Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 172005

Area Forecast Discussion
National Weather Service Marquette MI
405 PM EDT Mon Apr 17 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 403 PM EDT MON APR 17 2017

Wave of low pressure that brought rain/snow showers last night into
this morning is over southern WI. Still rather cloudy though as
northerly flow btwn the low and high pressure over northern Ontario
keeps a cool/moist flow off Lk Superior across Upper Michigan.
Clouds will try to break up some this evening but where winds remain
onshore that may be tough proposition. Best chance of low clouds and
maybe some fog developing as well will be closer to Lk Superior
northwest and north central with NE to E low-level winds providing
upslope flow. Where skies can clear out temps may drop well into the
20s. Best chance attm for that looks to be over interior east
forecast area. Attn late tonight is on vigorous shortwave over
northern Rockies this aftn. System moves across northern Plains
tonight and tracks over Lk Superior and northern Upper Michigan on
Tue. Swath of showers tied to shortwave in area of strong q-vector
convergence should arrive over west around daybreak and spread over
rest of forecast area late morning into the aftn. Despite strong
warm layer aloft leading to liquid think most of precip arrives
later in the morning to prohibit any chance of fzra or sleet. Just
went with mostly rain for ptype. Shortwave strong and tracking north
of forecast area led to addition of thunder with increasing ribbon
of steeper lapse rates toward WI border in the aftn.

Last issue to not be discounted will be strong wind field with the
system. H85 winds are progged up to 60 kt fm SSW. Stability will
play a factor with warming aloft limiting extent of mixing to sfc.
Strongest SE winds will be over Lk Superior and toward tip of
Keweenaw and also near Lk Superior from Marquette to Grand
Marais. Potential for gustiness over west half though especially
in the aftn if rain showers diminish for a bit as shown by latest
GEM model. That idea would also allow for better chance for
thunder as there could be a least some daytime heating. Overall
expect temps in the upper 40s to possibly upper 50s if there is
enough breaks of sunshine in the aftn for the west and near the WI
border. Coolest readings on the Keweenaw with the persistent
onshore winds.

.LONG TERM...(Tuesday night through Monday)
Issued at 342 PM EDT MON APR 17 2017

A split upr flow wl dominate the wx pattern acrs NAmerica during the
medium/extended range. After a period of drying on Tue ngt into Wed
following the passage of a srn branch disturbance, how far to the n
the next srn branch shrtwv wl track on Wed ngt/Thu wl be the main
fcst concern. Some of the medium range guidance, especially the 12Z
GFS, indicates this disturbance wl move far enuf to the n to bring
some wet, heavy sn that would require some headlines to at least a
portion of the cwa. But the 12z NAM shows the shrtwv tracking
sufficently far to the s to have relatively ltl impact. Since the
responsible disturbance is still over the data spase Pacific, wl
have to wait and see. Hudson Bay hi pres expanding into the area in
the wake of this disturbance wl then bring a period of dry wx for at
least Fri and Sat. Although a cold fnt associated with a nrn branch
shrtwv may pass Upr MI on Sun, accompanying pcpn should be on the
light side. Expect temps to run aob normal during the medium/
extended range.

Tue ngt...The area of showers, mainly over the e half, associated
with the passing shrtwv/sfc lo pres and cold fnt wl diminish and end
by about 06Z as larger scale subsidence/mid lvl drying follow the
exiting disturbance. Some of the models show some lingering upslope
pcpn over the w thru the ngt with cyc nnw h925 flow following the
cold fropa. But sinking subsidence invrn under 5k ft agl indicates
any ra that mixes with sn as h85 temps fall toward -2C wl be on the
light side.

Wed...Large scale qvector dvgc/subsidence supporting sfc hi pres
bldg into nw Ontario wl bring dry wx to Upr MI. But some mstr
trapped blo lo subsidence invrn in the shallow n veering ne flow on
the srn flank of the sfc hi pres wl probably maintain a good deal of
cld cover, especially over the hier terrain near Lk Sup. The best
chc for some clrg of the lo clds with more vigorous downsloping/
daytime mixing wl be over the scentral, but mid/hi clds wl be on the
increase there ahead of the next srn branch disturbance aprchg fm
the w.

