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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1203 PM EST Sat Dec 20 2014



A thin layer of moisture trapped under a deep low level inversion
with very little flow in the boundary layer will support a
persistence forecast through the remainder of the afternoon. So the
MVFR stratus deck that has been covering the metropolitan Detroit and ptk
tafs should last well into the evening. Fnt will remain on the edge
of this cloud deck leading to the potential for more variability in
cloud coverage. The expectation for some continued diurnal
enhancement will support a predominate broken coverage at fnt.

For dtw...there has been a region of clearing across south central
lower Michigan as some drier air has funneled through the inversion. A
weak low level west-SW flow will develop later today...which may bring
some degree of this clearing into metropolitan late this evening or
overnight. The expectation for at least some diurnal enhancement
this afternoon and the persistence of the low level inversion
tonight will support maintaining continuity with the previous
forecast in holding an MVFR ceiling through the taf period.

//Dtw threshold threats...

* high confidence in ceilings below 5000 feet this afternoon and
evening. Medium confidence overnight and Sunday morning.


Previous discussion...issued 300 am EST Sat Dec 20 2014

Short and tonight

Impressive upper level ridge over Quebec with bit more chaotic
pattern over central North America...with at least three distinct
upper waves seen on the latest water vapor imagery. These
features have little moisture to work with...and are mostly
resulting in just middle/high clouds. Majority of upper level pv will
be tracking south of Michigan border today/tonight. Main impact
will be modest moisture advection through tonight...with the very
light wind field shifting to the southwest. This is expected to
allow the low clouds (2-3 kft) currently along the I-94 corridor
to creep north through the day into tonight. Even if low clouds
fail to make much progress past the M-59 corridor...high clouds
will be plentiful...assuring mostly cloud skies for majority of
the County Warning Area. Due to the increasing clouds and with the current
temperatures in the middle teens to around 20 degrees across the
northern two thirds of the County Warning Area this morning...would expect maximum
temperatures to struggle to reach the freezing mark. Clouds and modest
moisture advection (surface dew points rising into the low/middle 20s)
tonight should lead to small diurnal swing...holding temperatures/mins
in the middle 20s.

Long term...Sunday through next Friday

The quiet weather pattern continues into Sunday as high pressure
begins to build off to the east of the region. With the beginning of
return southerly flow on the backside of this high...temperatures
will rebound into the middle 30s even under mostly cloudy skies as middle
to upper level moisture streams into the central Great Lakes within
southwest flow regime developing in advance of developing storm
system over the northern/Central Plains.

The stream of moisture in advance of this developing upper
trough/low will continue into early next week with precipitation
chances increasing steadily from Monday into Tuesday. All of this
precipitation should fall as rain showers as the steady stream of
mild and moist air quickly replaces any remaining cold air that is
positioned over the area early Monday.

The forecast remains quite difficult heading into the middle of next
week as medium range model runs vary on both the west/east position
of this aforementioned upper trough within the northern stream and
also with the degree of shortwave energy that will wrap into this
trough from the southern stream. Trends since 00z yesterday have
most definitely shifted westward with the track of any storm during
this time frame...with the eventual low pressure center now tracking
northward over or west of the area. In fact...GFS/Gem track a
rapidly deepening low pressure system straight north into lower
Michigan with European model (ecmwf) taking a similar deep low pressure along the
eastern edge of the County Warning Area into Lake Huron in the Wednesday/Wednesday
night time frame. It should also be noted that a majority of GFS
ensemble members focus primarily on the Pivot Point low pressure
well to the northwest under the northern stream upper low pressure
with only minor/modest development of the secondary low from the
southern stream system.

All in all...this steady trend westward suggests a better chance at
rain well into Wednesday before the system lifts north of the area
and cold air wraps into lower Michigan from the south/southwest and
changes any wraparound deformation precipitation to snow with time
Wednesday evening/overnight. Will make shifts in both the precipitation
type and temperature forecasts into Wednesday with a transition to
snow Wednesday night. Even with the uncertainty in precipitation
type/ is becoming increasingly likely that the track
and strength of this system will be conducive to rather strong winds
as it passes by/through and then north of the region.


Surface high pressure will remain over the area through much of the
weekend...providing generally calm conditions. This high will then
track into the eastern Great Lakes by Sunday night with southerly
winds increase somewhat Sunday night into Monday as a weak middle-level
trough then tracks through the northern Great Lakes. In addition to
increased winds...the trough as the potential to bring some light
snow showers to Northern Lake Huron by Sunday night. Active weather
will return to the Great Lakes for most of next week as a large
storm system develops to the west and eventually shifts into the
area. While uncertainty in the details of this storm remain rather
high...expect precipitation chances to increase substantially from
early to middle week with strong winds possible by middle week as low
pressure lifts through the region.


DTX watches/warnings/advisories...
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.


short term...sf
long term....dg

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at www.Weather.Gov/Detroit (all lower case).

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