Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 041726
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1226 PM EST Mon Dec 4 2017


.AVIATION...

BKN/OVC VRF conditions are expected to persist throughout the
afternoon and evening hours as an advancing warm front continues to
produce expansive cloud cover across Michigan. As the surface
continues to heat, daytime mixing will promote southerly wind gusts
up to 20 - 25 knots throughout the afternoon. Low level moisture and
associated pop-up showers will then start to develop throughout the
evening hours, with rain showers becoming likely throughout the
early morning hours as better forcing associated with the warm front
edges into southern Michigan. Thunderstorms will also be possible
with scattered rain coverage mainly between 02 - 06Z, however,
confidence is low regarding thunder chances, thus have opted to
leave out of the TAF. A strong cold front will then sweep through
the region overnight and exit eastward by sunrise Tuesday. WSW gusts
around 35 knots will be common through Tuesday morning and
afternoon as the inflow of colder air helps promote mixing.

For DTW... Increasing south wind will be favorable for operations
through the day under VFR above 5000 ft. Ceiling is projected to
drop below 5000 ft and then in MVFR during the early morning hours
as a strong cold front approaches tonight.

//DTW THRESHOLD PROBABILITIES...

* Low for ceiling 5000 feet or less during the afternoon, medium by
  mid evening, high overnight.

&&

.PREV DISCUSSION...
Issued at 422 AM EST Mon Dec 4 2017

DISCUSSION...

NEAR TERM / TODAY THROUGH TONIGHT

As of 420 AM EST...After an extended period of abnormally quiet
weather for late November/early December, a major pattern shift will
begin today as a powerful low pressure system develops over the
northern Plains. This low pressure system will usher in a wintertime
airmass across the region that will remain locked in place through
next week.

For the remainder of tonight, a few more hours of dry weather is
anticipated as light southeast flow begins to ramp up, especially
towards daybreak in response to the developing surface
low/tightening pressure gradient. With slowly increasing boundary
layer moisture, some patchy fog will be possible through daybreak,
although generally only reducing visibilities into the 2-4 SM range
at worst. Low temperatures have likely bottomed out as low-level
warm air advection begins in earnest as the system`s developing warm
front surges northeastward across southeast Michigan this morning,
and will slowly rise through sunrise. Despite the top of the upper-
level ridge moving overhead, increasing diffluence aloft will
continue to allow high clouds to spill over Michigan, gradually
lowering to an overcast mid deck.

Heading into the daytime hours, the low pressure system will rapidly
develop as it tracks northeast towards western Lake Superior.
Impressive dynamics will accompany the low, as phasing of the
northern and southern streams ensues throughout the day. Shortwave
energy ejecting out of the northern Rockies will open up the door to
abundant northern stream PV energy, leading to increasing QG forcing
that will first manifest itself in a surge of low-level warm air
advection/isentropic ascent this morning along the warm front. Low-
level southeast flow and moisture transport vectors keep the best
condensation pressure deficits mainly to the north and west, and
given antecedent dry airmass in place it will be hard to see showers
develop across most of southeast Michigan through the morning hours.
Enough saturation may occur towards the Saginaw Valley where chance
PoPs are carried.

Tightening pressure gradient will lead to increasing south/southeast
winds throughout the day as the low rapidly deepens to sub-990 hPa
as it approaches western Lake Superior. As 850 hPa temps surge to 10-
12 C today, resulting inversion will limit mixing depths generally
below 2000 ft, but gradient flow will be more than sufficient to
yield frequent gusts of 25-35 mph. Strongest winds will be mainly
north of M-59, as areas further to the south are modulated somewhat
by a more stable marine layer off western Lake Erie. The southerly
flow will lead to one last day of mild temperatures, with highs
easily reaching into the mid 50s for most areas.

Rain chances increase heading towards this evening on the nose of an
impressive 60-70 knot LLJ. Aloft, some marginal elevated instability
will be present, along with Showalter indices dipping below 0 C.
Likely ongoing convection further south across Illinois and Indiana
will preclude the best boundary layer moisture from advecting into
the region, but steepening midlevel lapse rates in response to
impressive midlevel height falls will still support a mention of a
slight chance of thunderstorms from about mid afternoon through
midnight from the Thumb/Saginaw Valley and points south. Latest high-
res model guidance depicts scattered showers and a few storms
developing during this timeframe, with a possible pre-frontal broken
line of low-topped convection developing near the US/Canadian border
this evening ahead of an approaching strong cold front. With the LLJ
overhead, there is the concern for possible convective gusts tapping
into the jet and exceeding 50 mph, and this trend will need to be
closely monitored as the day progresses. Best potential for this
initial scenario looks to be in the 00z-08z timeframe.

More persistent showers move across the region with the cold front
tonight. Another line of enhanced showers looks to accompany the
cold frontal passage wind shift late tonight, with frontal density
circulations leading to another burst of convectively-driven wind
gusts that may once again exceed 50 mph. Cannot rule out a few more
rumbles of thunder with the front, as some guidance hints at some
surface-based instability developing. With the parent low so far to
the northwest, best moisture convergence will be across the upper
Midwest, so only expecting around a quarter to half of an inch of
rain with the cold front passage. Latest timing on the front has it
entering western areas 06z-09z, and exiting east of the Detroit
metro 10z-13z. Winds will shift to the southwest behind the front
and continue to remain gusty as cold air advection begins in earnest.

