Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 201523

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1123 AM EDT Thu Apr 20 2017


Convective trends and severe weather potential remain on track with
refinements mainly tied to the northward progress of the warm front
and development timing of surface based storms.

Morning hourly mesoanalysis already indicates surface based CAPE
exceeding 1000 J/kg over northern Indiana where the warm sector is
quickly maturing due to diurnal heating. The main question is how far
north the warm front will move into Lower Michigan during the
afternoon, along and south of which will be the severe weather target
area. The latest observations and and rapid update model guidance
support recent expectations that the surface low will reach a
position over central Lake Michigan by mid afternoon. This is not far
enough north to overcome the aggregate influence of cool easterly
flow off Lake Huron, St. Clair, and Erie which will tend to stall the
northward progress of the warm front and/or produce a hybrid
boundary linked to the front near the shorelines. Expect the warm
front to surge into southwest Lower Michigan during early afternoon
and initiate or continue surface based convection. The front will
then more stubbornly move to a line roughly from Detroit to Howell to
Lansing by mid afternoon. No problem with the SPC slight risk
extending farther north to encompass hail potential for any strong
updrafts north of the front and to account for a late northward surge
of the front before the cold front moves in toward evening. The
enhanced risk will then cover the mid to late afternoon tornado
potential along the front/lake breeze hybrid and with discrete storms
in the warm sector where flow is locally backed. The instability and
wind shear profile elements and influence on convective mode
outlined in the previous forecast discussion remain firmly valid.
This will be followed by a second round of initiation along the cold
front over western Lower Michigan which will move in behind the early
to mid afternoon activity. These storms will have a chance to mature
into linear modes that will present a greater straightline damaging
wind threat before exiting/ending from west to east during the 7-9 PM
time period.


Issued at 748 AM EDT Thu Apr 20 2017


A complex aviation forecast exists today. Lead area of rain
containing embedded thunder arriving in advance of a warm front will
maintain the potential for isolated tsra through roughly 14z. In
addition, existing low stratus anchored north of a frontal boundary
draped across the northern Ohio valley may yet become problematic as
the warm front lifts back northward during the latter half of the
morning and early afternoon. There remains the potential for
additional lowering of cigs and/or the development of fog during this
time. Next window for possible thunderstorms will then exist from
mid afternoon through early evening /18z-00z/ preceding a cold front.

For DTW...Inbound line of showers and thunderstorms may tend to
weaken upon arrival, but will maintain a TSRA mention through 14z
given radar trends.  Low stratus remains prevalent just upstream
this morning.  Potential for this stratus to fill back in as a warm
front lifts into the airspace, before lifting with the frontal
passage by early this afternoon.  The passage of the front will be
marked by an increase in southwest winds.  Next window for
thunderstorms centered mid afternoon through early evening /19z-23z/.


* High in ceilings below 5000 ft today.

* Medium for thunderstorms today.

Issued at 338 AM EDT Thu Apr 20 2017


50-kt nocturnal LLJ ahead of the surface low lifting into southwest
Wisconsin this morning is fueling elevated convection across the
Great Lakes. Cloud canopy is rapidly becoming high overcast with
convective debris while fog product shows a rapid expansion of low
stratus north of the surface warm front over SE Michigan early this
morning. In the near term, the LLJ will continue to veer and
gradually weaken as it translates eastward toward the CWA. This may
support some increase in coverage locally but should, at least,
allow for existing activity to propagate through the northern CWA
where categorical pops remain in place. Pervasive chc of showers for
all locations through mid-morning as the convergent flank of the jet
slides through. Several hundred j/kg MUCAPE based off the frontal
inversion will support a hail threat as this elevated convection
moves through early this morning. Expecting predominantly pea/nickel
size type of threat due to relatively low CAPE density owing to some
capping in RAP/HRRR soundings. However, marginally supportive shear
in the effective layer precludes completely ruling out a severe hail
threat this morning. A particularly vigorous updraft could penetrate
the stable layer to the tune of a 35 kt wind gust through 15z.

