Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
348 am EST Wednesday Dec 18 2013
Short term...this morning through tonight
Despite a shrinking boundary layer upstream...lake effect snow from
Lake Michigan continues to maintain itself as it heads east...making
it all the way through Southeast Michigan. Hi-res model runs such as
the arw...nmm...hrrr and even the NAM indicate these conditions are
expected to last through much of the morning hours until surface
ridging turns winds from a westerly direction to a southerly
direction and the attendant warm air advection squashes the boundary
later to a mere 1000 feet. These light lake effect snow showers and
flurries are expected to mainly affect areas south of the Interstate
69 corridor with snowfall amounts limited to a dusting up to a half
of an inch in locations with more persistent snowfall.
Although the aforementioned surface ridging will bring an end to the
snowfall and bring warm air advection with it...the shallow boundary
layer and abundant cloud cover are expected to limit high
temperatures to the middle to upper 20s during the afternoon hours.
Warm air advection will continue through the overnight hours...with
low temperatures in the low to middle 20s expected to occur during the
early evening hours with slowly rising temperatures through the
overnight hours. Temperatures by the end of the night are expected
to be in the upper 20s to around 30 degrees.
As a warm front approaches late tonight...soundings indicate
increased low level moisture and weak isentropic lift associated
with the front could bring some freezing drizzle to Southeast
Michigan as soundings indicate very dry air above the low level
moisture with no ice crystals seeding the low cloud layers. Although
temperatures will be on their way towards freezing the ground should
still be cold enough for freezing drizzle. However the coverage and
confidence are fairly low given the weak isentropic lift and the
warm front itself weakening as it approaches from the west.
Long term...Thursday through next Tuesday
Most of the attention in the long term remains on a pair of low
pressure systems that will impact the Great Lakes Thursday through
Sunday night. The first system will occur Thursday evening through
Friday. This will be followed closely by the second system Saturday
through Sunday night. Both of these systems look will bring a mixed
bag of precipitation to the area making for a very challenging
A strong jet diving down the West Coast will form a closed low over
Baja California California. This will allow the upper level ridge to slide
through the Central Plains into the Midwest. All the while...strong
northern stream energy will help produce a split flow pattern over
the mountain west keeping a progressive and active pattern across
Canada. The first system coming on Thursday will be the result of a
northern stream wave phasing with a stronger shortwave ejecting out
of the base of the West Coast trough. This system will ride along
the strong thermal gradient laid out by the last few systems that
have passed through the northern Great Lakes this week. Models have
been fairly consistent with the timing of this system for a few
models runs now which is always favorable. The big problem and lack
of confidence comes with precipitation type. The trough will produce SW
flow and a period of warm air advection into Southern Lower Michigan. A warm layer looks
to develop from 950-700mb with below 0c temperatures at the surface on Thursday
as high pressure over the northern plains keeps a feed of cold air
into the region. This thermal profile combined with a moisture
profile saturated up to about 850mb and quite dry above
that...moisture not close to -10c or below...should lead to drizzle
or freezing drizzle into the day until the surface temperatures warm up enough
to put an end to it. The sheared out low will track from about chi
to phn or possible southward. The best play looks to be a mix of
rain and snow hitting heavier on the snow to the north and heavier
on rain to the south as surface temperatures will make a run at 40f. Have
included modest snowfall accumulations of 1 to 2 inches across the
Saginaw Valley and thumb Thursday night into Friday but expect this
to change as the thermal gradient location is better resolved. Model
quantitative precipitation forecast is right around 0.2 to 0.4 inches as the system is able to draw
some moisture from the gom and favorable middle/upper level dynamics
line up overhead helps realize this moisture.
The next system will occur Saturday evening through Sunday night.
This system looks to be stronger and better organized overall. The
vorticity maximum at the base of the trough will release as the trough drifts
eastward across the plains. Once again the thermal profile will
produce an increased lack of confidence with precipitation type over the
area. Models want to develop a fairly deep warm layer...0 to
+2c...from about 750-900mb as isentropic ascent draws mild air from
the warm sector up into the system. At this point will highlight a
potential mix of rain/snow as the warm layer is not that much above
0c and a simple shift in the track north or south could change this
easily. It should be noted that the NAM warm layer is much
warmer...almost to 7c...leading to much more liquid precipitation falling
but this is at the end of the run so will just make note of it at
this point. Models are showing an impressive amount of quantitative precipitation forecast as this
system originating in the Southern Plains is loaded with Gulf
moisture. For what its Worth the GFS is currently advertising over a
half inch of quantitative precipitation forecast which make the ptype that much more important. The
system is strong...with good upper level dynamics...strong 850-700
fgen...with a very favorable track for Southeast Michigan to get a good swath of
accumulating snow. More details and refinements to come.
Yet another low pressure system will track across the northern Great
Lakes tonight. Just ahead of the system the southwesterly flow over
the lake will increase with gusts around 25 to 30 knots. Winds
across Central Lake Huron will peak around midnight tonight making a
run toward low end gales but they are not quite there yet.
Increasingly warmer air advecting into the region overnight will
increase the stability over the lake helping to limit gusts to
below gale force. The short duration of a couple hours would likely
prevent a watch or warning from being hoisted regardless. Winds will
decrease over the southern Great Lakes on Thursday another low
pressure system begins to track up from the south. Extended period
of active weather will begin late Thursday lasting through the
weekend as a pair of low pressure systems work through the area.
Aviation...issued 1203 am EST Wednesday Dec 18 2013
A pattern of lake effect snow showers will affect the terminal
corridor mainly south of fnt through the dtw area during the early
morning. Lake effect will then diminish quickly before sunrise as
warmer air arrives over Southern Lake Michigan. The warmer air will
will shut down the snow showers but trap stratus over the region and
force an MVFR ceiling in the dtw area while any VFR ceiling that can
develop at fnt and mbs will lower back to MVFR as the low level flow
backs toward the southwest during the morning. Dry and even warmer
air will then continue to advance into lower Michigan during the
afternoon which will scour out the low clouds at all locations by
middle to late afternoon. VFR will then persist through Wednesday
evening as low level flow increases toward 20 knots.
For dtw... snow showers from Lake Michigan will diminish quickly
after 08z but not before intervals of IFR restriction in the heavier
activity. A dusting of accumulation could result if more than one
cluster can make it this far east before the pattern weakens.
//Dtw threshold threats...
* high confidence in ceiling below 5 kft through morning.
* A low probability of visibility below 1/2 mile between 6z and 8z
within a burst of moderate snow.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).