Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171146
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
746 AM EDT Mon Jun 17 2019

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 304 AM EDT Mon Jun 17 2019

- Gradual warming conditions are expected today through Wednesday
  with continued overcast to broken cloud cover.

- A swath of heavy rain is possible Wednesday evening into
  Thursday, especially along and south of I-96.

- A shift to a summer pattern is looking increasingly likely this
  weekend and beyond.

&&

.DISCUSSION...(Today through next Sunday)
Issued at 304 AM EDT Mon Jun 17 2019

We continue our stay in a stagnant airmass with mid-level zonal
flow, residual low-level moisture, and debris cloud cover from
convection well to our south and west. Patchy mist/fog with low
clouds continue along I-94 where easterly flow off Lake Huron
has been supplying an extra oomph of moisture. Somewhat "better"
conditions can be found along and north of I-96 where low-level
dry air continues to slowly drift southward (as confirmed by the
00Z APX RAOB).

A "persistence" forecast is favored over the next 24-48 hours but
with gradual improvement. Temperatures this afternoon have a good
shot of hitting the 70 degree mark thanks to thinning cloud
cover, with light east winds prevailing. As a high pressure
system slides overhead tonight, winds will become calm likely
leading to pockets of shallow fog (perhaps dense along rivers and
where the heaviest rain has fallen over the past week). Tuesday
doesn`t look half bad with breaks in cloud cover and highs in the
mid to upper 70s. There may be just enough differential heating
along the lakeshore to allow for a lake breeze to develop and
surge toward US-131 given weak prevailing winds. No precipitation
is expected through Tuesday night.

The main forecast concern over the next 7 days is the potential for
a fairly wound-up low pressure system to swing through the Lower
Great Lakes around the Wednesday night/Thursday timeframe.  Forecast
model guidance has been consistent in showing a compact mid-level
wave and accompanying surface low pressure system, both perhaps
convective in origin, developing in the Plains Wednesday afternoon
and shooting eastward along a line generally from southeastern
Iowa through northern Illinois along the Michigan/Indiana/Ohio
borders by Thursday morning. As the low deepens on its trek
through the Lower Great Lakes, strong frontogenetical forcing
along and north of the low in combination with PWATs of 1.5-1.8"
will lead to a band of heavy rainfall as demonstrated by a
concerning signal in deterministic model runs of a swath of 2"+ of
rain in 6 hours through a part of Lower Michigan. To be honest,
this has the "SHARS" look (Subtle Heavy Rainfall Signature). If
this trend continues, some sort of flood watch may become needed
given the current "soaked sponge" state of our soil.

It`s also worth noting deepening low pressure systems in June with
abundant low-level moisture is always concerning from a severe
weather perspective, with forecast kinematic fields from the
southeastern quadrant of the system in Illinois/Indiana Wednesday
evening suggesting the potential for considerable directional and
speed shear in the lowest 3 km. While the current track of the low
would indeed focus any severe weather threat south of Lower
Michigan, any shift northward would become quite problematic. We
also can`t forget about the potential for gusty winds Thursday
shifting from the northeast to northwest, potentially posing a
threat for a brief period of lakeshore flooding/beach erosion
mainly south of South Haven. And, this is all assuming the system
takes the track as currently advertised. Considering the
aforementioned apparent convective origins of the low, it`s
entirely possible that Lower Michigan will sneak by without any
impacts. So, the message at this point is to keep a close eye on
the forecast as the Wednesday/Thursday timeframe could be quite
active in the Lower Great Lakes.

Looking toward this weekend and beyond, there are bonafide signs of
a more summer-like pattern setting up in the Great Lakes. Both ECMWF
and GFS ensemble forecast guidance (aided by detail within
deterministic counterparts) suggest a strong pacific jet will
transition into semi-persistent troughing over the western US and
ridging across the east by this weekend. Rising heights and
southwesterly low-level flow will allow for temperatures to return
to (and stay in) the 80s potentially as early as next weekend.
However such a pattern may also come at a cost; the ECMWF daily mean
instability from next weekend and beyond exceeds 500 J/kg
signaling what may be the start of active convective period.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 745 AM EDT Mon Jun 17 2019

LIFR conditions continue at KAZO with MVFR weather at most other
TAF sites at 12z. The trend should be toward improvement through
the course of the day, with sites becoming VFR from north to south
this afternoon. Both the ceilings and visibilities (where fog
remains) should trend upward/improving. There may be a widely
scattered shower this morning, but for the most part we are
expected dry weather through 12z Tuesday.

Tonight we will start off VFR, but there is potential for some
stratocumulus to reform with bases in the 2000-3000ft range late,
after 09z.

Winds will be from the northeast and east today at 5 to 10 knots.
A lake breeze looks to develop at KMKG this afternoon though.

&&

.MARINE...
Issued at 304 AM EDT Mon Jun 17 2019

Easterly winds this morning will back northwesterly but remain light
at or below 10 kts. Winds and waves will remain calm through about
the middle of the week before a low pressure system swings through
the Lower Great Lakes. Gusty easterly winds Wednesday and early
Thursday will back northerly and eventually northwesterly. The
potential exists for a brief period of large waves, beach erosion,
and lakeshore flooding, on Thursday especially south of South
Haven. Chances for thunderstorms will return this weekend.


&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...Borchardt
DISCUSSION...Borchardt
AVIATION...Duke
MARINE...Borchardt


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