Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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634
FXUS63 KGRR 281600
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1200 PM EDT Sun May 28 2017

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 330 AM EDT Sun May 28 2017

Showers and thunderstorms are likely this afternoon and evening
across Southwest Lower Michigan. A few storms may reach severe
levels with large hail and damaging winds. The best chance for
storms will be this afternoon for areas along and east of U.S,
Highway 131. The region will become more influenced by an upper low
over the next couple of days with a fair amount of cloudiness and
chances for showers especially during the afternoon and evening
hours of Memorial Day and Tuesday.

&&

.UPDATE...
Issued at 1200 PM EDT Sun May 28 2017

Forecast looks fine, with only minor tweaks made. Thunderstorms
and showers will stay over the NW CWA for the next few hours, then
the cold front will begin to sweep them east. A fairly solid band
of showers and storms should then traverse the CWA between 18Z
and 00Z.

CAPE values as of 15Z were still below 1000 J/kg, so the severe
threat is minimal at this point. However instability will continue
to climb with fairly clear skies SE of a Grand Haven to Mt.
Pleasant line. Some of this area will likely see CAPE readings
climb toward 2000 J/kg by mid/late afternoon. Expect the line will
intensify into the afternoon, along with additional cells
developing ahead of it.

0-6km bulk shear values will be around 40 knots over the SE CWA,
which should support organized cells. However the 0-1km shear is
in the order of about 10 knots, which is quite low. Seems the
tornado threat is very low, unless we happen to see a right-
turning super cell develop. So wind, hail and lightning will be
the primary threats.  The left exit region of the upper jet come
overhead this afternoon, which will help maintain the showers and
storms, however a solid low level jet is missing, with the front
mainly working with it`s own lift/convergence.

Bottom line, we will continue to see showers and storms increase
through the afternoon. A few storms could become severe between 1
and 8 PM, but a significant outbreak is not likely. This said
however, folks enjoying outdoor activities this holiday weekend
will also need to heed the risks from lightning, and take cover as
storms approach.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 330 AM EDT Sun May 28 2017

Main short term focus is on the chances for thunderstorms today and
the threat for severe weather. Otherwise, we become under the
influence of an upper low for Memorial Day and Tuesday.

Perimeters seem to be coming together today to make showers and
thunderstorms likely with the chance for severe weather as well.
Main feature to key on is a compact, potent shortwave seen in the
water vapor imagery over Minnesota. This shortwave is making steady
progress our direction and timing tools bring it into Western Lower
Michigan near peak heating. So, convection will get a synoptic scale
boost despite cloud cover which may be present. Next feature of note
today is a weak surface low centered in Southern Illinois this
morning. The low lifts northeast through the day moving through
southern portions of the forecast area this afternoon. Low level
convergence aided by the low and its weak fronts may tend to focus
convection, especially from Grand Rapids to the south and east.

Feel the last few runs of the HRRRx have had too little in the way
of CAPE, with the 3km NAN being more realistic. The simulated
reflectivity off of the 3km NAM over the last few runs has
consistently shown storms firing midday in the 16-18z time frame and
gradually sweeping to the east during the afternoon. CAPE values are
forecast to reach the 1000-2000 J/KG range much of this owed to
cooling in the mid levels via the approaching shortwave. Any breaks
that occur resulting in diurnal heating will only be a bonus to the
convection. The 850mb LLJ is lacking, but that is made up for with a
40-50 knot mid level jet at 500mb`s.

Bottom line, expecting scattered storms to develop in the early
afternoon becoming more numerous in the 18z to 21z time frame before
sweeping east towards evening. Severe weather appears possible given
decent instability and deep layer shear values on the order of 30-40
knots. Isolated large hail and damaging winds will be the main
threats. Feel much of the area has a possibility of severe weather
with the highest likelihood being from the Grand Rapids metro area
to the east and south. These areas will have the best moisture
return at the surface and be in closest proximity to the surface low.

Beyond this evening, we move into our typical Memorial Day upper low
regime it seems. Memorial Day into Tuesday we are looking at a fair
amount of cloud cover associated with the upper low with diurnally
driven showers. So, the showers and even an isolated thunderstorm
will be concentrated in the afternoon and evening hours of both
days. High temperatures will cool slightly from the 70s today, to
near 70 tomorrow and into the 60s for Tuesday.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM EDT Sun May 28 2017

A deep upper low will swing through the Great Lakes Tuesday night
and Wednesday. This will bring cooler air with highs in the mid 60s.
The best chance of rain will likely come toward the end of the week
and the weekend. A cold front is progd to move south from Canada
Thursday night and the GFS and ECMWF show a wave developing on the
western flank over the eastern Dakotas. Showers/storms are possible
as the cold front moves through the cwa Friday and then again
Saturday as the wave moves east.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 745 AM EDT Sun May 28 2017

VFR weather is expected this morning. Aviation conditions will
deteriorate for a time this afternoon. Showers and thunderstorms
will become more numerous this afternoon between 17z and 23z
across the TAF sites. Locally, MVFR conditions are possible as the
storms roll through any given TAF site, with some IFR for a short
time as well. Conditions should pop back to VFR this evening
after about 23z, which should persist through the night.

A few strong storms are possible today with hail and winds in
excess of 40 knots possible.


&&

.MARINE...
Issued at 330 AM EDT Sun May 28 2017

Winds and waves will remain below small craft advisory criteria
today and tonight. The gradient tightens a bit though into Memorial
Day and Tuesday and we may see waves on the increase. Winds in the
lowest few thousand feet are in the 15-30 knot range over the lake
out of the west.


&&

.HYDROLOGY...
Issued at 210 PM EDT Sat May 27 2017

River levels in the upper Grand and the Kalamazoo basins are running
above normal for late May while elsewhere the levels are closer to
normal. Rain totals around a half inch on Sunday will likely not be
enough to produce flooding. Occasional showers over the upcoming
week will be rather light.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

UPDATE...JK
SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...04
AVIATION...Duke
HYDROLOGY...CAS
MARINE...Duke



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