Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201607

Area Forecast Discussion
National Weather Service Grand Rapids MI
1207 PM EDT Tue Aug 20 2019


Issued at 1206 PM EDT Tue Aug 20 2019

- Threat of storms increasing as well as a chance for severe.

- Dry weather Wednesday night through the weekend into next week

- Cooler temps late in the week into the weekend


Issued at 1206 PM EDT Tue Aug 20 2019

Primary concern for the morning update is the threat of
thunderstorms this afternoon/evening and the potential for severe
weather. Instability parameters are continuing to increase
despite a cirrus debris canopy that covers much of the forecast
area. MUCAPE values have already hit 3,000 j/kg at midday, with
LIs of -8C over Southern Lake Michigan. The instability is
occurring due to two main features. The first is low level
moisture transport ahead of the the Mesoscale Convective Vort
(MCV) moving east across Northern Illinois. The second factor
aiding instability is cooling aloft associated with an upper
shortwave moving east into the area.

Deep layer shear continues to look not very impressive, but could
increase if the MCV trends into our southern CWA, which it appears
to be doing. We are watching the line of convection moving east
across Northern Illinois and have it timed into far Southwest
Lower Michigan around or slightly below 3pm. The threat for severe
is certainly possible given increasing moderate instability and
forced ascent via the MCV and line of storms already in place.
Large hail and damaging winds will be the primary threat along
with hydro concerns if we get training along and south of I-96.
I-96 to the south is the area of primary concern.


.DISCUSSION...(Today through Thursday)
Issued at 300 AM EDT Tue Aug 20 2019

The primary fcst challenge is to assess convective potential for
today through Wednesday.

A consensus of latest CAMs and HREF members suggests that an MCS
will develop over IA early this morning and become well
organized/forward propagating as it moves ese across IL later
this morning and IN later this aftn. So most of the organized
convection will stay south to sw of our fcst area later today.

However it is noted that a consensus of latest HREF members
suggests that fairly strong instability will develop over our
southern fcst area late this morning and aftn (ensemble mean sb
cape values reaching 2-3k j/kg). Fortunately deep layer shear
later today is rather weak and there is a lack of a stronger
forcing mechanism for convective initiation to occur.

Nevertheless isolated strong to marginally severe storms seem
possible given strong instability and rather dry air aloft and
steep mid level lapse rates. Some guidance fcst soundings suggest
DCAPE values may reach around 1k j/kg this aftn too so isolated
damaging wind gusts also seem possible in addition to hail. A
shortwave moving in from the nw and possibly an mcv will be the
forcing mechanisms for sct convective development later today.
Convective potential will increase if an mcv over IA moves further
north than currently expected.

Just a smaller chance for showers and storms will linger tonight.
Then on Wednesday some stronger convection could potentially
develop over our se fcst along and just ahead of the cold front.
The latest ECMWF is concerning as it suggests SB cape values could
reach around 3k j/kg toward KJXN late Wed am/early Wed aftn along
and just ahead of the cold front. Dry wx is fcst Wednesday night
through the weekend into early next week.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 658 AM EDT Tue Aug 20 2019

The scattered thunderstorms this morning should continue to
weaken as they move inland. Still local IFR  conditions will
be possible for a few more hours. A few additional thunderstorms
may develop as well for this afternoon with the relatively best
chance for a storm at our southern terminals from mid afternoon to
early Tuesday evening. Storms and heavier showers could cause
brief reductions to MVFR/IFR conditions. There could be an IFR fog
event later tonight as the low level moisture values will be
elevated. I put some decreasing conditions in the TAFs then.


Issued at 300 AM EDT Tue Aug 20 2019

Rather minimal wind speeds and wave heights are fcst today through
tonight. Winds will veer to the sw and increase a bit to around
10 to 15 kts this aftn.

Winds will veer to the nw and increase to around 15 to 25 kts
Wednesday after cold frontal passage and cause wave heights to
build to around 3 to 5 feet. A small craft advisory could be
needed for Wed aftn/eve.




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