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Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
619 PM EST sun Dec 8 2013
Light snow continues to move in across Southeast Michigan from the
southwest this evening. Radar imagery shows most of the snow this
evening will be limited to ptk north to mbs...with very little
additional snow at the metropolitan airports. Upstream observations and
forecast soundings continue to indicate the possibility for freezing
drizzle as middle level dry air strips away the ice crystals. However
given the scattered nature of these observations and how quickly the
precipitation is ending will continue to leave out of the tafs this
cycle. Ceilings should eventually settle to MVFR...although a few bumps
up to VFR is not out of the question this evening before low level
moisture advects in from the southwest. Visibilities will improve
late tonight and Monday morning as winds increase behind the warm
front. Breezy conditions will develop by Monday afternoon...with
southwesterly gusts to 25 knots.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet through the taf
* High confidence of snow as precipitation type tonight.
* Low confidence with development and potential timing of freezing
Previous discussion...issued 344 PM EST sun Dec 8 2013
Short term...this afternoon and tonight
Upper level wave positioned at the base of a longwave trough
covering much of the country will lift from Colorado/Kansas to Michigan
tonight. At the surface...low pressure will deepen as it moves from
southern Missouri to the southwest corner of Michigan by 06z.
Surface pressure falls will become refocused over the northern Great
Lakes by morning however as the upper wave moves into Michigan and a
larger Canadian trough moves into the upper Midwest and western
Elevated 925-850mb warm front will continue to lift through the area
this afternoon and the first half of the night as the low
pressure system tracks northeast. Radar imagery has lit up over the
past few hours from western Michigan back through northern
Illinois...indicative of the more widespread snow accumulations.
This coincides nicely with where models place fgen along the 850mb
front. Best accumulations this afternoon/evening will occur along
this front and area of fgen as it lifts north and east...spreading
highest accumulations of 1-2 inches north of the M-59 corridor.
Further to the south...just expecting from about 0.2 to one
inch...tapering from north to south...with just some light
isentropic ascent present as the front lifts through.
Models show drier air arriving in the middle-levels late this evening
and overnight. NAM/GFS forecast soundings show this chipping away at
moisture within the dendritic growth zone and limiting ice nuclei
starting as early as 00z for dtw. Satellite imagery with ice
enhancement really illustrates this nicely...with cloud tops warming
upstream over Iowa and north. Illinois...and falling short of -10c over
southern Missouri/Illinois/Indiana. Snowfall should taper off from
south to north as the surface warm front lifts through and drier air
at the middle-levels arrives. Some low-level lift looks to still be
present after ice nuclei strips away...with saturation and an
inversion/shear layer first up to about 700mb...then falling to
800mb by morning. This supports the threat for freezing drizzle...at
least for patchy areas after the snow exits...and forecast does
carry a mention.
Temperatures will not change too dramatically through the evening
and overnight...remaining in the 20s. Will see min temperatures in the
evening before they remain steady or rise overnight as the surface
warm front lifts through. Frontal passage will also switch winds
around to the south...with an increase in speed towards daybreak.
Long term...Monday through Saturday
The story of the long term is the cold Arctic airmass sinking into
the Great Lakes. The longwave trough over the central Continental U.S. Will
broaden and expand eastward this week with several reinforcing shots
of Arctic air to keep Southeast Michigan 10 to 15 degrees below normal. In
addition to the cold...there may be some wind on Tuesday to deal
with and a couple clipper/shortwaves through the week.
Monday will start off with a cold frontal passage which will veer
the surface winds to the SW in the morning and closer to
westerly later in the day. After some brief warm air advection advection
from the south today which raised 850mb temperatures into the single
negative digits...Post frontal cold air advection will quickly drop them back to
near -15c. Model soundings are advertising mixing depths increasing
toward 4kft Monday which would likely bring some gusts approaching
30 miles per hour. The increased wind field will help bring lake effect from
Lake Michigan into the County Warning Area. Not looking for much in the way of
accumulations this far east though.
A surface ridge axis will slide through the area late Monday night which
will disturb the gradient flow and combined with drier air should
put an end the lake effect activity. A clipper will then drop into
the trough and slide through northern Michigan on the nose of 115kt jet
streak. This will produce some snow showers over the area but will
also introduce the next shot of Arctic air. 850 mb temperatures will be
around -18c but even colder temperatures are not far off with northern Michigan
being closer to -25c. Mixing depths will increase further
approaching 5kft which combined with strong cold air advection and a tight pressure
gradient should allow wind gusts to approach 35 to 40 miles per hour during the
afternoon. This will be monitored closely as it is nearing Wind
A shortwave trough and resulting weak surface low pressure system
will move through the Great Lakes on Wednesday. This will bring
another chance for some snow showers to Southeast Michigan. There is
some disagreement among the models regarding the strength and track
of this disturbance. Therefore...have only included low chance probability of precipitation
for the Wednesday timeframe. High pressure returns to the Ohio
Valley and southern Great Lakes on Wednesday night and into Thursday
while another clipper system tracks across the northern Great Lakes.
This system will bring yet another chance for some snow showers on
Thursday night and into Friday to mainly the northern County Warning Area /Saginaw
Valley and northern thumb/. West-northwest winds will prevail though
the extended timeframe keeping high temperatures in the low to middle
20s through the end of the week.
Potential for a couple rounds of unsettled weather over the Great
Lakes the next few days. Combination of clipper systems diving
through the area with cold Arctic airmass in place will lead to
periods of gale or near gale force winds over Lake Huron. A low
pressure system lifting north through the area today will pull a
cold front through the region Monday morning. A gale watch is in
effect for most of the Lake Huron zones starting Monday afternoon.
Will hold off on upgrading to a warning as it is still a marginal
event with only sporadic pockets of 35 knot gusts. Have expanded the
watch to encompass outer Saginaw Bay and the nearshore zone from
Port Austin to Harbor Beach to account for channeling effects. A
second chance for gales will come on Tuesday with the next low
pressure system and reinforcing shot or Arctic air. In
addition...conditions will become favorable for freezing spray
across northern and central sections of Lake Huron early in the
Previous discussion...issued 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
gale watch...lhz361-lhz362-lhz363-lhz421-lhz441-lhz462...from 3 PM
Monday to 4 am Tuesday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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