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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
638 am EST sun Feb 1 2015

Aviation...

Warm advection light snow with IFR visibilities and ceilings has become
establsihed over all of southeastern Michigan this morning. The
light snow is expected to persist through the middle of the
afternoon. The strong winter storm system will then become in close
enough proximity by the 20z timeframe to allow for a change in
snowfall rates. Moderate to heavy snow is expected to develop...
impacting the Flint terminal southward between 22-4z. LIFR to vlifr
conditions are now forecasted in the tafs with the most impactful
portion of the winter storm. Reduced visiblities in blowing snow may
added in subsequent tafs.

At dtw...light IFR snow is expected to persist throughout the
mornign and into the middle afternoon hours. Snowfall rates are
expected to increase around 20z...to as high as in inch per hour
through 4z.

//Dtw threshold probabilities...

* high confidence in ceilings below 5000 feet through the taf period.

* High confidence in precipitation type of snow.

* Medium confidence in visibility dropping less than 1/2sm due to
heavy snow between 22 and 4z.

&&

Previous discussion...issued 418 am EST sun Feb 1 2015

Short term... storm term today and tonight

Satellite imagery and National radar mosaic this morning shows the
major winter storm system becoming increasingly more organized over
the central United States. Overall...the governing dynamics at the
hemispheric and synoptic scales has been addressed in earlier
discussions and remains valid. A check through iniitializations and
first 6 hours of nwp output yields no glaring deviations from
reality. Given the consistency of the model trend for
slower...wetter...and farther north...the changes this forecast
package were to increase forecasted snowfall amounts for all
areas...particularly during the 20-05z timeframe. Headline changes
are: a Winter Storm Warning is now in effect for the
shiawasee/Genesee tier of counties and the issuance of a Winter
Weather Advisory for the Saginaw Valley and northern thumb counties.

The light snow shield lifted across the in/Ohio borders this morning
on cue with precipation onset meeting earlier forecasts. This
initial light snowfall is being driven by outstanding isentropic
lift in combination with very favorable right entrance region
dynamics. In addition...it appears the lead shortwave energy that
was earlier associated with the north half of the Desert Southwest
circulation arrived on a good trajectory and is at least partially
responsible for the impressive Arch of 20-30 dbz echoes that is in
place over northern sections of Illinois and Indiana. Right
now...difficult to see this moderate to heavy snow band persist as
the cyclonic vorticity feature lifts steadily northward because of
the dependence on the strong low to midlevel Theta-E
gradient/baroclinic zone that is also in the same location. As the
vorticity feature lifts northward it will outpace the Richer
moisture. As a result...looking at a long duration of light snow to
persist through the noon hour/17z. This is supported well by the
forecasted soundings with a very stable thermodynamic profile
through 25 kft...but note the awesome depth of moisture. It will be
a real grind on amounts during the next 9 to 10 hours with 3 to 4
inches expected around Detroit and 2 to 3 inches for the northern
suburbs of Detroit up to Saginaw.

The high impact portion of the winter storm for southeastern
Michigan is expected between roughly 20-05z. It is at this time the
phased upper level potential vorticity anomaly and associated 850mb
cyclonic circulation will track through northeastern Illinois and
northern Indiana. Models forecast a contraction of the main
baroclinic zone downstream pivoting and lifting into the vicinity of
the far west end of Lake Erie...the rivers and Lake St Clair. Models
continue to highlight a well developed and long duration deformation
response striping directly over the area from 21-09z. Bulk of the
model guidance is tightly clustered with quantitative precipitation forecast over the County Warning Area...while
the NAM is on a total island. Have discounted the anomalous NAM
values...but did increase quantitative precipitation forecast amounts across the board per trends.
Snowfall totals are now expected to range 10 to 14 inches across the
far south...7 to 11 inches along the M 59 and I 69 corridors...5 to
7 inches Tri Cities...and 3 to 5 inches far northern
Thumb/Bay/Midland.

