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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
703 am EDT Sat may 30 2015



Aviation...

Showers will spread into the kmbs/kfnt corridor through the morning
with a partial coverage of showers developing further southeast as
diurnal heating increases ahead of the front. Diurnal
destabilization will support a chance of thunderstorms from around 18z
until cold frontal passage. Frontal passage will be marked by an abrupt shift
to northeast wind gusting to 20 to 25 knots. Showers will likely
contain heavy rain at times during the afternoon.

Upstream wave presently over northern Arkansas will bring continued
widespread showers into tonight. Showers will be heavy at times from
kptk into the Detroit area and will persist into Sunday afternoon.
Prolonged period of IFR likely.

For dtw...ceilings will intermitently fall below 5kft through the
morning before settling below 5kft as deeper moisture moves in. Peak
time for tstorm potential will be focused in 18-22z period.

//Dtw threshold probabilities...

* high for ceiling at or below 5kft.

* Medium confidence that a tstorm will impact kdtw airspace this afternoon

&&

Previous discussion...issued 254 am EDT Sat may 30 2015

Short term...today and tonight

Deep layer southwest flow continues to intensify over the central
United States and Great Lakes in advance of digging polar low over
eastern Manitoba and western Ontario early this morning. Corridor of
strong moisture transport evident on North American water vapor loop
already contains precipitable waters approaching 1.5" as far north as western
Quebec. Strong surface cold front stretches from northern Oklahoma
across The Straits at press time and can be easily identified on
regional radars. Slowly weakening mesoscale convective system over Missouri and Arkansas is
in the process of being swept northeast within prevailing
southwesterlies.

The surface cold front will be augmented by cold marine air over the
next several hours, which will subsequently augment the temperature
gradient and boundary layer convergence. Precipitable waters are forecast to
increase to near 1.7" by late morning immediately in advance of the
cold front as aforementioned upstream convective remnants are
ushered northeast into lower Michigan. Strengthening ageostrophic
frontal circulation will support these showers as they lift into County Warning Area
and will be supportive of an increase in coverage and intensity
along and north of the I-69 corridor through the morning. Warm high
dewpoint airmass will not require much heating to elicit a
convective response, so there is at least some expectation for
prefrontal showers to carry a heavy rain threat as early as 15-18z.

Nam12 and rap13 indicate MLCAPE approaching 800-1000 j/kg during the
afternoon within the prefrontal warm sector. Low-level flow will be
sufficiently veered to allow best destabilization across eastern
areas. Weak shear and poor middle-level lapse rates will continue to be
a prevailing theme, but ongoing Theta-E advection within this
environment supports increased probability of precipitation and mention of thunder well in
advance of the front from late morning through the remainder of the
afternoon. Frontal passage itself will be accompanied by strong boundary layer
convergence and a high probability of thunderstorms containing heavy
rainfall, especially by late afternoon/evening. An isolated severe
wind gust is not out of the question, but the primary threat will be
torrential downpours and localized flooding/roadway ponding. Best
chance for any higher impact thunderstorms containing said downpours/wind
will be eastern areas where frontal passage will occur later in the peak
heating cycle and instability will be maximized. Target time for
frontal passage in the Detroit metropolitan is 21-00z but prefrontal showers may
train across the area earlier in the day depending on degree of
destabilization. In the meantime, elevated portion of the front will
undergo considerable strengthening as strong right entrance dynamics
spread across all of Michigan 18-00z. This will be sufficient to
sustain showers in the wake of surface frontal passage for all areas.

Frontal passage will not go unnoticed as model cross-sections
suggest the front will maintain density current characteristics even
through peak heating, which is impressive by late may standards. The
temperature will drop several degrees with passage and cold
advection will continue through the night. Temperatures will fall
well into the 40s for most areas and winds will gust to 25 kts,
higher along the Lake Huron and Saginaw Bay shoreline.

850 mb-700 mb frontal zone will remain active and will only slowly drift
south through Saturday night as strong right entrance dynamics
remain in place over Southeast Michigan. Showers will therefore
persist over most of the area through 06z, though a lull in forcing
should keep affected areas confined to 0.1 to 0.25" quantitative precipitation forecast during this
period.

