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area forecast discussion
national weather service detroit/pontiac mi
1239 pm est mon jan 18 2021


broad low pressure and moist cyclonic flow keep a healthy coverage
of lower clouds over southeast michigan this afternoon with mvfr
ceilings remaining predominant heading into this evening. a midlevel
disturbance tracks across the region this afternoon, helping to
steepen lapse rates and generate enough surface convergence to result
in an expanding coverage of snow showers for the early taf period.
the showers will be light to moderate in intensity with pockets of
ifr visibilities in any more robust activity. the passing of the
midlevel wave will lead to decreasing snow shower coverage after 00z,
but plenty of low-level moisture will remain in place with continued
mvfr conditions likely. model soundings show a deep enough layer of
saturation that light snow showers or just flurries may continue into
the overnight period, especially for locations along and north of
m-59. surface winds remain on the order of 10 kt from the wsw this
afternoon and evening.

for dtw... window for best snow shower activity remains focused on
19z to 22z with brief periods of ifr possible in any higher
intensity showers. with temps near or just above freezing,
accumulations will be limited but any more robust showers may bring
a quick dusting to a tenth of an inch. snow showers generally taper
off this evening with the brunt of any lingering showers/flurries
tracking north of dtw tonight.

dtw threshold probabilities...

* high in ceilings below 5000 feet through tonight.

* high in precip type as all snow today.


.prev discussion...
issued at 352 am est mon jan 18 2021


eastward advancement of low-mid thetae plume that had been in place
sunday within weak stretching deformation axis has now advected
eastward into portions of far sw ontario and lake erie. the net
result will be modest but steady cold advection within the column
today. h85 temperatures are forecasted to be some 3 to 4 c cooler
than yesterday. fstill relatively modest for what is the
climatologically coldest part of the year, forecast soundings show
the cold air is fairly shallow today, including the northern
areas/northern thumb. top of the boundary layer/convective layer
today is again expected to reach -10 to -12c. this will be good for
ice nucleation, but progged midlevel stability caps access to the
dendritic growth zone. additionally, models show sharpening of
surface based trough northward into the northern great lakes region
with most enhanced 1000-850mb convergence consolidating from the a
marquette mi - gaylord mi - buffalo ny line. projected uvvs within
boundary layer are notably weaker in the latest runs for this monday
period which tempers expectations for breadth and duration of snow
shower activity today. however, a weak meridional wave will push
across lake michigan by this afternoon, providing dcva synoptic
ascent to an already cyclonic flow environment. will likely see some
lake michigan activity break away and push eastward late this
afternoon/early evening. minor january snow accumulations will be
possible (dusting/less than an inch) particularly north of i 69
quite possibly along m 46. however, favorable lake effect
convergence axis that had been hinted on for the far northern thumb
is not expected.

a secondary cold front, 850mb temperatures of -21c - 925mb
temperatures of -19 c are expected to push southward into lake
superior by 12z tuesday. a strong composite mid-upper trough lobe
will then swing in line with lower michigan and lake huron during
the day tuesday. strong planetary vorticity advection with lower
tropospheric instability will cause cyclonic shear side cyclogenesis
confined to the northern great lakes. latest survey of moisture/low
level convergence keeps activated lake effect to the north and east
of the forecast area. with a stronger zonal pressure gradient
expected for tuesday under some modest cold air advection did go
ahead and used hand/blend edits to the wind grids for tuesday (west
20 to 25 mph possible) for tuesday night, rapid eastward pivot to
composite trough will allow low-mid arctic air to push through all
of the area. temperatures tuesday are expected to be some 2 to 4
degrees cooler than monday. will need to monitor how fast surface
ridge builds in for tuesday night. if winds totally decouple than
the going min temperature forecast is too warm.

chilly day wednesday as cold cyclonic flow does not get flushed out
until really late in the day. progressive upper level ridge will be
in place through wednesday night. models remain very consistent with
the trajectory of inbound absolute vorticity maxima and broader mid-
upper level trough for thursday. models continue to keep height
falls and low pressure track well north of lake superior and lake
huron. only real thing to watch is how far south the forward warm
advection wing will extend. virtually all model data has kept the
qpf north of the thumb out across northern lake huron through friday


a weak upper-level disturbance will continue tracking east across
the eastern great lakes the remainder of tonight and today. light to
moderate north northwesterly winds will prevail over much of the
local waters, except for lake st. clair and western lake erie which
will experience wind directions out of the southwest south of the
frontal boundary. snow showers will continue to become more
scattered in nature with drier weather returning by tuesday as the
disturbance departs further east and the front lingering over the
region washes out. by midweek, a ridge of high pressure will
approach from the midwest and work to increase the pressure gradient
across the central great lakes, resulting in fresh moderate
northwesterly flow across the local waters with occasional gusts in
the 25 knot range. the offshore component of the wind is expected to
keep nearshore waves below small craft criteria.


.dtx watches/warnings/advisories...
lake huron...none.
lake st clair...none.
michigan waters of lake erie...none.


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