Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 170741
AFDMQT

Area Forecast Discussion
National Weather Service Marquette MI
341 AM EDT Wed May 17 2017

.SHORT TERM...(Today and tonight)
Issued at 340 AM EDT WED MAY 17 2017

...Flash Flood Concerns for NW Half of U.P. into early Thu...

Radar trends have shown convection weakening over south central
Upper Mi early this morning and thus severe t-storm watch for MNM
county has been cancelled. Satellite imagery shows a vigorous
shortwave rapidly ejecting ne from mid-upper trough over the
Western Plains and pushing the next wave of convection north thru
into north central WI. Models don`t appear to have a real good
handle on the timing of this next round of convection based on
regional radar trends. Looks like it will be moving into Upper Mi
quicker than any of the models suggest...NAM may be handling it
the best but even it is too slow. Based on tracking tools leading
edge of convection should reach into sw Upper Mi just aft 08z
which is an hour or two quicker than the NAM. So long story short
there won`t be as much of the break in shra/tsra early this
morning as previously thought. Convection over northern Wi
supported by MUCAPE gradient of 500-1000 j/kg just to the south,
h85 jet max of 40 kts and strong 850 mb moisture transport.

Today...RAP and NAM shows the MUCAPES in 500-1000 j/kg range along
the sfc trough/cold front into central Upper Mi through much of the
day today while MUCAPE values taper off to 200-400 j/kg over western
counties. Therefore, the best chance for thunderstorm activity will
be over the central portions of Upper Michigan. Best effective shear
will remain farther to the west where instability is weaker which
should limit chances for severe during the day especially with
expected cloudy skies.

Tonight...vigorous, but weakening shortwave, will be approaching
Upper MI this evening and passing across the area Thu morning/early
aftn. Associated sfc low pres will track along a sw-ne oriented
frontal boundary extending somewhere across the se half of Upper MI.
This still points toward the nw half of Upper MI for the area to see
widespread moderate to heavy rainfall. Combination of fgen, vigorous
deep layer forcing per q-vectors, diffluent flow aloft with some
coupling of upper jets, and precipitable water upwards of 1.5 inches
(250+pct of normal) feeding across frontal boundary will support a
good potential of moderate to heavy rainfall. Certainly could see
widespread 1-2 inches of rainfall with some local 3 inch amounts
across roughly the nw half of Upper MI. Given model trends of
heavier pcpn shifting slightly e have decided to include Iron,
Baraga and southern Houghton counties into flash flood watch which
extends into Thu morning. Likely in response to vigorous shortwave
dynamics models show MUCAPEs ramping up a bit along frontal boundary
into 1000-1300 j/kg range this evening as deep layer shear increases
to least 40kt, which will support the potential of isold svr storms
near the front at least during the evening hrs over s central Upper
MI.

.LONG TERM...(Thursday through Tuesday)
Issued at 325 AM EDT WED MAY 17 2017

Shrtwv/sfc lo pres riding enewd along stalled fnt in the ncentral
Great Lks wl bring the threat for more moderate to heavy ra on Thu
mrng before this disturbance eixts to the e in the aftn and large
scale subsidence in its wake brings a return of drier air. Gusty nne
winds wl prevail on Thu especially near Lk Sup under the sharp pres
gradient btwn the departing lo and aprchg hi pres over scentral
Canada. Depeninding on much cold air spills into the area in this
flow, the ra could at least mix with sn over the hier terrain of the
nw before the pcpn ends. The hi pres wl then bring a period of moclr
skies/cool conditions for Thu ngt/Fri. Another disturbance/lo pres
aprchg fm the sw wl bring a return of shower chcs for the weekend.

Thu...Shrtwv now lifting newd out of the srn Rockies and
accompanying sfc lo pres are fcst to be over nw WI at 12Z Thu. As
these features continue to the ene along the stalled frontal bndry
lying over the se cwa, widespread rain on the cold side of the bndry
wl persist into the aftn, when the shrtwv/sfc lo pres are progged to
exit to the e and allow for larger scale subsidence/drying to
oversrpead the area w-e. Axis of pwat up to 1.25 inches (about 200
pct of normal) over the area, passage of area of deep lyr qvector
cnvgc on the cyc side of the shrtwv, and track of the shrtwv/sfc lo
pres parallel to the upr flow/stalled fnt/band of fgen indicate some
mdt to heavy ra could continue in the mrng before the arrival of the
subisdence diminishes the rain. A sharp cyc ne flow under the tight
pres gradient btwn the departing sfc lo pres and trailing hi pres
over scentral Canada that supports h925 winds up to about 40 kts wl
cause a period of gusty nne winds, especially at exposed locations
near Lk Sup. As h85 temps fall toward -2C over the w later in the
day, temps may fall into the 30s before the return of much drier
air/some sunshine over the w allows recovery later in the day. NAM
fcst sdngs hint the arrival of the colder air could turn the pcpn to
sn over the hier terrain of the nw before the pcpn end.

