Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181000
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
500 AM EST Sat Jan 18 2020

.DISCUSSION...

The first band of snow lifted into SE MI right on schedule with KADG
(Adrian) reporting first -SN obs in the 03Z METAR. Mosaic MRMS
composite reflectivity continues to show various waves of banded
snowfall moving from SW to NE as KDTX indicates some +25 dBZ returns
correlating with reductions in visibilities down to 1/2SM. Several
early reports over 1 in/hr rates have come in as a good 1-3 inch
base holds over portions of the Warning area.

With regard to the surface low track, models remain fairly
consistent with placing the primary center near where the Upper
Mississippi intersects the 43rd parallel at 12Z. Surface dewpoint
depressions have been slightly subsaturated this morning which has
helped maintain relatively normal snow character (neither wet nor
dry). 925-850mb moisture surged in along 20C gradient over the span
of 30 miles when crossing through the low-levels earlier this
morning. Thermal profiles will start to become more isothermal in
the lowest 300 mb before the warm nose punctures the 850 mb level
signaling the transition away from snow. Limited refinements were
made to snowfall amounts for the rest of the event as uncertainty in
SLR reductions linger. Generally, 10:1 to 14:1 rates should hold
across the region through 12Z with perhaps an earlier degradation
for southern Lenawee and Monroe counties as mixed precipitation and
rain is being reported slightly south of the MI/IN/OH borders.
Periods of heavier snowfall have been noticed early today within
occasional convective structures which have the full support of
lower static stability and favorable ascent. A FGEN band is
currently moving in displaying impressive returns, thus will
continue to hang onto the mention of +SN over the WS.W counties.
With the exception of localized areas seeing 8 inches (perhaps 9),
most localities will end up in 5-7 inches range. Accumulating
snowfall will continue over the Thumb into the early afternoon hours
given the delayed WAA intrusion, thus the uptick in storm totals
there.

The converged wave aloft will pivot into the Ohio Valley forcing the
low to make landfall near Ludington shortly after 18Z by which time
the warm sector will be in full force. The progression of the
changeover looks to be perhaps be delayed by an hour or so with most
areas holding onto snow until 13Z. Areas south of I-96 will see at
least some rain or drizzle mix in from 12-15Z as the warm/occluded
front lifts north. All areas will see some degree of rain after 18Z.
Expanded the rain chances further north (and earlier) with this
forecast cycle even as temps were pulled back a bit. Surface
temperatures will still climb above freezing area wide late today as
southeasterly flow veers southerly then back to the west as the dry
slot fills in with stronger winds as the gradient is maximized. 20-
30 mph sustained winds will be possible.

Temperatures will fall into the upper teens to lower 20s overnight
as confluent anticyclonic flow advects colder Canadian air into the
Great Lakes for the next several days. Moisture will be re-sourced
from the Central Lakes producing scattered lake effect snow showers
on Sunday. Hi-Res guidance is already hinting at a decent leading
convergence band pushing south from Saginaw Bay by Sunday morning.
Given the resident airmass and cold temperatures, could see a
coating up to an inch for areas near the Thumb. This strong high
will take hold over the central Plains under an expanding ridge
which will maintain general NW flow pattern through Tuesday.

&&

.MARINE...

A low pressure system will deepen while tracking northeast through
the central Great Lakes on Saturday bringing a period of heavy wet
snow through Saturday morning before exiting off to the east on
Sunday. Winds gusts will likely reach low-end gales across Lake
Huron with a a Gale Warning in effect on Saturday across the open
waters of the lake. Winds will then switch to strong westerly late
Saturday behind a cold front. A Gale Warning will go into effect for
western Lake Erie and Lake St. Clair Saturday afternoon through
early Sunday.

&&

.PREV DISCUSSION...
Issued at 1151 PM EST Fri Jan 17 2020

AVIATION...

A widespread accumulating snowfall on pace to advance into the SE
Michigan airspace overnight. Steadily deteriorating conditions as
intervals of moderate to heavy snow maintain IFR to LIFR
restrictions throughout the early-mid morning hours. Winds from the
southeast near 10 knots during this time. Snow rapidly diminishes in
both coverage and intensity just as milder air lifts into the
region. This will transition precipitation type to simply areas of
drizzle from south to north late morning and early afternoon.
Temperatures hovering near the freezing mark may provide a brief
window for freezing drizzle at MBS. Winds strengthen from west-
southwest during the late afternoon, reaching in excess of 30 knots
at times through early Saturday night. Colder air accompanying this
wind shift will bring improving ceiling heights, while introducing
the potential for lake effect snow showers throughout the Saturday
night period.

For DTW...A steady increase in snowfall intensity expected
throughout the early morning hours. Peak intensity centered 09z to
13z, with snowfall rates up to 2 inches per hour. Potential exists
for some sleet to mix in with the snow around daybreak. The heavy
snow will abruptly transition to light rain or drizzle between 14Z
and 16Z. A gusty southerly wind during the early-mid afternoon
period expected to veer to westerly and increase after 20z-21z.
Gusts in excess of 30 knots at times into the evening period,
leading to crosswind considerations.

 DTW THRESHOLD PROBABILITIES...

* High for ceilings under 5000 feet overnight through Saturday.

* High for precipitation type as all snow through daybreak Saturday.
  Moderate for some sleet to mix in Saturday morning. High that
  precip will transition to all light rain/drizzle Saturday
  afternoon.

* Moderate in exceeding crosswind threshold from 250-270 degrees
  late Saturday afternoon through early Saturday night.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Warning until noon EST today for MIZ068>070-075-076-
     082-083.

     Winter Weather Advisory until noon EST today for MIZ047>049-053>055-
     060>063.

     Lakeshore Flood Warning from 10 AM this morning to 10 PM EST this
     evening for MIZ063-070.

     Lakeshore Flood Advisory until 4 PM EST this afternoon for MIZ049-
     055-076-083.

Lake Huron...Gale Warning until 4 AM EST Sunday for LHZ441>443-462>464.

     Small Craft Advisory until 4 AM EST Sunday for LHZ421-422.

     Gale Warning until 4 PM EST this afternoon for LHZ361>363.

Lake St Clair...Gale Warning from 4 PM this afternoon to 4 AM EST Sunday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 4 PM this afternoon to 4 AM EST Sunday for LEZ444.

&&

$$

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


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