Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 181741

Area Forecast Discussion
National Weather Service Marquette MI
141 PM EDT Tue Jul 18 2017

.SHORT TERM...(Today and tonight)
Issued at 508 AM EDT TUE JUL 18 2017

WV imagery and RAP analysis indicated wsw mid/upper level flow from
the Pacific Northwest into nw Ontario and the nrn Great Lakes
between a deep low north of Hudson Bay and a ridge over the s cntrl
CONUS. Upstream shortwave troughs supporting area of tsra were
located over n cntrl MN and cntrl ND. At the surface, a cold front
extended from srn Hudson Bay into nw MN. Weaker outflow boundaries
from convection Monday evening also remained over nw WI into wrn
Upper Michigan.

Today, although MUCAPE values in the 1000-1500 J/Kg over over ne MN
drop to less than 1k J/Kg over wrn Upper Michigan, expect 35 knot sw
925-850 mb winds with favorable theta-e advectoin to sustain the
convection into the wrn cwa this morning and then diminish as it
moves away from the higher instability.

There is more uncertainty with the evolution/redevelopment of tsra
this afternoon into this evening as the ND shortwave approaches.
Expect that tsra coverage/intensity will increase again after 18z
west and spread through much of the cntrl cwa.  With continued
moisture advection and even limited heating pushing temps into the
lower 80s and dewpoints into the upper 60s, MLCAPE values to near 2k
J/Kg combined with 0-6km shear values in the 30-35 knot range will
support a few strong and possibly severe storms with the potential
for damaging winds and large hail. In addition, locally heavy
rainfall may develop as pway values climb to around 2.0 inches.

The shra/tsra will end from nw to se late this evening and overnight
as the front sags to the south and the supporting shrtwv also shifts
to the east.

.LONG TERM...(Wednesday through Monday)
Issued at 339 AM EDT TUE JUL 18 2017

Upper-level ridging building over the Central and Southern Plains
will result in zonal flow dominating the northern Plains and
northern Great Lakes regions allowing for a number of shortwaves to
track across the area from mid to late week. There is quite a bit of
uncertainty in how precipitation chances will play out towards the
latter half of the week and weekend, but there should be a few days
that remain mostly dry as transient areas of high pressure quickly
move across the area in between systems. Temperatures should be
seasonable with highs in the 70s to lower 80s through much of the
extended period, but then cooling down a bit Sunday into Monday.

Beginning the wake of Tue`s cold front, a transient area of
high pressure will push into the area bringing dry conditions for
much of the day.

Wednesday night through Thursday brings another potential for
showers and possibly some thunderstorms as a shortwave is progged by
models to move across the Upper Great Lakes region. There has been
quite a bit of spread among the medium range models in regards to
how precipitation chances will play out as this wave moves across
the area. The Canadian strongly builds surface high pressure
building across the region on Wednesday and pushes the frontal
boundary and precipitation chances well south of the area Wednesday
night through Thursday. The 00z GFS and NAM show the frontal
boundary setting up over southern WI with a complex of storms
initiating along the boundary and dropping out of the northern
Plains and into mainly southern and central portions of Wisconsin.
The big question will be if any of this convection reaches far
enough north to bring showers into our fcst area. Given the model
uncertainty, will continue at least chance pops for mainly south
central portions of the fcst area.

Fri into Mon...For the most part, Friday looks dry as another
transient area of high pressure moves into the area. Given the
number of shortwaves progged to move across the area throughout the
week, it is not surprising that the models are diverging in the
speed/track of additional waves through the weekend. Right now,
given the considerable model uncertainty in regards to the strength,
location, and track of shortwaves moving thru the area will continue
to keep chc pops in the fcst for Sat into Sun with the best chcs for
shra/tsra (likely pops) probably late Sat into Sat night. Monday
looks drier and cooler as Canadian high pressure builds back over
the Upper Great Lakes.

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 140 PM EDT TUE JUL 18 2017

Challenging aviation forecast with the arrival of a cold front and
expected showers and thunderstorms. These storms will impact
KIWD/KCMX this afternoon and then KSAW later this afternoon/evening.
There is some uncertainty in regards to the exact location of where
the stronger convection will move; therefore, amendments may be
needed to fine tune as the afternoon/evening progresses. The main
threats will be locally heavy rain, which will likely lead to
reduced visibilities, and gusty winds. Later tonight as the cold
front pushes south of the area, showers and storms will vacate the
area. There is the potential for some fog in areas that see heavier
rainfall this afternoon, but with a drier air mass progged to move
into the region wasn`t confident enough to include mentions at this

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 314 PM EDT MON JUL 17 2017

Winds will remain southerly tonight, with speeds ranging from 10-
15kt and occasional gusts to 20kt until sunset. High pressure will
continue to sag south away from the U.P. and the approach of a
frontal boundary will shift winds to the southwest Tue and
eventually northwest late Tue ngt into Wed. Winds will then mostly
be under 15kt from Tue aftn thru Fri as pres gradient will be weak
across the Upper Lakes.

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


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