Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 252001

Area Forecast Discussion
National Weather Service Marquette MI
401 PM EDT Mon Sep 25 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 401 PM EDT MON SEP 25 2017

Water vapor imagery and RAP analysis show that pattern progression
is beginning. Deep trof that has been over the western CONUS is
weakening and shifting downstream. Strong ridge that has been over
eastern N America remains. However, main positive anomaly center
that had been s of James Bay has started to weaken and drift se.
Btwn the trof and ridge, deep sw flow is continuing into the Upper
Great Lakes today. At the sfc, nearly stationary front that has
lingered generally just w of Upper MI for the last couple of days
has slipped e into western Upper MI. Lake breeze component has also
assisted the front in dropping s into n central Upper MI this aftn.
Under mostly sunny skies ahead of the front, MLCAPES have increased
to 500-1000j/kg over central Upper MI per latest SPC mesoanalysis.
The instability and low-level convergence provided by front will set
the stage for convection, which is just starting to develop, to
expand over the next few hrs. Farther w behind front, areas of shra
have been spreading from IA to western Lake Superior today. Ahead of
the front, temps are again unseasonably warm with mid/upper 80s away
from Lake Michigan moderation. Behind the front, current temps are
in the 60s far w.

Instability along with convergence provided by frontal boundary will
generate sct to nmrs shra/tsra in the vcnty of the front this
evening across basically the nw half of the fcst area. With deep
layer shear on the order of 25-35kt and mlcape upwards of 1000j/kg,
an isolated svr storm or two will be possible for the next few hrs.
Otherwise, very hvy rainfall is likely with any of the storms thru
the evening due to precipitable water around 1.5 inches (upwards of
250pct of normal) and freezing level at 12000ft or higher. The
evening convection will tend to fade away late evening into the
overnight as instability diminishes. Focus of shra should then tend
to shift farther w where fgen and better deep layer forcing will
provide support for pcpn just ahead of and w of sfc low pres wave
which will be tracking nne toward nw WI late tonight.

On Tue, as sfc low pres lifts across far western Upper MI and out
over Lake Superior, cold front will sweep e thru the day. Thru the
morning, shra will continue to be most widespread over far western
Upper MI along and w of sfc low track. If there is enough heating
ahead of the front, isold svr storms are not out of the question
over the eastern fcst area in the aftn given 30-40kt of deep layer
shear. Will be something to keep in mind if there isn`t much in the
way of cloud cover thru the morning to hold temps/instability down.
High temps should range from the 60s w to the upper 70s/around 80F

.LONG TERM...(Tuesday night through Monday)
Issued at 242 PM EDT MON SEP 25 2017

Nam shows a shortwave over the upper Great Lakes 00z Wed and the
area remains in upper level troughing through 00z Fri. Nam shows
some 850-500 mb q-vector convergence and some moisture moving
through the area on Thu. Will keep some pops in for Tue night
through Wed morning and again for Thu afternoon.

In the extended, GFS and ECMWF show a 500 mb trough over the ern
U.S. with a ridge over the plains and a trough off the west coast
12z Fri. Ridging moves into the upper Great Lakes 12z Sat with
troughing in the western U.S. 12z Sat. Troughing moves into the
Rockies and northern plains 12z Sun and into the upper Great Lakes
12z Mon. Looks dry for most of the extended period with temperatures
starting to below normal and warming to above normal. Chances for
pcpn look to be Fri morning and again on Mon.

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 141 PM EDT MON SEP 25 2017

A cold front slowly moving across Upper MI combined with a wave of
low pres moving nne along the front will bring lowering cigs. with
front already passed KIWD/KCMX, lower cigs have already developed.
At KIWD, expect low MVFR cigs to fall to IFR later this aftn, then
to LIFR tonight as -shra coverage increases. At KCMX, MVFR cigs
should prevail into the evening before falling to IFR late tonight
as -shra coverage increases in nw Upper MI as well. At KSAW, VFR
conditions initially will likely fall to low MVFR or IFR this
evening under upslope nne winds. Further reduction to LIFR will
occur overnight thru mid morning. There could be some -shra around
KSAW beginning late this aftn. Not out of the question that there
could be a tsra at any of the terminals, though KSAW is more favored
due to build up of instability this aftn. As a result, mention of
thunder was only included at KSAW for the late aftn/early evening.

.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 401 PM EDT MON SEP 25 2017

A wave of low pres will lift nne late tonight/Tue along a frontal
boundary extending from the Plains to the Upper Great Lakes. This
will result in ne wind increasing to 20-30kt over western Lake
Superior tonight into Tue morning before diminishing in the aftn.
Winds will remain lighter over eastern Lake Superior. As the low
lifts ne of Lake Superior Tue evening, nw winds will increase to 20-
30kt across the lake Tue night into Wed with the strongest winds
over eastern Lake Superior. Winds will diminish to mostly under 20kt
Wed night into Thu. A low pres trof swinging se across the Upper
Great Lakes Thu night will usher in another round of 20-30kt winds
on Fri. Not out of the question that there could be a few gale force
gusts to 35kt over mainly the central portion of Lake Superior.
Winds will then diminish to under 20kt for Sat.

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


SHORT TERM...Rolfson
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