DTX pulls trigger on advisory, no areas under warning despite 5-7" predicted

Update: Other regions have since followed suit, so the advisory is essentially for all of lower Michigan at this point. DTX's forecast discussion blog hints that some areas may yet be warned on if it looks like higher end snowfall totals will come into play:

...will be issuing winter weather advisory area-wide, with the heaviest snowfall expected to angle
southwest to northeast through the middle of the cwa. Within this corridor, future shifts can
monitor trends to determine if a narrow band of enhanced snowfall accumulations might occur and
warrant an upgrade to a winter storm warning for a few counties.

This is probably the highest end winter weather advisory I've seen issued to date, and I'm a bit confused as to why a warning wasn't put out for 5-7" predicted areas, especially given that it's the first storm of the year. Nevertheless, a winter weather advisory has replaced the winter storm watch for all areas of southeast Michigan.

Text of the advisory for the counties under the higher totals:

URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
325 PM EST FRI NOV 20 2015

...ACCUMULATING SNOW EXPECTED ON SATURDAY...

MIZ049-054-055-060>063-068>070-075-210500-
/O.UPG.KDTX.WS.A.0002.151121T1000Z-151122T0200Z/
/O.NEW.KDTX.WW.Y.0009.151121T1000Z-151122T0200Z/
HURON-TUSCOLA-SANILAC-SHIAWASSEE-GENESEE-LAPEER-ST. CLAIR-
LIVINGSTON-OAKLAND-MACOMB-WASHTENAW-
INCLUDING THE CITIES OF...BAD AXE...CARO...SANDUSKY...OWOSSO...
FLINT...LAPEER...PORT HURON...HOWELL...PONTIAC...WARREN...
ANN ARBOR
325 PM EST FRI NOV 20 2015

...WINTER WEATHER ADVISORY IN EFFECT FROM 5 AM TO 9 PM EST
SATURDAY...

THE NATIONAL WEATHER SERVICE IN DETROIT/PONTIAC HAS ISSUED A
WINTER WEATHER ADVISORY...WHICH IS IN EFFECT FROM 5 AM TO 9 PM
EST SATURDAY. THE WINTER STORM WATCH IS NO LONGER IN EFFECT.

HAZARDOUS WEATHER...

 * SNOW WILL DEVELOP ACROSS THE REGION EARLY SATURDAY MORNING AND
   INCREASE IN COVERAGE AND INTENSITY SATURDAY AFTERNOON AND
   EVENING. THE HEAVIEST SNOWFALL RATES ARE EXPECTED SATURDAY
   AFTERNOON AND EARLY SATURDAY EVENING. SNOWFALL TOTALS OF 5 TO 7
   INCHES ARE EXPECTED.

 * THERE REMAINS SOME UNCERTAINTY AS TO THE TRACK OF THIS
   SYSTEM...WHICH MAY IMPACT WHERE THE HEAVIEST SNOW OCCURS.
   TEMPERATURES NEAR THE SURFACE WILL ALSO AFFECT HOW EFFICIENTLY
   SNOW IS ABLE TO ACCUMULATE.

 * THE SNOW WILL RAPIDLY TAPER TO LIGHT SNOW SHOWERS BY LATE
   SATURDAY EVENING. TEMPERATURES WILL DROP BELOW FREEZING
   SATURDAY NIGHT WHICH COULD MAKE WET OR SNOW COVERED ROADS VERY
   SLIPPERY.

IMPACTS...

 * TRAVEL MAY BE ADVERSELY AFFECTED...ESPECIALLY SATURDAY
   AFTERNOON AND EVENING.

 * THERE IS A CHANCE OF A PERIOD OF HEAVY WET SNOW ACCUMULATION
   WHICH MAY BE ABLE TO BRING DOWN SMALL TREE LIMBS.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

 * A WINTER WEATHER ADVISORY MEANS THAT PERIODS OF SNOW WILL
   CAUSE TRAVEL DIFFICULTIES. BE PREPARED FOR SLIPPERY ROADS AND
   LIMITED VISIBILITIES...AND USE CAUTION WHILE DRIVING.

Other areas are forecast to receive 3-5 and 1-3 inches respectively (see previous outlook).

This is Michigan, so warning criteria tends to be a bit higher here than elsewhere. My thinking is that NWS's thinking is that this will be on the lower end of the scale in the most affected areas, with the "5" being favored over "7". I'd still advise major caution out on the roads tomorrow, and take all necessary preparations for a major storm.