Attention then turns to this afternoon as the cyclone lifts into
northern Lower Michigan. Warm front will gradually lift to north,
likely slowed by existing stratus inhibiting insolation. However,
this effect will also serve to strengthen the boundary via
differential heating wherever it ultimately sets up.
Reintensification of the low-level jet and increasing flow from the
southwest will build dewpoints into the low 60s and temperatures
well into the 70s south of the warm front. At the same time,
increasing convergence within the strengthening mid-level flow is
expected to support scattered deep convection within the warm sector
environment well ahead of the cold front. 0-6km bulk shear around 40
kts/MLCAPE surpassing 1000 j/kg will favor some degree of
organization, potentially including supercells, carrying a wind/hail
threat as early as noon local time. This period is expected to
represent the best potential for a supercellular mode before wind
field ramps up mid/late afternoon yielding an increasing
favorability for bowing segments and the subsequent transition a
fairly widespread severe wind threat south of the warm of the front.
At this time, confidence in the front lifting much of anywhere north
of Metro Detroit is relatively low and coincides with the SPC
depiction of a transition from ENH to SLGT in that vicinity.
Probability of primarily a hail increases with northward extent as
the tendency for convection to be elevated increases.

Best estimate for evolution of afternoon convection is for the
aforementioned initial round /roughly 16-20z/ along the leading
edge/nose of the LLJ to be followed by a strongly forced line along
the cold front roughly 21-01z. RAP cross-sections indicate a
significant bulge in the frontal structure at 850-700mb meaning the
the convective line associated with cold fropa will likely lead the
surface cold front by a decent margin.

Overall, despite the widespread wind threat, the magnitude of the
ambient wind field is forecast to be on the average side and the
threat should largely remain confined to gusts up to around 60 mph.
Ample shear, adequate MLCAPE of 1500 j/kg, and favorable CAPE
density throughout the event will favor quarter to golf ball sized
hail - any larger hail particularly likely with any rotating
updrafts ahead of this evening`s line. Tornado threat will be
minimal over most of the area owing to high LCLs, storm motions of
35-40kts which will inhibit any respectable storm-scale modification
of the environment. The exception will be along the surface warm
front, and thus its position during each round of convection will be
important to monitor. Locally lower LCLs and enhanced 0-1km SRH in
excess of 250 m2/s2 may pose an enhanced risk for a tornado or two
most likely along or north of the M59 corridor, though it is worth
noting that stretching potential in the lowest half to 1km will be
limited invof the front.

In the wake of this evening`s cold front, high temperatures dip into
the 50s for the remainder of the week and through the weekend.
Confluent northwest flow aloft will support persistent high pressure
and quiet/dry conditions through this time. Moderation back into the
60s as southerly flow increases in advance of a Pacific wave early
next week.


A gale warning remains in effect today for northern Lake Huron.
Arrival of colder air under the existing easterly flow ahead of low
pressure moving into the western Great Lakes will provide enough
instability to bring a period of gales over this region.  This
pattern will also maintain small craft conditions for Saginaw Bay
and southern Lake Huron.  Numerous to widespread showers and
scattered thunderstorms will also accompany the system before it
exits tonight. Moderate west to northwest flow is projected to
follow by Friday which could build waves in excess of advisory
criteria before cooler high pressure arrives to start the weekend.


A warm front will lift across southern Michigan today.  Showers will
expand over the region during this process with a chance of
thunderstorms. Coverage is expected to be greatest near the Tri
Cities and northern Thumb early in the day before developing
southward toward Detroit during the afternoon. Widespread rainfall
totals around 0.75 inch are expected with localized totals over 1
inch again possible in thunderstorms before activity exits the
region by Thursday evening. The potential for some minor flooding
will exist, particularly given the wetter than average conditions
noted in recent weeks.




Lake Huron...Small Craft Advisory until 10 AM EDT Friday for LHZ421-441.

     Small Craft Advisory until 10 PM EDT this evening for LHZ442-443.

     Gale Warning until 8 PM EDT this evening for LHZ361-362.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.




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