Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 161713
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 354 AM EST Sat Dec 16 2017

DISCUSSION...

Southeast Michigan residing within an energetic mid level northwest
flow early this morning, sandwiched between an exiting clipper
system shearing away from northern Lake Huron and shortwave ridging
easing into the plains. Weak frontal boundary embedded within the
resultant contracted height gradient responding much more
dynamically than anticipated 24 hours ago, as fgen forcing reacts
favorably to a period of left exit region upper jet dynamics and
weak isentropic ascent within the 925-700 mb layer. The net result
is a widespread area of light to briefly moderate snowfall impacting
a broad corridor between I-94 and the Tri-cities this morning.
Latest hi-res and local probabilistic model output direct the
greatest response across western portions of the I-69 and M-59
corridors /areas west of I-75/, where accumulation potential upwards
of 2 inches will be plausible through the morning.  Elsewhere,
accumulation within the half to inch range certainly in play, with
some southward drift of the frontal slope back down into the ARB-DTW
corridor expected as forcing starts to wane late morning/early
afternoon.

Large scale ascent becomes quite limited this afternoon. However,
moisture remains plentiful up through the nearly stationary 850-925
mb frontal slope parked overhead. This suggests there remains some
potential for a more contracted region of light snow showers or
flurries to persist this afternoon and evening, as continued mid
level southwest flow sustains very weak moist isentropic ascent
across the frontal slope. Worth noting that a warming mid level
environment above the boundary will start to introduce concerns for
potential loss of ice nucleation, as column saturation may
eventually shift above the -8C isotherm. Thus, not out of the
question for a period of patchy freezing drizzle to emerge directly
below the frontal interface. Favored locations for late day activity
aligned somewhere within the M-59 to I-94 corridor, where the
boundary is projected to reside.

Fairly neutral pattern for near surface thermal advection today,
given retention of a light easterly flow and the extensive cloud
deck. A general modification of the existing air mass under building
heights off the midweek arctic plunge will still bring afternoon
highs back to near or a touch above those noted yesterday. Readings
will range from mid 20s north to near freezing close to the Ohio
border.

Pesky frontal boundary lifts north tonight in response to increasing
mid level warm air advection as shortwave ridging folds into the
area.  Some light snow showers/flurries/patchy freezing drizzle
remains a possibility as this process unfolds, focus shifting north
of M-59 overnight.  Little to no accumulation anticipated. High
pressure anchored north of lake Huron will retain easterly flow at
the surface, ensuring a standard nocturnal cooling response
commences as opposed to a warming surface layer. Lows bottoming out
in the mid teens to lower 20s Sunday morning.

Relatively benign conditions for the bulk of the Sunday period, the
region positioned between departing upper ridge and a deamplifying
southern stream shortwave shearing out of the southern plains.
Recent model guidance trending toward the south with this wave,
simply providing far southern lower Michigan with a glancing shot
late Sunday. Weak but sustained warm air advection preceding this
system will lift enough of the thermal profile above freezing to
support mainly light rain should the northern extent of any
attendant forcing manage to clip the region late afternoon/early
evening. Highs on Sunday lower to middle 30s.

A brief transition of the larger scale pattern toward lower
amplitude westerlies will provide a period of milder conditions -
relative to this recent stretch - for the early week period.  The
modest recovery in upper heights combined with continued low level
southwest flow will bring highs to around or just above normal on
Monday /upper 30s-lower 40s/. Northern stream wave shearing across
southern Canada will send a moisture starved weak cold front through
the region sometime within the Monday night or Tuesday period.
Perhaps a few light rain or snow showers depending on the timing,
but largely looking at benign stretch of weather given the lack of
meaningful forcing or cold air advection to generate a potential
lake effect contribution. High pressure dominates Wednesday, with
attention then turning to a possible stronger system to finish the
week.

MARINE...

A cold frontal trough is currently draped across the central Great
Lakes and is slowly drifting southward this morning. Outside of some
snow showers, the front will feature a wind shift from southwesterly
to easterly this afternoon. Winds on either side of the boundary are
less than 20 knots with waves on their way down. High pressure will
slide across the northern Great Lakes today and tonight leading to
continued light winds through the rest of the weekend.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....BT
DISCUSSION...MR
MARINE.......DRK


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