Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 011746
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1246 PM EST Wed Mar 1 2017

.UPDATE...

Attention is now focused on a rapidly developing cold front that is
at the doorstep of southeastern Michigan. The feature, currently
along a line from Kalamazoo to Fort Wayne, is exhibiting a wind
shift from approximately 240 to 270 degrees. Already at this early
hour, have been seeing surface observing sites reporting wind gusts
of up to 40 knots over portions of NW Indiana. A tremendous amount
of synoptic scale support will now exist this afternoon for the
rapid development of the cold front. Water vapor imagery matches the
observed placement of shortwave energy to the progged potential
vorticity fields, which increases confidence that a increasing
ageostrophic response will become in phase with the circulation along
the juvenile front. System relative flow and orientation of the
isobars on the 284K surface continue to suggests the deepening
isentropic downglide will impact southeastern Michigan. Splitting
hairs but most pristine of down the slope support appears to be
lining up immediately south of the Michigan Stateline, directly
across far northern Indianan and Ohio.

There are a few specific items along with a few trends to
communicate. 1. Will watch for low-topped convection breaking out
along this wind shift as it pushes through between 17-20Z. Currently
seeing a few showers initiating along this line. Very meager
instability available perhaps a few hundred J/kg. However, some
buffer soundings suggest high enough near surface theta e content to
support some very brief potential for convective heights to reach 10
kft agl. Given the convective component could certainly wind gusts
during the next 0-3 hrs of up over 50 mph. Will monitor and issue
convective warnings as needed. 2. RAP, HRRR, and new NAM are now
suggesting a slight northward shift to the stronger wind potential
early with winds extending into far southern portions of Shiawassee,
Genesee, and Lapeer counties. Attm, models suggest winds there of up
to 40 mph for a duration of only 3 hours from 18-21Z. After that
time, the thermal trough and lighter wind field is expected to push
southward. Will forgo the expansion of the Wind Advisory northward
now and will continue to monitor during the afternoon. 3. The last
item that will be looked at more closely this afternoon will be
strong wind potential that is expected to develop along the Lake
Huron shoreline in the Thumb tonight. Wind trajectories are
forecasted to become solidly north northwesterly which will
essentially allow the wind field to scrape the coast. Right now, the
forecast is close to calling for 40 to 45 mph immediately along the
shoreline, but will evaluate the full 12Z guidance for the 4 PM
package. 4. The most recent of HRRR and RAP trends suggests some
loss in signal for +40 mph this afternoon. Instead, the signal
firmed up on a longer duration of 35 to 40 mph that should persist
right throughout the evening right to midnight.

Snow squall potential for southeastern Michigan including the Detroit
Metro this evening....

Main feature to key off during the 01-05Z timeframe is along the
main cyclonic midlevel trough axis that is expected to swing across
the state. Forecast soundings do show an increase in convective
depths from approximately 2500 ft to 5000 ft agl. Supersaturation
with respect to ice will also increase which lends increased support
for ice nucleation. Attm, there is little expectation for any
snowfall accumulations outside of a dusting.

Accumulating Snow Potential northern portion of the cwa...

No real surprises here in the last guidance with regards to the main
synoptic scale forcing for widespread precipitation today. Models
have converged on a solution that keeps main 700-500 mb deformation
axis over northern Lower Michigan this afternoon. Per regional
reflectivity mosaic, surface observations, traffic cams widespread
light to moderate to heavy snow is now ongoing. It still appears that
slight a southward expansion of the light snow axis will occur into
areas north of M 46 during the late afternoon. In-house probabilities
have trended with the latest HRRR and RAP which has trended downward
with amounts. Seeing much less support for higher liquid equivalent
falling in the northern cwa. Will adjust expected snowfall amounts
down into the 1 to 2 inch range.

&&

.PREV DISCUSSION...
Issued at 1110 AM EST Wed Mar 1 2017

AVIATION...

MVFR/IFR stratus engulfs the area with the exception of a well-mixed
dry slot surging toward the Detroit area at issuance time. Although
this dry slot may clear out cigs for an hour or two, the trade off
will be earlier introduction of wind gusts. Lead edge of cold air
carrying threat of convective shower development and high end wind
gusts toward 40 kts is currently over SW Lower MI and western IN.
This will reach locations from PTK south by approx 19z. FNT expected
to be on the periphery with gusts topping out nearer 30 kts. There
is at least some potential for a deeper shower to produce a higher
end wind gust...especially in the Detroit area. Wind veers toward
NWly as cold front works through this evening. Light snow showers
with minimal vsby restriction anticipated as forcing departs.
Exception will be KMBS possibly KFNT where snow squalls with periodic
IFR vsbys may prove problematic.

//DTW THRESHOLD PROBABILITIES...

