Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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853
FXUS63 KDTX 150745
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
245 AM EST Sun Dec 15 2019

.DISCUSSION...

Cold frontal passage early this morning will bring a period of light
snow shower activity to the area, enhanced to some degree by plume
of low level moisture off Lake Michigan. This activity will settle
south through the forecast area during the pre-dawn hours and then
generally be limited to areas south of M-59 or so by 12z. Any
lingering lake effect plumes within cold air advection will also
orient northwest to southeast into the far southern portions of the
forecast area. This will be minor snow shower activity given rather
shallow low level inversion heights. Drier low level airmass with
building surface ridge will also limit (and eventually end) this
activity.

Main focus of this forecast will then turn to synoptic system(s)
late tonight into Monday night. Model solutions generally remain
progressive with the upper flow pattern as 150 knot sub-tropical
jet arcs east/northeast into the eastern CONUS around shortwave now
over the central plains and well in advance of a notable Pacific
shortwave now progressing onshore over Oregon and northern
California. The lead jet dwarfs energy digging in behind the Pacific
system (which is on the order of 100 knots in terms of the jetlet
speed). This further supports a progress track for this system

The trajectory of the lead jet would keep the right entrance region
south of the forecast area and support a general model consensus of
keeping all/most of snow from these shortwaves to the south of lower
Michigan. This will certainly be the case for the lead shortwave as
snow shield expands east into the Ohio Valley late today/tonight
with little to no pivot north as the wave tracks into zonal and
slight confluent upper level flow.

There remains some uncertainty with regard to the second more
dynamic shortwave now over the western CONUS which will generally
maintain its amplitude a bit better than the lead wave. Shortwave
energy will also be pivoting around upper low positioned over the
far north central portions of Canada and digging through central
Canada (and eventually into the northern Great Lakes). The initial
interaction of these streams will allow the upper pattern to buckle
over the plains with an upper trough developing. Shortwave ridging
downstream of this, while subtle, may be enough to coax entrance
region of the subtropical jet closer to the forecast area as the
southern wave approaches.

So far, the satellite representation (including a decent look from
GOES-15 in this fairly data void satellite area) of the lead
vorticity spokes pivoting around northern upper low suggest this
energy may be digging a bit sharper southwest into the Northwest
Territories. If the trend were to persist, a slightly deeper mean
upper trough would result and the odds that some light deformation
snow from the southern system would lift into southern parts of the
forecast area would increase. Given this, will not abandon the idea
of some light accumulating snow over far southern areas completely
despite a definite consolidation of most model solutions in that
direction.

That said, there is likely a rather hard limit to now far north the
deformation can stray as the overall pattern, including the northern
stream energy, remains very progressive and will tend to support
better phasing of the southern/northern streams to the east of the
region. So, feel comfortable leaving northern portions of the
forecast area dry with increasing chances of light accumulating
southward (and particularly closer to the Michigan and Ohio state
line).

A colder pattern becomes established into the middle of the week
once this southern shortwave passes and the northern vorticity digs
through Ontario and allow for a brief intrusion of arctic air into
the Great Lakes. The coldest day will be Wednesday when temperatures
hold in the 20s throughout the day. The overall upper pattern does
remain progressive though and this bubble of arctic air moderates as
the northern upper low pivots into the Canadian maritime. A wavy
zonal pattern then expands east over the CONUS in the wake of this
system and brings moderated Pacific air back east through the CONUS
and cuts off any further intrusion of arctic air for the remainder
of this forecast period (and beyond). This will result in moderating
temperatures back to around 40 for week`s end.

&&

.MARINE...

A Gale Warning will remain in effect through the morning hours as
marginal gales affect the northern half of Lake Huron. Increased
northwest flow is also bringing hazardous boating conditions for
small crafts impacting the Thumb nearshore zones with elevated wave
action. These conditions will persist into the afternoon. Winds and
waves begin to ease up throughout the afternoon today as the low
pressure system lifts further off to the northeast and replaced by a
weak high pressure moves into the northern Great Lakes. Winds remain
light across the lakes on tomorrow into Tuesday while a low pressure
system stays to the south across the Ohio River Valley, but brings
chances for snow showers to the southern lakes on the northern
fringe of the system. A more impactful surge of arctic air with a
polar low pressure system will arrive late Tuesday into Wednesday.

&&

.PREV DISCUSSION...
Issued at 1154 PM EST Sat Dec 14 2019

AVIATION...

A steady flux of low level moisture within cold west-northwest flow
will maintain widespread MVFR stratus through Sunday morning.
Potential for some lake effect snow showers will exist through the
early-mid morning hours. Outgoing forecast continues to highlight
the most likely window for development, calling for a modest
response with limited accumulation potential. Gradual low level
drying will ease ceilings into VFR as stratus lingers throughout
Sunday. Moisture may prove sufficient to offer some flurries into
the afternoon, but additional meaningful activity is not expected.

For DTW...Extensive lower stratus will hold firm through the night.
Window for snow shower activity centered mid morning Sunday /09z-
13z/. Any accumulation will remain minor.

DTW THRESHOLD PROBABILITIES...

* High for cigs below 5000 ft through Sunday

* High for ptype as snow.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 5 PM EST this afternoon for LHZ421-441-
     442.

     Gale Warning until noon EST today for LHZ361>363.

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

$$

DISCUSSION...DG
MARINE.......DG
AVIATION.....MR


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