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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
904 PM EST Monday Dec 22 2014

Update...

Forecast is largely on target. Minor changes were made with the
most recent update. Increased probability of precipitation for the southwestern portion of
the forecast area closer to 100 percent. They are certain to get
the light showers as they are closer to being saturated with the
first waves of light rain/virga and given the showers over
Indiana that will be moving north-northeast into the southwest third of the
forecast area. Made the temperatures more steady instead of rising
a bit given the lack of warming this evening to our south in
northern Indiana...SW lower and Northern Ohio.

As for the freezing rain potential...left a chance thorugh 06z in
the Tri Cities with the lighter precipitation. Temperatures remain 32 to
35 degrees with dew points are around 30. Expect a little wet
bulbing at the surface at or just below freezing before a rebound
with the slight heavier rain. At the same time it will continue to
warm above the surface layer and that falling rain will also
become increasingly warmer too.

&&

Aviation...issued 615 PM EST Monday Dec 22 2014

Moisture continues to increase this evening...but the dry air under
10k feet is taking a little more time to overcome. Will push back
the ceilings below 5k feet to later this evening. Deeper plume of
moisture will lift north tonight mainly after 06z which will
introduce MVFR/IFR ceilings and visibilities as rain showers spread across the
state. Conditions will deteriorate to MVFR between 04-08z with IFR
possible around 10-14z. One additional concern remains for mbs...and
possibly fnt...is that sub freezing temperatures at the surface may lead
to a couple hours of light freezing drizzle at the onset before
rainfall rates increase and temperatures rise slightly. Best chance
for this would be between 03-06z. Confidence remains low at the
moment so will continue to leave out of the tafs.

For dtw...ceilings around 4000ft will lift north over the terminal
this evening as deeper plume of moisture lifts into the region. MVFR
rain showers will begin around 06-08z with IFR ceilings/visibilities possible
between 11-14z. Flow will be out of the east/southeast as the surface warm front
remains to our south.

//Dtw threshold probabilities...

* high in ceilings at or below 5000 feet after 03z tonight.

&&

Previous discussion...issued 337 PM EST Monday Dec 22 2014

Short term...through tonight

The beginning stages of the active upper level trough will commence
later this evening as the isentropic ascent along the elevated warm
front starts to ignite over the area. An initial ribbon of moisture
is currently working northward but with deep layer of dry in the
lowest 10kft of the column...this moisture is only acting to prime
the upper levels for the deeper plume of moisture further south and
west. This enhanced plume on the nose of the 850mb Theta-E gradient
will lift north through the area between about 03-06z. Deep layer of
southerly flow will lead to strong warm air advection with 900mb temperatures approaching
5c by 00z Tuesday. Warming continues with temperatures above 0c extending up
to 800mb. Temperatures at the surface will be slower to climb as the surface warm
front remains to our south through the overnight. This will mainly
be a non issue with dewpoints climbing into the low/middle 30s and warm
air aloft...precipitation should fall as rain for the majority of Southeast Michigan.
The area of concern for some frozen form of precipitation will up across
our northern counties...parts of Saginaw/Midland/Bay. This area will
possibly see surface temperatures hold at or below freezing at the onset of the precipitation
which produces a thermal profile which could result in some light
freezing drizzle. If the rainfall rates get high enough at the start
it may decrease chances of refreezing on cold surfaces. At this
point...will go with a light rain or freezing drizzle forecast as it
is not a sure thing by any means and will then monitor observation this
evening and handle accordingly.

Long term...

Focus continues to be on impacts associated with strong cyclogenesis
Christmas evening. Strong upper jet /E.G. 175 knots sampled by Spokane Washington
12z radiosonde observation/ will amplify the central Continental U.S. Longwave trough already in
place. Weaker upper wave will shear out while associated surface
reflection dissipates on Tuesday...giving way to stronger energy
rounding the base of the trough. This energy will lift
northward...becoming negatively tilted and closing off Wednesday.
Associated surface cyclogenesis will take advantage of preexisting
baroclinic zone...deepening around 20 mb from 06z Wednesday-06z Thursday
as it lifts from the deep south toward Lake Huron.

