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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
134 PM EDT Sat Jul 26 2014



Aviation...

//discussion...

The combination of an upper level disturbance passing through
northern Michigan today and a warm front approaching southern
Michigan will bring clouds below 5000 feet and a chance for showers
and thunderstorms over Southeast Michigan this afternoon and this
evening. There is low confidence however in activity hitting any of
the terminals as coverage is not expected to be widespread. Recent
model trends show a burst of rain showers/thunderstorms and rain developing across the County Warning Area
around 2300z...therefore included a tempo group at all sites
highlighting the most likely timeframe for storms. Despite the
higher confidence in rain showers...thunderstorms and rain coverage is still expected to be
too isolated to include in any of the tafs at this time. Drier air
will work into the area at the middle levels during the evening and
overnight reducing chances for precipitation. Calm winds and an
additional increase in near-surface moisture will however allow some
light MVFR fog to develop overnight.

For dtw...although showers and thunderstorms will be possible
anytime from this morning through mid-evening...the best potential
will be in the early evening (20-23z).

//Dtw threshold threats...

* medium confidence in ceilings at or below 5000 feet this afternoon
and this evening.

* Low confidence in scattered thunderstorm development near terminal
this afternoon and evening.

&&

Previous discussion...issued 353 am EDT Sat Jul 26 2014

Short term...today and tonight

A complex scenario looks to be in place today...but overall chances
for thunderstorms and severe weather look low for most of the area.
First...a warm front currently lies stretched from Iowa and southern
Illinois. The elevated portion of this front now looks to just
barely lift into Southern Lower Michigan today. Without the
low-level influx of warmer air models had previously showed with a
faster and further north arrival of the elevated front...forecast
soundings now indicate a cap will remain over most of the area into
the afternoon...and possibly not erode until this evening. The
exception may be near the Ohio border (closer to the front).
Next...a piece of energy that sheared off the upper low over
Saskatchewan is now moving into western Ontario per water vapor
imagery. This wave and associated surface trough are forecast to
drop across northern Michigan late this afternoon and evening.
Cooler air aloft associated with this feature may help to erode the
cap late this afternoon and evening...especially closer to the
shortwave over central Michigan. Lift from the surface trough and
broader left exit region forcing over the southern and central Great
Lakes may help scattered showers/thunderstorms form as this wave
approaches and moves across the area this afternoon and evening. So
overall...probability of precipitation are fairly low...but highest across the Saginaw
Valley and thumb. Severe weather potential is conditional today. 0-6
km bulk shear still look to reach 40-50 knots as an upper speed maximum
moves across the area. Good instability however is in question with
the middle-level cap expected to linger through much of the day and as
moisture transport likely becomes impeded by convection firing in a
more favorable environment to our south. This should hold surface
dewpoints down into the low/middle 60s north of the Detroit area...with
some higher values near the Ohio border helping to boost cape values
there. Temperatures should continue their warming trend as slightly
warmer air does arrive today...with highs reaching near 80.

A plume of drier air looks to work into the area by this evening and
overnight. Cooler air advection aloft should ramp during this same
timeframe as the Canadian upper low drops into Minnesota and surface
troughing ahead of this feature pushes into the western Great Lakes.
Thunderstorm potential will depend on whether we can destabilize
before the drier air works in. Better potential for storms will
therefore be in the very late afternoon through mid-evening. The
overnight hours then look fairly quiet. The warm front lifting up
into southwest Michigan...and associated Theta-E push...may allow a
few showers/thunderstorms to develop near the Ohio border late tonight
however.

Long term...

The compact and unseasonably strong upper level circulation over
south-central Canada today will begin to affect Southeast Michigan Sunday.
This system will present both the potential for severe weather by
Sunday afternoon and for heavy rainfall by Sunday night. The
convective complex expected to develop tonight will reach the
virginias judging from model mean relative humidity clustering there by Sunday
morning. That should be enough separation to allow some low level
moisture to return into lower Michigan ahead of the Canadian system
as it settles into the upper Midwest. The problem is with the
magnitude of the moisture transport depending on the eventual
character of the low level wind field. The NAM and GFS have been
mostly consistent over the last few cycles of output with the
indication of lower pressure over Illinois/Indiana by 12z Sunday.
This is likely a reflection of the approaching upper low and exit
region of the upper jet but could also be embellished by influence
of the mesoscale convective system source region. Assuming larger scale support of the
surface pressure reflection...the resulting hybrid warm front/triple
point would then direct the deepest part of the low level moisture
axis through Ohio. The NAM is most aggressive with this process but
still indicates upper 60s surface dewpoint over our area during
Sunday afternoon...enough to generate cape near 2000 j/kg to
interact with the cold front as it moves southward through lower
Michigan later Sunday afternoon and evening. Model soundings
indicate the magnitude of the wind field in the low to middle levels is
less than ideal for strong contributions to convective organization
but the potential for boundary interaction and the dynamic strength
of the upper low continues to demand careful monitoring for severe
weather potential as the system moves through the region.

While surface based convection runs its course Sunday afternoon and
evening...a mature conveyor of moisture will develop around the east
and north flank of the upper low. There is good model agreement on a
well organized Theta-E trowal axis wrapping around the system in the
850-700 mb layer indicative of strong deformation forcing through
the north flank of the circulation. The GFS indicates the deepest
moisture with modest elevated instability within the deformation
axis highlighted by 700 mb dewpoint near 3c reflected in upper 30s
k-index...precipitable water near 1.5 inches...and 850 mb based Li
around -1c. Plan to continue the inherited categorical forecast for
showers as this system will be very efficient at wringing moisture
out of the pattern. Localized rainfall totals of 2-3 inches would
not be a big surprise which could cause minor flooding in prone
areas...but concerns for more widespread excessive rainfall are
eased slightly by the relatively quick movement of the system
through lower Michigan by Monday morning.

The small but strong low pressure system Sunday will move eastward
by Monday and become absorbed into the long wave upper trough over
eastern Canada. This will open the door for another surge of high
pressure with polar origins that will bring below normal
temperatures back into the Great Lakes. Forecast confidence is
enhanced by high predictability in rock solid maintenance of the
amplified long wave pattern with a ridge anchored over western North
America and northwest flow aloft over the Great Lakes. This will
contribute to maintenance of cool conditions through much of the
upcoming week.

Marine...

Scattered coverage of thunderstorms today and tonight will become
more widespread across the Great Lakes Sunday and Sunday night.
Severe weather potential remains greatest Sunday in the area of Lake
St Clair and Western Lake Erie. Wind and wave action will otherwise
be favorable today through Sunday before a cold front moves through
the region Sunday night. Northerly winds will ramp up behind the
front...peaking around 30 knots over Southern Lake Huron Monday.
Small craft advisories will likely be needed as the long north fetch
will be favorable for building large waves that will impact the
south half of Lake Huron and nearshore marine zones. Cool high
pressure will then build into the northern plains and Great Lakes
and return benign marine weather for the middle week period.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.

&&

$$

Aviation.....Kurimski
short term...hlo
long term....bt
marine.......bt



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