706
FXUS63 KGRR 131935
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
335 PM EDT Mon May 13 2024

.KEY MESSAGES...

- On and off showers/storms through Tuesday evening

- Dry with seasonable temperatures Wednesday...

- Mainly low risks for showers late week and into upcoming weekend

&&

.DISCUSSION...
Issued at 335 PM EDT Mon May 13 2024

- On and off showers/storms through Tuesday evening

We have scattered to broken coverage of mainly showers late this
afternoon firing close to a cold front that is slowly sagging south
through the area. Instability ahead of the front has increased to
500-1000 J/kg ahead of the front over our area. It is more
favorable/higher across the Lake over WI and IL where more storms
have fired with better low level moisture, and closer to the upper
trough aloft.

Instability progs show that we are peaking right about now with
regards to instability. Part of this is that they are forecasting
dew points to mix out/down a bit for the next few hours. Then we
obviously cool down as the sun sets. We do anticipate more showers
than storms through sunset. Any storms that form should not cause
too much trouble with a lack of deep layer shear around. This was
evident in the storms over WI and IL as they were very much of the
pulse variety.

We will see additional showers and embedded thunder into tonight,
even as we lose diurnal instability. This is because we see the
upper low to our west approach the area and help out with broad
scale lift and interacting with the front.

We could see a break in the activity late tonight and Tuesday
morning, before another round affects the southern portion of the
area. The sfc front is expected to sink south of the forecast area
by tomorrow morning. We will then see the deformation zone of the
upper low closing off provide the threat for additional showers. The
trend over the last 24 hours has been for this to be a little
further south. This looks to be the result of the upper low going
just a little further south, and drier NE flow eating away at
lighter rain on the northern edge.

All of this should come to an end Tuesday evening, with drier
conditions spreading over the entire area. One thing to mention for
starting Tuesday night. Some of the smoke aloft over the far Upper
Midwest and Canada from Canadian Wildfires may start to move overhead
in the wake of the system Tuesday night.

- Dry with seasonable temperatures Wednesday...

The upper level disturbance over the Ohio Valley Tuesday will push
east toward the mid Atlantic states by Wednesday. This will allow
ridging at the surface and aloft to overspread the Great Lakes. With
a relatively dry E/NE flow and strong May sunshine, a good deal of
sunshine is expected Wednesday. Temperatures will be near normal, as
morning lows in the 40s rebound to around 70 during the afternoon.

- Mainly low risks for showers late week and into upcoming weekend

Models/ensembles are in general agreement that a progressive and
quasi-zonal upper level flow pattern will impact the northern CONUS
late in the week. This set up will likely result in multiple short
wave troughs quickly traversing the northern part of the country. In
the absence of strong meridional flow, temperature changes behind
any associated surface fronts should remain small/negligible.

The strongest short wave /relatively speaking/ is expected to impact
West Michigan late Thursday into Friday, and will likely provide the
best chances /50-70%/ for showers/storms. Additional low chances
/20-30%/ for showers will remain the forecast through the upcoming
weekend, as the next short wave potentially impacts the area. Common
of this synoptic pattern, the speed and strength of these short
waves has a high degree of uncertainty -- and consequently,
uncertainty also revolves around the precise timing and coverage of
any associated showers. So, in spite of the low risk for showers
that`ll be reflected in the forecast, the upcoming weekend likely
won`t be a horrible one /i.e. no prolonged or heavy rain expected/.

Temperatures Thursday through the weekend will average near to
somewhat **above normal**, with plenty of 70s expected for highs,
and 40s and 50s for lows.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 150 PM EDT Mon May 13 2024

VFR conditions along with brisk southwesterly breezes /gusting to
around 20 kts/ will continue through the afternoon. Then, as a
cold front over northern Lower Michigan moves slowly south late
this afternoon and interacts with daytime heating and increasing
moisture coming into region from the SW, scattered showers and
thunderstorms will develop after 20-22Z (a bit earlier at MKG).
The risk for showers/storms will continue overnight, and as the
front and cooler air drops southward, MVFR ceilings /1.5 to 2.5
kft/ are expected to develop area-wide after midnight. Winds will
be light and variable along and immediately behind cold front,
before becoming northeasterly 5-10 kts in the post cold front air
mass on Tuesday.

&&

.MARINE...
Issued at 335 PM EDT Mon May 13 2024

We may not need any Small Craft Advisories for much of this week.
There are some better winds aloft this afternoon that are being
realized inland where there is better mixing. Over the cooler waters
of Lake Michigan, the winds are not mixing down nearly as much. We
expect this to continue until the core of winds aloft move out this
evening.

We do see some winds aloft ramp up some Tuesday night into Wednesday
once again. These winds will be offshore in nature, and are more
likely to stay below criteria. We will have to watch this period, as
if it looks like the winds will be realized, an advisory will be
needed.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
MARINE...None.

&&

$$

DISCUSSION...NJJ/Smith
AVIATION...Smith
MARINE...NJJ