Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 180140

Area Forecast Discussion
National Weather Service Grand Rapids MI
940 PM EDT Wed May 17 2017


Issued at 329 PM EDT Wed May 17 2017

The unseasonably warm temperatures are about to come to an end as
a strong cold front comes through the area during the afternoon
hours of Thursday. Till then scattered showers and thunderstorms
will be around the area. Lows tonight will be in the 60s, which is
nearly 20 degrees above normal The afternoon high Thursday will
be near 80 inland. Once the cold front comes through though it
will be much cooler. Lows Friday morning will be in the upper 30s
to mid 40s. Highs on Friday will struggle to make 60 degrees. It
will be windy this evening and again during the day Thursday.

The next threat for thunderstorms will be Saturday night into
Sunday. This storm will bring widespread rain across the area.
Beyond that we will have more spring like temperature than normal
temperatures with some scattered light rain showers from time to
time through at least Wednesday.


Issued at 940 PM EDT Wed May 17 2017

It remains quiet in sw lwr MI this evening with a dry air mass in
place and little to no instability. The main item of interest is
the 60 knot low level jet spreading into the region from the
southwest over the next few hours.

Synoptic scale sfc gusts tonight should remain under advisory
criteria due to limited mixing after dark, but some ocnl/brief
gust to 40 mph will be possible. It`s possible we`ll need an
advisory however on Thursday, mainly southeast of GRR, once
deeper mixing develops by mid morning.

Latest RAP guidance shows about 500 j/kg of elevated cape coming
in toward midnight and even some sfc based cape after 06z. However
fcst soundings have a bit of a cap and stay rather dry through
the night. That said, if any convection were to move into the area
or develop, some strong wind gusts would be possible given how
strong the low level jet is.

The severe cell earlier west of MKE dissipated quickly as it moved
northeast. We`ll keep an eye on the bow echo currently approaching
Madison WI. Whatever is left of that line extrapolates into
Ludington and Muskegon toward 1 am and could bring some strong
gusts if it holds together. Suspect it will weaken considerably
before it gets here.


.SHORT TERM...(This evening through Friday night)
Issued at 329 PM EDT Wed May 17 2017

The primary question for tonight into Thursday is the risk for
thunderstorms. It will be windy this evening but not enough
widespread wind gusts over 40 mph to issue an advisory. The winds
will diminish by mid evening.

There is an upper low going negative tilt this evening as it
tracks northeast across Iowa to the western Upper Peninsula by
mid morning Thursday. Most of the strong mid level dynamics stay
west of Lake Michigan which means a significant mid level dry
layer making it hard to get any organized convection to reach
Southwest Michigan even with the strong cold front Thursday.

We have significant instability moving into the area this evening
and overnight but instability is elevated and we an elevated
mixed layer of very dry air between 750 mb over 300 mb (from
subsidence due to us being just a touch to far east of the strong
mid level dynamics) . This will make it hard for any organized
convection to impact the area tonight or Thursday. Even so, we
have a 50 to 70 knot low level jet crossing the area between 9 pm
and 2 am on a track from Benton Harbor to Alma. The other problem
is we do not have a good coupling of the upper jet lift area over
the low level convergence area tonight or Thursday. Still we do
have a strong cold front coming through the area and there will be
good surface convergence along the cold front Thursday. So I up
chance pops and will call that good.

Beyond that much cooler air moves in Thursday night into Saturday
as Canadian High pressure builds in behind the front. This air is
unseasonably chilly and that will mean highs will be at least 25
degrees cooler Friday than today. It would not be out of the
question to see patchy frost Friday morning over our northeast
CWA. I do believe the air will be dry enough that we will see some
sunshine on Friday too.

.LONG TERM...(Saturday through Wednesday)
Issued at 329 PM EDT Wed May 17 2017

Main upper low over the Central Rockies today will pivot into the
plains states later this week and move through the Great Lakes this
weekend. This system is the main feature in the long term. The
associated surface low will move through the Western Great Lakes
from Saturday into Sunday. The weekend looks to be wet as a good
surge of moisture moves into the region. PWAT values reach the 1.5
inch range which is fairly high for this time of year. CAPE values
are not overly impressive so have rain showers with a chance of
thunderstorms wording. Do not think we are looking at widespread
storms, but definitely widespread rain showers. Best chance for
precipitation will be from Saturday afternoon into Sunday morning.

Upper troughing will linger across the Great Lakes into the first
have of next week. Scattered showers look likely in a return to more
spring like weather.

High temperatures around 70 this weekend will cool slightly back
into the 60s for early next week.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 755 PM EDT Wed May 17 2017

The primary aviation focus over the next 24 hours continues to be
the winds as a southwesterly low level jet ahead of an
approaching cold front impacts the area.

Have included LLWS in the TAFs for much of the night since the
sfc winds will probably diminish a bit after sunset while winds at
2000 ft will be 45-50 kts.

Sfc winds will increase and become gusty again toward morning as
the cold front gets closer, and sfc gusts on Thursday could
exceed 35 kts, especially south and east of GRR.

Guidance and upstream trends suggest our thunder threat is
probably too low to put in the TAFs at this time, but will AMD if
that changes. Will keep VFR conditions going.


Issued at 329 PM EDT Wed May 17 2017

I will maintain the small craft advisory through Friday night but
it will likely have to be extended into at least Sunday as the
next storm moves through the area.


Issued at 1241 PM EDT Wed May 17 2017

Streamflow is currently normal for most sites. Although a few
storms may move through the region through Thursday, they won`t be
widespread enough to impact the main basins. Still looking at
basin average rainfall amounts this weekend of 0.50"-1.00". Can`t
rule out some locally higher amounts. That shouldn`t be enough
rain to cause more than some within bank rises, though above bank
rises can`t be ruled out. Additional rainfall is possible by the
early and middle portions of next week.


LM...Small Craft Advisory until 6 AM EDT Friday for LMZ844>849.



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