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Area forecast discussion 
National Weather Service Grand Rapids Michigan
657 am EST Wednesday Jan 28 2015

Latest update...
aviation

Synopsis...
issued at 326 am EST Wednesday Jan 28 2015

After a quiet day today...a wintry mix will be arriving late tonight
across southwest lower Michigan...while mainly snow is expected for
central lower. Some areas across southwest lower will see a coating
of ice through Thursday morning...before the precipitation changes
over to snow Thursday. Meanwhile...1 to 2 inches of snow can be
expected across much of central lower.

It will turn colder with lake effect snow showers for Thursday night
and Friday. There will be another good chance for snow from Sunday
into Monday. It should get even colder behind this system will
highs only in the teens Monday and Tuesday.

&&

Short term...(today through friday)
issued at 326 am EST Wednesday Jan 28 2015

Will go ahead and issue a Winter Weather Advisory starting late
tonight and extending into Thursday afternoon.

The clipper system will arrive late tonight and bring a mixed bag of
precipitation types. The system will bring in warmer air aloft...while the
boundary layer holds on to shallow cold air. This will produce
mostly freezing rain and sleet for the southern and central County Warning Area.
Precipitation amounts seem modest on the models...but there is potential that
this system may over-achieve. A 45-50 knots low level jet noses into
lower Michigan after 06z and could cause the precipitation to go somewhat
convective...producing locally heavier precipitation. For now expect most
areas within the advisory area will see around a tenth of inch of
ice...along with some sleet accumulation.

Further north...expect the precipitation to be mainly snow and will therefore
not have this area in the advisory. 1 to 2 inches of snow can be
expected. This could come in a quick burst late tonight given the
dynamics of the low level jet. This area may see a brief period of sleet too.

By Thursday we will gradually transition to mostly snow from northwest to
southeast as the warm pocket aloft moves east. However this may take most
of the morning until this occurs in the southern County Warning Area. Snow
accumulations should generally be less than an inch across the
central and...and very little across the south.

Colder air arrives Thursday night and Friday bringing in some lake
effect snows. Inversion levels should be 4k feet or less...so accums
should be on the light side...mainly an inch or two along the
Lakeshore.

Long term...(friday night through tuesday)
issued at 326 am EST Wednesday Jan 28 2015

Main impact weather in the long term will be the opportunity for
accumulating snow starting Saturday night and possibly lasting into
Monday. At this time the potential exists for several inches of snow
across Southern Lower Michigan during this period. This is not a
certainty yet but numerous indications are present in the latest 00z
nwp suite.

Some features to watch for Saturday and Sunday will be the
positioning of a developing upper level jet (120-140 kt) over the
Great Lakes region and associated h500 shortwave trough. If the jet
can get far enough north (ie. GFS and Gem solutions) then we could
tap into some right entrance region divergence Saturday
night/Sunday. The stronger this divergence is...the deeper the h500
shortwave and associated surface low development...which trends northwest in
these situations.

Regarding model trends...decided to look at h500 heights and surface
mslp dprog/dt plots of the GFS...ECMWF...and Gem (dprog/dt reveals
past model runs for each respective model at a chosen forecast
time). For the forecast time I chose 00z Monday. The 00z GFS h500
heights are deeper and further west (trough axis along a line from
msp to kci) than the 12z Jan 27 run...but similar to the 00z Jan 27
run. The 00z European model (ecmwf) h500 heights are slightly deeper than past
available runs (trough axis near a line from mke to stl) but similar
in nature going back through the 00z Jan 26 run. The 00z Gem is
deeper and further west than the last 2 runs. At the surface...the GFS
and Gem are showing much deeper lows and further west than the European model (ecmwf)
(gfs has a 994 mb low over Cincinnati at 12z Monday...the Gem has a
990 mb low over Cleveland...and the European model (ecmwf) has a 998 mb low off the
coast of new jersey). With the GFS ensemble mean mslp similar in
nature to the European model (ecmwf) I am not totally confident in a deeper low
across Ohio just yet.

All this being said...a stronger solution with a further northwest track is
suggested with the 00z model runs...but would like to see a few more
iterations of this before confidence becomes greater. At this point
the consensus model quantitative precipitation forecast which weights the medium range models does
bring in a third of an inch or more of precipitation to most of the
County Warning Area from 06z sun to 18z Monday...which would yield several inches of
snow. Until a storm track is locked in by the models...uncertainty
does exist whether this will materialize...but there is a 50/50 shot
or slightly better that it would happen.

&&

Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 657 am EST Wednesday Jan 28 2015

Two significant issues to address for the taf package...icing due
to freezing rain and also low level wind shear. The first problem
to deal with will be a strong low level jet tonight as shown in
the 2k feet to 4k feet layer. Sref and NAM model winds at the 2k feet
layer show around 50 kts out of the south/SW from 02z to around 10z
or so. This occurs as surface winds will be out of the southeast. This is a
setup for rather significant low level wind shear. As such...made mention of this
occurring at all taf sites with slightly varying times of
occurrence.

In regard to the freezing rain threat...it should start
materializing around or shortly after 06z but is expected to be
light in nature. The layer of concern for icing will be from the surface to
around 3k feet. The melting layer will occur roughly from 3k feet to
7k feet. The freezing rain at the surface may mix with sleet at
times as the cold layer near the surface may be significant enough
to refreeze the water droplets into sleet pellets. Suffice to
say...less than ideal conditions for aviators starting very late
tonight and lasting at least through the middle to late morning
hours. Beyond 12z...expecting cold enough conditions aloft to keep
the precipitation all snow through Thursday afternoon although
heavy snow is not forecasted.

&&

Hydrology...
issued at 1215 PM EST Tuesday Jan 27 2015

A few area rivers showed minor fluctuations over the past 48 hours.
None of this was too concerning. The activity has seemed to level
off and stabilize for the most part. Colder temperatures will hold
on through much of the week with the warmest conditions being felt
Thursday...to the tune of low to middle 30s.

A couple rounds of wintry precipitation are possible. The two main
events anticipated are late Wednesday into Thursday and again by
Sunday. Moisture will be spread out over time and colder temperatures
do not look to be cause for concern. Rivers will continue to be
monitored in the case that any fluctuation does occur.

&&

GRR watches/warnings/advisories...
Michigan...Winter Weather Advisory from 1 am to 4 PM EST Thursday for
miz037-043-044-050>052-056>059-064>067-071>074.

Lm...none.
&&

$$

Synopsis...jk
short term...jk
long term...Hoving
aviation...Hoving
hydrology...jam

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