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fxus63 kdtx 111211 

Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
711 am EST sun Dec 11 2016


Snow has become widespread across the forecast area. Fairly high
liquid to snow ratios at this time have led to a very dry and fluffy
snowfall, with morning amounts of 1 to 2 inches already. Given the
expectation for some compaction during the course of the day,
total forecast snowfall amounts for the event will remain
unchanged attm. A forecast update will be issued simply to remove
the early morning wording.


Previous discussion...
issued at 556 am EST sun Dec 11 2016


Snow over Southeast Michigan will become more intense by afternoon,
with all sites expected to drop to LIFR visibilities for much of the
afternoon and evening. Snowfall rates will likely reach one-inch per
hour at the Detroit-area terminals during this time. Some
improvement in visibility is expected during the late evening, as
snowfall intensity begins to wane. Snow should exit by 08z. Total
accumulations at the terminals are expected to range between 5 and
10 inches, with heaviest amounts the further south you go.

For dtw...IFR conditions are expected to persist through the morning
as intensifying snowfall keeps visibilities below 2 miles. LIFR
conditions are expected during the peak of the snowstorm during the
afternoon and evening. Improvement should occur after 05z, with snow
tapering off by 08z. Total accumulation at dtw is expected to range
between 7 and 10 inches.

//Dtw threshold probabilities...

* high for ceilings below 5000 feet through the taf period

* moderate for visibility below 1/2 mile in snow 20-02z.

* High for snow as precipitation type through the taf period

Previous discussion...
issued at 405 am EST sun Dec 11 2016


As of 330 am, light snow stretches from ern South Dakota all the way across
srn lower mi. This snow is occurring within a broad region of moist
isentropic ascent along the 290-300k surfaces. This ribbon of ascent
will be persistent through the duration of the morning, while the
mid level front and associated stronger frontal forcing slowly lifts
toward the north. The overall weak nature of the forcing this
morning, with the region of lift not always conducive to good
Snowflake size, will limit accumulations to around an inch (maybe
two) by early afternoon.

Conditions are then expected to become much more conducive to heavy
snowfall during course of the afternoon. The mid level wave now
located over the Montana/North Dakota border will track across Minnesota
into nrn WI/wrn upper mi this evening, spreading mid level height
falls downstream across lower mi. In response, sfc low pressure will
develop across the wrn Great Lakes this afternoon, then lift across
northern mi tonight. The key feature of interest is the rapid
development of an upper jet Max downstream across se Canada this
afternoon. Upper divergence will rapidly increase over lower mi,
within the entrance region of this jet. The corresponding
ageostrophic response will lead to an increase in the isentropic
ascent from the nrn Ohio Valley into srn lower mi. The 00z model
suite indicate redevelopment of the mid level front/deformation
region across Metro Detroit this evening. Moist ascending inflow
into the deformation will support an uptick in snowfall intensity in
the 18 to 21z time frame, with peak forcing now expected from 21z to
03z (during the evening hours). Model cross sections depict some
decreasing instability aloft atop the strong inflow. Moisture
transport still looks quite good as the bulk of the 00z model suite
indicate mixing ratios of 3 to 4 g/kg in the 850-750mb layer lifting
across the Ohio/Michigan state line this evening.

Always the most challenging aspect in winter systems is where the
meso scale forcing will support the more intense snowfall. This
system is no different. The bulk of the 00z suite, including the
hrrr and arw, suggest the mid level deformation will set up
somewhere across Metro Detroit/Ann Arbor up into Port Huron later
today. Storm total quantitative precipitation forecast among the 00z suite generally range anywhere
from 7 tenths up to an inch. Previous runs of the European model (ecmwf) have been on
the lower end with respect to total liquid quantitative precipitation forecast. The noted uptick in
the 00z run lends support to the rest of the model suite. Model
cross sections indicate the depth of the lift will support better
Snowflake size during the afternoon and evening. Of some concern is
an isothermal layer in the -5 to -10c range at times during this
evening, which would support good snowfall aggregation.

Taking these factors into consideration and with snow to liquid
ratios from Metro Detroit south expected to slowly drop from around
15:1 down to 8:1 by late tonight, total snowfall of 6 to 10 inches
look possible south of a Howell to Port Huron line. It is very
possible that the bulk of this could fall in a 6 hour window
between 4 PM and 10 PM. Therefore, the advisory for these locals has
been upgraded to a warning. Farther north, forcing is expected to be
weaker. However, liquid to snow ratios will be higher which will
still support advisory type accums. Based on the latest guidance,
fcst snow amounts across the Saginaw Valley and thumb will be
tapered to 4 to 6 inches, with 6 to 8 holding along the I-69
corridor from Lansing to Lapeer. One of the main concerns during
this event will be where the more persistent mid level deformation
and Pivot Point set up. A slight deviation from the current forecast
thoughts will require some adjustments to fcst snowfall amounts
during the day.

A punch of mid level dry air arriving overnight will result in an
abrupt end to accumulating snowfall prior to the Mon morning rush
hour. The departure of the mid level wave to the east of the Great
Lakes on Mon will support prolonged subsidence. Low level cold air
advection on Mon will be fairly weak. This and with steep low level
lapse rates, temps should hold in the 30s on Monday. A deep upper
low is still forecast to rotate across cntl and ern Canada next
week, driving Arctic air into the Great Lakes by midweek and
resulting in much below normal temperatures.


Winds will turn southerly today, becoming a little gusty during the
afternoon and evening, in advance of low pressure moving through the
Great Lakes. The southerly winds will allow wave heights to build to
about 4 feet along the nearshore waters of Southern Lake Huron for
several hours this evening between Port Sanilac to Port Austin. The
low will also spread steady snowfall across the area through much of
tonight, with poor visibilities below one mile expected through much
of the afternoon and evening. Winds will then shift westerly and
gust up to 30 knots early Monday morning behind a cold front, with
another reinforcing shot of cold air early on Tuesday maintaining
moderate speeds. Arctic air spilling into the region will then bring
the threat for freezing spray, renewed lake-effect snow squalls, and
potentially gale force wind gusts over Lake Huron Tuesday night
through Thursday.


Low pressure lifting across the area will spread a long-duration
snowfall into Southeast Michigan today through late tonight. Storm
total snowfall is expected to reach between 6 and 10 inches along
and south of the M-59 corridor. Totals north of there will reach 4
to 8 inches, with amounts showing a decreasing trend as you move
from south to north. Snow intensity is expected to peak during the
afternoon and early evening. Rainfall equivalent of the snow that
will fall is expected to range between one-third and three-quarters
of an inch. Snow will taper off after 1 am, with only a few light
snow showers expected by sunrise.


DTX watches/warnings/advisories...
Michigan...Winter Weather Advisory until 4 am EST Monday for miz047>049-

Winter Storm Warning until 4 am EST Monday for miz063-068>070-075-

Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.



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