Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 161739
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1239 PM EST Mon Jan 16 2017


.AVIATION...

Waves of precipitation are expected to continue this afternoon into
Tuesday morning as they slowly lift northward ahead the approaching
upper wave. The initial influx of moisture has already produced
sleet and freezing rain for the Detroit area TAFs. The degree of wet
bulb cooling continues to suggest freezing rain and sleet for the
afternoon and early evening for the Detroit area. Expect a slightly
deeper cooler air mass and wet bulb potential to bring some snow
chances for FNT and MBS before changing over to all freezing rain
this evening. Freezing rain changes over to all rain by 01z for
Detroit TAFs but lingers to around 06z for PTK, FNT, and MBS. Should
produce a light glaze of ice...especially for the northern TAF
sites. Rain will then continue through daybreak Tuesday

Ceilings and visibilities will continue to lower through the
afternoon and evening. After 06z tonight and into Tuesday morning
all TAF sites will have LIFR Ceilings with visibilities between 1/2
mile and one mile.

For DTW...The potential for freezing rain through about 00z to 02z.
Still some concern for a little ice accumulation. The degree of warm
air aloft and rising temperatures through the night will sustain
mostly rain. There will however be a period this evening, especially
between 5 PM and 9 PM, where sfc temps may drop to freezing or a
touch below. This will warrant at least a mention of freezing rain
in the TAF through evening. Around midnight and later, ceilings
lower below 1000 feet and will remain there through Tuesday morning.
Ceilings could get close to the 200 ft thresholds toward daybreak
Tuesday morning.

//DTW Threshold Probabilities...

* High in ceilings 5000 ft or less by mid afternoon through Tuesday.

* Moderate for freezing rain this afternoon and evening.

* Low for ceilings and/or visibilities below 200 ft/one half mile
  Tuesday morning.

* Low in thunderstorms Tuesday morning.

&&

.PREV DISCUSSION...
Issued at 1010 AM EST Mon Jan 16 2017

UPDATE...

Arc of mid level forcing pivoting out ahead of the main system
dynamics and moisture advection proving adequate enough to sustain a
narrow stripe of high based activity late this morning.  Upstream
observations suggest that despite the presence of a deep dry layer
across the lowest 5000 ft, this precipitation will survive at least
long enough to impact a portion of southeast Michigan thru early
afternoon.  Recent HRRR guidance sufficiently handling the evolution
and placement of this band, suggesting a corridor between I-94 and I-
69 will see the greatest increase in coverage of a light wintry mix
into the afternoon.  While temperature are responding diurnally, the
evaporative cooling process may begin to retard this response,
effectively holding temperatures at or below freezing well into the
afternoon. Given a poorly saturated low level profile carrying sub-
freezing wet bulb temperatures, expectations are for a
combination/mix of light freezing rain, sleet and snow to occur
through this time. Upstream reports indicate precipitation intensity
may prove sufficient to create a hazard on our dry/cold road
surfaces.  Given this growing concern, electing to move up the start
time for the freezing rain advisory for the mid tier of counties /M-
59 to I-69 corridor/.

Subsequent trends may eventually necessitate a similar adjustment
for the Tri-Cities/Thumb, but will hold off for now as both model
guidance and radar trends indicate the bulk of this lead activity
will hold south.  Trends in surface temperatures locally and
immediately upstream to be closely monitored moving forward,
particularly given the insistence by short range guidance /HRRR and
RAP/ in working mid 30s temperatures up into a good portion of the
forecast area by mid-late afternoon.  Continue to note a colder
projection by the ARW/GEM/NAM, lagging any meaningful climb above
freezing until this evening once evaporative cooling ceases and the
profile saturates. This could inevitably leave the ARB-DTW
corridor more susceptible to a period of freezing rain. Will
reevaluate this afternoon the need to include Washtenaw/Wayne
counties in a short duration headline for the evening period,
affording a bit more time to evaluate near term temperature/dewpoint
trends.

PREV DISCUSSION...
Issued at 331 AM EST Mon Jan 16 2017

DISCUSSION...

Upper wave as seen on dynamic tropopause coming out of
northern Texas, to be positioned over/near Kansas City Missouri
late today as the northern shortwave trough axis coming out of the
northern rockies helps draw the system northeast. Shearing out
upper level PV anomaly center looks to be over/near southern Lake
Michigan early tomorrow morning.

