Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 291943

Area Forecast Discussion
National Weather Service Marquette MI
343 PM EDT Sat Apr 29 2017

.SHORT TERM...(This evening through Sunday)
Issued at 342 PM EDT SAT APR 29 2017

The main weather concerns during this time period will be an
approaching low pressure system from the southwest, introducing a
chance of rain very late tonight through Sunday.

Tonight: Low pressure over the Southern Plains is expected to slowly
lift northward toward the Upper Great Lakes region through this time
period. As this happens, the surface low and upper level 500mb low
are both expected to intensify. The main impact that this will have
on the U.P. tonight, will be a steady increase in moisture flow into
the area. Initially, high clouds will be on the increase this
evening and then steadily increase in depth through the overnight
hours. Isentropic upglide will slowly increase across far south
central portions of the U.P. overnight, which may give a few rain
showers. The precipitation will likely have trouble lifting too far
to the north as dry high pressure remains in place over southern
Hudson Bay, helping to continue pumping low to mid level dry air
into the area. In fact, models continue to trend a bit slower with
the onset of the precipitation.

Sunday: This system will continue northward, reaching southwest Iowa
by 00Z Monday. Again, the main concern will be the continued/steady
increase in moisture across the area as the high over southern
Hudson Bay continues to shift to the east and the low slowly
approaches. A majority of the system forcing will remain to the
south of the U.P. through late Sunday afternoon; however, the
isentropic upglide will continue to steadily increase throughout the
day. This will lead to at least scattered rain showers building
across mainly the south half of the area, with cloud cover
increasing across the north half. Precipitation totals through
Sunday afternoon will be between a tenth to a quarter of an inch
south half with a tenth or less north half.

.LONG TERM...(Sunday night through Saturday)
Issued at 342 PM EDT SAT APR 29 2017

The active wx pattern that has dominated most of Apr wl continue
into May. Main focus for early this coming week wl be the impact of
a slow moving, negatively tilted closed lo lifting newd fm the srn
Plains into the wrn Great Lks on Mon. Although the bulk of the
heavier pcpn is likely to fall as rain thru Mon in the strong waa
ahead of the aprchg lo, there could be some sn over mainly the hier
terrain of the nw half Mon ngt into Tue as some colder llvl air
arrives in the cyc nw flow behind the departing closed lo. The upr
pattern wl then change to one featuring a nw flow alf btwn a bldg
rdg in the Rockies/wrn Plains and lingering troffing over the e.
While disturbances embedded in the nw flow alf could bring some
showers to Upr MI later this coming week, the primary result of this
pattern change wl be a drying trend for Upr MI.

Sun ngt/Mon...The negatively tilted closed upr lo in the srn branch
flow is progged to move from near Kansas City at 00Z Mon to the
MI/WI border by 00Z Tue. In general, the models have continued to
trend toward a bit farther w track for this feature, not surprising
considering the upr rdging present just off the se CONUS. The
arrival of a ccb/trowal with vigorous isentropic ascent best shown
on the 300K sfc (h725-8) that lifts pwat to 1-1.25 inches under
dpva/diffluent flow alf ahead of the aprchg closed lo wl bring a
period of at least briefly heavy pcpn to the area on Sun ngt.
Although some recent model runs, especially the 00Z and 12Z Cndn
models, indicate the pcpn could be at least mixed with sn over
mainly the nw half with h85 temps as lo as -2C to -4C, the model
trend toward a farther w track and vigorous waa supports the warmer
scenarios/rain as the primary ptype best illustrated by the
consistently warmer ECMWF and recent GFS runs. However, as the pcpn
begins on Sun evng, evaporative cooling of the lingering llvl dry
air could result in a mix with some sn over the hier terrain of the
nw cwa. Since the closed lo has trended to move a bit farther to the
e, the more recent models show more aggressive mid-lvl dry slotting
wrapping arnd the aprchg closed lo arriving fm the se earlier and as
soon as late Sun ngt over the se cwa, resulting in diminished shower
intensity into Mon. Shower coverage will probably increase again on
Mon as the closed lo/deep cyc flow/backwash mstr under the lower
hgts arrive fm the sw and the dry slot shifts to the ne. Although
daytime max temps on Mon wl be warm enuf to support rain, the
readings wl be cool for late Apr. Some colder llvl air in the nw
flow behind the passing lo pres could arrive over the far w near
Ironwood late in the day on Mon to at least cause the pcpn to mix
with sn there.

