Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 130739
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
239 AM EST Sat Jan 13 2018

.Discussion...

Lake effect snow showers continue to buffet the immediate Lake Huron
shoreline with streamers extending south along the St Clair River.
The intensity of these snow showers has remained generally on the
light side with visibility restrictions at Port Huron and Narnia
fluctuating in the 3 to 5 mile (at worst). Hires models in general
maintain this activity roughly along the shoreline into the early to
mid morning before low level flow backs more to the northwest and
allows for a steady shift eastward out onto the lake. Given current
intensity trends, do not expect any more than an inch (locally two?)
of powdery snow over far eastern parts of St Clair/Sanilac counties.

Otherwise, very cold conditions will persist over the weekend as a
sprawling arctic high pressure builds aggressively east/southeast
through the eastern CONUS. Highs will range in the teens to near 20
within this airmass with lows in the single digits. With fresh snow
pack and relatively light winds Saturday night, would not be too
surprised to see a few locations below zero.

Attention then turns to a rather vigorous arctic shortwave which is
forecast to dig sharply south-southeast into the area from the
Arctic Circle, arriving in the upper midwestern states on Monday. As
would be expected with a system originating from this region,
moisture quality is a bit suspect with H85-H7 mixing ratios of only
1.0-1.5 g/kg. However, broad area of isentropic ascent in advance of
this feature will likely lead to widespread light snow across the
region. Models are actually in rather good agreement with both the
placement and strength of this system early next week, so confidence
in a light snow event is rather high for Monday with a few inches of
snow quite possible.

Surface troughing, enhanced by lake aggregate response, will focus
continued snow showers activity into Monday night and Tuesday as
this shortwave, by that time the major part of a high amplitude
positive tilted upper trough, works steadily east through the area.
With mesoscale factors beginning to come more into play in any
additional snowfall accumulations, confidence in the location of
best low level convergence to enhance/focus snow shower activity
decreases during this time frame.

This feature also ushers in more arctic air, sustaining very cold
conditions into the middle of next week as highs remain in the teens
and lower 20s and lows bottoming out in the single digits to lower
teens. Moderating temperatures, however, are expected from late next
week into the weekend as upper troughing over the eastern CONUS
weakens as best negative height anomaly pivots east/northeast into
the Canadian Maritime. This allows for a "wavy" quasi-zonal upper
pattern to set up over the northern CONUS which brings more Pacific
maritime air east over the CONUS. This would that temperatures will
return to at least the 30s for highs by Friday/Saturday.

&&

.MARINE...

North northwesterly winds will gradually weaken with time today as
high pressure building eastward from the northern Plains slackens
the pressure gradient. Elevated seas and wind gusts approaching 30
knots at times will still be possible for much of the day across
southern Lake Huron, but should remain below gale force. Continued
filtering in of cold air over ice free areas of Lake Huron will lead
to areas of freezing spray which may be moderate to possibly heavy
at times. Winds will decrease below 20 knots tonight and into
Sunday, becoming southwest ahead of a clipper low pressure system
that will track across southern Michigan Sunday night into Monday.
Increasing winds are expected with the arrival of the clipper
system, especially across northern and central sections of the Lake
Huron basin.

&&

.PREV DISCUSSION...
Issued at 1153 PM EST Fri Jan 12 2018

AVIATION...

Much drier low level conditions will exist through the period as
arctic air takes firm residence under prevailing northerly flow.
Western edge of the lake Huron moisture plume currently glancing
across the region will maintain a window for periodic MVFR stratus
early this morning. Otherwise, VFR expected through at least the
morning period. Existing gusty northerly conditions will gradually
ease through the morning period.  Some downstream penetration of
lake Michigan moisture may translate into a lower coverage of
stratus Saturday afternoon/evening as winds turn northwesterly with
time.

For DTW...Terminal will reside along the western edge of the lake
Huron moisture plume, maintaining the potential for pockets of MVFR
stratus during the early morning period.  Gusty northerly winds will
gradually ease throughout the morning, but keep sufficient magnitude
to maintain northeast operations well into Saturday.

//DTW THRESHOLD PROBABILITIES...

* Low for ceiling at or below 5000 ft early this morning.  Low
  again Saturday afternoon and evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Heavy Freezing Spray Warning until 11 AM EST this morning for
     LHZ362-363-441>443-462>464.

     Gale Warning until 11 AM EST this morning for LHZ441>443-462>464.

     Gale Warning until 7 AM EST this morning for LHZ362-363.

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

$$

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


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