Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201718

Area Forecast Discussion
National Weather Service Grand Rapids MI
118 PM EDT Thu Jul 20 2017


Issued at 330 AM EDT Thu Jul 20 2017

A warm and humid period will continue into the weekend as a frontal
boundary meanders across the Southern Great Lakes region. Periods of
showers and storms can be expected from this morning right through
the weekend. Highs the next couple of days will be well into the 80s
if not near 90 in spots.


Issued at 1014 AM EDT Thu Jul 20 2017

We will issue a fcst update to remove mention of thunderstorms
from the fcst late this morning/today and to lower pops. Will
only hold onto relatively higher pops for showers for our extreme
ssw fcst area where showers will linger until around 15Z late this
morning before moving south of our fcst area.


.SHORT TERM...(Today through Saturday)
Issued at 330 AM EDT Thu Jul 20 2017

Main concern in the short term is what has been the concern the last
couple days, coverage and intensity of thunderstorms. At the
surface, a frontal boundary will sag south of the area this
afternoon. The front remains south tonight and Friday, approaches as
a warm front from the southwest Friday night and potentially moves
into the area on Saturday with an approaching low pressure system.

The main time frames for storms looks to be this morning and again
Friday night into Saturday. These are the time frames when mid level
shortwave troughs affect the area and push low level jets through
the region.

Still expecting a push of showers and storms into the area early
this morning between 10z and 15z most likely. The short range models
all still advancing a low level jet into the region and its rare
that a formidable low level jet does not produce precipitation in
these regimes. At this point not expecting severe storms given
somewhat limited instability.

This afternoon, there is some question as to where additional storms
will fire based on where the front is located and how this morning`s
precipitation affects the instability. Cannot argue with the day one
outlook at this point with a Slight Risk to the south/east of GRR.
One negative factor of note however is that the low level jet peels
away this afternoon which is never a good thing when expecting

Tonight into Friday we may be somewhat of a lull as the front sags
south into IL/IN and OH. Southern portions of the forecast area
towards I-94 may see some storms however.

The next focus is on the Friday night and Saturday time frame as the
front tries to lift back north as a warm front. On Saturday there is
varying placement of a low that moves towards Lake Michigan.
Thinking Friday night into Saturday morning may be the most likely
time frame for additional storms as the nocturnal low level jet is
overhead. Severe weather chances are tough to determine given how
earlier convection may affect this time frame.

.LONG TERM...(Saturday night through Wednesday)
Issued at 330 AM EDT Thu Jul 20 2017

The heavier rain potential looks like it should be mostly done by
the beginning of the long term on Saturday night. The weather does
look to remain unsettled into Monday before it quiets down quite a
bit from later Mon through the end of the period on Wednesday.

The series of sfc lows will be mainly east of the area by Saturday
night. A northern stream upper trough will drop over the area and
keep conditions unsettled through Sunday with short waves moving
through. A small chance will linger into Monday as the trough
remains close enough. This trough and NW flow aloft will bring cool
temps across the area, especially on Mon.

We are looking for mainly dry conditions along with warming
temperatures for Tuesday and Wednesday next week. Upper heights will
build as lows over the Northern Pacific and Gulf of Alaska dig in
over the Western U.S.. Temperatures will warm to well back into the
80s for highs.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 112 PM EDT Thu Jul 20 2017

Overall the risk for storms today is somewhat diminished because
of the convection earlier today. The atmosphere will still be
unstable and a local shower or thunderstorm could develop. Based
on the limited potential...I did not included any storms in the
TAF forecast.

Fog and Stratus could become an issue tonight. Models have been
showing the low level humidity values increasing later tonight
into early Friday. The wind will be relatively weak...thus any low
conditions that develop may be slow to diminish


Issued at 330 AM EDT Thu Jul 20 2017

Wake low winds have moved through southern zones early this morning
with winds in the 30-35 knot range. Waves have briefly spiked up
this morning to around 4 feet. Have opted not to issue a Small Craft
Advisory at this point though given winds are already dropping, but
we will be monitoring. A low level maxima of wind will move through
this morning as well around 12z to 15z. If these winds prove to be
stronger than thought, which is possible, we may need to issue a
headline. After 18z, winds should be fairly light through Friday.


Issued at 330 PM EDT Tue Jul 18 2017

Although locally heavy rain is likely with thunderstorms moving
through the region into Thursday morning, the progressive nature of
the storms will limit the flood potential. What these storms may do
is set the stage (via antecedent conditions) for a possibly bigger
hydrologic event Friday night into Saturday. It is possible that PW
values will be near the maximum July values that have been recorded
going back to the 1940s (reference: SPC sounding climatology page
for DTX). Those maximum values are in the 2.00"-2.25" range. GFS,
GEM, and ECMWF guidance are strongly indicating a large area of over
2.00" PW for southern and central Lower MI for approximately a 24
hour window from Friday night into late Saturday, with some
solutions indicating over 2.25". This would equate to high rainfall
rates within any convection and an increased flood potential, at
least on local scales.

In terms of river flood potential, the main stem rivers would need
quite a bit of rain to go above flood stage, based on some ensemble
QPF analysis for the next 72 hours. Smaller rivers and streams would
be more prone to reaching flood stage with this setup. Still,
depending on where the storms move, the larger basins could be at
risk for considerable rises and possible flooding at certain points.




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