Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211756
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
156 PM EDT Sun May 21 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 302 AM EDT Sun May 21 2017

A cold front will push eastward through the region through early
afternoon. A few showers and thunderstorms may accompany the
passage of this feature. A weak area of high pressure tracks up
the Ohio Valley for tonight and Monday diminishing the risk for
precipitation. Later Monday another low pressure system moves in
our direction...and then tracks through the state on Tuesday. This
will result in wet weather.

&&

.UPDATE...
Issued at 727 AM EDT Sun May 21 2017

With the convection more widespread ahead of the front I did
update the forecast to reflect this. Looks like the front will
continue advancing eastward through the CWA...so by early
afternoon the storms should be east of Lansing and Jackson.
Some of storms could have gusty winds and small hail.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 302 AM EDT Sun May 21 2017

Will need to monitor the thunderstorm potential later this morning
and into the early afternoon. If we do see decent heating ahead of
the cold front... storms could accompany the passage of the front.
Wind shear values are rather strong so organized convection could
develop. However...it looks like it will stay fairly
cloudy...limiting this potential. As the wind veers this morning
with the front moving in...low clouds and fog could occur. So I
added this to the forecast mainly for western locations. Strong
subsidence moves in behind the front this afternoon quickly
diminishing any remaining showers.

The upper low tracks through Lake Superior and into Ontario
tonight. Some wrap around moisture could result in a few showers
north later tonight.

For Monday a shortwave digs down the backside of the departing
low...resulting in a surface wave that develops upstream on the
frontal zone over in Iowa and Illinois. This surface low then
moves in for Monday night into Tuesday. Timing the onset of the
precipitation is uncertain. Some high res models are trying to
trigger thunderstorms as soon as late afternoon on Monday. For
now I will add a low risk for showers to the late afternoon Monday
forecast...then increase the POPs Monday night into Tuesday.

.LONG TERM...(Tuesday night through Saturday)
Issued at 302 AM EDT Sun May 21 2017

Another upper low is progged to be over the Lower Great Lakes into
the middle of the week.  This will keep the area cool and showery. I
added thunder to the Tuesday night forecast and bumped up POP as
this appears to be the period when the surface low comes through
with solid lift and somewhat of a low level jet. With more of a
northwest flow behind the system into Wed the thunder risk drops off.

The showers should continue through at least Wed eve as the low
heads northeast into Canada, then the pcpn appears to lessen over SW
MI into Thu, but we may still hang on to a couple showers with cool
air aloft producing isolated diurnal showers.

Warmer air finally appears to move into the area by Friday night and
into Saturday with upper ridging developing and a southerly flow at
the surface.  Both the GFS and ECMWF show a short wave running into
the upper ridge, so there is the outside chance of a thunderstorm or
two Friday night/Saturday.  However a pattern change, with warmer
temps and less rain seems in the offering by the end of the week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 155 PM EDT Sun May 21 2017

Showers and isolated tstms are now exiting to the east with a
cold frontal passage. Rapid improvement of cigs, from IFR/LIFR to
MVFR/VFR, takes place behind the front along with a wind shift to
westerly.

For tonight more clouds rotate in from the west with more MVFR
cigs possible. Those clouds lift out of the area on Monday
morning, leading to VFR but breezy conditions for the rest of the
day. Winds on Monday will be from the southwest at 15-25 kts.

&&

.MARINE...
Issued at 302 AM EDT Sun May 21 2017

The cold front tracks through the nearshore waters this morning.
The wind will shift resulting in building waves. Right now it
looks like conditions will remain below criteria through tonight.
Fog may have impacts this morning...as abundant low level moisture
flow in over the colder waters.


&&

.HYDROLOGY...
Issued at 1005 AM EDT Sat May 20 2017

Rivers levels have returned to near normal. Expecting rain totals
ranging from a half inch in Central Lower Michigan to around an inch
in Southern Lower Michigan today through Sunday with locally higher
totals possible. Additional precipitation is possible Monday night
through Friday. Some river rises are likely with this rain, but we
are not expecting flooding at this time. Smaller streams and rivers
may need to be watched for above bankfull rises. Main stem rivers
should be able to handle this rainfall fine unless guidance trends
upward.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory from 8 PM this evening to 10 PM EDT Monday
     for LMZ844>849.

&&

$$

UPDATE...MJS
SYNOPSIS...MJS
SHORT TERM...MJS
LONG TERM...JK
AVIATION...Meade
HYDROLOGY...63
MARINE...MJS



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