Thu ngt thru Fri ngt...As the sfc hi pres over scentral Canada on
Thu shifts to the e under shrtwv rdging trailing the departing
shrtwv/sfc lo, much drier air wl diminish lingering clds over the e
on Thu evng and then result in moclr skies thru Fri. Thu night could
turn quite chilly especially over the interior w, which wl be under
the slacker pres gradient closer to the aprchg sfc hi pres and more
sheltered fm the llvl ne flow that wl be slowly diminishing. Tended
toward the lo end of guidance for min temps in this area. H85 temps
up to 4C on Fri aftn wl support max temps rebounding into the 60s
with mixing up to h8-85 on the fcst sdngs away fm cooling off the
Great Lks in areas exposed to the llvl ne flow. Although lingering
dry air associated with the sfc hi pres drifting toward wrn Quebec
wl bring quiet wx on Fri ngt, expect hi clds to overspread the area
in the sw flow alf ahead of the next shrtwv lifting out of the wrn
trof following the shift of the upr rdg axis to the e. These hi clds
and an incrsg se h925 flow wl limit the diurnal temp fall. Expect
the lowest min temps over the interior e closer to the departing hi
pres/weaker gradient.

Weekend...Although there remain disagreements on the timing, pcpn
chcs wl return fm the sw on Sat/Sat ngt as a diffluent, incrsgly
moist upr flow with pwat rising aoa an inch along with waa head of
the shrtwv/sfc lo pres lifting toward the Upper overspread the cwa.
Despite the differences on timing, most of the models show the sfc
lo pres moving thru wcentral Upr MI, so temps wl run aoa normal. Not
out of the question there could be a rumble of thunder over the e
half with fcst Showalter Index as lo as -2C on the e side of the lo
track.

Mon/Tue...The longer range guidance shows quite a bit of uncertainty
on the fcst early next week, related to how quickly the shrtwv/lo
pres responsible for the weekend pcpn exits to the ne and on the
depth of that lo pres/potential for a sharper cyc wnw flow to linger
in its wake and possibly result in some lingering showers. Looks
like cooler wx wl follow whether more mstr lingers or not.
&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 824 PM EDT TUE MAY 16 2017

The combination of warmer and more humid air across the area and
rain from showers this evening should lead to cigs/vsbys lowering to
MVFR at IWD and CMX later this evening and to LIFR and possibly
VLIFR late tonight as winds shift more westerly (onshore) behind
passing trough/cold front. There wl also be a period of llws
especially at SAW tngt under a strengthening llj ahead of the
approaching cold fnt. With nocturnal cooling at SAW, some low
clds/MVFR cigs wl probably impact that site late tonight into Wed
morning and conditions will likely continue to fall to IFR at SAW
especially if winds shift northerly behind cold front later Wed
afternoon. Expect some modest diurnal improvement to IFR at IWD
and CMX Wed afternoon behind the front.

Isolated thunderstorms will be possible this evening into early
overnight but given poor confidence on whether storm will directly
hit TAF sites, did not include anything more than VCTS in TAFS.
Expect another round of showers and isolated TSRA on Wednesday,
but again not enough confidence on areal coverage to include in
TAFS.
&&

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 325 AM EDT WED MAY 17 2017

Relatively light winds this morning will become nne this afternoon
following a cold frontal passage and then increase to gale force
tonight over the w half of the lake as the gradient tightens between
hi pres over scentral Canada and a lo pres moving northeast along
the stalling front over se Upper MI. These gales will overspread the
e half of the lake on Thu morning as the sharper pres gradient
follows the lo pres shifting to the e. The closer approach of the
trailing hi pres will then cause diminishing winds w to e on Thu,
with winds diminishing under 20 kts on Thu night even over the
eastern portion. These lighter winds under 20 kts will then persist
through Sat as hi pres/flat pres gradient dominates. The arrival of
a tighter pres gradient with lo pres moving into the Upper Lakes
might allow for winds to increase back up to 25 kts on Sun.

Combination of showers and increasingly humid air arriving over the
cold lake waters will result in fog over the entire lake into this
evening. This fog may become dense at times. The fog will dissipate
late Wed night/Thu morning as the strong n winds behind the lo
pres passing to the s draw drier air over the area.
&&

.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
  Flash Flood Watch through Thursday afternoon for MIZ002-004-009-
     010-084.

Lake Superior...
  Gale Warning from 8 AM to 8 PM EDT Thursday for LSZ267.

  Gale Warning from 4 AM to 4 PM EDT Thursday for LSZ265-266.

  Gale Warning from 11 PM EDT /10 PM CDT/ this evening to 11 AM
     EDT /10 AM CDT/ Thursday for LSZ162.

  Gale Warning from 11 PM this evening to 2 PM EDT Thursday for
     LSZ263-264.

Lake Michigan...
None.
&&

$$

SHORT TERM...Voss
LONG TERM...KC
AVIATION...Voss
MARINE...KC



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