599
FXUS63 KGRR 221753
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
153 PM EDT Sun Sep 22 2024

.KEY MESSAGES...

- Showers and isolated storms this afternoon, then dry and cooler

- Rain chances Monday Night through Wednesday

- Model differences in latter half of work week

&&

.UPDATE...
Issued at 1050 AM EDT Sun Sep 22 2024

The rain is moving in and the hourly POP grids were adjusted to
account for the slightly quicker eastward movement, along with
the band of warm advection showers currently extending across the
northern forecast area.

Extensive cloud cover across the forecast area should reduce
heating/instability and thus the chances of strong convection,
but some isolated storms or line segments are still possible
especially across the eastern zones.

&&

.DISCUSSION...
Issued at 339 AM EDT Sun Sep 22 2024

- Showers and isolated storms this afternoon, then dry and cooler

A cold front is slowly moving east across central WI early this
morning. Showers and thunderstorms continue to develop ahead of
this front and we`ll likely see rain this afternoon as the front
moves over the cwa.

Based on forecast soundings, instability is the lacking
ingredient this afternoon. LI`s are largely above 0c and CAPE
values are generally a couple of hundred joules/kg as the front
moves through. There is good moisture convergence along the front
thanks to a 40 kt LLJ and precipitable water values that climb to
near 2 inches. Shear values are decent, though, at 40 kts. We`ll
continue with the idea of most locations seeing rainfall. Roughly
a quarter to half inch of rain is expected before the front moves
through and precipitation moves east.

RH progs suggest that clearing will occur from the northwest late
tonight and spread across the cwa Monday. After highs in the mid
70s today, highs will be around 70 Monday.

- Rain chances Monday Night through Wednesday

Shortwaves in both the southern and northern streams will converge
on the Great Lakes in the early to mid week time frame. A surface
low will move into the area on Tuesday lingering in the area Tuesday
night. Deeper moisture can be seen on GFS Bufkit overviews from
Monday night through much of Wednesday. We have 30-60 pct chances
for precipitation (highest south) as the low moves through the area.
PWAT values reach the 1.00 to 1.50 inch range with the low, so
precipitation amounts could be locally towards 1.00 inch, but most
areas will stay well below that.

- Model differences in latter half of work week

Differences between the ECWMF and GFS begin to show up Wednesday
night in terms of how the upper pattern evolves. The GFS has more in
the way of ridging through the Thursday through Saturday time frame,
while the ECMWF has an upper low nudging our direction from the mid
Mississippi valley. Bottom line at this point is we will feature dry
weather for much of the Wednesday and Thursday time frame. Model
blends give us isolated chances for rain Friday and Saturday. We
will roll with that at this point, as confidence is low in the most
likely solution at this point for day 6 and 7.

Temperatures will be near normals early in the week with the clouds
and precipitation, warming to above normal once again by the end of
the week. 70s for highs look likely Thursday and Friday.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 153 PM EDT Sun Sep 22 2024


Showers have begun as rainshowers are overspreading the region
from the west to the east. Periods of heavy showers have brought
predominately MVFR cigs and vsbys with periods of IFR cigs and
vsbys to MKG and BIV. Expect cigs and vsbys to continue to lower
through the afternoon and into the early evening hours. As the
cold front swings east through the TAF sites today there remains
some chance for isolated thunderstorms, mainly 20Z to 00Z with
the best chances at the southern TAF sites. IFR and LIFR
conditions will become the main aviation weather category between
00z and 06z tonight.
 Cigs will slowly improve to MVFR tomorrow morning between 11Z at
15Z with VFR predominate after 16Z.

&&

.MARINE...
Issued at 339 AM EDT Sun Sep 22 2024

North winds will increase behind the cold front today. Wave progs
indicate waves will increase to 4-7 feet north of Whitehall this
afternoon. The higher waves will work south with time. We issued a
Small Craft Advisory and Beach Hazards Statement from this
afternoon through Monday morning for the hazards.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through Monday morning for MIZ037-043-
     050-056-064-071.
MARINE...Small Craft Advisory until 11 AM EDT Monday for LMZ844>849.

&&

$$

UPDATE...Ostuno
DISCUSSION...Duke/04
AVIATION...Duke/Ceru
MARINE...04