Very dry airmass currently in place, per 00z dtx sounding, making for
difficult call when exactly overrunning precipitation will develop
over southeast Michigan today as low level jet remains well to the
west through the day. Will be favoring a drier/virga/spotty
precipitation scenario during much of the daytime hours with lead
850-700 mb Theta-E moisture gradient lifting north. On flip side,
surface/low level anticyclonic flow and wet bulbing effects should
be able to hold surface temps around the freezing mark through
much of the evening hours as deeper moisture arrives (PW values
pushing up to 1 inch), and will be going ahead and issue a winter
weather advisory for mainly freezing rain for northern two thirds
of CWA (North of Eight Mile Road). Deep enough cold air at the
start to even offer up a period of sleet/snow (mainly north)
before max warm layer exceeds 3 C, and thus have issued winter
weather advisory north of I-69, but snow/sleet accumulations
expected to be less than 1 inch, with freezing rain the bigger
concern.

Overall, surface temperatures look to be more in the marginal 30-32
degree range, which should help diminish/limit impacts (icing on
roads) somewhat. Although, have noted the colder raw regional GEM
and ARW surface temps. Local probabilistic SREF guidance indicating
low chance of a tenth of inch of ice accumulation across most
locations, but preference was to go a bit more aggressive with the
favorable sunset/post sunset timing and just enough of dry/cold
southeast feed from southern Ontario to help offset the self
limiting freezing rain sensible heat release. Light glaze of ice
accumulation south of M-59, with a tenth to two tenths north before
the transition to all rain occurs overnight. Most of ice
accumulations occuring on the higher/elevated surfaces, such
as tree tops/wires.  Will issue 2 segment advisory for differing
transition timing from south to north, 2 AM, 6 AM. Day shift
will have to watch southern four counties (especially
Washtenaw/Lenawee) for short period of light icing, and can handle
it with SPS or short fused advisory (4-10 PM).

Moist/Warm conveyor belt clipping southeast Michigan late Tonight,
as 850 mb dew pts of 5 to 6 C move through. Steep mid level lapse
rates of 7 C/KM from 700-500 MB with showalter index near zero
support slight chance of a thunderstorm.

SFC-700 mb lows/circulations tracking close to the Tri-Cities region
during the day on Tuesday, and should see good deformation rain
shield sweeping across southeast Michigan during the day.  Still
some mixed signals in how much warm air at the surface infiltrates
across the southern Michigan border as triple point may not get past
western Lake Erie, and will continue to highlight just 40s, but 50+
degrees possible in/around Monroe county.

Modest low level cold advection and cyclonic flow Tuesday night
supporting low clouds and possible drizzle/light rain showers, along
with light fog continuation. Not so sure there will be enough cold
air around to support much of a changeover to snow/sleet, as 00z EURO
maintaining 850 mb temps above 1 C, with any precipitation likely
shutting down before this marginally colder air arrives.

Polar jet displaced north of the CONUS will result in much above
normal temperatures for southeast Michigan to end the work week
as southwest flow increases, with another moisture laden
low potentially approaching from the south by Friday morning (per
00z Euro). The Canadian and GFS keep the system just to our south of
Friday, but it looks to be a real close call.

MARINE...

A ridge of high pressure will hold across srn Lower Mi and Lake Erie
today, keeping winds light across the southern Lake Huron basin and
points south. While southwest winds will hold through the day across
central and northern Lake Huron, fairly stable conditions over the
lake will keep gusts under 20 knots. Low pressure will move into
southern Lake Michigan from the southwest late tonight. While this
will lead to the development of east-southeasterly winds tonight
into Tuesday morning, strong over lake stability and a relatively
weak sfc low will keep peak wind gusts at or below 20 knots. The low
will track across southern Lake Huron Tues afternoon, supporting a
brief uptick in northeast winds across northern Lake Huron. A
weakening gradient will ensue Tues night as the sfc low departs the
region to the east, leading to weakening winds.

HYDROLOGY...

An strong influx of moisture into Southeast Michigan will occur late
today into Tuesday morning, supporting widespread precipitation.
Most of this will fall as rain. There is a high probability that 24-
hour QPF totals will range between a half an inch and an inch. The
period of heaviest rain will occur between midnight and noon on
Tuesday. With no snow cover and frozen ground, there should be good
run off, so rises in area rivers are expected. Based on forecast
precip totals, river levels will remain below flood stage.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 7 PM this evening to 5 AM EST Tuesday
     FOR MIZ047>049-053>055.

     Freezing Rain Advisory until 2 AM EST Tuesday FOR MIZ060>063-
     068>070.

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

$$

AVIATION.....RBP
UPDATE.......MR
DISCUSSION...SF
MARINE.......SC
HYDROLOGY....SC


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at www.weather.gov/detroit.



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