Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 222255
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
555 PM EST Tue Jan 22 2019


.AVIATION...

A mix of freezing rain, ice pellet and snow will expand over the
area this evening and persist into the overnight before changing to
rain from south to north. FZRA will be the primary precipitation I-
94 with a mix of FZRA/PL KPTK and a transition to more PL/SN KFNT and
especially KMBS. Icing on the order of 0.1"-0.2" appears a pretty
good bet KPTK south with more of a mixture of SN/PL/ICE accums to
the north become largely SN accums KMBS. Temperatures should edge
above freezing I-94 by 07z- 09z and several hours later KFNT/KMBS.
Cigs will quickly fall thru MVFR into IFR (and quite possibly LIFR
later tonight). Vsbys in IFR range will also become common as the
evening progresses. The low pressure system bringing this wintry mix
will pass the area during the day Wednesday with rain ending during
the afternoon and cigs slowly edging up late in the forecast period
as drier WSW flow ensues in the wake of the low.

For DTW...FZRA will become increasingly persistent this evening and
may be mixed with PL early in the forecast. A change to rain will
then occur overnight as surface temperatures edge above freezing.
Timing remains uncertain, but will probably occur in the 07z-09z
time frame. MVFR conditions will trend to IFR tonight and then
improve with passage of the system late in the forecast period.

.DTW THRESHOLD PROBABILITIES...

*  High in precip type as freezing rain this evening and tonight.
   High in precip type as all rain by Wednesday morning with the
   transition roughly 07z-09z.

*  High for ceilings below 5000 tonight and Wednesday morning.

&&

.PREV DISCUSSION...
Issued at 409 PM EST Tue Jan 22 2019


NEAR TERM / REST OF TODAY THROUGH TONIGHT

Winter Weather Advisory in effect for all of southeast Michigan...

As of 405 PM EST...Sprawling low pressure system continues to take
shape across the central Plains and is on track to bring a period of
mixed wintry precipitation to all of southeast Michigan beginning
this evening and continuing through tonight. Latest water vapor
satellite imagery continues to depict a deepening longwave trough
over the central CONUS that has become more meridionally-oriented as
the day has progressed. The end result will be an impressive mass-
flux adjustment downstream over our region in the next 24 hours.

Rapid low-level warm air advection continues this afternoon across
the region, as 850 hPa temps have warmed to near 0 C near the Ohio
border, a change of almost 15 C in the past 18-24 hours. This warm
air advection will be riding on the heels of an impressive 80+ knot
LLJ that will pump anomalous moisture (5 g/kg specific humidity in
the 850-700 hPa layer) over the region late this evening and through
tonight. Also a signal to the change in airmass will be the rapid
intensification of the upper jet that will increase to near 180
knots by Wednesday morning; placing southeast Michigan in favorable
diffluent flow aloft. Combined with the juxtaposition of the left
exit region of the intensifying LLJ, southeast Michigan will be
placed in an extremely favorable region for enhanced lift as the
night progresses.

Before any precipitation can occur however, remnant dry near-surface
arctic air will need to take some time to saturate as afternoon
dewpoints across the region largely remain in the single digits to
lower teens with temperatures in the 20s. Current thinking is that
precipitation will begin overspreading the region during the 5pm-7pm
timeframe (as supported by weak radar returns across lower Michigan
as of 3pm) in the form of light snow showers as the thermal profile
wet-bulbs in combination with evaporative cooling effects. A quick
few tenths of an inch of snow accumulation will then be possible for
the latter half of the evening commute, with totals closer to an
inch in the Tri-Cities region possible.

Developing warm nose in the 925-850 hPa layer will begin to lift
northward into lower Michigan in the 00z-03z timeframe, allowing for
a transition south of the I-69 corridor to freezing rain. Depending
on intricacies with the thermal profile, intermittent sleet will
also be possible, with the best chance of this occurrence being
along and to the north of the Irish Hills where a slightly deeper
near-surface cold layer will temporarily reside.

However, thermal profiles become increasingly favorable for freezing
rain by 03z-04z, with a colder profile supporting a primarily all
snow event north of the I-69 corridor. Latest trends in model
guidance continue to hone in on the region between the M-59 and I-94
corridors as the greatest potential to see a prolonged period of
accumulating freezing rain at the nose of the LLJ and low-level
frontal slope. In fact, there are some signals in the hi-res
guidance that there is a double frontal slope regime, with the lower
levels activating closer to the Ohio border, and a slightly higher
zone activating closer to the Tri-Cities/northern Thumb region.

Peak of the event looks to occur in the 04z-09z (11pm-4am)
timeframe, as surface cold air remains tough to dislodge despite the
lowering warm nose warming to 2-4 C. Regardless of the air
temperature, prolonging the impacts will be road temperatures likely
lingering below freezing after surface temperatures warm above
freezing. The transition to rain from south to north looks to begin
occurring as early as 3am near the Ohio border, and eventually
working its way up to the M-46 corridor by 7am-8am. With the warm
nose approaching 4 C by this time, freezing rain/snow will simply
transition to rain.

