Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 140156
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
956 PM EDT Wed Mar 13 2024

.KEY MESSAGES...

- Showers and Risk for stronger thunderstorms Thursday

- Chance of rain Saturday, then colder with snow showers

&&

.UPDATE...
Issued at 956 PM EDT Wed Mar 13 2024

A developing warm front will be located over portions of Illinois
and Indiana tonight. The front will be positioned over northern
portions of those states by daybreak. Aloft, an 850mb warm front
will develop by morning as well located over southern portions of
the Lower Peninsula by daybreak. Both of these features will
provide a focus for rain shower development as moisture transport
lifts our direction on southerly winds. Precipitable Water values
by morning will have risen to near 1.0" along the MI/IN border.
There is just enough instability for a few embedded thunderstorms
near and south of I-96 overnight. Not expecting any severe weather
tonight given marginal instability. Chances for precipitation
tonight range from 30 pct across the north up along U.S. 10 to
60-70 pct in the far south near I-94.

&&

.DISCUSSION...
Issued at 233 PM EDT Wed Mar 13 2024

- Showers and Risk for stronger thunderstorms Thursday

SPC`s meso analysis page shows the MU CAPE values upstream near
Chicago of around 100 J/kg. The MU CAPE forecast from the SPC HREF
page indicates southern zones may see values approaching 250 J/kg
tonight. Thus we could see a few showers and perhaps an isolated
storm moving in/developing overhead overnight. Given that the low
levels will be relatively dry to start the night, some of the rain
may initially struggle to reach the ground leading to only
limited qpf. However the atmosphere is expected to saturate by
daybreak. Also a weak 15 to 20 knot low level jet is shown to
reach into southern parts of the CWA after 09z. Thus the heaviest
showers are expected then.

Looking at Thursday, in general the main window for strong to
severe storms is 3 pm to 8 pm. This is when a strengthening mid
level jet moves, especially over the southern zones. Wind speeds
may top 70 knots within this jet core. As the same time, the warm
front will be lifting northward through northern IN. Models vary
as to whether the warm sector makes into the CWA and that will
make a difference as far as how much if any severe weather we see.
Ensemble mean MU CAPE values from the SPC HREF model keeps nearly
all of the CWA under 500 J/kg. The ensemble mean surface based
CAPE tops out at 250 J/kg at the southern CWA border, south of
I-94. Based on this, the severe weather risk is limited for our
CWA. However a slight adjustment northward of this instability
would raise the severe weather risk quickly as the deep layer
shear will be quite favorable for organized convection. For now we
will maintain the high POPs across the region and focus the
elevated thunderstorm risk over southern parts of the CWA. Locally
heavy rain will be possible as the LPMM values from this model
indicate an increased potential for some areas to see more than an
inch.

-Chance of rain Saturday, then colder with snow showers

No major storms are heading toward southwest Lower but there are a
couple of opportunities for some light rain and/or snow.

Saturday night is the first opportunity for precipitation. A cold
front with associated upper trough will drop south Saturday.
Moisture is not abundant along the front, given the placement of the
low over Ontario. However, the southward advance of the upper trough
will help to generate some light rain Saturday. Highest chances will
be over the northern cwa, north of M-46. The cold front will clear
the cwa by 06z Sunday and h8 temps will fall to -8c. Thus, any
precipitation that develops will be light snow.

Another surge of colder air will arrive Sunday night and Monday and
probably generate some light lake effect snow. If there are any
accumulations, the favored areas will be adjacent to the lake shore.
Once the core of the colder moves east, the lake effect threat will
end and we`re mainly looking at dry weather from Monday night
through Wednesday.

Highs in the 50s are expected Friday and Saturday before to 35 to 40
Sunday through Tuesday, then back to the mid 40s Wednesday.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 749 PM EDT Wed Mar 13 2024

This forecast period will start out with VFR conditions and no
impacts. We will see conditions deteriorate overnight and early
Thursday, and persisting through the end of this forecast period.

We currently have mid level cloud cover moving through with some
sprinkles reaching the ground. Lots of dry air to overcome, so no
conditions less than VFR through 04z. We will see showers
increase in coverage after 04z, and become widespread toward
10-12z from south to north. There may be some scattered MVFR in
the heavier showers. We also believe some thunder will become
possible, especially across the southern terminals.

Widespread MVFR and eventually IFR will spread across the region
by 18z Thursday. It will persist through the remainder of the
forecast period. Rain will become more drizzle across the southern
terminals, while steady rain will continue along the I-96
terminals.

&&

.MARINE...
Issued at 233 PM EDT Wed Mar 13 2024

The winds and waves to start the period are weak/low. The flow
will increase tonight and be offshore so that will keep the wave
heights limited. The flow backs to the north Thursday night and
into Friday so the waves will be building, especially across the
southern zones. Small craft advisories will likely be needed
Saturday as the winds continue to back to the southwest and
increase to over 20 knots.

&&

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

&&

$$

UPDATE...Duke
DISCUSSION...MJS/04
AVIATION...NJJ
MARINE...MJS


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