Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 162043
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
343 PM EST Sat Dec 16 2017

.DISCUSSION...

Southeast Michigan will be under the influence of NW to W flow at
mid and upper levels today as the low pressure system that brought
snow showers yesterday through this morning departs to the east.
With it will go a lot of large scale ascent that would provide any
greater chances for substantial precipitation. This could be noted
in the gradual weakening trend from west to east on radar earlier
this afternoon. A stalled frontal boundary will linger around the
southern Michigan state line today with light easterly surface flow
north of the boundary associated with a high pressure system moving
across the northern Great Lakes. This is also leading to a
temperature gradient with slightly above freezing to the south of
this boundary this afternoon and 20s to the north. There is good
boundary layer moisture present, so some weak moist isentropic
ascent under W-SW flow in the lowest levels could contribute to some
snow flurries throughout the day. This SW flow is also leading to
some warm air advection aloft, which will likely lead to the loss of
ice nucleation with temperatures warming above -8C within the
saturated column. This will result in a tricky part of this forecast,
which is the potential for freezing drizzle to develop along and
just north of the boundary later this afternoon and early evening
where surface temperatures remain below freezing. Chances this
afternoon and evening remain very low and current trends do not look
to support any development, but the presence of the boundary
lingering around keeps a non zero chance of freezing drizzle. The
most likely locations for potential freezing drizzle would be within
the I-94 corridor and south given location of the boundary. Since
little impact is expected if enough local forcing occurs to squeeze
out precipitation, will leave out mention of any in the grids.

A southern stream shortwave ejecting out of the southern plains and
shearing out as it nears Michigan will bring continued chances for
precipitation along the boundary as it lifts north tomorrow. Forcing
along this front remains fairly weak and is lacking any deep
moisture above boundary layer. This will lead to a potential for
more light snow showers, flurries and patchy freezing drizzle again
through the morning and early afternoon. Little accumulation is
expected with this round as well. The warm air advection will help
bring surface high temperatures up to lower to mid 30s tomorrow.
More moisture will arrive along with attendant PV clipping the area
keeping precipitation chances in the forecast for tomorrow evening.
The warmer thermal profiles should result in mainly liquid
precipitation, though some snow flakes may mixed in cannot be ruled
out completely.

The pattern becomes less amplified for Monday and the passing wave
will help push some of the colder air out briefly allowing
temperatures to reach well above freezing for the entire area.
Potential for locations south of I-96 to reach into the low 40s and
and upper 30s to the north into the Thumb and Tri-Cities. Low end
chances for precipitation will remain across much of southeast
Michigan for Monday, but conditions look to be rather tame compared
to the last several days.

Benign, albeit breezy, weather expected Tuesday, and highs will
reach a few degrees above normal - high 30s north, low 40s south -
before cold frontal passage occurs in the late afternoon/evening.
The parent low pressure system will be located well to the north
over Hudson Bay where the best forcing and moisture convergence will
remain; not much more than a subtle shift in the wind from SW to W
or WNW and a decrease in temperatures expected with the front. Lows
Wednesday morning will round out in the low-mid 20s. Another quiet
day Wednesday as surface ridge traverses across the Great Lakes.
Highs will reach the low 30s around SE Michigan in the cooler
airmass.

The second half of next week will be impacted by a low pressure
system that will develop over the central plains and push
northeastward starting on Thursday. The warm front will lift through
SE Michigan during the afternoon and lead to a chance of showers
that will increase in likelihood and coverage overnight as the low
pressure center approaches. Precipitation type forecast confidence
is still low as models continue to diverge on placement and timing
of the low. If the low passes to the north, a rain to snow evolution
will be most likely, but if the low passes over Ohio, we will stay
on the cold side and an all-snow solution is more likely.
Regardless, widespread precipitation is expected Friday. Another
cold airmass will settle into the region for next weekend in the
wake of this system.

&&

.MARINE...

Generally favorable marine conditions will continue tonight through
Monday in terms of wind and waves. A weak front moving south of the
Michigan border tonight will return back northward during Sunday as
weak low pressure moves through the central Great Lakes. This system
will bring a light rain and snow mix to marine areas Sunday and
Sunday night before giving way to a larger and stronger low pressure
system early next week. The potential for westerly gales is
beginning to show with this system by Tuesday.

&&

.PREV DISCUSSION...
Issued at 1213 PM EST Sat Dec 16 2017

AVIATION...

The weak front stalled between PTK and DTW separates borderline
IFR/MVFR to the north from VFR to the south during early afternoon.
As light snow continues to diminish over the region, the front will
drift back southward during mid to late afternoon and bring renewed
coverage of MVFR ceiling into the DTW area while some breaks of VFR
occur toward MBS. The front is not projected to move much farther
than the Ohio border and so light drizzle and freezing drizzle
cannot be ruled out while the boundary is nearby, however upstream
observations support very low probability of occurrence for late
afternoon and evening. This will continue to be monitored tonight
through Sunday morning as the front moves back northward. Otherwise,
a mix of IFR and MVFR ceiling and possibly some fog will remain the
primary aviation concerns through the period.

For DTW... The front stalled just north of the terminal will slide
back southward during the afternoon. This will turn the wind back
around to NE and lower ceiling back down into MVFR during the
evening and into IFR late tonight and early Sunday morning. There
remains low potential for freezing drizzle while the front is nearby
to the south and as it moves back northward during Sunday morning.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less tonight through Sunday morning.
  Moderate Sunday afternoon.

* Low for freezing drizzle precip type tonight through Sunday
  morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...AA/TF
MARINE.......BT
AVIATION.....BT


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