Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 122057
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
357 PM EST Fri Jan 12 2018

.DISCUSSION...

Model data has the main southern stream upper level PV anomaly now
lifting northward out of the far Southern Mississippi River Valley
and into portions of the Tennessee Valley this afternoon. Water vapor
imagery suggests that some absolute vorticity/shortwave energy is
shearing out from the northern part of this anomaly and surprisingly
still tied to the greater system warm conveyor that is just edging
up to southeastern Michigan from the south. Conceptually, this fits
with the reconfiguration and reorientation of the upper level jet
entry region that is in place over the Great Lakes. Needless to say
that some d/dt of the upper level mass fields is still going on and
the weather is not completely over with. This is evident on the
regional mosaic radar that shows some increasing moisture streaming
northward out of portions of IN and OH. Locally, have even seen a
redefining of the back edge west of Washtenaw Co. Given the
evolution of the all of the observational datasets, it appears that
some additional light snow will be possible throughout the remainder
of the afternoon and into the evening commute, particularly south of
the M59 corridor. The main negative moving forward for additional
precipitation is that a significant amount of low level dry air as
undercut the system and has advected into the area. Little
additional snowfall is expected over the area. Additional snowfall
accumulations of a half inch or less are still possible for metro
Detroit, with perhaps an inch or so possible in sections of Monroe
County near Lake Erie and Ohio. Despite traffic data suggesting that
flow rates have been relatively ok for far southern sections of the
forecast area today, surface temperatures in the low 20s to upper
teens and the potential for a few additional hundredths of liquid
supports maintaining the Winter Weather Advisory. The decision was
made to cancel the Winter Weather Advisory north of the I69 corridor
with no additional precipitation chances.

Attention turns to the southern Lake Huron basin as some hi
resolution models suggest that some lake effect snow potential in
northerly flow will exist near the Lake Huron shoreline. From a big
picture perspective, there are a couple of items that favor the
development of some lake effect, the depth of cold arctic air and
steep low level lapse rates, and both the integrity of the surface
lake aggregate trough, and the depth of the upper level cyclonic
flow. However, some major deterrents exist as well, including the
lack of respectable lower tropospheric convergence signature, and no
shortwave energy or cloud seeding. There is some question as to
exactly where the activity will develop as models are trying to show
development at or within the shoreline. Question is whether or not
the models are handling the ice cover and resultant land breeze
circulations well. Have included PoPs for snow along the shoreline 6-
12Z and 12-18Z Saturday with accumulations of an inch possible for
each 6 hour period. Something to monitor overnight, no lake effect
headlines anticipated.

Dominant, arctic anticyclone shown to bulldoze directly across the
Lake region on Saturday. Some lake aggregate signature exists of
depth of midlevel stability and depth of dry air should really limit
sky cover. Given some fresh snow and 850mb temperatures, lowered
highs a degree or so with teens all areas. Very cold, with
windchills struggling to rise out of the low single digits above
zero.

Model guidance is now coming into better agreement that a clipper
system will track through Lower Michigan on Monday. With
temperatures well below freezing (highs in the low to mid 20s) there
will be no precip type issues with this system. Currently looking
like an all day light snow event with a few inches possible across
SE Michigan by Monday evening.

In the wake of this system, a reinforcing shot of cold air will
pivot through the region as 850-mb temps approach -18 C on Tuesday.
This will keep highs in the upper 10s to lower 20s Tuesday into
Wednesday with nighttime lows in the low single digits Tuesday
night. Snow showers will also be possible on Tuesday as a low-level
trough lingers over SE Michigan before high pressure builds into the
region on Wednesday. Upper level ridging will then nudge eastward
bringing a warming, drying trend through Friday.

&&

.MARINE...

Northerly gales will hold through tonight as the gradient between
the exiting low and approaching high remains fairly tight with
strong cold advection. The cold temperatures moving back in will
likely result in a period of heavy freezing spray as well. Headlines
will remain intact to account for gales and freezing spray tonight.
Winds will relax on Saturday as the arctic high drifts over the
region. They will remain below 20 knots through Sunday before
increasing once again Sunday night ahead of a clipper that will
track through the region on Monday.

&&

.PREV DISCUSSION...
Issued at 508 AM EST Fri Jan 12 2018

AVIATION...

