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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
641 PM EST Sat Nov 22 2014



Aviation...

//discussion...

Low pressure organizing over southern Canada and the plains states
tonight will maintain milder southerly wind over Southeast Michigan through
Sunday. Low level wind shear remains in place tonight below 2000
feet in this pattern enhanced by limited boundary layer mixing.
Ceilings will be no better than MVFR through the period combined
with intervals of IFR in both ceiling and visibility depending on
mild air interactions with snow cover. With snow cover basically
gone at the metropolitan airports conditions have improved vastly. The
latest observational data suggests the next significant drop in
conditions is most likely Sunday morning as increasing surface
dewpoint combines with the next surge of rain showers. Expect the
showers to last off and on into Sunday afternoon with a modest
improvement to low end MVFR ceiling during the day.

For dtw... precipitation type will remain firmly in the form of all
rain going forward in the forecast through Sunday. Temperatures will
continue a steady rise even after sunset tonight with readings in
the lower to middle 40s before the next surge of rain showers begins
Sunday morning.

//Dtw threshold threats...

* high confidence in ceilings below 5000 feet.

* High confidence in rain as precipitation type today through
Sunday.

&&

Previous discussion...issued 420 PM EST Sat Nov 22 2014

Short term... late afternoon through tonight

Low pressure developing in central Canada into the northern plains
states will maintain southerly wind and a supply of mild air into Southeast
Michigan through tonight. After a slight early evening drop back
toward the upper 30s...temperatures will easily be able to remain
steady or slowly rise through the lower and middle 40s during the
night. This mild air is also moisture laden which will keep clusters
of showers going amid cloudy sky with fog and drizzle through the
region.

Middle afternoon observational trends favor the greatest coverage of
showers during late afternoon through middle evening followed by a
break during the overnight hours. Satellite and model analysis
Fields Point to another weak wave aloft helping to support clusters
of showers moving in from northern Indiana and SW lower Michigan.
The short wave forcing will interact with the low level Theta-E
ridge being drawn into Southeast Michigan around the 850 mb level. The
associated moisture transport and elevated instability is modest
which will limit shower intensity...and the wave is progressive
enough to slide through our area by midnight. The passage of this
feature will then limit precipitation to patchy drizzle within areas
of fog likely to form as increasingly mild air flows over snow
covered ground. Plan to monitor observational trends for dense fog
potential which should be limited to the lake effect enhanced M-59/I-
69 corridor and possibly points north. Snow cover is reported to be
diminishing rapidly to the south through the Detroit metropolitan area
toward the Ohio border.

Long term...Sunday through next Saturday

Warm and rainy pattern will continue Sunday and early Monday before
a cold front Ushers in strong winds and colder air later in the day.

Large upper trough over the Pacific northwest and northern rockies
will progress east towards the Great Lakes...scooping up the upper
low now seen on water vapor over Texas in the process. The remnants
of the Texas wave look to lift northward across lower Michigan
Sunday night...with the upper trough axis following by Monday night.
Models continue to key in on a band of isentropic lift progressing
northward over Southeast Michigan...with timing now ranging from
morning to afternoon. This band of ascent combined with ample
moisture remaining over the area should provide a good coverage of
rain showers tomorrow. Other concern will be continued warm air
advection over the cold ground where at least some snow cover will
remain. This could lead to some fog formation...but thickness may be
enough to have an impact to interests other than aviation. Better
influx of moisture and additional warm air looks to occur later in
the day and overnight as the upper wave from Texas approaches and
the low-level jet ramps up over the Ohio Valley ahead of the
advancing upper trough. Positive vorticity advection ahead of the approaching wave combined
with ascent along the warm front and a coupled jet structure should
provide deep layer forcing. This will combine with precipitable water values in
excess of 1.0 inches to provide a widespread rain event during the
evening and overnight. Quantitative precipitation forecast amounts look to range between 0.6 and 1.0
inch. Flooding does not look to be a concern given low current low
river levels and only a small amount of snow cover remaining.
Temperatures will remain very mild...supported by h850 temperatures
reaching around 8c in the warm sector. This should support maximum temperatures
in the upper 40s to low 50s tomorrow...with only a small drop into
the middle 40s for Sunday night.

Models solutions continue to diverge with the track and strength of
the surface low as it lifts across western and northern lower
Michigan Sunday night into Monday. Biggest implication with this is
the affect on and uncertainty with the wind speed forecast. Current
preference GOES to the last few Euro/Gem solutions due to good run
to run consistency and good overall consensus within the model
guidance envelope. This has an average low pressure of 974mb. 12z
NAM was not too far off this and shows good descent behind the cold
front along isentropic surfaces between 280-290k (into the ground).
This descent combined with steepening lapse rates/deepening boundary
layer from strong cold air advection...a tight surface pressure
gradient...and a core of 40-55 knot winds between 900-800mb may
allow very strong wind gusts to reach the surface. Current forecast
highlights gusts 40 to 45 miles per hour...with potential to reach 50 miles per hour.
Headlines are a possibility...but there is still plenty of time for
things to change. A few rumbles of thunder may also be possible
early on Monday in the warm sector as instability increases...and
possibly becomes surface based. Rain will change to snow behind the
front as temperatures fall during the late afternoon and
evening...with light accumulations possible in the evening and then
overnight as lake effect processes ramp up. Drier air will filter
into the region...potentially clearing moisture out of the dgz in
the evening per 12z NAM. Gem/Euro try to brush the northwest corner
of the forecast area with deformation lingering behind the departing
system...something to keep an eye on for the Saginaw Valley but
accumulations look light at this time.

Low pressure tracking northeast of the region will continue to bring
a low chance for snow showers to Michigan on Tuesday. Breezy
conditions will continue through the day with gusts still in the 20
to 30 miles per hour range. High pressure creeping in from the southwest will
bring dry conditions to the area...but Arctic air intrusion comes
back in place with temperatures 10 to 15 degrees below normal. A clipper
system could bring accumulating snow on Thanksgiving...more details
when nwp comes closer to a consensus.

Marine...

Southwest winds will continue to decrease through the early evening
across the central Great Lakes...allowing winds waves over Lake
Huron to gradually subside. Winds will shift more southerly Sunday
and Sunday night as a large low pressure system begins to lift
towards the area from the plains. Warmer air will also continue to
surge into the area...providing a fairly stable environment over the
Warm Lake waters. This should help limit gusts to the 15 to 25 knot
range Sunday and Sunday night.

The low pressure system will lift across the western and northern
Great Lakes Sunday night through Monday...sending a strong cold
front across the area Monday afternoon. Gale force winds look likely
behind the cold front Monday and Monday night as a tight pressure
gradient develops and much colder air rushes into the area...with
potential for wind gusts to exceed 45 knots. There is still some
uncertainty with how much this system will deepen and the exact
track it will take across the Great Lakes. Further adjustment to the
forecast is possible. If the system deepens further than currently
anticipated...storm force wind gusts could develop over portions of
Lake Huron. Those with interests in the Great Lakes Monday and
Monday night should follow changes in the forecast closely. A gale
watch is now in effect Monday and Monday night for all marine areas.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...gale watch from Monday morning through Tuesday morning for
lhz361>363-421-422-441>443-462>464.

Lake St Clair...gale watch from Monday morning through Tuesday morning for lcz460.

Michigan waters of Lake Erie...gale watch from Monday morning through Tuesday morning for lez444.

&&

$$

Aviation.....Kurimski
short term...bt
long term....hlo/mm
marine.......hlo



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