Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 171658
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1158 AM EST Fri Jan 17 2020


.AVIATION...

There remains a few areas of MVFR based strato cu in the vicinity of
southern Ontario and Lake Erie. As the low level east-southeast flow
increases this afternoon/evening, some of these clouds may continue
to be ducted under neath the shallow inversion, thus temporarily
impacting the terminals. A region of moderate snow will overspread
Se Mi in the 04Z to 07Z time frame tonight. Some fluctuations in
intensity are likely during the overnight before an additional
influx of moisture supports some heavy snow. This is most likely in
the 09Z to 14Z time frame and will primarily be focused south of
FNT. Snowfall rates through this period will range between one to
two inches per hour.

For DTW...The onset of light to moderate snow is expected between
04Z and 05Z at metro, with the peak intensity likely to occur
between 09Z and 13Z, with snowfall rates up to 2 inches per hour.
Some sleet may mix in with the snow around daybreak. The heavy snow
will abruptly transition to light rain or drizzle between 14Z and
16Z. Strengthening south winds during the early afternoon on
Saturday will veer toward the west toward the end of the forecast
period. Gusts by early Sat evening may exceed 30 knots.

 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.

&&

.PREV DISCUSSION...
Issued at 451 AM EST Fri Jan 17 2020

DISCUSSION...

An impactful winter storm is expected to produce periods of heavy
accumulating snowfall for Southeast Michigan Friday night through
Saturday.

For this morning, mid-level isentropic downglide will facilitate
increasing dewpoint depressions for much of the daylight hours
keeping conditions dry and chilly. 850mb temperature spread ranges
from -8C to -12C translating to surface lows hovering in the upper
teens by daybreak. Decentralized surface pressure field over lower
MI will prohibit much of a gradient keeping winds benign as
tropospheric ridging allows for anticyclonic flow induced subsidence
paving the way for occasional cloud breaks. High cloud will advect
en-masse aloft ahead of the next disturbance. In fact, current GOES-
E RGB channel depict the ridge making progress into SW lower MI with
incoming cirrus. High temperatures will remain in line with normal
climatological values today, just below freezing.

Attention turns to the upcoming winter storm starting late Friday...
Upper-level dynamics fields continue to suggest the merging of
northern and southern streams of the split jet by 18Z Friday. This
will support the rapid intensification of the combined upper-level
wave as heights plummet and geostrophic velocities surge over the
Missouri basin Friday evening. The feature will induce rapid
cyclogenesis on the lee-side of the Rockies kicking a surface low
eastward as deep layer cyclonic flow ensues. Given the broad reach
of this system, the 925-850 mb layer will quickly become saturated
as efficient moist WAA converges up the Mississippi valley
characterized by strong low-level ThetaE advection rates (over
20C/12hr) across the MI/OH border after 00Z Saturday. The moisture
gradient remains very sharp along the affiliated warm layer such
that 850mb dewpoint temperatures will jump 25C in less than 3 hours
late Friday night as the event gets underway.

Shifting focus to amounts, SLRs appear to fluctuate near
climatological values (about 12:1) with the first round of snow.
Forecast soundings illustrate excellent supersaturation (wrt ice)
prior to 12Z across the southern counties. Given sufficient cold air
in place ahead of the moisture plume, a well aligned DGZ aloft will
work in tandem with efficient riming process to created large and
fluffy flakes overnight. Model QPF pushes liquid equivalent values
into the 0.75-1.00 inch range through Saturday evening south of the
M-59 corridor favoring periods of heavy snowfall as rates approach
the 1 in/hr mark. Deformation will play a two-fold role with this
event as the NAM depicts an initial deformation axis along the I-94
corridor followed by a second period as the low lifts north later on
Saturday. This will help further enhance areas of synoptic ascent
and mesoscale vertical motions to produce localized areas up to 8
inches. Snow totals fall off heading north of M-59 as moisture
diminishes, thus the current breakout between Warning/Advisory.
Overall, 4-7 inches of total snowfall is the most likely scenario at
this juncture, before the melt-off begins.

The largest source of uncertainty with this event lies in timing the
arrival of warmer air Saturday morning. Surface temperatures will
climb above freezing on Saturday from southwest to northeast as
easterly flow veers southerly. Precipitation will still be on-going
in some areas which increases the likelihood of a snow-sleet-rain
progressing phase change. Once the snow is undercut by warmer air,
the melting and runoff processes that follow could make for areas of
localized ponding or flooding in poor draining areas by the
afternoon. Temperatures late Saturday afternoon could even approach
40F in the Metro area (accelerating snow-melt) before temperatures
retreat again Saturday night once the dry slot works around the
back side of system delivering revived zonal flow.

Upper low lingers on Sunday and Monday with limited as chillier
temperatures persist with limited potential for precipitation.
Meanwhile, strong high pressure will take hold over the central
Plains under an expanding ridge which will generally maintain a NW
flow pattern through Tuesday.

MARINE...

Winds will relax through the morning as a strong area of high
pressure passes overhead weakening the pressure gradient. Wind
through the afternoon will be relatively light (under 20 knots)
before ramping up again tonight ahead of the next low pressure
system. This next low will deepen while tracking northeast through
the central Great Lakes on Saturday bringing a period of heavy wet
snow Friday night into Saturday morning. Winds will likely become an
issue as well as initially they increase Friday night ahead of the
warm front. Winds could briefly touch gales across central Lake
Huron but confidence is low at this point. Winds will then switch to
strong westerly late Saturday behind a cold front with a good chance
of reaching gale force across western Lake Erie where a Gale Watch
will be issued. Gale potential is slight less north through Lake St
Clair and southern Lake Huron and will forgo a watch at this time.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Warning from 10 PM this evening to noon EST Saturday
     for MIZ068>070-075-076-082-083.

     Winter Weather Advisory from midnight tonight to noon EST Saturday
     for MIZ047>049-053>055-060>063.

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

     Small Craft Advisory from 4 AM Saturday to 4 AM EST Sunday for
     LHZ421-422.

     Gale Warning from 4 AM to 4 PM EST Saturday for LHZ361>363.

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

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

&&

$$

AVIATION.....SC
DISCUSSION...KK
MARINE.......DG


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