A few more important items to discuss. 1. Very cold surface
temperatures in the teens during the height of the event this
evening. As a result...salt preperation will likely have little
impact on treating surfaces. 2. Low confidence in snowfall ratios.
Generally do not like to forecast very high ratios north of 15:1 in
winter storm scenarios. The reason is wind will likely lead to
Crystal fracture and settling. Could also see a 10:1 bias in
accumulation reporting because of the blowing of snow/required
estimation. 3. Will reiterate one more time the height of the event
is expected this evening and may test some patience due to possible
headline fatigue.

Long term... Monday through Friday

Rapid departure of the middle level deformation to the east by late
Monday morning...with a corresponding increase in upper heights
filling eastward into the afternoon. Substantial drying through the
column under increasing middle level subsidence and deeper northwest
flow indicates a transition toward mostly sunny skies through the
day. The resident Arctic air will struggle to respond despite the
added insolation...temperatures Monday afternoon reaching the lower
and middle teens. Gradient will ease with time as surface ridging
builds into the Ohio Valley...but there will be a noted wind chill
element /readings below zero/ particularly through the first half of
the day. Some increase in high cloud will dampen the temperature
drop to some degree Monday night. Strength of the existing air mass
will still support lows making a run toward the lower single
digits...coldest locales perhaps dipping below zero.

Active northern Pacific upper jet will anchor several pieces of
shortwave energy into the existing quasi-zonal west-northwest upper
flow positioned across the northern Continental U.S.. lead wave remains
forecast to pass harmlessly to the south Monday night...with
attention focused on a trailing pv filament backed by more favorable
left exit region upper jet support. Some increase in large scale
ascent will result in a weak surface response over the Midwest by
late Tuesday...the attendant cold frontal boundary then lifting into
Southeast Michigan by Tuesday night. A modest period of moist
isentropic ascent combined with a quick but solid shot of left exit
jet dynamics will support an area of snowfall along the northern
flank of the surface wave. Some variability yet at this time scale
with the placement of the main stripe of middle level dynamics and a
corresponding ribbon of accumulating snow...the main feature of
interest still a good 36 hours away from proper sampling. Current
forecast will hold the line with a general 1 to 2 inch swath
centered on the Tuesday night/Wednesday morning period.

Stronger shortwave energy of Arctic origin will drive a more
pronounced southward plunge of lower heights into the Great Lakes by
middle week. This will result in the next Arctic intrusion...the
ensuing cold air advection bringing 850 mb temperatures below -20c
by Thursday. Low level anticyclonic flow filling in behind the
exiting middle level trough suggests a relatively quiet stretch of
weather to finish the work week.

Marine...

Northeast winds will strengthen today and tonight across the region
as a strong winter storm tracks through the Ohio Valley and lower
Great Lakes. This increase in the wind field will now bring the
potential for a period of gales tonight across southern and Central
Lake Huron...including Saginaw Bay. Increasing confidence in both
the duration and maginitude of these gusts now support the issuance
of a Gale Warning across the aforementioned locations. Gale force
gusts will remain too infrequent across the Michigan waters of Lake
Erie and will hold with gusts to 30 knots at this time. Winds will
diminish on Monday as the storm system exits to the east and high
pressure builds into the region.

&&

DTX watches/warnings/advisories...
Michigan...Winter Storm Warning until 7 am EST Monday for miz060>063-068>070-
075-076-082-083.

Winter Weather Advisory until 7 am EST Monday for miz047>049-
053>055.

Lake Huron...heavy freezing spray warning from 7 PM this evening to 10 am EST
Monday for lhz362-363-462-463.

Gale Warning from 7 PM this evening to 10 am EST Monday for lhz363-
441>443-462>464.

Gale Warning from 5 PM this afternoon to 4 am EST Monday for lhz421-
422.

Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&

$$

Aviation.....Cb
short term...cumulonimbus
long term....mr
marine.......mr

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