Mon into Tue ngt...Although the intense dynamic support will
diminish as the deep closed lo exits slowly to the ne, lingering
sharp cyc flow shifting to the nw/persistent lo-mid lvl mstr in its
wake wl maintain at least hi chc/likely pops into Tue. The arrival
of some drier air and weakening cyc flow on Tue ngt under steadily
rising hgts ahead of aprchg sfc hi pres wl then cause diminishing
pops. With the wshft to the nw and arrival of some colder air, the
showers will at least mix with some snow, especially over the hier
terrain of the nw half, where there could be some sn accums on Mon
ngt into Tue mrng. Tue max temps wl run well blo normal.

Wed thru Sat...Upr MI wl be dominated by a nw flow aloft ahead of a
bldg upr rdg in the Rockies/wrn Plains. While disturbances digging
into the Great Lks under this flow could bring some clds and perhaps
a few showers especially on Thu into Fri, overall dryness of the
airmass and timing uncertainty wl limit fcst pops. There is some
disagreement among the longer range guidance on the amplitude of the
bldg rdg and upr troffing that wl prevail to the e. The models that
show a hier amplitude flow indicate cooler wx wl persist into the
weekend under a more persistent nnw flow alf. On the other hand, the
guidance featuring a lower amplitude/more progressive flow hint the
upr rdg wl build into the wrn Great Lks toward the weekend, which
would result in dry/ warmer wx.

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 149 PM EDT SAT APR 29 2017

VFR conditions will prevail over the next 24 hours; however, there
may be lower clouds that try to build into the KSAW TAF site toward
the very end of the TAF period. There may be a few wind gusts this
afternoon as mixing increases through the day, but not expecting
much more than 15 to 20 knot gusts.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 342 PM EDT SAT APR 29 2017

High pressure will remain in place across Lake Superior through
tonight, keeping winds light northeast in the evening, turning
easterly overnight. Winds will remain at or less than 15 to 20
knots. Sunday, low pressure will approach the Upper Great Lakes from
the southwest allowing the pressure gradient to tighten once again.
This will allow winds to increast to 20 to 30 knots over much of
Lake Superior, with the strongest gusts expected over the west half
of the lake due to terrain enhancements on a favored norhteast wind.
As the pres gradient tightens over the Upper Lakes on Sun night
ahead of deep lo pres approaching from the sw, ne winds will
increase to gale force over at least portions of the w and central
lake. Expect the strongest gales up to 40 kts over the w, where the
terrain will funnel the ne flow and further enhance the wind speeds.
Issued gale warnings for the areas to be impacted. As the pres
gradient begins to weaken on Mon night into Tue behind the departing
lo pres, winds will diminish and back toward the n ahead of
approaching hi pres that will build over the Upper Lakes on Wed and
further reduce wind speeds under 20 kts thru Thu.

Upper Michigan...
Lake Superior...
  Gale Warning from 2 AM to 2 PM EDT Monday for LSZ265-266.

  Gale Warning from 8 PM EDT /7 PM CDT/ Sunday to 5 PM EDT /4 PM
     CDT/ Monday for LSZ162.

  Gale Warning from 2 AM to 8 PM EDT Monday for LSZ263-264.

Lake Michigan...
  Gale Warning from 2 PM Sunday to 8 AM EDT Monday for LMZ221-248-



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