Strong arctic cold front has swept through all terminals leaving
strong northwestly winds in its wake. The cold air advection is
allowing for a 1-3 hour period of -FZRA still to come at the
southern terminals beginning around 13Z while the transition to snow
showers has begun for the northwest terminals. IFR conditions
accompany this system and will likely hold through most of the
morning into the afternoon. LIFR conditions still remain possible
within the heavier snow bands if any were to impact a terminal.
Expect gusty winds throughout today ranging between 20-25 knots and
remaining out of the NW through this TAF period. Snow showers that
follow the wintry mix will result in 2-5" of accumulation as it
tapers off from northwest to southeast through the late afternoon.

For DTW...A window of 1-3 hours for freezing rain potential remains
this morning centered around 13z-15z. Transition to accumulating
snowfall thereafter through early afternoon. Current forecast calls
for total accumulation of 2 to 4 inches in the metro area. IFR CIGS
have entered the area and will remain through at least the afternoon.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling at or below 5000 ft through Friday.

* Moderate for the occurrence of light freezing rain between 13z-
  15z.

* High in the transition from -FZRA to -SN late morning after 14z-
  15z.

PREV DISCUSSION...
Issued at 450 AM EST Fri Jan 12 2018

Discussion...

Latest runs of hrrr/ruc13 suggest the best forcing early this
morning will run southwest to northeast through the northwest
quadrant of the forecast area (including Midland/Bay counties). The
change in precipitation type from rain to freezing rain/sleet to all
snow remains somewhat tricky, but general trend suggests this will
be a relatively quick transition of the order of 1 to 3 hours. While
this will minimize ice accumulations, it may very well enhance local
snowfall amounts where the changeover coincides with best lift.

Will be adding Midland/Bay to the ongoing Winter Weather Advisory.
Given the aforementioned trends, this seems most prudent for Bay
county where it appears a quick several inches of snow (2-3 inches)
will be quite possible at least in portions of the county, but
really see know reason to split hairs at this point and forgo only
one county in the forecast area. Will also increase accumulations in
a band running from Howell/Flint on northeast into the Thumb where
precipitation rates after the switch to snow suggest the potential
for 3 to 4 inches of snow. Would not be at all surprise to see a
very narrow swath of 5 inches somewhere in this general vicinity.

While the trend for better forcing is displaced northwest in recent
hrrr/ruc13, 00z HiRes model suite suggest a decent potential of 2 to
3 inches of snow further southeast into Metro Detroit and points to
the south. Will pretty much leave these snowfall totals alone given
the relatively high uncertainty even this close to the event.

In all cases, a period of minor ice accumulations can also be
expected in advance of the snow as shallow cold air advection
outruns the deeper surge of cold air by several hours. This will
generally be less than a tenth of an inch, and in many cases no more
than several hundreths of an inch. Very difficult to say, but
perhaps the best potential for ice accumulations will run along the
Irish Hills into the southern Thumb. All in all, a messy forecast,
and day, across the region.

Precipitation will end rather quickly northwest to southeast across
the area in the late morning to early afternoon period. At the same
time, a sharp temperature drop into the 20s (and teens) will occur,
leading to the freezing of any liquid on surface areas.

A cold period of weather will then set up in the wake of this system
through the weekend into much of next week. This period will not be
quite as cold as the previous cold snap from late December into
early January, but expect a period into the middle of next week
where highs only reach the teens to lower 20s and lows drop into the
single digits in most cases.

One more low pressure system, this one a more classic clipper
system, will traverse the area roughly in the Sunday night to Monday
period and bring the potential for a few more inches of snow and the
renewed push of cold air to keep temperatures in the teens/20s next
week. Some decent snow shower activity looks possible into Tuesday
as surface troughing with this secondary surge of arctic air helps
to focus lake effect activity.

MARINE...

NNW winds with gusts to gales will develop across Lake Huron as an
arctic front moves across the Great Lakes resulting in Gale Warnings
that will last through Saturday morning. Due to the frigid
temperatures returning to the region, a Heavy Freezing Spray warning
is also in effect to go along with the Gale Warning. Areas in the
warnings are central/southern Lake Huron and the nearshore zones
along the northern and eastern Thumb. Winds will begin to decrease
through Saturday afternoon as a cold arctic high pressure takes
hold. A clipper system tracking through the Great Lakes early next
week will bring the next chance for increased winds across the
lakes.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 7 PM EST this evening for MIZ055-
     061>063-068>070-075-076-082-083.

Lake Huron...Heavy Freezing Spray Warning until 7 AM EST Saturday for LHZ362-363-
     441>443-462>464.

     Gale Warning until 7 AM EST Saturday for LHZ362-363-441>443-462>464.

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

$$

DISCUSSION...CB/TF
MARINE.......DRK
AVIATION.....AA


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