Area Forecast Discussion

FXUS63 KMQT 201939

Area Forecast Discussion
National Weather Service Marquette MI
339 PM EDT Thu Sep 20 2018

Issued at 512 AM EDT THU SEP 20 2018

...Wind, Waves, and Wet Today into Tomorrow...

The well-advertised dynamic low pressure system slated to rock Upper 
Michigan and lakes Superior and Michigan is currently hanging out in 
the NW KS/SW NE/E CO area, waiting for an upper trough to eject out 
of the Rockies to give it the jump start it needs. In the meantime, 
a stationary boundary that will become the warm front that will lift 
north into the forecast area is draped across E central NE through 
central IA, with showers and embedded thunderstorms ongoing ahead of 
it across the MN/IA border through S into central WI. Other than 
some light rain showers and widespread drizzle, the U.P. is in a 
state of calm before the storm, with the first sign of the incoming 
low being the rapid increase of winds over western Lake Superior 
later today. Gale Warnings have been issued as a result of the 
expected winds; see the Marine discussion below for more details. 

The aforementioned ongoing showers and thunderstorms currently well 
to the south of the U.P. will lift north ahead of the warm front 
today, with CAPE values creeping up by this afternoon, along with 
decent WAA. SPC has downgraded the Enhanced risk that had previously 
included Menominee County to a Marginal for all of Upper Michigan. 
Additionally, WPC has included nearly all of Upper Michigan in a 
Slight risk for excessive rainfall. The bottom line is that given 
enough instability (should some mentionable CAPE be able to work its 
way in with the warm front), some thunderstorms could become strong 
to marginally severe. The primary threat would be gusty to perhaps 
damaging winds, particularly this evening as the low approaches and 
the nocturnal LL jet gets cranking. This would be contingent on how 
much an updraft would be able to tap into the ~60+ knot 850mb jet 
and mix it down to the surface. 

As alluded to (re: WPC's Slight risk), heavy rain will be another 
hazard that portions of the U.P. will likely have to contend with. 
Most of the models have kept a western trend of the heaviest 
rainfall amounts, which many of the 00Z runs have depicted as 
occurring over western Lake Superior. However, there isn't enough 
confidence to say that the western U.P. into the Keweenaw 
Peninsula and Isle Royale will be spared. It is quite possible for
1"-1.25+" of rain to fall within a 6-hour window this evening, in
addition to nearly three-quarters of an inch that will fall on 
either side of this 6-hour timeframe (00Z - 06Z tonight). Total 
amounts of about 2+" will be possible in the western third of the 
U.P., with 1"-2" elsewhere. Locally higher amounts will definitely
be possible, especially if there's any eastward move in the 
current forecast. While the latest FFG is above these forecast 
amounts, there could still be some localized flooding concerns, 
particularly in prone locations. 

Although the winds will be on the rise over Lake Superior as early 
as later today, the synoptically-driven winds over the land areas 
will be temporally displaced, with the noticeable uptick holding out 
until tonight with the approach of the low. Friday will be dominated 
by breezy conditions, especially for locations along the unsheltered 
shoreline of Lake Superior. On top of these winds and temperatures 
dropping from west to east on Friday, showers may be possible on the 
backside of the exiting low, with perhaps some enhancement thanks to 
the lake. All of this activity is currently anticipated to settle 
down by Friday night (i.e. winds dying down, clouds clearing out, 
and temps continuing to drop), which could allow for the development 
of frost in some interior sections. It's not out of the question 
that some locations, particularly in Iron County, could flirt with 
or drop slightly below the freezing mark by early Saturday morning.

Surface high pressure will briefly take control of the forecast 
area, with a weak disturbance dropping south out of Ontario in the 
middle of this weekend. However, any precipitation that does occur 
will be relatively light with otherwise benign weather conditions 
anticipated. It isn't until early next week with the 
approach/crossing of another deepening low pressure system that more 
active weather will dominate. Although widespread showers, along 
with gusty winds is expected with this next system, model trends 
have depicted this one to not be as robust as the one knocking on 
the door today. Temperatures will generally be near normal for mid- 
to late September through much of the forecast period (beyond 
tomorrow) with a slight warm-up possible on Monday ahead of this 
next system.