SHORT TERM / TUESDAY THROUGH THURSDAY NIGHT

Coupled upper-level jet on Tuesday will allow the low pressure
system to reach its maturity, nearing a minimum pressure close to
970 hPa as it stalls in central Ontario. Southern stream component
of the jet will reach 160+ kt as it moves overhead, with the
northern stream branch ushering in the wintertime airmass and
completing the pattern shift to a lake effect snow regime. At least
through the morning hours Tuesday it looks to be dry with the dry
slot moving by overhead. Continuing cold air advection will lead to
increasing mixing depths of 4-5 kft and efficient momentum transfer
within the boundary layer as low-level lapse rates steepen in an
unstable static stability profile. This will lead to continued gusty
winds across the region and a double punch first with the rush of
cold air compounded by wind chills plummeting into the 20s, and wind
gusts of 35-40 mph. Latest probabilistic guidance keeps gusts below
advisory criteria, but will have to see just how deep mixing depths
actually get with 40-50 kt winds near the capping inversion. High
temperatures Tuesday will peak early in the morning with low 40s
northwest to upper 40s southeast, falling into 30s throughout the
day.

High-res guidance and even some higher resolution global models are
signaling at the potential for snow squalls downwind of Lake
Michigan Tuesday evening, possibly impacting the evening commute in
a corridor mainly confined to between I-96 and I-69. Model soundings
indicate a favorable profile with steep low-level lapse rates
beneath a sharp inversion in the 7-10 kft layer, and a 1-2 kft
saturation layer with respect to ice in the -10 to -15 C layer. Will
need to pay close attention to this evolving forecast over the next
24 hours, as the potential for a high impact event, especially given
this being the first good setup of the season. Will raise awareness
in the HWO.

Lake effect snow pattern will continue into Wednesday and Thursday
as cyclonic flow lingers and thermal trough passes overhead (850 hPa
temps dropping to -13 C Wednesday). Scattered snow showers will be
possible across much of the region, with the potential for a more
significant band bringing a few inches of accumulation Thursday.
Wednesday will mark the first day in a stretch of below normal
temperatures, with high temperatures both days struggling to reach
freezing with lows into the 20s. It will remain breezy into
Wednesday, helping to reinforce the feeling of winter.

LONG TERM / FRIDAY THROUGH SUNDAY

The long term will feature continued amplified longwave troughing
across the eastern half of the CONUS, reinforced by additional
pieces of northern stream shortwave energy diving southward from
Canada. Lake effect pattern looks to continue, with a weak clipper
system possibly affecting the region this weekend. With the highly
amplified flow in place, difficult to ascertain much precision in
the timing and placement of activity, but the main thing is it will
continue to remain cold with chances of snow showers. Temperatures
will on average run about 5-10 degrees below normal.

MARINE...

Low pressure organizing over the Midwest today will move across Lake
Superior tonight and strengthen considerably while moving into
northern Ontario by Tuesday morning. This system will produce a long
period of gales and very high waves across Lake Huron today through
Tuesday night and into Wednesday morning. Farther south, the wind
field will be a little weaker with gusts around 30 knots expected
over Lake St Clair and western Lake Erie. A brief gust to gale force
is possible there, especially with the passage of the cold front
tonight.

The first phase of the gale event will be from the south today and
tonight peaking at high end gales with gusts near 45 knots over
central and north sections of Lake Huron during the afternoon. The
system will then bring a strong cold front across the region after
midnight followed by moderate southwest gales for the second phase.
Gusts near 40 knots are expected over all marine areas behind the
front while more concentrated over Saginaw Bay and central Lake
Huron Tuesday through Tuesday night. Strong westerly flow will
continue Wednesday while diminishing below gales during the morning.
In addition to the wind, an active pattern of rain and scattered
thunderstorms today and tonight will transition to lake effect snow
squalls Tuesday which will last through the end of the week.

HYDROLOGY...

Low pressure developing over the northern Plains Monday will rapidly
deepen into a powerful storm system as it tracks northeast across
Lake Superior and into northern Ontario Tuesday. Moisture from the
Gulf coast will stream northward into the Great Lakes ahead of this
system and interact with a strong cold front tonight. Rain showers
and scattered thunderstorms will become widespread over southeast
Michigan tonight before being swept eastward by the front toward
Tuesday morning. Rainfall totals will range from a quarter to a half
inch. Locally higher amounts are possible in any thunderstorms
during the peak of the event, mainly after midnight through 6 AM
Tuesday. Ponding of water on roads and in prone areas will be
possible but no other flooding concerns are anticipated.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning until 4 PM EST Wednesday for LHZ462-463.

     Gale Warning until 10 AM EST Wednesday for LHZ441>443-464.

     Gale Warning until 10 AM EST Wednesday for LHZ361-362.

     Gale Warning until 4 PM EST Wednesday for LHZ363.

     Gale Warning from 4 AM Tuesday to 7 AM EST Wednesday for LHZ421-422.

Lake St Clair...Small Craft Advisory until 4 AM EST Wednesday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 AM EST Wednesday for LEZ444.

&&

$$

AVIATION.....AM
DISCUSSION...IRL
MARINE.......BT
HYDROLOGY...BT


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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