Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 191908 CCA

Area Forecast Discussion...Correction
National Weather Service Grand Rapids MI
300 PM EDT Wed Apr 19 2017
...Correction for typo...

Synopsis/Short Term/Long Term/Marine

Issued at 300 PM EDT Wed Apr 19 2017

A northward moving warm front will bring showers and thunderstorms
tonight. More showers and thunderstorms may redevelop Thursday
morning and early afternoon along and just ahead of a cold front.
A much cooler and drier airmass will move in behind that front
Thursday night through Saturday.


.SHORT TERM...(This evening through Friday night)
Issued at 300 PM EDT Wed Apr 19 2017

Primary short term fcst concerns involve determining convective
potential late this afternoon through mid afternoon Thursday.

At 1830Z showers are moving across far southern lake Michigan and
also developing over southern lower MI south of I-96 out ahead of a
weak mcv. Instability should increase a bit from mid to late
afternoon out ahead of this feature. Therefore showers will continue
to develop late this afternoon south of I-96.

In addition to showers a few thunderstorms may also develop over
that area as instability increases a bit. The relatively best chc
for convection late this aftn will be over our far south/se fcst
area near to south of a line from KAZO to KJXN where relatively
stronger instability will develop. No severe wx will occur late
this afternoon and early evening.

Widespread rain and convection will develop late this evening and
overnight over our northern forecast area mainly north of I-96 as
1000-850 mb moisture transport ramps up significantly. The llj will
also increase as does deep layer shear. Some strong to potentially
severe storms are possible overnight with large hail being the
primary svr wx threat with the elevated convection. Any storms could
bring locally heavy rainfall as well.

Sfc based convection may develop along and just ahead of the cold
front Thursday morning through mid afternoon. Plenty of moisture
will be in place just ahead of the front with forcing from the
cold front and potential for an axis of sb cape values to reach
1000-1500 j/kg just ahead of the cold front. Deep layer shear is
strong so there is potential for a few strong to potentially
severe storms through mid afternoon.

The relatively best chc for severe wx will be over our se fcst
area just ahead of the cold front from around 17Z-20Z where weak
to potentially moderate instability could develop just ahead of
the front. This notion is supported by 12z nssl wrf sim z
guidance. However the severe wx potential is mitigated by
uncertainty about instability as extensive morning clouds/showers
would keep instability in check.

.LONG TERM...(Saturday through Wednesday)
Issued at 300 PM EDT Wed Apr 19 2017

The next major storm will likely be later next week, with a weaker
system in the Tuesday time frame. From Sat into Monday expect dry
weather (no rain) and temperatures close to normal.

Overall our weather pattern will remain largely unchanged as the
northern stream stays north and the quasi-zonal primary polar jet
remains near 40 N (more or less). The polar jet is expected to stay
from near Japan then eastward across the central Pacific, then
across the CONUS.  I do not see a major change in this pattern into
at least early May.

We did however get a break in the East Asian Jet this weekend. That
in turn allows the northern stream to dig farther south toward the
Great Lakes this weekend. That blocks the next Pacific storm system
from getting this far north. As a result we end up with near normal
temperatures and no rainfall for Saturday through Monday. Frost and
freezing temperatures are possible over northern areas of our CWA on
Saturday and Sunday morning.

By early next week the East Asian Jet feature comes back and that
will lead to a warm up and stormy weather just beyond this forecast
period. There is a weak system that shears out of the Pacific wave
stream late in the weekend and crosses the Great Lakes in the Monday
Tuesday time frame. This does have enough instability for
thunderstorms but it is moving quickly and will not have time to
pull in a lot of Gulf moisture so at this point I do not expect a
significant rainfall or thunderstorm event as it moves through.

Bottom line is near normal temperatures and little if any
precipitation till at least Tuesday.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 154 PM EDT Wed Apr 19 2017

Currently all TAF sites are VFR (as of 1745z) and for the most
part I expect it to stay that way this afternoon. There is an area
of showers an a cluster of thunderstorms from near MKE south to
CHI heading northeastward. There is also an area of MVFR cigs and
visibilities assoicated with this area of showers/thunderstorms.
I expect the thunderstorms to stay south of I-94 but I could see a
few showers between 19z and 23z across our taf sites as this area
moves through.

Early tonight I expect VFR cigs to prevail but after 06z a warm
front pushes northeast across the area and will see IFR cigs
spreads across the area. I also believe there will be showers
around between 09z and 16z but the thunderstorms should miss this
area. Even so thunderstorms are not out of the question so I put
VCTS at all taf sites tomorrow morning. The low clouds will last
through most if not all of the day.


Issued at 300 PM EDT Wed Apr 19 2017

Wind speeds and wave heights will remain below small craft
advisory criteria in the near term. Winds will veer to the east to
southeast overnight and increase to around 15 to 20 kts. After
passage of a cold front winds will veer to the west at 15 to 25
kts Thursday but wave heights should remain below small craft
criteria. However thunderstorms will pose a hazard to mariners
tonight through Thursday morning.


Issued at 1118 AM EDT Wed Apr 19 2017

River levels have returned closer to normal in many locations but
still remain a little high for the time of year. Rivers are above
bankfull and falling at Ionia, Maple Rapids, and Eagle, while
Evart is above bankfull and rising. Rain totals Thursday in excess
of an inch are possible upstream of Evart, which may boost the
crest over the comping days. Croton may reach bankfull by Friday
also. More spotty rain totals are expected in the Grand and
Kalamazoo basins, which should have a limited effect on main stem
rivers. Smaller river basins that receive locally heavy rain
(including Eagle) would be susceptible to rise again.




SHORT TERM...Laurens
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