.LONG TERM...(Friday night through Thursday)
Issued at 336 PM EDT THU SEP 20 2018

After the late week shortwave slides east with a frontal boundary 
following close behind, optimal radiational cooling conditions 
develop Fri ngt across the Upper Peninsula. High pressure arrives 
and should erode any lingering clouds with much cooler air aloft and 
light winds. Current temperature guidance suggests temps could fall 
into the lower 30s across the Central/Western portions of the Upper 
Peninsula, and could easily see the traditional cold spots falling 
into the upper 20s before daybreak Saturday. 

The mid-lvl trough mainly stays north of the area; however, a 
secondary frontal boundary is just north of Superior Saturday. This 
could introduce additional clouds and a slight chance for showers. 
The longwave pattern begins to flatten into a quasi-zonal 
orientation, which may produce a day or two in the extended with 
quiet weather. Although due to the faster mid-lvl flow the ability 
for systems to easily traverse the Northern CONUS will bring small 
chances for showers with temps in the 60s and cool overnights in the 
30s to low 40s closer to Lake Superior and Michigan. 

Tue/Wed of next week will start to focus on a system expected to 
develop over the Central Rockies. This system will have much cooler 
air sliding south from Canada mid-week, and will eventually arrive 
across the Great Lakes. While we anticipate some moderation to the 
airmass by late next week, guidance has been suggesting much colder 
air to arrive for a brief period late next week.

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 143 PM EDT THU SEP 20 2018

Sticking with IFR conditions, with a chance for temporary LIFR, 
through this forecast period. A low pressure system, and associated 
warm and cold fronts, with ample moisture will make their way from 
the Central Plains into Upper Michigan tonight. Patchy fog is 
sticking around SAW this morning, and there are slight chances later 
on for fog to redevelop after the first round of showers moves 
through. Thunderstorm chances still remain for this evening into the 
extreme early hours of Friday. If thunder does develop, expect 
embedded thunder as the most likely scenario instead of discrete 
thunderstorm cells. Confidence for thunder has increased some, but 
will include as a vicinity for now as current high-res models have 
not come to the same solutions for timing and coverage. Look for a 
low-level jet with winds up to 60 kts to come in tonight which will 
likely cause stronger wind shear for all TAF sites.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 440 AM EDT THU SEP 20 2018

Today through tonight, low pressure currently ejecting out of the 
central Plains will track northeast up towards, and across Lake 
Superior into early Friday morning. As the area of low pressure 
strengthens along its track, expect the first round of gales to 
develop across Lake Superior later today ahead of the main system. 
Across western Lake Superior, winds will funnel from Isle Royale 
down into Duluth, with northeast gales up to 40-45 knots at times 
later this evening and tonight. Across eastern Lake Superior, winds 
ahead of the system will start off easterly, increase in speed and 
then becoming more southerly as the low tracks across central Lake 
Superior tonight. A screaming low-level jet of 50 to 60 knots out of 
the south-southwest tonight into early Friday morning will allow the 
gales to hold on across eastern Lake Superior into Friday morning. 
Therefore, have extended the gale warning across all of the open 
waters for this first round of gales. During this time period, the 
large waves are expected across the west half Lake Superior, and 
will build upwards of 12 to 15 feet across far western Lake 

As the low pressure system exits Lake Superior early Friday morning, 
winds will shift around and become north-northwest on Friday and 
produce yet another round of gales across all of Lake Superior. The 
stronger cold air advection and pressure rises look to arrive Friday 
morning into early afternoon. With winds just above the surface 
pushing 35 to 45 knots, the increasing lake induced instability will 
easily allow these stronger winds to mix down. Large waves across 
Lake Superior will continue, but as we move into Friday afternoon 
expect the axis of larger waves to shift towards eastern Lake 
Superior, where fetch will be maximized under north-northwest flow. 
Late Friday into Saturday, high pressure will gradually slide 
eastward, reducing wind speeds across Lake Superior. On Saturday, 
expect the winds to become southwest between 20 and 30 knots as a 
weak low pressure system tracks across Lake Superior. On Sunday, 
winds will switch around and become northeast around 20 to 25 knots.

Upper Michigan...
  Lakeshore Flood Advisory from 2 PM to 11 PM EDT Friday for 

Lake Superior...
  Gale Warning from 8 PM this evening to 10 PM EDT Friday for 

  Gale Warning until 6 PM EDT /5 PM CDT/ Friday for LSZ162-263.

  Gale Warning from 11 AM to 10 PM EDT Friday for LSZ250-251.

  Gale Warning from 11 AM to 8 PM EDT Friday for LSZ242>245-248-

Lake Michigan...
  Gale Warning from 11 PM this evening to 8 PM EDT Friday for 



LONG TERM...Beachler