Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 230612
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
212 AM EDT Fri Jun 23 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 300 PM EDT Thu Jun 22 2017

A cold front will continue to advance southeastward through the
Great Lakes region and result in development of showers and
thunderstorms across lower Michigan tonight. A few showers and
thunderstorms will linger into Friday morning. A much cooler
airmass will move in for this weekend into early next week.

&&

.UPDATE...
Issued at 958 PM EDT Thu Jun 22 2017

Flood risk has been increasing tonight. The combination of a very
moist airmass...slow moving front...instability and upper level
divergence exists tonight. The 00z sounding from DTX has lots of
instability and PWAT values around 1.75 inches. GRB has deep
moisture and thin deep cape along with low LCL`s. Thus efficient
rain will occur with the passage of this front. In addition a low
level jet is shown to strengthen overhead tonight. So plenty of
ingredients exist overnight.

The convection that we had earlier this evening generated some
rainfall amounts over 1.5 inches leading to saturated soils.
Highest amounts were over Allegan county. Another swath of heavy
rain was just south of Mount Pleasant.

Flooding is not certain to happen. Some high res models diminish
the convection as it moves in...while others keep it vigorous.
Given the unusual setup will go with a watch as there is an
increased potential for flooding

UPDATE Issued at 626 PM EDT Thu Jun 22 2017

I increased POPs for this evening based on radar trends. Lots of
low level convergence and instability over the region so the
storms should continue to increase. Plus it looks like an MCV
lifting out of IA. Zones and Nearshore were updated.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 300 PM EDT Thu Jun 22 2017

Primary short term concerns involve determining convective/svr wx
potential tonight.

At 18Z the sfc wx map shows the cold front extends from near KMQT
southwestward across northwestern WI to near KMSP. The cold front
will continue to advance southeastward and result in convective
development across our fcst area tonight. Before that occurs an area
of mainly light rain showers over central Lake MI will continue to
advance eastward into our nw/nrn fcst area during the next couple of
hours.

Isolated convection may develop very late this aftn/early this
evening. However there really is not much of a forcing mechanism for
convective initiation to occur until around mid to late evening
through the overnight period along and out ahead of the cold front.

Some strong to potentially severe storms are possible from mid
evening through the overnight period as a result of forcing from the
front/llj and deep moisture and strong deep layer shear on the order
of 40-55 kts. We expect a line of strong to potentially severe
storms to move into our far nw fcst area by around 01Z-02Z as
suggested by latest 3km nam high res sim z guidance. The line will
gradually advance to the southeast across most of our fcst area
overnight.

Although a few storms may become severe there are several factors
going against severe potential including unfavorable cold frontal
timing and lack of stronger instability. Upper level
forcing/dynamics are also unfavorable as strong pva does not move
in until tomorrow after fropa has already occurred.

Some heavy rainfall is likely tonight with ample low level
moisture in place out ahead of the cold front with precipitable
water values tonight in the 1.75 to 2 inch range along with
vigorous 1000-850 mb moisture transport. A consensus of short
range guidance qpf amounts and our fcst rainfall amounts are not
sufficient enough to warrant an areal flood watch headline.
However some localized flooding is certainly possible in areas
that receive the most rainfall.

Showers and a few storms will linger Friday morning especially
over our southeastern fcst area before a cooler and drier airmass
begins to advect in after fropa. A few instability showers are
anticipated Saturday as a result of daytime heating and with the
pool of cooler air aloft under the upper trough axis. Several
upper disturbances rotating around the base of the trough axis
will enhance the potential for a few showers from time to time.

.LONG TERM...(Sunday through Thursday)
Issued at 300 PM EDT Thu Jun 22 2017

Sunday and Monday will see cooler than normal weather with diurnal
enhancement of showers and a few thunderstorms as upper level
troughing brings steep lapse rates. No severe weather is expected
although the storms could bring small hail, especially on Monday
afternoon as freezing levels are unusually low.

The trough axis moves east by Tuesday with shortwave ridging
returning Wednesday and Thursday along with temperatures warming to
near normal.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 212 AM EDT Fri Jun 23 2017

A cold front stretches from a little north of KMKG to near KMOP at
06z. Showers and thunderstorms are located along and north of the
front. The front will sag south through the night and be near the
I-94 TAF sites in the 12z to 15z time frame. MVFR ceilings will
become more widespread through the remainder of the night and
possibly even dip into the IFR category towards 12z. Showers and
thunderstorms will gradually drop south through the area between
06z and 12z. The showers and storms should be pushing south and
east of the area around 15z or so. After 18z, VFR weather is
expected.

&&

.MARINE...
Issued at 300 PM EDT Thu Jun 22 2017

Thunderstorms will pose a hazard to mariners tonight with wave
heights in the two to four foot range. Wave heights will remain
below small craft criteria Friday.


&&

.HYDROLOGY...
Issued at 300 PM EDT Thu Jun 22 2017

Locally heavy rainfall and perhaps minor areal flooding appears
possible Thursday evening/overnight. PW values will rise over 1.75
inches this evening as sharply higher-dewpoint air at 850 mb is
advected from the south. These PW values are in the range of
daily records for June in the DTX-area sounding climatology. Higher
moisture content in the column of atmosphere will favor more
efficient rain production. With a tendency for storms to repeat over
parts of the area tonight, it seems reasonable that some locations
could receive over 2 inches of rainfall within a few hours. The most
favored area for significant rain totals Thursday night appears to
be north of I-96.

The tendency for flooding to develop will depend on local antecedent
conditions. While May and early June were rather dry, last week`s
rainfall ranged from 1.5 to locally over 5 inches in areas north of
South Haven, Hastings, and Lansing. In addition, areas along and
north of M-20 saw between 0.5 and 1.5 inches of rain Thursday
morning. Places that saw heavy rain last week will be more prone
to having at least minor areal or street flooding. As for rivers,
the the Flat near Smyrna and the Maple near Maple Rapids are running
high after last week`s rain, and will be less tolerant of any
additional heavy rain that falls within their basins. Above bankfull
rises are possible, so people with interests along those rivers
should monitor the situation.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch until 6 AM EDT early this morning for MIZ037>040-
     043>046-050>052-056>059-064>067.

LM...None.
&&

$$

UPDATE...MJS
SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...Ostuno
AVIATION...Duke
HYDROLOGY...CAS
MARINE...Laurens


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