Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 181936
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
335 PM EDT Thu May 18 2017

LATEST UPDATE...
Synopsis/Short Term/Long Term/Marine

.SYNOPSIS...
Issued at 335 PM EDT Thu May 18 2017

A cold front will progress south of the area this evening and end
the chances of rain for the night. The front will become stationary
across portions of Illinois, Indiana and Ohio on Friday before
returning north as a warm front on Saturday. Friday and Friday night
will be mainly dry but there may be a few stray showers in southern
portions of the state. Rain chances increase substantially for
Saturday into Sunday as a low pressure system lifts out of the
plains and into the Great Lakes. Showers and storms are possible
over the weekend as this low moves through. Early next week
seasonable temperatures and scattered showers will be common as the
pattern remains unsettled.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 335 PM EDT Thu May 18 2017

Prospects for precipitation are small this evening as a westerly
wind off the lake has swept the deeper moisture east and south of
the area. Surface dew points have fallen off into the middle 50s
which has diminished CAPE values considerably. Most areas have seen
CAPE values dip below 500 J/KG, the exception being down in the
southeast towards Jackson. Have some small pops in the forecast for
a stray shower to the north and a chance of a shower or a storm
towards Jackson through about 8pm or 9pm.

Otherwise expecting partly cloudy conditions tonight with some lower
stratocumulus trying to thicken up behind the frontal passage.

A low moving our direction from the plains may spread some light
rain showers into the area Friday afternoon into Friday night with
the bulk of the precipitation holding off until over the weekend.

Showers and a few embedded thunderstorms are expected from Saturday
into Saturday night as the low moves up through the Western Great
Lakes. The heaviest precipitation is expected from Saturday
afternoon into Saturday night in the short term period. 60-80pct
pops are in the forecast during these periods. Rainfall totals of a
half inch to and inch are quite possible in most areas.

.LONG TERM...(Sunday through Thursday)
Issued at 335 PM EDT Thu May 18 2017

An active weather pattern will bring wet conditions to the area.
There will be a couple of chances for thunderstorms with much cooler
air settling in through the middle of the week.

A cold front will extend south of a surface low, which will be
situated over western Lake Superior Sunday morning. Some leftover
convection from Saturday night`s activity over Illinois will likely
be pushing through early Sunday morning. The front then swings
through the area from mid-morning to early afternoon. Instability is
marginal with LIs around -4 and 500-700 J/kg of CAPE. The front
should clear the area by early afternoon with only showery activity
behind the upper low into the day Monday.

The next features will be in the form of a shortwave developing on
the back side of our original upper low. This wave will hinder much
of Lower Michigan through next week. As the shortwave rounds the
western side of the upper low, the polar jet digs south, eventually
merging with the subtropical jet. A 140 kt jet core will allow the
shortwave to deepen into a closed upper low, as it curves around the
southern end of the Great Lakes.

Showers will begin to develop Tuesday with the better probability
for thunderstorms Tuesday afternoon. At least the northern half of
the state will see better chances for shower activity continue into
Wednesday. At the same time, cooler air spills in from the north.
850 temps Wednesday into Thursday may barely be above freezing.
Cloudy, wet weather may have high temperatures struggling to get out
of the 50s Wednesday with low 60s Thursday. Normal highs are in the
upper 60s to lower 70s... so it will feel less like late spring, to
say the least.

The system will rotate off to the northeast for Thursday. While low
POPs are in the forecast, dry conditions should win out a majority
of the time. Another shortwave develop on the western periphery of
this low, but should hold off until Friday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 141 PM EDT Thu May 18 2017

West and southwesterly gusts of 35-40 kts have been observed
throughout the area. Gusty conditions will continue to be an
issue through the remainder of the afternoon. Cigs look like less
of an issue with VFR/MVFR through the period.

A cold front that is currently over eastern Wisconsin will push
eastward into Lower Michigan this evening. A few showers are
possible along the front. However, the environment does not look
favorable for development. So, any shower activity would likely be
spotty and brief... hit-or-miss. Winds will shift out of the
northwest behind the front with gusts of 20-25 kts possible
through early evening. Wind is expected to slow to 10 kts or less
after sunset, becoming northerly into early Friday.

&&

.MARINE...
Issued at 335 PM EDT Thu May 18 2017

Small Craft Advisory remains in effect through tonight, before the
flow goes east/off shore. There will be a brief lull in the winds
and waves early this evening in the slack gradient near the front,
but north winds and larger waves pick up considerably after sunset.
Waves should peak in the middle of the night in the 4 to 6 foot
range.

A quieter period on the lake is expected from Friday morning into
Friday night. The next system quickly affects the lake however with
an increasing easterly wind late Friday night into Saturday. Winds
may be strong enough on Saturday especially to go with a Small Craft
Advisory even with the off shore flow. Winds will likely touch the
20-30 knot range at times Saturday.

&&

.HYDROLOGY...
Issued at 1241 PM EDT Wed May 17 2017

Streamflow is currently normal for most sites. Although a few
storms may move through the region through Thursday, they won`t be
widespread enough to impact the main basins. Still looking at
basin average rainfall amounts this weekend of 0.50"-1.00". Can`t
rule out some locally higher amounts. That shouldn`t be enough
rain to cause more than some within bank rises, though above bank
rises can`t be ruled out. Additional rainfall is possible by the
early and middle portions of next week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory until 8 PM EDT this evening for MIZ040-044>046-
     050>052-056>059-064>067-071>074.

LM...Small Craft Advisory until 6 AM EDT Friday for LMZ844>849.

&&

$$

SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...JAM
AVIATION...JAM
HYDROLOGY...Hoving
MARINE...Duke



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