Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 161744

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1244 PM EST Tue Jan 16 2018


Passing low-level theta-e ridge axis within a background of moist
cyclonic flow and mid-level PV advection is supporting light snow in
the Pontiac to Detroit corridor. Expect generally borderline
MVFR/IFR vsby with a brief window of solid IFR possible as strongest
forcing passes in the next couple of hours. Lake moisture
entrainment in the surface trough over the Saginaw Valley is
supporting increasing snow coverage and reduction in vsbys. Surface
trough will sweep through the area with diminishing convergence as it
moves southeast. Snow will gradually diminish after 00z with
incoming shortwave ridging. Brief window for clearing late before
debris releases from Lake Michigan. Gusty southwest winds and
borderline MVFR cig develop during Wed morning.


* High for cig aob 5kft into tonight.


Issued at 939 AM EST Tue Jan 16 2018


A sfc trough axis currently extends from sfc low pressure over
southern Lake Huron across the Saginaw Valley. Meanwhile, an
elongated upper low is positioned across Lake Huron and Southern
Lower Michigan. Weak large scale ascent will persist across Se Mi
today as the upper low slowly rotates toward the ern Great Lakes.
Enhanced boundary layer convergence will also persist along the sfc
trough axis. There has already been some increasing radar returns
across srn Lower Mi this morning. This trend should continue into the
afternoon with increased boundary layer convergence associated with
daytime heating, particularly in the vicinity of the sfc trough axis.
NAM and RAP soundings suggest steepening low level lapse during the
course of the day, with convectively induced lift falling within the
region of good dendrite production. This and supersaturation with
respect to ice in the convective cloud layer should result in decent
snowflake size, while limited moisture will keep the snow dry and

The sfc trough axis is forecast to shift from the Saginaw Valley and
thumb region down toward metro Detroit late this afternoon and
evening before pushing off to the east overnight. Given the above
factors, coverage of snow showers for this afternoon and evening will
be increased in the forecast with minor total snow accumulations (an
inch or less this afternoon and evening). There have been multiple
lake vorticies across southern Lake Huron this morning given the sfc
low overhead. Some of these are likely to brush across the eastern
thumb, producing locally higher snowfall totals. The expected
transient nature of the forcing in the thumb suggests holding a more
conservative snowfall forecast attm (a couple inches total).

Issued at 334 AM EST Tue Jan 16 2018



As of 330 AM EST...Clipper low that brought light snowfall to the
region yesterday continues to linger across southern Lake Huron early
this morning, with a bent-back occluded front draped near the M-59
corridor. Convergence along this front, coupled with lingering large-
scale ascent from a closed upper low in place over southern
Wisconsin/northern Illinois has resulted in periods of light snow
continuing through much of the night across portions of southeast
Michigan. Local topographic effects have resulted in localized areas
of more enhanced radar reflectivities, particularly along the glacial
ridge, but overall flake size is small with weak omega in near the
DGZ. Thus, accumulations overnight will likely be on the order of a
few tenths to around a half of an inch for most areas. Light snow is
expected to expand gradually northward into the morning hours,
although most areas of southeast Michigan will still be capable of
seeing flurries and occasionally more steady light snow showers.

The exception will be across far northeastern Huron county, where a
convergent near surface wind field will allow for lake enhanced
activity through the morning and into the afternoon. Tough call on
whether the meat of the band actually makes it onshore, as hi-res
guidance indicates a largely offshore component to the low-level
flow, but this could be modulated by more persistent onshore
north/northeasterly flow developing later this morning as the
decaying clipper tracks slightly further east. Will continue to
monitor closely, but given the light rates expected, will hold off
on an advisory for now.

Later this morning, northern stream PV streamer moves across the
region as the upper low becomes elongated and eventually opens up as
core of 140+ kt jet streak moves off to the northeast of the trough
axis. This lingering synoptic scale lift coupled with increasing
boundary layer forcing due to diurnal effects will promote a
potential round of more persistent light snow this afternoon and
evening with slightly higher rates (0.2 to 0.4 inch per hour) and
better dendrite production, especially east of the I-75 corridor.
These trends are supported by multiple hi-res model sources, as the
trough axis swings through.

Given all these factors in play, another inch or so of fluffy
accumulation will be possible through this evening for most areas,
with locations closer to Lake Huron possibly seeing as high as 2
inches in spots. Highest totals are expected over northeastern Huron
county, where 2-4 inches are possible, especially if the core of the
aforementioned band can move onshore. After low temperatures in the
upper single digits to mid teens (warmest between M-59 and I-94),
highs will reach into the lower 20s for most areas.

Snow chances end by late this evening as flow trajectories become
increasingly anticyclonic. With light winds, favorable radiational
cooling will allow low temperatures tonight to fall into the single
digits for most areas.


A period of more tranquil weather will occur during the short term
period, as the upper-level trough axis exits east of the region and
is replaced by rising heights and increasingly anticyclonic
northwest flow aloft. At the surface, strong high pressure will
crest across the Tennessee River Valley. This pattern will usher in
drier air through the column resulting in a period of dry weather
and partly cloudy skies, although pesky residual boundary layer
moisture trapped beneath the subsidence inversion could lead to
periods of more cloudy skies. Highs on Wednesday will continue to be
below normal and in the 20s, but the onset of weak return flow
around the surface high will allow temperatures to moderate
beginning Thursday and continuing into Friday. Highs Thursday will
make a run at the freezing mark and warm into the mid and upper 30s
for Friday. A passing northern stream shortwave trough Thursday
night into Friday will do nothing more than increase cloud cover as
limited moisture will accompany it.


Unsettled weather returns for the weekend and into early next week
as vigorous Pacific energy moving ashore the west coast carves out a
deep upper-level trough and eventual lee cyclogenesis in the south-
central Plains. Both the GFS and ECMWF are in fairly good agreement
with the PV energy moving onshore late Friday into Saturday, and the
downstream synoptic pattern favoring the aforementioned lee
cyclogenesis. The low pressure will deepen as it tracks northeast
towards the Great Lakes region Sunday into Monday, with a slowing
trend noted in the last few runs of model guidance. Broad
southwesterly flow ahead of the low, aided by strengthening return
flow around high pressure situated across the southeast states will
overspread the region ahead of this system allowing for a warmup in
temperatures with daytime highs reaching into the 40s. Despite the
milder temperatures advecting in, so will the moisture along with
increasing rain chances for the latter half of the weekend into


Low pressure will track into southern Ontario this morning wind
winds veering to the north-northeast in its wake. While colder air
will spill across the lake in the wake of this low, a decreasing
gradient will limit peak wind gusts at 20 knots or less on Lake

A strengthening southwesterly gradient will develop by mid week as
strong high pressure slides across the southern United States and a
slow moving cold front pushes across northern Ontario. Warmer air
advancing across the lakes will inhibit mixing depths, thus the
chances for gale force wind gusts on Wednesday and Thursday still
appear too low to carry a Gale Watch. The most likely period for
marginal gale force gusts seems to be limited to the late afternoon
and early evening period on Wednesday before stable conditions fully
develop. Central Lake Huron, downwind of the funneling effect of
Saginaw Bay, will be most vulnerable to a period of gusts pushing 35


Lake Huron...Gale Warning from 10 AM to 10 PM EST Wednesday for LHZ362-421-441-

     Gale Watch from Wednesday afternoon through Wednesday evening for

     Gale Watch from Wednesday morning through Wednesday afternoon for

     Gale Warning from 10 AM Wednesday to 4 AM EST Thursday for LHZ363.

     Gale Watch from Wednesday morning through Wednesday evening for

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



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