Models generally in good agreement through about 06z Wednesday with
initial wave and accompanying moisture surge. Expect an area of rain
showers associated with a warm front to be ongoing at 12z Tuesday
and translating northeastward across the local area. This shower pattern
should diminish from SW to NE from the late morning into the
afternoon as we briefly get into the warm sector of the decaying
initial cyclone and deep-layer moisture decreases. As such...tapered
probability of precipitation to chance by afternoon. Could still be some areas of drizzle
around though with plentiful lower clouds. Mild high temperatures in the
40s.

After 06z Wednesday...well-advertised model differences in the secondary
/stronger/ surface low have caused headaches and uncertainty for
previous forecast packages. The good news is...at least for the 12z
model suite...a consensus appears to be emerging. The
consensus track...typified by the 12z GFS/ECMWF/ggem...takes the
strengthening surface low from around Nashville Tennessee at 12z
Wednesday...northward to Lake Huron by around 06z Thursday...straight through
or just west of DTX County Warning Area. The 12z European model (ecmwf) especially increases
confidence in this warm solution as it has trended toward the
GFS. Basically disregarded the 12z NAM eastward cold outlier.
Despite this emerging consensus...there are still differences in
the timing and strength of the surface low.

Gulf moisture will re-enter the region late Tuesday night with 850
mb dewpoints increasing to 7-10c during the day Wednesday and
precipitable waters around 1 inch per the GFS. Lift will increase as well due to
system-relative isentropic ascent...fgen forcing with baroclinic
zone tightening with time...and DCVA with deepening negatively-
tilted upper wave. The result will be a wet and mild Christmas evening.
With the track of the low-level cyclone over or to the west of our
region...precipitation will be solidly liquid during the day.

After 00z Thursday any precipitation will change to snow as the low moves
off to the north and colder air comes in on the southern flank of
the low. The question is...how much forcing/moisture will be
available on the back side of the low. 12z suite has cast doubt on
snow potential with a dry slot over much of Southeast Michigan and the main
deformation snow just to our north and west. However...with well-
publicized model consistency issues...will continue with the 1 to 3
inch idea until we can get a little more run-to-run consistency. A
little eastward shift is definitely not out of the question...which
would put the deformation snow over Southeast Michigan. Regardless...low temperatures
should still be rather mild...mostly around freezing as the cold air
on the backside of this system is not all that cold...with 850 mb temperatures
around -6. Other story will be the winds...with the low still
deepening to our north. Kept the idea of 35 miles per hour wind gusts Wednesday
night.

Kept dry forecast Christmas day as aforementioned cold sector air
that is not actually that cold should not cause much of a Lake
Michigan response...and we actually get 850 mb warm advection later in
the day. Rather breezy conditions can be expected much of the
day...though winds will decrease with the diminishing pressure
gradient through the day.

Marine...

Southeast winds will increase during the night and into Tuesday
morning as low pressure moves from the northern plains into the
western Great Lakes. This system will draw warmer air atop the
lakes...thus limiting the mixing potential. This will generally hold
peak wind gusts below 25 knots. The exception will be north of
Presque Isle light where channeling and a little better over lake
instability will push wind gusts toward 30 knots.

The gradient will relax Tuesday night into Wednesday...leading to a
substantial decrease in the winds. An intensifying low pressure
system is then forecast to lift into the Great Lakes region from the
deep south Wednesday and Wednesday night. There remains some uncertainty as to
whether or not this low will track across lakes Erie and Huron or
track across lower Michigan late Wednesday/Wednesday night. In addition...there is
also a good deal of uncertainty as to how quickly this system will
lift north across the region. There is however growing confidence
that this low pressure system will rapidly deepen as it tracks north
across the region. So a gale watch will be issued Wednesday night and
Thursday. Further refinements in timing and wind directions are likely
over the next two days as confidence increases on the timing and
track of this forecast low pressure system.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...gale watch from late Wednesday night through Thursday afternoon for
lhz363-421-422-441>443-462>464.

Small Craft Advisory until 7 PM EST Tuesday for lhz421-441-442.

Gale watch from late Wednesday night through Thursday afternoon for
lhz361-362.

Lake St Clair...gale watch from Wednesday evening through Thursday morning for
lcz460.

Michigan waters of Lake Erie...gale watch from Wednesday evening through Thursday morning for
lez444.

&&

$$

Update.......rbp
aviation.....Rbp
short term...drk
long term....dt
marine.......SC



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