Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 131417 AAA

Area Forecast Discussion...UPDATED
National Weather Service Detroit/Pontiac MI
1017 AM EDT Tue Mar 13 2018


As of 1015 AM EDT...Made a few adjustments to the ongoing forecast
based on latest trends in regional radar and hi-res model guidance.
Main update to the forecast for today and into this evening was to
bump up snow totals across the eastern Thumb and southward into St.
Clair and portions of eastern Macomb county. Hi-res guidance and
radar support a more persistent band of snow showers to affect these
areas, with embedded heavier bursts of snow and possibly snow squalls
moving through especially this afternoon and into this evening
coinciding with peak daytime heating. Exeter, Ontario radar also
depicts a potential band beginning to set up currently along the
northern Thumb in eastern Huron and Sanilac counties, with regional
observations supporting a good, long area of fetch and convergence
reaching up as far north as the Thunder Bay area. Will be monitoring
this band closely over the next few hours for a potential short-fused
Winter Weather Advisory.

Elsewhere across southeast Michigan, ongoing forecast looks to be in
good shape, with scattered to numerous snow showers through much of
the day, with periods of heavier bursts of snow and possible snow
squalls mainly in the 3pm-7pm timeframe as convective depths are
maximized (reaching 5-8 kft). There are some signals in some
convective instability rooted in the boundary layer as well which may
boost the potential for bursts of snow, especially north of the I-94
corridor and east of US 23. Model soundings also depict periods of
supersaturation with respect to ice that, if can coincide with peak
boundary layer convergence and lift, could lead to brief snowfall
rates as high as a half an inch per hour (possibly up to an inch per
hour in the eastern Thumb and St. Clair/Macomb counties).
Accumulations will generally be an inch or less across the rest of
southeast Michigan, with highest totals north of M-59. The bursts of
heavy snow will also lead to rapid reductions in visibility below 1
mile at times and with high temps struggling to reach above freezing,
could lead to a quick coating of snow on roadways and slippery travel


Issued at 645 AM EDT Tue Mar 13 2018


Cold air aloft associated with an upper low rotating across the
eastern Great Lakes today and daytime heating under a March sun
angle will sustain periodic snow showers across Se Mi. While snow
showers will be possible at any time this morning into this evening,
the most probable time period for higher intensity snow showers will
be from mid afternoon into the early evening. This will be due both
to the peak in the diurnal heating cycle and added convergence from
a weak sfc trough pushing into Se Mi off Lake Huron. The snowfall
rates within these snow showers will be quite high. The result will
be large variations in cigs and vsby within the snow showers. The
intensity may also be enough to drop a quick coating of snow over a
very short duration. Current indications suggest the more frequent
intervals of heavier snow showers will be at PTK and FNT where low
level convergence is expected to be a little greater. The deepening
of the mixed layer this afternoon will also result in an increase in
nw winds this afternoon, gusting at times over 20 knots.

For DTW...Recent radar trends suggest some light snow showers
tracking just southwest of metro. Can not rule out some snow showers
prior to mid afternoon. The focus for snow showers in the TAF will
be late afternoon into the early evening, coinciding with peak
daytime heating and remnant low level convergence expected to slide
into the area from the northeast. Accumulations may be difficult
this afternoon as temps warm above freezing. However, any higher
intensity snow showers after 21 or 22Z may be able to quickly drop a
dusting of accumulation.


* Moderate for ceiling 5000 ft or less today. Low tonight.

* High for snow as precipitation type.

* Low for northwest wind exceeding crosswind threshold this

Issued at 345 AM EDT Tue Mar 13 2018


Trough axis and elongated mid level low pressure residing across the
central Great Lakes will transition into a more roundly defined mid
level circulation by this morning. This closed low will pivot from
Lake Huron to western New York throughout the day putting southeast
Michigan in another favorable position to experience scattered to
numerous snow showers. Increasing northwest winds on the western side
of the low will accompany a mid level cold pool with temperatures
nearing -40C at 500 mb overhead by this afternoon. Low level moisture
will be sufficient with specific humidity values of 1.50-1.75 g/kg
at 850 mb over the area throughout the day and a little help from
Lake Huron. Favorable diurnal timing will result in steep low to mid
level lapse rates and create an environment with impressive
convective depth nearing 7-8 kft. Good instability will also reside
within the dendritic growth zone (DGZ). Similar to yesterday,
potential exists for transient, yet heavy bursts of snowfall given
the favorable convective environment. Snow will originate from the
north and move southward in association with a lake aggregate surface
trough and expand coverage of snowfall through the morning and
maintain into the evening. The intense snowfall will have the
potential to bring significant reductions in visibility, but is not
expected to lead to significant accumulations due to the transient
nature of these bursts of snowfall. Accumulations are expected to be
generally around an 1 inch or less for much of southeast Michigan.
However, portions of the Thumb will have potential to exceed an inch
if a convergence band sets up within the long fetch of north winds
over Lake Huron during the morning. Confidence is not high in this
occuring though, given the convective nature of this event. The
expectation will be for snow showers to be more cellular and quick
moving rather than banded structures that would result in locations
experiencing longer duration of snowfall.

The mid level trough axis and cold pool will begin to pivot
southeast of Michigan by tonight resulting in a decrease in snow
shower activity with the exception of the potential lake effect band
setting up across portions of the northern Thumb shoreline under
north northwest winds. This will provide additional opportunity for
accumulating snowfall. A trailing secondary wave riding the backside
of the mid/upper level trough will dive south out of Ontario into the
central Great Lakes on Wednesday. However, moisture will be lacking
with the timing of this wave as specific humidity values within the
850-700mb layer will drop to around 0.50-0.75 g/kg. This wave will
also lack the favorable diurnal timing and instability of the
previous two days. This combination will lead to low confidence at
this time in mentioning any snow for Wednesday. As the trough exits
further to the east, so does some of the colder air associated with
the cold pool. Granted the overall change will not be much with an
increase in a few degrees at 850 mb for Wednesday resulting in high
temperatures slightly warmer into mid to upper 30s.

The eastern CONUS troughing pattern will continue through the end
end of the week. This will maintain northwest flow and persistent
temperatures characterized by highs mostly in the 30s and overnight
lows in the 20s. Not much in the way of snow for much of southeast
Michigan with a dry airmass moving in, but the favorable northwest
flow will result in slight chances for lake effect snow to affect
portions of the Thumb on Thursday and Friday.

Heading into the weekend the lake effect snow chances will drop off
as winds begin to shift to the west as weak mid level ridging take
over and a surface high pressure drifts across the Plains. This will
affectively cut off the Lake Huron moisture source and also allow
some weak warm air advection. Temperatures look to warm up into the
40s by Saturday and even make a run at 50 degrees for Sunday. Will be
keeping an eye on the next chance for precipitation early next week
as a low pressure system looks to move out of the Central Plains and
through the Ohio Valley.


A surface trough will push south across Lake Huron this morning. An
increasing north-northwest gradient and deep over-lake mixing depths
will support an increase in the winds during the course of the day
in the wake of this trough. Wind gusts across the southern half of
Lake Huron will approach 30 knots late this afternoon into the
evening. The northwest winds will slowly weaken late tonight into
Wednesday as a ridge of high pressure pushes into Lower Mi, allowing
the gradient to relax. Another weak surface trough/cold front will
traverse Lake Huron Wednesday night. Post frontal cold air advection
and a tightening gradient will support an uptick in the nw winds on


Lake Huron...Small Craft Advisory until 4 PM EDT Wednesday for LHZ442-443.

     Small Craft Advisory until 4 PM EDT Wednesday for LHZ421-441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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