Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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478
FXUS63 KDTX 162338
AFDDTX

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


.AVIATION...

A weak frontal boundary has become stationary over southeastern
Michigan this evening. Observationally, low level clouds have
cleared out south of Metro Detroit, while MVFR stratus holds strong
north of the frontal boundary which is along the I 94 corridor. The
cause of the clearing is the far southern forecast area has been
influenced by greater low-midlevel ridging and anticyclonic flow
trajectories. The forecast is a difficult one for clouds as the
frontal boundary is forecasted to move very little over the next 24
hours. There are three narratives the forecast will be based on. 1.
preference is to keep a prevailing MVFR cig height throughout the
period out of respect for this frontal boundary. Relying on the edge
of the MVFR to creep back southward tonight as the midlevel high
pressure center rolls to PA by morning. 2. Will be leaving all
freezing drizzle/fog out of the forecast tonight and Sunday morning.
Justification for this is lack of upstream observations/persistence
and there little significant low level moisture advection during this
time. 3. Introduced IFR and fog at the taf sites at the end of the
period. A surge of moisture is forecasted to reach southern Lower
Michigan after 00Z Monday as a cyclonic circulation center lifts out
of portion of MO and directly into southern Lower Michigan.

For DTW... Still expecting the front to drift back southward tonight
in response to high pressure pushing east of the cwa center. This is
expected to lead to an expansion of MVFR stratus. Little
observational support for any fog or drizzle precludes a mention in
negligible low level moisture advection setup. The potential for
drizzle and fog will exist with the frontal boundary in vicinity of
the airfield, better potential appears later Sunday.

//DTW THRESHOLD PROBABILITIES...

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

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

&&

.PREV DISCUSSION...
Issued at 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.

&&

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

$$

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


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