Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
728 am EST sun Dec 8 2013
VFR conditions will prevail this morning as dry air holds initially
in the lowest 8 kft of the atmosphere. A wave is expected to work
through the region this evening and tonight which will bring a
widespread accumulation of light snow. Generally...looking at
straight warm air advection event which should bring a longer
duration of light snowfall. Enough top down saturation should occur
to allow for IFR conditions in steadier light snow this evening.
//Dtw threshold threats...
* medium confidence in ceilings at or below 5000 feet by early
Sunday afternoon...high confidence later Sunday afternoon into
* High confidence of snow as precipitation type Sunday afternoon and
Previous discussion...issued 350 am EST sun Dec 8 2013
Short term...today and tonight
Upper level energy rounding down through The Four Corners this
morning will eject northeastward across the plains states this
afternoon...across the Great Lakes tonight. A main component to the
precipitation event is that geopotential heights at the longwave
will be on the rebound. These setups generally allow for a greater
displacement than lone isentropic lift/warm air advection processes.
Start times can also creep up and verify sooner than what is typical
from a straight front or gradient feature. This rebound or ridge
amplification will be a downstream response to the very strong
trough structure crashing down and into the western states.
Model plan views support broad warm air advection to begin this
morning for locations south of I 69 spreading north during the
afternoon. This initial surge will occur very high in column higher
than 12-14 kft above ground level as subsaturated conditions underneath/sub 10 kft
above ground level make it inhospitable for snowflakes. Beginning during the late
morning...Theta-E content will lift northward east of lower Michigan
which will actually help system relative lift in this instance. This
will set the stages for light snow chances to increase from south to
north during the afternoon hours. This is quite the departure from
model runs of the past few days which were advertising this to
solidly be a Sunday night event. The previous forecaster mentioned
that static stability will remain relatively high in a good chunk of
the column which is still unquestioned. This will ultimately be the
limiting factor in this event. However...there are a couple of
things Worth noting. The first is fairly high snow ratios which
should allow some snow amounts to stretch out. The second is we will
likely see the highest accumulations for those locations where the
vorticity maximum stripes directly overhead. At this time...this is expected
to occur from far southeastern Wisconsin across Central Lake
Michigan through central and northern portions of lower Michigan and
Lake Huron. Qpe is expected to be light in this longer duration
grinder event...generally .6 to .13 inches. This projects to snow
amounts that will range from 1 inch in many locations to 1 to 2
inches for portions of the Tri Cities. Snow amounts of a half inch
or so will be possible south of I 69 before 00z...with the remaining
light snow set to fall between 00-12z tonight (south to north).
Bottom line...enough snowfall to call it a widespread accumulation.
Long term...Monday through Saturday
Low amplitude impulse embedded within the controlling central Continental U.S.
Longwave trough will quickly exit to the northeast by Monday
morning. Some deepening of the surface low will commence upon the
exit leaving a strengthening westerly gradient to drive a period of
stronger cold air advection through the latter half of the day. The
advective process into a sufficiently deep moist layer and within a
background of weakly cyclonic flow continues to support a low chance
of snow showers/flurries. Overall strength of the wind field and
degree of mixing will result in some gusts within the 30 miles per hour range.
This combined with temperature readings hovering just below freezing
will make for a raw day...wind chills in the teens.
Main story for the remainder of the work week will be the cold.
Clipper system pivoting through the mean trough quickly skirts
across the northern Great Lakes on Tuesday. This will provide the
first of at least two reinforcing shots of Arctic air...850mb
temperatures currently projected to settle into the -15 to -17c
range. This will lock daytime temperatures in the 20s...and
dependent on the degree of cloud coverage will support lows anywhere
from middle teens to upper single digits Tuesday night. Noting the 40
knot wind on both the NAM and GFS sneaking down into the
Post-frontal 900 mb layer Tuesday afternoon...looking at a period of
stronger gusts under increasing mixing and steady isentropic
descent/caa. Not out of the question to see a marginal advisory
worthy wind event Tuesday afternoon/evening...although strong
Reliance will exist on the degree of mixing with the better
isallobaric forcing directed to the north. The additive wind
component will translate into single digit wind chill readings
Tuesday into Tuesday night.
A trailing northern stream wave anchoring the eastward propagation
of the entire mean trough axis into lower Michigan will solidify
this Arctic plunge heading into Wednesday...the thermal profile
taking yet another step downward. Middle level frontal dynamics will
undergo some degree of strengthening along the advancing surface
wave in the presence of favorable right entrance jet forcing.
Moisture depth may be an issue given the Arctic origin...but still
worthy of a chance mention for light snow showers accompanying the
uptick in ascent on Wednesday. Little change in expectations
through the end of the week...Arctic airmass remaining firmly
entrenched across the region within deep northwest flow.
High pressure settling across the region will maintain lower wind
and wave conditions today. Developing low pressure over the plains
today will lift across Northern Lake Huron early Monday morning.
Increasing over lake instability will contribute to a modest
strengthening in southeast winds tonight. Winds will shift to
southwest as the low lifts into southern Ontario on Monday. Arctic
air spilling into the region will bring the potential for gales late
Monday and Monday night across northern and Central Lake Huron.
Gusty winds will exist through the remainder of the week as
reinforcing surges of Arctic air move across the lakes. In
addition...conditions will become favorable for freezing spray across
northern and central sections of Lake Huron early next week.
gale watch...lhz361-lhz362-lhz363-lhz462...from 3 PM Monday to 4 am
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).