Wed ngt/Thu...A disturbance moving e thru the srn branch flow
dominating the n half of the CONUS and accompanying sfc lo pres are
progged to move thru the Great Lks area during this time. Since Upr
MI wl be near the sharp gradient of mstr btwn the two branches,
model fcsts vary considerably. The 12Z GFS shows a farther n track
for this disturbance, with some sharp deep lyr qvector cnvgc/mid lvl
fgen/upr dvgc in rrq of 110kt nrn branch h3 jet max near James Bay
and left exit of 100kt jet max supporting the srn branch shrtwv
impacting Upr MI. Fcst thermal fields fm the GFS indicate the pcpn
may begin as a mix of ra/sn but then chg to mainly sn as the sharp
uvv cools the column sufficiently. The GFS qpf aprchg an inch over
all but the Keweenaw would sug advy to perhaps warning sn amounts
could occur where the bulk of the pcpn falls as sn. On the other
hand, the 12Z NAM shows a farther s track, with accompanying pcpn
mainly over just the scentral. The 12Z Cndn and ECMWF models have
trended a bit to the n with the track of the shrtwv/sfc lo pres,
with some moderate qpf over mainly the s half. Still lots of
uncertainty with this fcst, especially with the sharp gradient of
deep mstr fcst and main disturbance still over the Pacific Ocean.
Although a lack of interaction btwn the srn branch and the zonal nrn
branch flows dominating scentral Canada favors a lower impact event,
if the track of the srn branch disturbance is farther to the n,
there could still be some sgnft pcpn/sn. Considering the nwd shift
by most of the models on the track of the srn branch shrtwv, raised
pops a bit above the model consensus and lowered fcst temps for Thu
to account for thicker clds/better chc for heavier pcpn and more
dynamic cooling.

Thu ngt thru Sat...Lingering pcpn over the se cwa on Thu evng wl
diminish following the exit of the srn branch shrtwv to the e and
arrival of large scale subsidence that wl support Hudson Bay hi pres
expanding over the wrn Great Lks into the weekend. This rdg and pwat
fcst to fall under 0.25 inch wl bring a period of moclr skies on
Fri/Sat. H85 temps btwn 0C and 2C on both days wl support max temps
peaking as hi as 55-60 away fm the cooling influence of the lks.
With lgt winds on Fri ngt under the hi pres rdg, some interior
locations may see temps dip as lo as the 20s.

Sun/Mon...The fcst during this time wl depend on the track of a srn
branch disturbance thru the OH River Valley/Lower Great Lks and a
nrn branch shrtwv passing thru Ontario/its attendant cold fnt. The
12Z GFS and NCEP manual progs indicate the cold fnt associated with
the passing nrn branch shrtwv wl swing thru the Upr Lks early on Sun
accompanied by some lgt pcpn, with trailing hi pres/dry air
following into nw Ontario/the Upr Lks on Mon. Although the extended
models show some variations on this theme, none of the longer range
guidance shows any interaction btwn the branches/ separate shrtwvs.
So expect no sgnft impacts/pcpn. Temps look to run mainly blo normal
during this time.

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 150 PM EDT MON APR 17 2017

Sfc ridging and associated drier air trying to push in from the
north. Should see improvement this aftn to MVFR or VFR at all the
TAF sites, but with onshore winds into this evening expect KCMX and
KSAW to develop low cigs again. Pattern would favor some fog but not
sure on how persistent/widespread so just went MVFR vsby for KSAW at
this time.

Winds becoming more variable overnight as low pressure system
crosses northern plains will result in less of an onshore wind and
probably will see cigs rise or even scatter out overnight.
Developing SE winds could lead to another period of low stratus late
tonight/daybreak Tue at KSAW. Otherwise skies should be VFR as mid
clouds lower with system moving in from west. Rain should arrive at
KIWD before end of TAF cycle and over KCMX and KSAW just beyond end
of this TAF period. Another hazard will be low-level wind shear with
very strong S-SSW low-level jet ahead of the sfc low.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 342 PM EDT MON APR 17 2017

Expect light winds this evening under passing hi pres to give way to
se winds increasing up to gale force over at least the e half on Tue
afternoon as the gradient tightens between the departing hi pres and
a lo pres tracking thru MN toward the Upper Lakes. Issued gale
warnings for the areas expected to be impacted. Winds will shift to
the nnw up to 25 to perhaps 30 kts on Tue night before further
diminishing on Wed as trailing hi pres crosses Ontario/Lake
Superior. Depending on the track of another lo pres approaching the
Lower Great Lakes on Thu, ene winds up to 25-30 kts will be possible
then. As hi pres builds back into the Upper Lakes late in the week
following this lo pres, winds will diminish under 25 kts.

Upper Michigan...
Lake Superior...
  Gale Warning from 2 PM to 11 PM EDT Tuesday for LSZ266-267.

  Gale Warning from 2 PM to 8 PM EDT Tuesday for LSZ244-245-264-

Lake Michigan...


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