Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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437
FXUS63 KGRR 292029
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
329 PM EST Tue Nov 29 2016

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

.SYNOPSIS...
Issued at 330 PM EST Tue Nov 29 2016

There is a chance for scattered showers and a slight chance for
thunderstorms this evening across the southern forecast area.
Colder air moves back in beginning Wednesday evening, and lake-
effect precipitation is likely Wednesday night through the rest
of the work week. Some wet snow will be possible over the northern
CWA, mainly at night with cooler surface temperatures in place.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 330 PM EST Tue Nov 29 2016

There is a chance for scattered showers and a slight chance for a
few rumbles of thunder this evening into late tonight mainly across
the southern half of Lower Michigan.

Weak potential vorticty advection is noted this evening as a 155 kt
jet streak lifts north over Lower Michigan. A 40 kt LLJ noses into
the southern CWA this evening and then pushes east across Southeast
Lower Michigan tonight. The stout SW flow provides modest moisture
transport just ahead of a weak cold front. Instability is marginal,
but decent by late November standards. MUCAPE is forecast to be on
the order of 100-200 J/kg, and steep mid-level lapse rates this
event support a slight chance of thunderstorms in the forecast.
Instability wanes rapidly after 03z.

The weather looks fairly quiet on Wednesday, with just a lingering
chance for some light rain in the far eastern CWA. Forecast
soundings indicate a lot of dry air on Wednesday before wrap-around
moisture and colder air arrive Wednesday night. Highs on Wednesday
will be in the upper 40s to low 50s.

Cold air advection ramps up Wednesday evening and continues
overnight. Moisture depth increases Wednesday night and Thursday,
and 850 mb temps fall to around -5 C. The southern mid-lake buoy
was reporting a water temp around 10 C, so lake-generated
instability should be sufficient. Favorable low-level omega is
also in place Wednesday night with troughing in place, so lake-
enhanced precip is likely. Precip will begin as all rain, but snow
should mix in for many areas away from the immediate lakeshore
early Thursday morning. Lake-effect rain showers will continue
Thursday with highs in the lower 40s. The potential for a rain-
snow mix or changeover to all snow increases once again Thursday
night as surface temps cool. Accumulations should remain very
light and confined to the far northern CWA.

.LONG TERM...(Friday through Tuesday)
Issued at 330 PM EST Tue Nov 29 2016

The long term period begins with a lingering upper trough over the
Great Lakes as the main upper low will have moved east and opened up
by this time. The thermal trough will be overhead early in the long
term with 850mb temps in the -5C to -10C range from Friday into
Sunday. Lake generated precipitation is expected during this same
period as there will be moisture in place and Delta T`s in the mid
to upper teens. So, we are looking at mixed rain and snow showers
through the period, with the snow a bit more favored at night and
rain more favored during the day as temperatures will be marginal
for both precip types. On Sunday a northern stream upper wave will
enhance precipitation with more rain and snow in the forecast.

Early next week a low confidence forecast remains as the ECMWF and
GFS continue differing solutions. The ECMWF has backed off on a deep
low for Monday and has fallen more in line with the GFS with surface
ridging nearby. On Tuesday both have a weak low moving through the
area.

Bottom line, the weekend looks cool with rain and snow showers,
mostly lake generated. Early next week 850mb temps warm which should
take lake precipitation out of play, but we may see mixed light rain
and snow from a weak synoptic system.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 110 PM EST Tue Nov 29 2016

VFR weather dominates the area at 18z, the exception being MKG
where some moisture off of Lake Michigan is producing a ceiling at
1900ft. Expect these clouds to dissipate or lift soon with MKG
becoming VFR as well by mid afternoon. A gusty south wind will
continue this afternoon with winds of 15-25 knots common.

This evening a disturbance will move into the area from the
southwest out of Illinois. The disturbance will bring clouds, the
chance for scattered showers and an isolated thunderstorm. At this
point have VCSH in the forecast as the precip should be scattered.
The threat for storms looks too low to include at this point, but
not out of the question. The main impact from the disturbance is
that it will likely slide MVFR clouds into the area overnight.
Some model indications even show an IFR deck developing late
tonight into Wednesday morning. Winds will be much lighter
overnight into Wednesday on the order of 5-10 knots.

&&

.MARINE...
Issued at 330 PM EST Tue Nov 29 2016

The Small Craft Advisory has been extended for Lake Michigan
nearshore waters from Grand Haven to Manistee until 1 AM tonight.
The Small Craft Advisory for St. Joseph to Grand Haven is in
effect until 7 PM this evening. South winds of 10-20 kts are
expected overnight with waves subsiding to 1-3 feet by daybreak.

After a brief lull, another Small Craft Advisory will likely be
needed beginning Wednesday night. West winds will increase to
15-25 knots as waves build to 3-6 feet.

&&

.HYDROLOGY...
Issued at 232 PM EST Tue Nov 29 2016

Rainfall amounts ranged from roughly a half inch in the greater
Lansing area to over 2 inches in Oceana county. The only rivers
making a run toward bankfull are Sycamore Creek at Holt (cresting
Wed morning) and Maple River at Maple Rapids (cresting this
weekend). Very minor flooding in low lying areas adjacent to the
rivers is expected. Heavy precipitation is not anticipated until
at least next week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 1 AM EST Wednesday for LMZ847>849.

     Small Craft Advisory until 7 PM EST this evening for LMZ844>846.

&&

$$

SYNOPSIS...EBW
SHORT TERM...EBW
LONG TERM...Duke
AVIATION...Duke
HYDROLOGY...CAS
MARINE...EBW



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