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Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
324 PM EST Sat Dec 10 2016

Discussion...

12z DTX sounding indicated 850 mb temp of -16 c, with inversion
height around 800 mb which has been able to support some
disorganized snow shower activity from Lake Michigan working into
Southeast Michigan. 12z NAM suggests a more focused band (seen in
925 mb omega) in between M-59/I-69 corridors will be lifting north
(toward Saginaw bay) through the night as winds back to the south.
This band will have potential to produce light localized accumulations
ahead of the main over-running snow shield arriving late tonight.

Upper level energy/wave now onshore, tracking through the Pacific
northwest. 12z NAM/Canadian/GFS consensus indicates strengthening
mid level wave/500 mb trough reaching the upper Mississippi River
vally by 00z Monday, with Max 6 hr 500 mb height fall center and
surface low tracking over/near Green Bay, and through far northern
lower Michigan/straights Sunday night. This is farther north and
west compared to 24 hours ago. This could impact distribution of the
snowfall across Southeast Michigan, as NAM suggests higher amounts a
bit farther north, as 700 mb fgen band looks to straddle I-69
corridor late in the day on Sunday. NAM also shows 4 g/kg of
specific humidity within the 850-700 mb layer reaching the southern
Michigan border late in the day. Unfortunately, placement and exact
southwest-northeast orientation of the developing 700 mb jet (50-70
knots) axis over the central/eastern Ohio Valley varies
significantly as an important weaker wave exits the Southern Plains.
Thus, GFS and UKMET suggest active frontal zone will reside along
and south of the Michigan border, robbing Southeast Michigan of some
moisture.

Isentropic ascent/fgen is very good, but relatively short lived for
any given location. Warm advection leading to average 850-700 mb
temps of -3 to -6 c south of M-59 Sunday afternoon, with -5 to -9 c
across the north, which will support higher snow ratios, and thus
thinking more uniform/widespread snowfall, as the lower qpf in north
is off set by higher ratios. Snow to qpf ratios quickly lowering to
10:1 down south is expected to cap higher totals. Have been debating
all day whether we are looking at more of 5 to 8 inch snowfall, vs 6
to 9 inch. Based off the 12z Euro still remaining more subdued with
moisture advection, only 3 g/kg of specific humidity in the 850-700
mb layer sneaking just past the southern Michigan border and 24 hr
qpf advertised to be in .3 to .6 inch range, prefer to go out with
high end advisory for 5 to 8 inches as suspect mid level fgen will
not stay parked over one particular location too long, but would
expected highest totals in around the Detroit Metro area toward the
border/along triple point. Backed surface-925 mb flow coming off
Lake Erie/Lake St. Clair could even give a modest boost to the low
level convergence as winds reach the higher terrain of the Glacier
Ridge. Can't rule out isolated/slightly higher totals exceeding 8
inches and possible upgrade to warning criteria, but uncertain on
location this far out. It continues to look like the
accumulating snow will be over before the Monday morning commute
as the aggressive dry slot punches in. Temperatures also look to
be around freezing mark Monday morning, so long as roads are
plowed and salted, should not have big issues on main roads.

Tuesday through Sunday extended forecast: scattered snow showers
will be possible mid-week as upper level Canadian low rotates
multiple weak waves through the Great Lakes region. It will also
help usher the descent of cold Arctic air into Southeast Michigan.
Temperatures will fall well below the norm, with high's in the teens
and overnight lows plunging into the single digits. On Friday,
models show a large, complex area of low pressure spreading eastward
across the central U.S. At present time, this low is expected to
reach the western Great Lakes Friday morning and Southeast Michigan
later that afternoon. Timing adjustments are likely as the low
develops and progresses.

&&

Marine...

Moderately gusty westerly winds this evening will gradually diminish
tonight, shifting to southerly by Sunday morning in advance of
approaching low pressure. Any ongoing lake effect squalls will
subsequently shift east into the Canadian waters through this time.
Southerly winds turn gusty late Sunday and Sunday night as the low
advances into the Great Lakes. A brief period of gusts into the 25
to 30 knot range over southern sections of Lake Huron during this
time. Otherwise, the main element with this system becomes a
widespread long duration accumulating snowfall that starts late
tonight and persists through Sunday night. Gusty westerly winds
develop behind this system Monday. The passage of an Arctic front
Monday night into early Tuesday will reinforce these winds through
the mid-week period, potentially leading to a period of gales during
this time.

&&

Hydrology...

Low pressure lifting across the area will spread a long-duration
snowfall into Southeast Michigan starting late tonight and lasting
through late Sunday night. Total snowfall of 4 to 9 inches is
expected at this time, with highest totals from the I-69 corridor
southward. Snow intensity is expected to peak during the afternoon
and early evening. Rainfall equivalent of the snow that will fall is
expected to range between one-third and three-quarters of an inch.
Snow will end very early Monday morning.

&&

Previous discussion...
issued at 1145 am EST Sat Dec 10 2016

Aviation...

Initial issue of concern is lake effect snow squalls kicking back up
off Lake Michigan. Bands have gotten a little more active this
morning with approach of next upper level short wave energy, but
low level winds over Southern Lower Michigan have essentially
backed to a more southwesterly direction. With changing low level
trajectories now expecting best threat for reduced IFR type
visibility in squalls to be from ptk north to around fnt this
afternoon. South of there... we've actually seen a fair bit of
scattering of low cloud deck which should fill in soon. With
continued backing of surface flow later this afternoon and into
the evening expect less organization of snow bands east of mid
Michigan and may see a period of no or minimal precipitation most
areas this evening. Widespread snow from next upstream low
pressure system still on target to impact Southeast Michigan after
about 06z with increasing intensity during the Sunday morning
hours. Expect widespread IFR conditions to develop during this
time period and continue through Sunday afternoon.

For dtw...bases will be mostly low end VFR for this afternoon...
with exceptions for MVFR where any snow squalls occur. Banding is
spreading off Lake Michigan further inland... but appears best
threat for stronger squalls will be north of dtw this
afternoon... with mostly just an occasional flurry at dtw/yip.
System snow overspreading the region from the west to impinge on
dtw around 06z... but intensity should be pretty light at the
onset. Expect more intensive snow to develop by late morning with
IFR conditions for both cig and vsby pretty solidly entrenched by
noon.

//Dtw threshold probabilities...

* moderate for ceilings below 5000 feet this afternoon... and
high below 1000 feet after 16z.

* Moderate to high for visibility below 3 miles after 09z... and
high after 16z.

* High for snow as precipitation type through Sunday.



&&

DTX watches/warnings/advisories...
Michigan...Winter Weather Advisory from 7 am Sunday to 4 am EST Monday for
miz047>049-054-055.

Winter Weather Advisory from 4 am Sunday to 4 am EST Monday for
miz053-060>063-068>070-075-076-082-083.

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

$$

Discussion...sf/de
marine.......mr
hydrology....mr

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