Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201636

Area Forecast Discussion
National Weather Service Grand Rapids MI
1236 PM EDT Sun Aug 20 2017


Issued at 258 AM EDT Sun Aug 20 2017

High pressure will bring one more day of fair weather then chances
for showers and thunderstorms increase early in the week. A cold
front moving in on Tuesday could bring a round of strong
thunderstorms. Fair and cooler weather will follow for the rest of
the week.


.SHORT TERM...(Today through Tuesday)
Issued at 258 AM EDT Sun Aug 20 2017

Two issues to focus on in the near term, first is the cloud
forecast for the partial solar eclipse that will be visible across
west Michigan Monday afternoon and then the potential for severe
thunderstorms on Tuesday.

For Monday, the sfc high is retreating and a broad area of warm
advection is setting up with showers and thunderstorms forecast to
form Sunday night across Wisconsin in area of mid level isentropic
ascent north of a warm front extending from Nebraska to southern
Wisconsin. These storms should dissipate as they move east Monday
morning with the weakening of the low level jet, but additional
storms are expected to fire up by the late morning across Wisconsin
and move east, reaching western zones during the afternoon.

The amount of debris cloud from the overnight convection combined
with high and mid clouds from the second round of storms could
obscure skies across much of Lower Michigan by the afternoon.
It is worth mentioning here is that even thick cirrostratus
clouds wouldn`t necessarily ruin the show as the partially
eclipsed sun may still be visible, but if combined with even a
thin deck of mid level clouds such as a layer of altocumulus could
obscure it.

For Tuesday, flat upper ridging begins to get replaced by an
amplifying trough and impressive height falls for this time of year
occur late Tuesday into Wednesday, along with favorable
positioning of the upper jet which will bring upper divergence
across Lower Michigan. This is attended by strong speed shear and
if there is enough instability Tuesday afternoon, a line of
strong to severe storms could form along or out ahead of the cold
front. The main threat will be strong damaging winds and flooding
rains as P-wats are close to 2 inches.

.LONG TERM...(Tuesday night through Saturday)
Issued at 258 AM EDT Sun Aug 20 2017

The long term portion of the forecast will see relatively quieter
conditions compared to the potentially active weather in the short
term on Mon night and Tue morning.

Drier weather is likely to have arrived before the forecast period
begins on Tue night. The cold front should have moved through the
entire area by then, sweeping all of the rich moisture out of the
area. We will be left with the upper trough lingering over the area.
This, along with a northerly low level flow holding on, will keep
cooler air in place for much of the week after Tue.

We have rolled with a dry forecast for now for Wed and beyond. This
is a cautiously optimistic forecast for now. The upper trough will
linger through next Fri, with the upper jet almost overhead through
the period. Moisture is forecast to be rather limited. This will
limit rain potential for short waves that could approach the area
from the NW. Toward the end of the week, the trough is expected to
get pushed out. This will allow for a moderating trend.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1236 PM EDT Sun Aug 20 2017

Expect VFR conditions to prevail through Monday afternoon into
Monday evening at all of our TAF sites. I strongly feel any
convection will miss our TAF sites to the north and south tonight
so we would get residual layered mid and high clouds. Another MCS
(shown by the ESRL HRRR and HRRR) head toward SW MI during the mid
morning Monday, however it dissipates before getting here. Once
again we get layered mid and high clouds. Just in case I put VCTS
for the MKG, GRR and AZO taf sites after 16z but that would be a
low probability event at best. If there were any precipitation
from that complex it would be sprinkles from high based mid
clouds. So, the bottom line is VFR rules at all of our TAF sites.


Issued at 258 AM EDT Sun Aug 20 2017

Winds and waves will be relatively light through Monday then
pick up Tuesday and Tuesday night with strong thunderstorms
possible at that time.


Issued at 315 PM EDT Sat Aug 19 2017

Our hydrologic focus remains on early this coming week. Confidence
is increasing that Monday into Tuesday could feature locally heavy
rains with a favorable setup for thunderstorm activity across our
region, more so than many of our events that we have had this
summer. Medium range guidance is generally indicating a potential
for 1" or more of rain with this system, with pockets of less than
1" mainly east of US 131. Keep in mind that medium range models are
non-convection allowing so these amounts will likely be locally
underdone. With PWAT values once again approaching 2.00" and
favorable H300 jet divergence / H850 LLJ of 30-40 kts and a sfc
frontal boundary draped across the Lower Peninsula, it is not hard
to envision localized flooding (ie. some water covered roads) with
this system. River responses depend on placement of rainfall and
duration of heavy rains. Small rivers and streams will be most
susceptible to quick rises.




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