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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
942 PM EDT Thursday Aug 21 2014

Update...

Ongoing activity...roughly along the I-96 corridor at press
time...blossomed on the outflow of the first cluster of convection
that dissipated over Washtenaw County earlier in the evening.
Expect this surface based activity to weaken during the late
evening and dissipate by midnight. New development will be
possible even before midnight due to elevated instability but this
will have greater likelihood during the overnight hours through middle
Friday morning. Persistent elevated instability will be tied to
the middle levels of the main frontal boundary stalled over Illinois
and Indiana. The 850-500 mb Theta-E gradient will then be
maintained over lower Michigan and subject to convective response
due to weak ripples of moisture advection combined with increasing
nocturnal destabilization aloft. There is also a possibility that
low level jet forced storms could make it from Wisconsin into Southeast
Michigan toward or after sunrise...but the instability axis is
oriented closer to the surface front which will make it difficult
for anything more than remnants to make it this far east through
Friday morning.

&&

Aviation...issued 707 PM EDT Thursday Aug 21 2014

//discussion...

The primary surface front will be stalled to the south and west of
lower Michigan tonight and Friday. This is expected to keep the
strongest thunderstorm activity away from the terminal sites but
with the possibility of renewed weaker activity developing overhead
later in the night. Any new shower/storm development will tend to
coincide with broader formation of MVFR ceiling and visibility north
of the surface front up to about ptk with fnt and mbs likely having
just a fog component...possibly IFR depending on higher cloud cover.
As long as the surface front remains to our south...weak easterly
flow from equally weak high pressure over Ontario will promote
improvement to VFR by Friday afternoon.

//Dtw threshold threats...

* low confidence on thunderstorm timing and coverage tonight.

* Moderate confidence on ceiling below 5000 feet tonight and Friday
morning.

&&

Previous discussion...issued 358 PM EDT Thursday Aug 21 2014

Short term...tonight

A few light showers have started to build across Southeast
Michigan...but have fizzled out as quickly as they developed due to
significant amount of subsidence across the region. Not expecting
much in the way of thunderstorm development the rest of the
afternoon...however a warm front across the Northern Ohio River
Valley will bring an increased chance of rain showers to Southeast
Michigan this evening. Precipitation chances will begin near the
Ohio border and slowly move northward as this front lifts slowly
across the County Warning Area.

Rain showers should continue overnight as models are continuing to
indicate an mesoscale convective system /mesoscale convective system/ to develop over the
western Great Lakes. This mesoscale convective system will track southeastward towards
Southeast Michigan. Current thinking is that the bulk...including
the most damaging wind gusts and severe potential...of the mesoscale convective system will
track south of the County Warning Area resulting in just rain showers across
southern Michigan. Have left in likely probability of precipitation across the southern two
tiers of counties to account for any deviation on the track of the
mesoscale convective system. Current Storm Prediction Center outlook has pulled back on the slight risk across
southern Michigan...adding to the confidence in the current southern
track of this mesoscale convective system. With most of the significant thunderstorm
development tracking south...the major concern for Southeast
Michigan will be the heavy rain potential. Current models are
showing precipitable water values in the 1.5 to 2.0 inch range. This will result
in nearly an inch or slightly more of rain to fall over a short
amount of time. At this time...flooding issues are not a concern.
This will have to be monitored closely as the event progresses as
little rain falling over an already saturated ground/urban areas
could quickly result in flooding concerns across metropolitan Detroit and
Southeast Michigan.

Otherwise...cloud cover overnight from the developing mesoscale convective system should
result in fairly overcast skies keeping temperatures on the mild
side. Lows should be in the upper 60s in the metropolitan Detroit area and
slightly cooler in the middle 60s across the rest of the County Warning Area. This is
slightly above average for this time of year where lows are normally
in the low 60s for Detroit and the upper 50s for Flint and Saginaw.

Long term...

No substantial changes from the previous couple of forecast
packages. Amplification of the subtropical ridge over the southern
United States is still forecast to result in significant warming of
the column over Southeast Michigan heading into the weekend. 850 mb temperatures both
Friday and Sat will be pushing the upper teens, which will support high
temperatures at least in the low 80s. There is some uncertainty
regarding the potential mixing depths on Friday owing to uncertain
evolution of tonight's convection, which correspondingly tempers
expectations for friday's maximum temperatures. Perhaps the primary reason for
abstaining from pushing for higher daytime highs, however, is
actually more of a credit to expected easterly winds at the surface
around the southern periphery of high pressure over Quebec. The
previous forecast handled this well and no changes were necessary.
Even with the tempered expectation for high temperatures, the lack
of any noteworthy exchange of airmass will ensure sticky dewpoints
in the middle 60s to near 70 each afternoon. Likewise, there remains
some concern over cooling potential at night due to antecedent moist
boundary layer conditions. Agree with previous discussion that the
presence of Theta-E gradient in vicinity of the lower peninsula immediately
downstream of the upper ridge axis and resident moist airmass make
it essentially impossible to rule out shower/T-storms during any one
period. However, until a well-defined opportunity for development
presents itself, the best approach remains to leave the forecast dry.

Sunday through thursday: rain chances will likely hang around
through much of next week as a series of shortwaves track over
Southeast Michigan. Model agreement is still quite low and have
therefore only put in low chance probability of precipitation into the forecast through
Thursday. Timing and coverage of the rain will be better defined
as models get a better handle on these disturbances. Temperatures
will remain fairly consistent with highs in the low 80s and low
temperatures in the middle to upper 60s through the period.

Marine...

A weak pressure gradient will maintain light winds across the waters
through the first half of the weekend. There is a chance for
thunderstorms for all marine areas late tonight into early
Friday...and severe weather is not out of the question during this
time...especially for Lake Saint Clair and Lake Erie. Moderate
southeast flow will develop by Sunday but wind gusts will remain at
a minimum.

&&

DTX watches/warnings/advisories...

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

&&

$$

Update.......bt
aviation.....Bt
short term...rk
long term....jvc/de
marine.......jvc



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