Total QPF through early Wednesday morning will range between a half
and three quarters of an inch north of the M-59 corridor, to
approaching 1 inch near the Ohio border. Luckily, low snow ratios
likely below 8:1 for the bulk of the event will limit snow totals
north of I-69 to generally the 2-4 inch range. If cold air can hold
out a little longer during the early morning hours, then locally
higher snow totals would be possible.

As for ice accumulation, expecting a pretty wide swath of around a
tenth of an inch accumulation south of the I-69 corridor. Between M-
59 and I-94, amounts will range between a tenth and two tenths of an
inch. With the cold road temperatures, very hazardous and slippery
travel conditions will result along with the potential for scattered
power outages. Winds look to be light enough with the surface trough
close by keeping from potentially more significant power outages.
There is a low chance that ice amounts could approach a quarter of
an inch in portions of the Detroit metro west to Ann Arbor.

SHORT TERM / WEDNESDAY THROUGH THURSDAY NIGHT

The low pressure center will track northeast across lower Michigan
during the day Wednesday, with latest trends favoring a tracking
line from Grand Rapids to Saginaw. Most of southeast Michigan will
briefly be placed in the warm sector of the system during the day as
temperatures warming to near 40 result in widespread rain showers.
Cold front sweeps through late afternoon bringing an end to
precipitation from west to east. Strong cold air advection (850 hPa
temps falling below -10 C by 12z Thursday) coupled with the dry slot
may allow for a good wind-pop Wednesday evening, and will need to
monitor these trends in upcoming guidance. Low temperatures
Wednesday night will fall into the 20s.

Little in the way of temperature recovery during the day Thursday
with highs in the mid/upper 20s and the potential for scattered snow
showers as lake effect activity is enhanced by a weak clipper-type
low passing north of Georgian Bay. Chilly night Thursday night with
lows falling into the single digits for most areas.

LONG TERM / FRIDAY THROUGH MONDAY

Another round of arctic air will be spreading across Michigan by
early Friday morning. This air is associated with a deep and cold
upper level low pressure system digging through Ontario.
Temperatures at 850 mb tank into the negative 20 to 25 C range,
which will help bring high temperatures for Friday down into the
single digit to low teens. As the base of the trough pivots across
the Great Lakes, cold westerly winds moving over Lake Michigan will
be able to pick up a little moisture boost and bring a chance for
some scattered snow showers across southeast Michigan Friday
afternoon into Friday night. Arctic airmass will be lifting the
northeast by Saturday night, but cold temperatures will linger
Saturday afternoon making for another very chilly day to start the
weekend. High pressure will lifting into the Ohio Valley, which keep
conditions mostly dry for Saturday. A series of waves will bring
chances for snow on Sunday and Monday with temperatures remaining
cold side of normal in the teens and 20s.

MARINE...

Low pressure will lift from the southern plains to northern Illinois
by Wednesday morning. The low will then traverse central Lower
Michigan during the day Wednesday. There will be a strengthening
southerly gradient ahead of this low pressure system. Gale force
winds will persist into early this evening. While probabilities
remain high that gusts will reach 35 knot gales, the probability for
gusts to 40 knots gales is low. The winds will gradually decrease
tonight due both to a subtle weakening of the gradient and low level
warming, thus reducing over-lake mixing depths. Winds will veer to
the west-northwest late Wednesday into Wed night as the surface low
departs east of Lake Huron.

Another intrusion of arctic air is forecast to overspread the Great
Lakes Thurs night into Friday. This will bring another round of
gusty winds (possibly gale force) and high probabilities of heavy
freezing spray.

HYDROLOGY...

There will be an influx of relatively high moisture preceding a low
pressure system that will traverse Lower Michigan on Wednesday
afternoon. The initial surge of moisture will result in a mix of
snow, sleet and freezing rain across the area late this
afternoon/evening before strong warm air advection transitions the
precip over to rain Tues night/Wed morning. The rain will come to an
end Wednesday afternoon. There is a high probability that total
liquid equivalent precipitation totals will exceed three quarters of
one inch. The potential exists for rainfall amounts to exceed 1
inch. If total precip does exceed an inch, there may be some
localized flooding concerns in urban Detroit where warming temps
will also result in some snow melt.

PREV DISCUSSION...
Issued at 358 PM EST Tue Jan 22 2019

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 8 AM EST Wednesday for MIZ047>049-
     053>055-060>062.

     Winter Weather Advisory until 8 AM EST Wednesday for MIZ063-068>070-
     075-076-082-083.

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

$$

AVIATION.....DG
LONG TERM...AA
MARINE.......IRL
HYDROLOGY....SC


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