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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
139 PM EDT Wednesday may 27 2015



Aviation...

Showers and thunderstorms have filled in over Southeast
Michigan...at least from fnt south...ahead of a surface trough and
cold front. Activity is expected to continue into the early evening
before the cold front works across the area and pushes storms east.
Severe weather is possible with these storms from ptk south. The
most likely threat is damaging wind gusts to 50 knots...however hail
to 1 inch is also possible. Otherwise...southwest winds will gust to
around 25 knots into the early evening before turning west and
decreasing behind the front. Surface high pressure building into
Southern Lower Michigan late tonight will allow winds to become very
light. This may promote light fog development after 08z.

For dtw...additional showers and thunderstorms may impact the
terminal before 22z. Some of these storms may be strong to severe.
Winds will continue to gust to around 25 knots through early evening
before turning west and decreasing.

//Dtw threshold probabilities...

* high probability for ceilings at or below 5000 feet this afternoon
into early evening.

* Medium potential for additional thunderstorms to affect the
terminal from 18z to 22z.

&&

Previous discussion...issued 1134 am EDT Wednesday may 27 2015

Update...
little change in thinking with potential for thunderstorms today
as the atmosphere continues to destabilize and a pre-frontal
trough and cold front move in ahead of the upper trough now
pushing across the western Great Lakes. Latest NAM sounding
modification for this afternoon (t/dew Point of 81/62 at dtw) yields
around 1500 j/kg of ml cape this afternoon. This should be enough
to support storms or small lines of storms given the strong
unidirectional wind field. Damaging winds to 60 miles per hour will be the
main threat.

Previous discussion...issued 348 am EDT Wednesday may 27 2015

Short term...today and tonight

Middle level trough will be swinging through the Great Lakes today
while a surface low over Ontario pulls a cold front across lower Michigan this
afternoon. This will finally provide some surface based forcing to a
warm and humid airmass thats been in place for several days now.

With the cold front off to the west at the start of the day...deep
layer swerly flow will once again rule over the region keeping
moisture levels elevated with dewpoints in the 60s. Overall the
moisture profile took a hit Tuesday when the wave lifted through the
state taking the >1.5 inch precipitable waters along with it...but still leaving
values over an inch to work with. Ll lapse rates will be decent once
again but we still struggle to get any notable in the middle levels.
The airmass looks to provide cape values of 800-1000 j/kg early in
the day but shear values are only 20-30 knots with veered low level
winds which will not help storm organization. The NAM/hrrr/gemreg
all suggest a prefrontal trough sliding through a few hours ahead of
the cold front. This looks to ignite a line of showers/thunderstorms
around 20-21z across far Southeast Michigan before the cold front slides through
around 22-00z producing another line. Not extremely confident in the
chances for severe weather as shear and midlevel lapse rates are
both weak and the prefrontal trough could work the atmosphere before
instability can maximize and it may not be able to recover for the
cold front. Regardless...there is enough potential to note a slight
chance of strong to severe thunderstorms this afternoon. Primary
threat will be damaging winds but slightly cooler middle levels will
allow for a hail threat as well. Primary area of concern will be
southeast of a line from Port Huron to Adrian due to the orientation
of the front...which lines up well with Storm Prediction Center and their new day 1
outlook.

Outside of thunderstorm activity...winds will be gusty today as we
mix up to 850mb with gradient flow around 30 knots just off the
deck. This could result in gusts at or above 30 miles per hour at the surface
if the possible lines of convection do not disturb the flow too
much. Cooler and drier air will move in overnight as the cold front
exits eastward after 00z. Overnight lows will return to the
middle/upper 50s vs the 60s we have been dealing with of late. Low
amplitude ridge beginning to work into the region overnight will
result in more of a zonal western flow vs a cool nwerly Post frontal
flow. This will limit the amount of cooling.

Long term...

Rising geopotential heights will push across the Great Lakes region
during the day on Thursday. Modeled absolute vorticity fields in
addition to bufr sounding data supports a signature for relatively
stable conditions due to subsidence. All of the County Warning Area begins the
Thursday period with no weather mention. The lower tropospheric
anticyclone center is prognosticated to slide eastward along/near Lake Erie
which will eventually southwesterly flow to spread across northern
lower Michigan Thursday afternoon and down into the Saginaw Bay/no
thumb vicinities towards the evening hours. Is is the higher Theta-E
content washing into the area from the west that allows the NAM to
generate some quantitative precipitation forecast for portions of NE lower Michigan and the tip of
The Thumb. Seems to be a classic example of the NAM convective
scheme kicking in...leading to a false representation of the
thermodynamic environment. Will include a chance pop for the areas
outlined...for the potential of some high based showers along lake
convergence boundaries. Warm advection...insolation...drier airmass
should allow temperatures to reach the lower 80s in many areas on
Thursday.

The high amplitude shortwave zipping quickly eastward across
northern Ontario on Friday will lead to a dominant 1000-500mb height
rise feature over much of the central United States. It is this far
reaching corridor of height falls that will strengthen...then drive
a strong surface cold front through southeastern Michigan during the
first half of the weekend. Still uneasy on when to begin
precipitation chances on Friday. Models are very bullish with
early quantitative precipitation forecast onset...but overall structure of the 300 mb heights and
some distance yet to the upper level jet axis suggests a lack of
forcing could yield quiet conditions. Soundings show no
capping...however...so confidence is low on timing. With that
stated...there is a strong confidence that overall frontal
dynamics will be high which brings a very good potential for
widespread meaningful precipitation amounts...primarily Friday
night and Saturday. The current forecast is including likely probability of precipitation.
With deep southwesterly flow...incrementally bumped Friday highs
upward into the middle 80s for much if not all of the County Warning Area.

There is high confidence in cooler temperatures for the weekend with
Saturday highs in the 70s...Sunday highs in the 60s.

Marine...

South to southwest flow will continue over the Great Lakes region
today...ranging from 10 to 15 knots with gusts to 20 knots. With a
persistent southwesterly wind forecasted for today...will issue a
Small Craft Advisory for Saginaw Bay and the nearshore waters of
Lake Huron from Port Austin to Harbor Beach to account for the
funneling of winds. There is another chance for showers and
thunderstorms this afternoon as a cold front moves through the
area. Dry conditions are the expected Thursday and much of Friday.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...Small Craft Advisory until 10 PM EDT this evening for lhz421-422-
441.

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

$$

Aviation.....Hlo
update.......hlo
short term...drk
long term....cb
marine.......cb



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