Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
712 am EDT Tuesday Mar 11 2014
VFR conditions will consist of high clouds thickening and lowering
during the morning after some MVFR fog/haze early due to melting
snow. Light wind will veer toward the northwest as weak high
pressure moves into lower Michigan from the Midwest. This will occur
as low pressure gathers strength over the Central Plains during the
day and tracks along the front stalled south of the Ohio border.
Precipitation will begin to impact Southeast Michigan with a rain/snow mix
during the late afternoon and evening before changing to all snow
after midnight at all locations.
For dtw... a light rain/snow mix during the evening will change to
all snow after midnight and likely accumulate around 3 inches by
sunrise Wednesday. The peak of the event is expected from about
sunrise to noon Wednesday with total accumulation likely exceeding 6
inches of wet snow before ending Wednesday afternoon.
//Dtw threshold threats...
* high confidence on ceiling below 5000 feet tonight.
* High confidence on rain changing to snow tonight.
Previous discussion...issued 403 am EDT Tuesday Mar 11 2014
Short term... today and tonight
Relatively mild air remains in place on the heels of the weak
front/pressure trough that moved through Southeast Michigan last evening.
That will allow a few degrees of temperature recovery during the
morning within the surface pressure col before winds veer more
toward the north during the afternoon. Thickening high clouds will
limit the morning warming trend and then surface cold advection on
the increasing northerly flow will force steady or even slowly
falling readings during the afternoon, especially in the Saginaw
Valley and northern thumb. Hourly temperatures are modified from the
usual diurnal trend in this manner from maximum temperatures in the lower half
of the 40s around noon.
The strong low pressure system developing in the plains will begin
to affect our area toward evening in terms of precipitation. The
combination of surface pressure falls and favorable positioning of
the upper jet will lead to middle level frontogenesis from the plains
into lower Michigan. Cross sections from the NAM and GFS both show
exceptional values of about 100 units of fgen in the 850-700 mb
layer developing between 00z and 06z tonight. The resulting
ageostrophic circulation will be plenty well organized to overcome
any lingering low level dry air with the main challenge in the
timing of rain/snow mix to all snow. The low level dry air in the
NAM implies enough wet bulb cooling to lean the precipitation type forecast
toward the colder GFS temperature and thickness profile. This sets
up the rain/snow line roughly along the Interstate 69 corridor but
with generally less than an inch of accumulation before midnight.
After that, precipitation rates will increase enough to force a change over
to snow all areas as the lead frontal forcing merges with the
dynamics of the low pressure system.
Long term...Wednesday through Monday
The current water vapor loop now shows the main pieces for the
Tuesday/Wednesday winter storm. The upper level trough is now over the
mountain west while the shortwave that will phase with it has just
released from the goa low. Overall thinking for the phasing systems
and resultant weather event for southern Michigan has not changed much
from previous forecasts. As much as some models try to deviate from
the consensus...they seem to work back toward it with the next run.
At this point it seems the devil is in the details which we will try
Early Wednesday morning southern Michigan will see a transition from a
lead frontal/fgen band rain/snow event to that of a strong surface low
induced snowfall event. With the Winter Storm Watch in place for
several counties...looked into current models for trends with
forcing location and timing to try to better define the event.
Regarding quantitative precipitation forecast...models have been on the bullish side to say the
least. The recent 00z Euro has stayed more subdued relative to the
NAM/GFS/hires models so have generally disregarded those amounts.
Looks like it may be an artifact from saturation higher up in the
column and models generating lift in those levels whereas we see the
event being a lower level based event tied to the strong forcing
closer to the frontal boundary itself. The watch was extended one
additional tier of counties to the north as a result of model
consensus trending that direction. 24 hours ago model relative humidity/specific
humidity bubble/and Theta-E gradient were all located south of a
line from about Ann Arbor to Pontiac to south of Port Huron. Current
models have all taken a little jog north thus raised probability of precipitation and quantitative precipitation forecast
values in that direction. The dry Arctic air surging in from the
north will create a rather sharp drop off in snowfall amounts when
it is all said and done but cannot be too certain where this will
line up. The northern most tier of counties from Genesee to Sanilac
will likely be the location of this drop off so there is slightly
lower confidence with these counties. The orientation of the wrap
around precipitation in the trailing deformation band warrants the SW to NE
angle of the watch area. Still concerned about the front and back
ends of the event. The front end concerns are how fast will the rain
change to snow early Wednesday morning and where will the band set
up? And on the back end...how long will the deformation band linger
over the area and how efficient will it be at snow production? A
system this strong deepening to around 985mb by the time it reaches
south of cle should have a rather pronounced deformation band which
models are hinting at. With the track heading to the NE that will
lead to longer residence times for our eastern counties. For this
reason the amounts have been raised slightly for the 18-00z
timeframe and the watch was extended to 00z Thursday as well. Overall
looking for the potential for 6+ inches south of a line from Jackson
to Port Huron and 4-6 inches to the I-69 corridor.
The trough rotating though the Midwest with the system will bring
850mb temperatures around -20c to Southeast Michigan Wednesday night. High pressure
building into the area will help create strong gradient of northerly
flow ushering in the cold air at the surface and a fresh snowpack should
help create favorable radiational cooling conditions and another
night with raw temperatures flirting with negative values.
High pressure will pretty much dominate the rest of the week and
weekend outside of a clipper tracking through Ontario. Outside of
producing SW flow and a brief warmup it looks like it will be pretty
Light wind will gradually veer toward the north during the afternoon
and tonight as a strong low pressure system moves into the Ohio
Valley. A Gale Warning is now in effect for Southern Lake Huron and
Lake St Clair for a slightly earlier onset compared to Lake Erie
where the watch remains in effect for now. The strong northerly wind
field will develop as the low moves into Pennsylvania by Wednesday
afternoon and then quickly to the Atlantic coast Wednesday night.
This system will be followed by polar high pressure and lighter
winds by Thursday, but the next Canadian low pressure system will be
close by and is expected to reach the North Shore of Lake Superior
by Friday morning preceded by moderate southwest flow and milder air.
Michigan...Winter Storm Watch...miz055-miz061-miz062-miz063-miz068-miz069-
miz070-miz075-miz076-miz082-miz083...from midnight Wednesday
to 8 PM Wednesday.
lhz464...from 8 am Wednesday to 8 PM Wednesday.
Lake St Clair...
Gale Warning...from 8 am Wednesday to 8 PM Wednesday.
Michigan waters of Lake Erie...
gale watch...from 8 am Wednesday to 10 PM Wednesday.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).