* High for cig aob 5kft

* Low for t-storm to impact KDTW airpsace

* High for exceeding crosswind threshold from 250 to 270 degrees

PREV DISCUSSION...
Issued at 238 AM EST Wed Mar 1 2017

DISCUSSION...

The initial wave of surface low pressure moving from Illinois into
central Lower Michigan will carry another round of storms through SE
Michigan with damaging wind still possible. This activity is closely
following the convection that occurred during the evening and early
overnight and is moving through a worked over atmosphere from an
instability perspective, although it will make the most of some
lingering mid level lapse rates hanging around 7 C/km within the
strongly forced larger scale environment. Current timing has even
trailing remnants exiting SE Michigan before sunrise.

Attention then turns to the effects of secondary low pressure
development moving across the south end of Lake Michigan and into
southern Lower Michigan during the morning. This low will be tied to
the upper level short wave moving into the western Great Lakes and
the associated additional dynamic forcing. The system presents some
lingering concerns for shallow/weak convection along the leading
cold front, strong gradient wind behind the front, and then a
transition from rain to snow along and north of the trailing cold
front all during the afternoon into early evening.

Recent runs of the HRRR are most bullish on afternoon convection
along the leading cold front as the surface low and upper wave bring
in steeper lapse rates, around 8 C/km in the 700-500 mb layer. This
is shown to produce surface based CAPE to around 500 J/kg with
reasonable surface parcel estimates around 63/55. Runs of the NMM,
ARW, 4km NAM, and 3km NAMX all share similar environments while
offering varying reflectivity output during the 15Z-21Z time period.
Model soundings indicate some capping temperatures to overcome in the
850-700 mb layer suggesting scattered coverage and/or later timing
and also indicate low freezing level that could make hail a concern
along with strong wind gusts.

Even if convection fails to materialize along the lead cold front,
strong gradient wind gusts are likely behind the boundary with the
greatest potential mainly south of the low track or along and south
of the I-69 corridor during the afternoon. The 00Z NAM remains
strongest with the wind field and builds the boundary layer enough
to tap into near 50 knot speeds aloft. Consensus of other model
soundings build the boundary layer up to about 875 mb tapping into
lower 40 knot winds. This, along with moderate cold advection and
isentropic downglide, will make surface gusts in excess of 45 mph
reachable from early to mid afternoon through mid evening. The
headline strategy at this point is to hold off for an early morning
update and issue a wind advisory once the early morning convective
potential diminishes.

Low pressure exiting the Thumb region will begin the rain to snow
transition through central Lower Michigan during late afternoon.
Model agreement is very good in terms of forecast soundings cooling
enough for all snow shortly after 21Z. The forecast challenge by
then will be the precise location of the deformation band and
associated higher snowfall rates. Recent model runs have been mostly
consistent with the bulk of the deformation setting up over northern
Lower Michigan during the afternoon. The southern fringe then
brushes the Tri Cities and northern Thumb during the early evening
before making a strong move southward toward midnight. The
progressive nature of the band during the overnight hours will also
be a limiting factor for accumulation that is expected to hold in
the 1 to 3 inch range mainly along and north of I-69 and an inch or
less south.

Cooler but quieter weather will then settle in for the end of the
week outside of sorting out the character of a clipper due in the
area Thursday night. Model solutions on this feature still range
from a couple inches of accumulation in the NAM to just a dusting in
other model solutions.

MARINE...

A series of low pressure systems with showers and thunderstorms will
track northeast along a stationary front across central Lake Huron
today. Winds will back to the north during the afternoon behind the
lows departure and increase to gale force gusts to 40 knots through
Thursday morning. Rain showers with this system will change to lake
effect snow showers. Northwesterly flow will linger through the end
of the week across the lakes then followed by a gradual increase in
southerly flow and a warming trend for the weekend.

HYDROLOGY...

Deepening low pressure moving through Lower Michigan will sweep
showers and out of Lower Michigan during the early morning. Until
then, total rainfall is expected to range from 0.75 inches to 1.25
inches. Bands of heavier showers and thunderstorms will have
potential to result in locally higher rates/amounts. Minor flooding
of low lying areas is possible along with rises in area rivers and
streams. Much colder air will filter in by the end of the day
through tonight, changing any precipitation to snow.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory until 10 PM EST this evening for MIZ063-068>070-075-
     076-082-083.

Lake Huron...Gale Warning until 4 AM EST Thursday for LHZ361>363-441>443-462>464.

     Small Craft Advisory until 4 AM EST Thursday for LHZ421-422.

Lake St Clair...Gale Warning until 10 PM EST this evening for LCZ460.

Michigan waters of Lake Erie...Gale Warning until 10 PM EST this evening for LEZ444.

&&

$$

UPDATE.......CB
AVIATION.....JVC
DISCUSSION...BT
MARINE.......BT
HYDROLOGY....BT


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