NWS Forecast Discussion

000
FXUS63 KDTX 260807
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
307 AM EST Tue Jan 26 2021

.DISCUSSION...

As of 07Z the mid level low is located over central Iowa. There has
been a steady intensification of radar returns extending from far SW
Lower Mi into the Mi/Oh border. These returns are located along the
lead edge of the mid level dry slot, within a zone of intensifying
mid level frontal dynamics. As the upper low advances into Western
Lower Mi by early afternoon the region of strong mid level frontal
forcing/deformation will advance toward the northern portions of the
forecast area. The upper low will then translate across central
Lower Mi this afternoon, becoming more sheared as it moves east.
This will cause the isentropic ascent within the mid level trowel
axis to slowly weaken as it lifts across the Saginaw Valley and
thumb region this afternoon.

It is notable that there appears to be a little lower static
stability, even some weak conditional instability, within and above
the mid level ascent within the trowal axis. This is likely to
support a rather brief period of intense snowfall rates and help
compensate for what otherwise are some less than ideal snow to
liquid ratios, at least across the southern portion of the forecast
area. While moisture within the mid level trowal is quite good
(specific humidities of 3 to 4 g/kg advecting into the mid level
front), the northward progression of the mid level front this
morning will allow the mid level dry push to advance into the
southern half of the forecast area during the course of the morning
and into early afternoon. This will strip ice nuclei from the clouds
and will transition the snow to areas of light freezing
rain/drizzle. Locations generally north of the I-69 corridor will
hold onto deeper moisture through the afternoon. So while the
intensity of the frontal dynamics wane, a colder profile with deeper
moisture across the north will support a longer duration of snow.

One item of concern will be the potential for some Lake Huron
enhancement across the Saginaw Valley this afternoon and the thumb
this afternoon/evening as the low level flow backs from northeast to
north. Hi res solutions are suggesting this will support a little
longer duration of snow in these regions. Although sfc-1km
differential theta e within the lake plume is not too impressive,
the deep layer moisture and synoptic scale ascent should allow at
least some added support. There was consideration for boosting
forecast snow amounts to low end warning criteria across the thumb.
A big limiting factor for going against this at this time is that
the depth of the cold layer is still not ideal for significant lake
enhancement.

Total liquid equivalent precip from this event looks to fall between
2 and 4 tenths of an inch. Overall, the total snowfall forecast for
the AM issuance remains in line with the evening forecast update; 3
to 5 inches total along north of the I-69 corridor, tapering to an
inch or two along the Mi/Oh border where mid level drying will be
faster to arrive and more pronounced.


Mid level confluence/subsidence will overspread Se Mi overnight into
the day Wednesday supporting a dry forecast. A compact upper low
will then rotate from northern Ontario Wednesday toward upstate New
York by Thurs night. This will drive a little colder air into the
Great Lakes on Wednesday, keeping temperature seasonally cold
through the end of the work week. A southern stream wave is forecast
by the medium range model suite to lift into the region late in the
weekend or early next week, brining the next chance for
precipitation to Se Mi.

&&

.MARINE...

Northeast winds have peaked out over Lake Erie early this morning
and will slowly weaken and become north-northwest today as low
pressure tracks into the Eastern Great Lakes. Northeast winds over
Lake Huron will still increase further however, with gusts of 25 to
30 knots, especially over Saginaw Bay where the fetch is maximized.
The prolonged onshore flow should have no problem with waves aoa 4
feet impacting the nearshore waters, and small craft advisories
remain in effect, with the exception of inner Saginaw bay due to ice
cover. In addition to the increased wind and waves, snow will
continue to lift from from south to north today, with visibilities
reaching 1 mile or less at times.

As the low exits east, winds become northerly Tuesday night into
into Wednesday, but should mostly hover around 15 knots before
increasing again Wednesday evening into Thursday as cold blast leads
to increased mixing depths. With these renewed northerly winds
reaching up to 25 knots and unstable low level profiles, waves
around 6 feet are expected to build over the southern Lake Huron
basin, and have extended the small craft advisories through
Wednesday night. Its possible the advisories may be extended further
in time as waves look to remain marginally elevated over southern
Lake Huron into Friday.

&&

.PREV DISCUSSION...
Issued at 1203 AM EST Tue Jan 26 2021

AVIATION...

A few flurries made it north of the Ohio border during the evening
only to be pushed back by low level dry air gaining traction from
Ontario. The dry air is now expected to hold VFR until after 09Z and
then get overwhelmed by the surge of snow moving in from the
Midwest. This brings an abrupt transition to IFR/LIFR mainly in
visibility and for a longer duration toward FNT and MBS after 12Z
through the morning. Upstream observations depict an impressive
pattern of snow moving in from the Midwest which is expected to
maintain intensity while setting up along and north of I-69 by mid
morning. To the south of this banded area, conditions for freezing
drizzle remain favorable as dry air surges into the mid levels over
the DTW corridor. It will be light using MVFR visibility as a proxy
for intensity until observations can offer some additional guidance
and before temperatures creep above freezing by afternoon. IFR
ceiling holds otherwise as the band of snow shifts northward toward
central Lake Huron into Tuesday evening.

For DTW... VFR occasionally reaches above 5000 ft as dry air fights
against snow moving in from the west. An abrupt transition to
IFR/LIFR visibility in a burst of snow occurs from about 10Z to 14Z
resulting in accumulation around 2 inches at the terminal. A
transition to freezing drizzle occurs by mid morning which brings
just trace amounts of icing on top of freshly fallen snow and before
temperatures rise slightly above freezing by afternoon.

DTW THRESHOLD PROBABILITIES...

* High for ceiling below 5000 feet late tonight.

* High for snow late tonight through morning.

* Moderate for freezing drizzle mid to late morning.

* Low for ceilings or visibilities to drop below 200ft or 1/2SM
  respectively.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 6 PM EST this evening for MIZ047-048-
     053-060-061.

     Winter Weather Advisory until 11 PM EST this evening for MIZ049-054-
     055-062-063.

     Winter Weather Advisory until 4 PM EST this afternoon for
     MIZ068>070-075-076-082-083.

Lake Huron...Small Craft Advisory until 5 AM EST Thursday for LHZ421-441>443.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 10 AM EST this morning for LEZ444.

&&

$$

DISCUSSION...SC
MARINE.......SF
AVIATION.....BT


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NWS DTX Office Area Forecast Discussion