Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 251928

Area Forecast Discussion
National Weather Service Grand Rapids MI
328 PM EDT Fri May 25 2018

Synopsis/Short Term/Long Term/Marine/Hydro

Issued at 326 PM EDT Fri May 25 2018

A weak area of low pressure will provide a chance for showers and
thunderstorms tonight and Saturday. Locally heavy rain and
lightning will be the main threats from storms that develop. Areas
of fog will also develop near Lake Michigan tonight and Saturday
morning. This fog may become dense. Very warm temperatures will
continue through the Holiday weekend, with highs reaching the mid
80s to low 90s. Temperatures will be cooler near Lake Michigan.


.SHORT TERM...(This evening through Sunday night)
Issued at 326 PM EDT Fri May 25 2018

Potential for convection, fog, and building heat will be the main
concerns in the short term.

Convective forecast for tonight remains a little challenging as we
watch several smaller features. For the evening, we will be
watching convection over Wisconsin, particularly that which is
trying to develop along remnants of outflow boundaries over
eastern Wisconsin and Lake Michigan. If anything survives the
journey over the lake, it could bring a quick shower or
thunderstorm north of Muskegon this evening. For the overnight
hours, an upper shortwave that pushed from KS into MO will lift
northeast into the Ohio Valley and far southern Michigan late
tonight and Saturday morning. Shot of lift and cooler air aloft
(steeper mid-level lapse rates) will help provide a chance for
showers and a few thunderstorms overnight, mainly south of I-96.
Models have been keying into into this feature more with runs
today, and a boosting of overnight pops into the chance category
was one of the biggest changes to the forecast for the short term.
If we can get a thunderstorm going, rain may be locally heavy due
to light steering flow and high PW values.

For Saturday, SB CAPE will build with heat and increasing
humidity, but unsure how widespread activity can become given lack
of a focusing mechanism. May be mainly reliant on small-scale
boundaries such as lake breezes (more likely over the eastern part
of the state) and outflow from other thunderstorms. Will keep
highest potential over the eastern portion of the forecast area
where upper shortwave passing through tonight will keep mid-levels
cooler. ML CAPE of 1000+ J/kg could allow for a brief strong
updraft, but shear values through the late afternoon do not look
high enough for much storm organization. Main threat will be heavy
rainfall with light winds aloft and PW values of 1-1.5 inches.
Quieter weather is expected on Sunday as high pressure begins to
rebuild over the area.

Fog may also become an issue near Lake Michigan both
tonight/Saturday morning and Saturday night/Sunday morning. Higher
dewpoints advecting in over the cold waters will allow fog,
possibly dense, to form over the lake. South to southwest flow may
then push inland and impact areas near the lake.

Temperatures will continue to run well above normal. Fog and
increased clouds tomorrow should keep temperatures in the mid 80s
tomorrow (cooler near lake). High pressure and increased sunshine
should then allow temps to warm to near 90 on Sunday.

.LONG TERM...(Monday through Friday)
Issued at 326 PM EDT Fri May 25 2018

Well above normal temperatures are expected for much of the period.

Models are in relative agreement in showing a mid level ridge across
Lower MI to start the forecast period.  Subsidence associated with
this feature will act to keep the weather relatively dry for Monday
and into Tuesday. The low level thermal ridge is shown to be located
over Lower MI on Monday. weather looks increasingly
likely. This thermal ridge is  shown to shift south of the area on
Tuesday...thus high temperatures are expected to end up lower but
still above normal.

Next..deep moisture will be streaming in from the south Wednesday as
the mid level ridge is shown to shift east of the area.
Lift...associated the the passage of a trough Wednesday into
Thursday is forecasted to trigger some showers and storms.  Another
ridge is shown to push in from the west by Friday which would
support less potential for showers.  If this feature slows
down...Friday could feature additional showers and thunderstorms.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 126 PM EDT Fri May 25 2018

Uncertainty with the aviation forecast revolves around the
potential for convection and for risks.

The atmosphere continues to destabilize upstream in WI and IL.
Thunderstorms will likely be developing in those regions before
heading toward Southwest Lower MI tonight. The thought is that the
atmosphere will generally stabilize as we go through the night.
Will feature the risk for thunderstorms around KMKG...then VCSH
for the remainder of the region tonight. A better chance for
storms exists Saturday...but that will likely require the
atmosphere to destabilize again...which should happen after the
18z end time.

With low level moisture on the increase...the potential for fog to
develop over Lake MI rises through the night. Will feature
visibilities being reduced at KMKG during the day. Will need to
monitor the IFR potential closely.


Issued at 326 PM EDT Fri May 25 2018

Winds and waves will not be high enough to pose a threat to
mariners over the Holiday weekend. Areas of fog are expected to
develop the next few nights and mornings over and near Lake
Michigan, however. This fog may become dense and significantly
restrict visibility. An advisory may be needed.


Issued at 326 PM EDT Fri May 25 2018

Rivers continue their decline across the region. Any weekend
rainfall is not expected to reverse this trend. The Grand River at
Ionia should return within banks by late tonight or Saturday.
Otherwise, all other rivers should be within their banks for the
entire weekend.




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