Intensity will ramp up south of the M59 corridor 06-12z as the
shortwave over Arkansas wraps into the longer wave trough over the
region. Although the bulk of the response and forcing associated
with this wave is forecast to take place after the short term
period...the beginnings of the associated increase in middle-level
deformation will still have an abundance of moisture within a deep
warm cloud layer to work with. Quantitative precipitation forecast is forecast to range from 0.5 to
0.75" during this time...highest amounts southwest. The potential
for embedded convective elements leaves the door open for locally
higher amounts. Total quantitative precipitation forecast for today & tonight should easily range
from 1 to 1.5" most places with higher amounts possible within
convective maxima.

Long term...Sunday through Friday

This forecast package will feature a significant change to the
Sunday forecast...basically in acknowledgment of the trends in the
model solutions over the last few cycles. In short...the Sunday
forecast has been adjusted toward wet and downright cold /for late
may standards/ in contrast to the cool and dry forecast which had
been advertised for the past several days.

It has become apparent that middle level height falls associated with
the upper trough moving across Quebec will not dive as far south as
earlier model solutions indicated...causing the upper jet maximum to be
situated farther north. This will allow a southern stream wave
/currently located over the OK/Arkansas border/ to lift into the Northern Ohio
valley and eastern Great Lakes Sun morning...with some possible phasing
with a northern stream short wave impulse. While the surface cold
front will be well south of the state by Sun morning...the middle level
front will remain over Southeast Michigan. The approach of the southern stream
wave will invoke a strong dynamic response along the middle level front
over Southeast Michigan. With 700mb dewpoints forecast to range from +1 to +4
c...the middle level deformation will lead to several hours of steady
rain /on top of what is expected today and tonight/. The main
forecast challenge this cycle is in determining where the more
persistent middle level forcing will be located. The expected compact
nature of the middle level wave supports the NAM/European model (ecmwf) Camp which keep
the axis of higher quantitative precipitation forecast across the southern half of the forecast area on
Sunday /closer to the middle level circulation/. There is a potential
for some locals to pick up over an inch of rain from early Sun
morning into the afternoon. The lack of middle level instability over
Southeast Michigan will negate a substantial convective component to the
rain and suggests rainfall amounts will not get too excessive.

Northeast winds will be quite gusty on Sunday due to strong high
pressure expanding across the northern Great Lakes and an associated surface low
tracking along the cold front over the Ohio Valley. Wind gusts may
actually exceed 30 miles per hour across the Tri Cities with the flow coming
off Saginaw Bay. This will sustain a Lake Huron modified boundary
layer across most of the forecast area in the Post frontal
airmass...meaning temperatures will be downright cold for may 31st
/daytime temperatures struggling to reach low 50s/.

A northern stream wave dropping into the eastern Great Lakes will
sustain middle level troughing over the region into early next week
even as the Sunday system shears off to the east. Drier air will
eventually make its way into the forecast area by Monday as high
pressure builds into lower Michigan. NE winds will persist on Monday
/albeit weaker than sun/...keeping temperatures on the cool side of normal
during the first half of the week. Middle level heights will rebound
during middle to late next week...offering more Summer-like
temperatures during the last few days of the forecast period.

Marine...

A cold front will push across Lake Huron this morning. Winds will
back toward the north-northwest with the passage of the front. The front will
then move across Southeast Michigan and Lake Erie this afternoon and will trigger
some thunderstorms. The Post frontal airmass will be rather chilly
for late may standards. A wave of low pressure is forecast to track
along the front over the Ohio Valley on Sunday. This along with
building high pressure across the northern Great Lakes will lead to
strengthening northeast winds tonight into Sunday. Small Craft
Advisory conditions will be likely. Given the northeast wind
trajectory down Saginaw Bay...there is a chance that wind gusts may
actually approach gale force on the Bay Sunday. This will be
something for later shifts to monitor. Winds and waves will diminish
Sun night into Monday as the low departs to the northeast and high
pressure builds into the region.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...Small Craft Advisory from midnight tonight to 8 PM EDT Sunday for
lhz421-422.

Small Craft Advisory from 4 PM this afternoon to midnight EDT
Sunday night for lhz441>443.

Lake St Clair...Small Craft Advisory from 10 PM this evening to 8 PM EDT Sunday for
lcz460.

Michigan waters of Lake Erie...Small Craft Advisory from midnight tonight to 8 PM EDT Sunday for
lez444.

&&

$$

Aviation.....Jvc
short term...jvc
long term....SC
marine.......SC



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