Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 241955
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
255 PM EST Fri Feb 24 2017

.DISCUSSION...

Impressive late February warm sector intrusion well underway this
afternoon, with temperatures at DTW tying the all-time record high
value for February /70 degrees/.  20z surface analysis places the
996 mb surface low at the southern tip of Lake Michigan, with the
attendant surface warm front now arcing along a line from roughly
south of Lansing into the I-69 corridor.  Modulating low-mid level
environment seemingly evolving as anticipated thus far, with steady
low level destabilization under very respectable warm/moist
advection translating into MLCAPE upwards of a 1000 J/KG across the
warm sector.  Lead pre-frontal trough now extending from north-
central IN into far south-central lower MI. Just enough ascent to
generate mainly low topped convective cells within this profile. The
exception now ongoing south of the I-94 corridor, where a dynamic
bow structure has recently emerged.  This activity proving a touch
more progressive relative to model guidance, but still leaves a
window for additional development/expansion through early evening as
this lead ascent works across the existing but clearly vulnerable
cap.  Additional development could also emerge along the immediate
northeast side of the low/near the triple point over southwest lower
MI, capitalizing on a localized axis of convergence. Pronounced deep
layer and low level shear parameters during this time remain
conducive to rapid organization into discrete/ supercellular
structures and/or line segments should any stronger updrafts take
hold.  This will maintain a tornado risk, particularly near the warm
front.

Greatest potential for organized convection still centered within
the 01z-05z window early Friday night, owing to a sizable increase
in mass/frontal convergence into an existing modestly unstable
profile.  Any nocturnal downturn in available instability at the
surface may become essentially neglible, given the high degree of
instability /for late February standards/ that will still exist just
off the surface.  General model signal suggests a lead pre-frontal
trough anchoring weak mid level cold air advection/steeper lapse
rates could become an initial focus, either just upstream or
directly overhead.  Development is expected either on this feature,
the trailing cold front or both.  In any case, magnitude of the wind
field across both the lowest 6 km and 1 km could support a mixed
mode, tending to be discreet/supercellular early before congealing
into linear segments. Greatest risk with southward extent, with
areas south of M-59 appropriately highlighted within an enhanced
risk.

A more seasonable air mass to take residence for the upcoming
weekend.  Extended period of cold air advection initiates overnight,
with a solid secondary response expected Saturday under substantial
height falls.  850 mb temperatures back below -10C by Saturday
afternoon, leaving temperatures nearly steady or even falling
through the 30s.  A period of enhanced cva accompanying the upper
trough passage may augment the remnant deformation/wrap around
moisture axis across the region during the afternoon.  This will
support a period of light snow showers/flurries, any accumulation to
remain localized and likely confined to grassy surfaces given the
recent warm spell. Some increase in the westerly gradient by
afternoon, with gusts into the 30 to 35 mph range.   Surface ridging
Saturday night, with emerging low level southwest flow in it`s wake
establishing a weak pattern of warm air advection into Sunday. This
pattern may yield a narrow corridor of weak ascent along the edge of
inbound northern stream wave.  Current model indications point to
the greatest potential for precipitation development to remain to
the west and north. Afternoon temperatures siding on the warmer side
of normal.

Surface high pressure in place over the Great Lakes region will keep
conditions relatively dry throughout the day on Monday, as S/SW
winds allow warmer air to pool into the region, pushing daytime
highs in the mid to upper 40s throughout the CWA. The chance for
precipitation will increase steadily early Tuesday morning into
Tuesday afternoon, as low pressure from Kansas/Oklahoma pushes
northeast into Michigan Tuesday into Wednesday. A very slight chance
to see snow/wintry mix will be possible at the onset of
precipitation, as temperatures are expected to sit just above
freezing values at the surface, with 850 mb temperatures averaging -
2 to 0 C with the latest 12Z GFS suite. However, rain will be
expected with the bulk of the event as WAA occurs throughout the
day, ushering in above freezing temperatures at 850 mb by the
afternoon, with surface temperatures peaking in the upper 40s to
lower 50s.

Low PoP chances will be possible throughout the remainder of the
week as a series of upper-level disturbances and weak surface lows
eject from the Northern Central Plains into the Great Lakes region.
Above seasonal temperatures look to continue for the early half of
next week, with a return to seasonal temperatures by the second half
of the week, as daytime highs peak in the mid to upper 30s by
Thursday and Friday.

&&

.MARINE...

Active stretch of weather across the eastern Great Lakes as a strong
low pressure system lifts northeast through the region today and
tonight. Already elevated northeasterly flow will continue to ramp
up this morning resulting in a period of gales across northern Lake
Huron and small craft advisories (for both winds and waves) around
the tip of the Thumb and into Saginaw Bay. These headlines will
continue through this evening until the center of the low reaches
the lake briefly relaxing flow a bit overnight until gusty
west/northwesterly winds behind the cold front ushers in colder and
more unstable air into the region. This may result in another period
of gales across much of the eastern lakes Saturday into Saturday
night. In addition to strong winds, this system will bring showers
and thunderstorms to the region today and tonight. Some strong to
severe thunderstorms may develop on the cold front later this
evening into the overnight hours.

&&

.PREV DISCUSSION...
Issued at 1143 AM EST Fri Feb 24 2017

AVIATION...

Warm front is expected to clear southern Taf site (dtw/det/yip) at
the start of the taf period, allowing for VFR conditions with
increasing southerly winds in the warm sector, gusting up to 30
knots late in the day. Anticipating the front getting hung up around
or just north of FNT this afternoon, maintaining lower ceilings
(MVFR), with MBS locked in IFR/low IFR with northeast flow off
Saginaw Bay. Increasing chance of thunderstorms this afternoon, with
numerous thunderstorms likely around early this evening. Storms will
have the potential to be severe. Activity looks to end by around
Midnight, as cold front moves through, resulting in wind shift to
the west-southwest. Frequent gusts in the 20 to 30 knot range
expected overnight, right into Saturday as colder air filters in,
resulting in flurries or light snow showers during the day Tomorrow.

For DTW...Carrying a 4 KFT warm sector ceiling this afternoon, but
possible there will be scattered periods. Southerly winds expected
to top out in the 25 to 30 knot range late this afternoon. Steep mid
level lapse rates moving through this afternoon could sustain
scattered showers and possible thunderstorms this afternoon, but
best chance of strong to possibly severe thunderstorms looks to be in
the early evening hours. Picked out 1-4Z window for thunderstorms in
TAF, but activity could arrive sooner. Cold frontal passage around
midnight, and expecting enough drying/subsidence to scatter out low
clouds, before cold advection allows for low ceilings (MVFR) to
return early Saturday Morning, with strong west-southwest (240-260)
winds gusting between 25 to 30 knots during Saturday.

//DTW THRESHOLD PROBABILITIES...

* Medium in ceilings aob 5000 feet through this evening, low
  overnight, and medium tomorrow.

* Low in thunderstorms this afternoon, medium this evening.

* Low in westerly cross wind threshold Tomorrow.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Saturday morning through late Saturday night for
     LHZ361>363-441>443-462>464.

     Small Craft Advisory until 10 PM EST this evening for LHZ421-441.

     Gale Warning until 10 PM EST this evening for LHZ361-362.

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

$$

DISCUSSION...MR/AM
MARINE.......SF
AVIATION.....SF

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