Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 072032
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
332 PM EST Thu Dec 7 2017

.DISCUSSION...

Lake effect snow regime will continue to affect parts of southeast
Michigan this afternoon into the evening as low level flow makes the
turn from the northwest to southwest. It will be likely to see snow
showers given the super saturation with respect to ice as the
streaming lake effect snow showers maintained in the 850/925mb
moisture axis move through southeast Michigan. Snow showers are not
expected to be widespread and cells will be transient in nature as
the main source of lift provided by the shortwave is quick to exit.
Areas along and south of I-94 will have the best shot at seeing these
snow showers. Accumulations will remain on the lighter side, but the
set up for good dendritic growth could produce around a half inch
with any quick snow shower bursts. The big moisture boost from Lake
Michigan will effectively be cut off with the wind shift to the
southwest later this evening and dry air will advects into southeast
Michigan. Temperatures overnight will dip into the upper teens and
low 20s with temperatures rebounding to slightly below the freezing
mark for tomorrow. Brief break in precipitation overnight until
tomorrow afternoon with some of the Hi-Res models hinting at some
lake effect streamers off Lake Michigan tomorrow afternoon within the
southwest flow. The Flint and Tri-Cities areas would be the most
likely candidates under this flow to potentially get some light snow
showers. Will keep slight chance of PoPs through tomorrow until
attention turns to tomorrow night.

Potential for the first widespread snowfall of this season is taking
shape as a low pressure system drops out of central Canada through
southeast Michigan tomorrow night into Saturday. There will be a
rather strong PV anomaly associated with the shortwave that is
embedded in the more amplified trough over the region. The track of
the surface low will play a roll in how much snow southeast Michigan
will get and confidence is increasing in a more favorable southerly
track putting much of the area on the northern side of the low.
Timing of this system will see light snow showers beginning late
Friday night around midnight or slightly after as the system will
work to combat existing dry air at the surface after saturation
occurs within the 850 to 700 mb layers. Saturday morning currently
looks like the peak of snow showers. Activity will continue through
the afternoon and taper off into early evening as the system exits
quickly to the east. Midlevel lapse through Saturday morning and
early afternoon should be around 6 C/km, which would help boost
snowfall rates. Some uncertainty still remains with this system as
models try and get a handle on the QPF amounts. Current QPF forecast
with the amount of forcing with the system would yield widespread
snowfall amounts of generally 2-3 inches with locally higher amounts
possible. These numbers would increase if QPF trends upward, so will
need to keep an eye on the QPF trends as the event nears.

Once the system moves east of Michigan on Saturday afternoon,
northerly flow will take over and set up the potential for lake
effect in the Thumb off of Lake Huron. One of the limiting factors
for accumulations will be the length of time for a more northerly
flow opposed to a northwesterly. Will not make much adjust from
previous forecast and keep minor accumulations for now in the Thumb.
Cold temperatures stay in place as 850 mb temps of around -12 to -15
C will again keep high temperatures just below the freezing for
Saturday.

An amplified upper-level trough will continue to sit over the
eastern third of the U.S., bringing multiple snow chances to the
Great Lakes through the extended period. Temperature highs will also
remain average to slightly below average as west/northwest flow
dominates through next week. To start, a clipper system pushing
southeast across Ontario throughout Sunday will provide the slight
chance for light snow, mainly across the Tri-Cities to Thumb, as the
low slides east into Quebec. Slightly better chances for snow will
then enter throughout Monday as a second potential clipper system
pushes in Monday through Tuesday. However, major discrepancies
regarding the strength of the low and thus snowfall potential exists
between the GFS and ECMWF. The GFS shows a strengthening low as it
pushes southeast from Manitoba into Michigan, strengthening down to
997 mb by early Tuesday morning. The ECMWF 12Z run fails to
materialize a defined low pressure system, but does push a weak cold
front through the Great Lakes providing enough forcing for potential
snow.

Upper-level troughing is expected to strengthen Tuesday into early
Wednesday and will allow northerly/northwesterly flow to dominate
across Michigan. This will raise some concerns for lake-effect snow
chances over the eastern side of the thumb. Zonal flow is expected
to relax late Wednesday into Thursday into more of a meridional
pattern which will diminish lake effect snow chances by the end of
the week.

&&

.MARINE...

Winds will remain elevated as westerly winds turn more southwesterly
through this evening. Surface pressure gradient will increase with
the passing of an embedded shortwave trough and lead to these
southwest winds remaining elevated into Friday. A short period of
gales remains possible over central Lake Huron late tonight into
Friday afternoon. A period of weaker southwesterly winds will last
Friday night before a quick hitting low pressure system dives out of
central Canada. Winds will quickly veer to the north Saturday
morning and increase to around 20-25 mph with gusts reaching near
gales again. Widespread snowfall will also accompany this low
pressure system. Active pattern continues Sunday night into Monday
with a front moving south and maintaining elevated winds.

&&

.PREV DISCUSSION...
Issued at 1151 AM EST Thu Dec 7 2017

AVIATION...

Lake effect clouds and snow showers/flurries will persist through
the afternoon under continued west-northwest flow across Lake
Michigan. This flow regime will continue to keep the higher
concentration of snow showers and MVFR clouds south of PTK. Hi res
model solutions suggest the snow showers releasing off Lake Mi this
evening, with the back edge of the snow showers pushing through Se
Mi in the 01 to 03Z time frame. This will be due to a surge of drier
air across the lake, which will then be followed by a backing of the
low level winds toward the southwest. The result will be not only
for a decrease in the snow showers, but also a decrease in the sub
6k ft cloud deck.

For DTW...Radar data along with the latest model guidance suggests
some break in the snow shower activity is possible during the early
afternoon. An enhancement in low level convergence however may lead
to some higher intensity snow showers advancing back into metro
later this afternoon and persisting off and on into the evening. The
most probable time window for this to occur will be between 20Z and
02Z. A dusting of dry powdery snow is possible during this time
frame.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet this afternoon and
  evening. Low overnight into Friday morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late tonight through Friday afternoon for LHZ362-
     363-421-441-462.

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

$$

DISCUSSION...AA/AM
MARINE.......AA
AVIATION.....SC


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