Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
353 PM EDT Sat Aug 1 2015
Short term...this afternoon and tonight
Isolated to widely scattered showers have worked out as expected
this afternoon as weak shortwave skirts the area to the north and
northeast. Slightly better low level convergence has focused from
near the Saginaw Valley east-southeast into The Thumb and activity has to this
point remains confined to this portion of the County Warning Area. Any stray light
showers or sprinkles to the SW of this main area will be trivial and
all activity will fade rather quickly from late afternoon into early
evening as both lift from shortwave and minor instability from day
time heating are lost.
With mostly clear skies overnight tonight...expect decent radiational
cooling as SW winds decrease to around 5 miles per hour and surface dew points
settle into the 50 to 55f range. This will produce a pleasant night
with low temperatures in the upper 50s to lower 60s. These lows may
occur a few hours earlier than usual as the low level gradient does
begin to tighten in the 09z-12z time frame...which will lead to wind
speeds increasing to some extent fairly early in the morning.
The upper shortwave now over northern Saskatchewan and Manitoba will
drop down into the Great Lakes Sunday and Sunday night as the upper
low over Hudson Bay sinks southward. A stream of smaller shortwaves
aligned along a jet axis stretching back through western and central
Canada will also feed into and potentially strengthen this wave as
it drops into the area tomorrow. At the surface...a trough will
swing across Michigan late Sunday through Sunday night. Models
continue to show a favorable environment for this trough to deepen
or even close off by Sunday night...as it becomes centered under
upper height falls and more favorable right entrance forcing of the
In advance of the trough...plume of higher moisture can already be
seen on water vapor imagery coming in off the Pacific and streaming
well into Canada. Models are in good agreement with this moisture
getting scooped up by the system...and pooling ahead of a cold front
that will drop through Southeast Michigan during late evening and
overnight Sunday. Latest runs show precipitable water values in the 1.0 to 1.5 inch
range...even slightly higher.
Potential exists for a few rounds of thunderstorms Sunday and Sunday
night. The first round will be possible during the morning and
through the early afternoon as an upper shortwave over Montana first
tracks through the area...coincident with a weak warm front and
initial Theta-E advection also lifting through the area in advance of
the approaching trough. Forecast soundings show instability will be
elevated for this round (if it occurs)...and with the better core of
winds still upstream...this should limit severe potential for the
first part of the day. Thought is that we will then see a break in
activity middle-day under subsidence behind the upper wave and while
instability recovers from early convection/clouds.
Strong low-level jet (50-60 knots) should surge into the area during
the later part of the afternoon into the early overnight hours. This
should provide additional instability and moisture...and allow 0-6km
bulk shear values to increase to 30-40 knots by evening. Still some
uncertainty as to whether the warmer air aloft will be too much and
keep a cap over the area...model forecast soundings are still
divided on this but may also be exhibiting some convective feedback.
Better push of moisture in the evening should align nicely with
forcing however (front..strong fgen...mid-level positive vorticity advection...right entrance
region) for showers and thunderstorms to become likely. Main
question will be strength and if we can get a more organized
convective system going. Preliminary thoughts for the evening are to
see a line of storms develop along the cold front upstream over
northern Michigan and Wisconsin during the evening (still some
timing differences in models however that have to be watched)
..with this line then moving across Southeast Michigan during the
later part of the evening and overnight hours. Will have to watch
potential for an mesoscale convective system to develop over the area...given strong inflow
from the low-level jet and strong upper divergence over the region.
Models continue to show between 1000-2000 j/kg of cape during the
evening/overnight as the front drops through. Combined with shear
values...this will support a threat for severe storms. Primary
threats will be damaging winds and large hail...but cannot rule out
tornado threat with local heights dropping overnight while 0-1km shear
increase. Very heavy rainfall can also be expected...but fast
movement of storms should keep flood potential localized.
Tuesday through Saturday. Breezy west to northwest winds will
accompany the lingering surface trough Tuesday afternoon. The minor
instability induced by the trough and afternoon surface heating
could trigger some scattered showers and a possible rumble or two
..especially over the northern thumb areas. Models indicate little
energy leftover to support more than isolated garden variety
thunderstorms. Wednesday and Thursday still look favorable for ample
sun amid drier/cooler conditions. Clouds begin increasing Thursday
evening ahead of friday's weather system...which is too far out to
put any Faith in timing or impact. Afternoon high temperatures
hitting below average all week with 70s persisting through Saturday.
Westerly winds will continue to gust to around 20 knots into this
evening over Lake St Clair and Western Lake Erie in response to a
weak trough moving through the central Great Lakes. A few
thunderstorms will also be possible over Lake Huron this
evening...before activity dissipates.
Winds will become light overnight before increasing from the
southwest by late Sunday morning as a deepening trough of low
pressure moves towards the area. Winds are expected to gust to
around 25 knots for most nearshore areas...but attain slightly
higher speeds...up to 30 knots...over Saginaw Bay where southwest
winds will funnel. Winds will begin to diminish late Sunday night as
the trough moves overhead. Small craft advisories are now in effect
for all nearshore areas from late Sunday morning through late Monday
night. Winds will turn west and then northwest behind a cold front
early on Monday. Speeds Monday look to be greatest over Lake
Huron...where they will reach 15 to 20 knots from the northwest. In
addition...thunderstorms will be possible Sunday and Sunday night.
Some of these storms may become severe...with damaging wind gusts to
50 knots the main threat.
Aviation...issued 110 PM EDT Sat Aug 1 2015
Passing weak shortwave will aid diurnal cumulus field and bring scattered
to broken lower VFR deck in the 5-7kt foot range. Scattered rain showers will remain
east of the terminals by and large. West winds will gusts to 20 kts
can be expected into 00z-01z time frame. Gusty SW flow will set up
by midday Sunday as gradient tightens in advance of approaching cold
front. Gusts may reach/top 25 knots at times by afternoon.
For dtw...sct-bkn cumulus field at 5kft may tend to lift to 6-7kft
as afternoon proceeds. Otherwise...winds from 270-280 degrees will
gust to near 20 knots today with 210-220 degree winds gusting to 25
kts or more by midday Sunday.
//Dtw threshold probabilities...
Lake Huron...Small Craft Advisory from 10 am Sunday to 4 am EDT Monday for
Lake St Clair...Small Craft Advisory from 10 am Sunday to 4 am EDT Monday for
Michigan waters of Lake Erie...Small Craft Advisory from 10 am Sunday to 4 am EDT Monday for
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