across the fcst area with mins the 60s, with some
readings around 70F in locations across the w and n central that see
downsloping under ssw winds.

On Mon, slight eastward drift of sfc front occurring late Sun night
will continue thru Mon as sfc low pres waves exits northern Ontario.
Low-level convergence along frontal boundary should help to initiate
shra/tsra into western Upper MI on Mon, mainly during the later
aftn. Conditions will continue to be unseasonably warm/humid ahead
of the front into the central and eastern fcst area, though not as
warm as previous days with highs in the mid 80s. Will be in the 70s

As main energy in the western trof lifts ne, reaching the Dakotas/MN
by Tue evening, another sfc low pres wave is fcst to lift up the
frontal boundary. Eastward progress of the sfc front prior to
arrival of the sfc low pres wave will be important in determining
where more widespread and potentially mdt/hvy pcpn will occur.
Consensus of recent model guidance is for frontal boundary to reach
central Upper MI prior to sfc wave arriving. This would place the
western fcst area under more widespread and heavier shra/embedded
thunder late Mon night/Tue. Given the high precipitable water up
around 1.6 inches (near record values for this time of year), heavy
rainfall is a possibility, especially if sfc wave is a little
stronger. Right now, none of the model runs show anything unusual
for pcpn amounts. Pcpn and progress of frontal boundary will lead to
cooler conditions Tue than Mon. Max temps should range from the
lower 60s w to low/mid 70s e on Tue.

In the wake of fropa, cooler air will follow on Wed with a trend
toward dry weather. Max temps should be near seasonal late Sept
readings, in the upper 50s to mid 60s.

Thu-Sat, there is good agreement among the GFS/ECMWF/CMC for a
shortwave to drop se into the Upper Great Lakes Fri morning. Main
disagreement is on how strong this wave will be. The GFS is notably
stronger than the ECMWF and especially the CMC, though the 12z ECMWF
has trended stronger. This would result in a small, blustery fall
sfc low pres dropping se thru the northern and eastern parts of the
Great Lakes, accompanied by shra that would transition to lake
effect/lake enhanced shra for a time in its wake. At this point,
there`s not much support from the CMC/GFS ensembles for the stronger
shortwave. Nonetheless, the wave will support development of sct
-shra on Thu that will transition to -shra that have some lake
component off Lake Superior as 850mb temps drop to around 0C on Fri.
Dry weather will return for Sat with sfc high pres building into the
western Great Lakes. Coolest day will be on Fri under clouds/some
shra and arrival of 850mb thermal trof. Max temps on Fri may not get
out of the 40s in the higher terrain of the w and n central. A sharp
contrast from the current unseasonable, record breaking heat.
Otherwise, 50s will be the rule on Fri.

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 147 AM EDT SUN SEP 24 2017

There is a chance of fog tonight at KCMX and maybe KIWD. Did not
have enough confidence in fog at KIWD, so left it out there.
Otherwise, VFR conditions are expected.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 239 PM EDT SAT SEP 23 2017

Low to mid 60s dewpoints sliding across Lake Superior are expected
to linger into Tue bringing patchy to areas of fog across the lake.
Some of the fog could be locally dense at times. Expect winds to
generally be below 20 knots through the early part of next week
under a weak pressure gradient. As a cold front moves across Lake
Superior Tue night into Wed with colder air coming in, winds will
increase to 20 to 30 knots for gusts and these would be the strongest
winds for this forecast period.

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


LONG TERM...Rolfson
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