Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
343 PM EDT Friday Jul 31 2015
Short term...late afternoon through tonight
Modestly sufficient moisture in the presence of steep low level
lapse rates have resulted in congested cloud development and
embedded isolated high based showers this afternoon. The
clouds/showers are confined within a couple of surface trough
features pivoting through the Central Lakes region - subtly driven
mainly by the settling of the middle-level jet core south over the
region. The showers and clouds will quickly relent once the boundary
layer mixing ceases early this evening. Likewise...the breezy
conditions accompanying the well mixed boundary layer will lay down
toward sunset as well.
Moonlit skies can be expected tonight with slightly lower
dewpoints...which should allow temperatures to drop a few degrees lower
than last night.
Long term...Saturday through next Friday
Another surface trough is expected to push across Michigan tomorrow
as the next in a series of upper waves rotates around the upper low
sitting over Hudson Bay and tracks across the northern Great Lakes.
The position of this trough looks to be slightly further south when
compared to today per latest model guidance. This should again allow
a few showers and thunderstorms to fire north of M-59 as diurnal
heating allows lower-level lapse rates to steepen and middle-level
lapse rates remain just cool enough to support activity. Activity
should end by early evening as drier air and subsidence arrives on
the backside of the trough. The boundary will become well-mixed once
again tomorrow...especially on the southern fringe of the trough.
This will help to boost temperatures into the 80s once again and
provide gusty winds during the afternoon and early evening.
Upper low over Hudson Bay will finally drop south on Sunday...
allowing a stronger upper shortwave now north of Saskatchewan /
Manitoba to swing down across the Great Lakes. Models are still in
disagreement with whether the resulting surface feature will
remain an open wave or deepen sufficiently to close off...but they
do all show the system deepening in response to upper height falls
and more favorable jet dynamics (right entrance region and upper
diffluence). GFS/Euro have a nice 40-50 knot low level jet surging
up into lower Michigan by Sunday afternoon...which should serve to
increase moisture and instability in advance of the cold front.
The front and the unstable airmass preceding it should provide a
nice focus for showers and thunderstorms to develop...and probability of precipitation
were increased Sunday into Sunday night. Models are still
struggling a little to pin down the timing of the front...and also
show potential for an mesoscale convective system to develop upstream and then move in
overhead Sunday night. As a result of the uncertainty...a large
window of probability of precipitation still spans both of these periods. Still early to
have a good feel for storm strength and evolution given model
uncertainties. Instability could be limited by clouds or
convection firing earlier. If we can generate enough instability
ahead of the front (and keep it if early Sunday night timing pans
out)...strong to severe storms could occur. Day 3 Storm Prediction Center outlook from
this morning did include lower Michigan in a marginal threat...
which looks appropriate for now.
Monday through Friday. Sunday's upper level low continues to migrate
through Southeast Michigan Monday with the brunt of the
precipitation expected to fall between midnight and about 8 am
Monday. Widely scattered showers will linger through Monday
afternoon with skies generally clearing in developing northwest
flow. At present it looks like fair weather will follow through
Wednesday for the majority of Southeast Michigan. The exception will
be some scattered showers and thunderstorms over the southern most
counties during the afternoon. Scattered showers and thunderstorms
are anticipated to spread to include most of areas of the County Warning Area for
Thursday and Friday. Temperatures will run below normal with highs
in the 70s.
Westerly wind gusts will increase late this afternoon and evening
behind the passage of a surface trough. Gusts are expected to reach
around 25 knots...and small craft advisories remain in effect for
all nearshore waters. Winds will remain elevated over the open
waters of Lake Huron tonight...with sustained speeds between 15 and
20 knots...but will become lighter over the remainder of the area.
Another trough is expected to work east across Michigan on
Saturday...again increasing westerly winds for the afternoon and
evening. With the position of the trough expected to be a little
further south tomorrow...the strongest gusts should remain over Lake
St Clair and Western Lake Erie...and are expected to remain under 25
knots. Isolated showers and thunderstorms will be possible both for
the remainder of today and on Saturday as the two weak troughs pass
through the area...mainly over Lake Huron.
A larger trough is forecast to move across the area on Sunday. This
should first provide a period of stronger southwest winds on Sunday...
and small craft advisories may be required. There is also a good
chance of showers and thunderstorms on Sunday and Sunday night...
some of which could be strong. Northwest winds will follow a cold
front late Sunday night into Monday. At this time speeds look to
remain below any headlines.
Aviation...issued 1248 PM EDT Friday Jul 31 2015
More diurnal cumulus developing this afternoon compared to yesterday-
bases remain at or above 6kft. Expect scattered coverage to continue
through the afternoon. Additionally...scattered showers and isolated
thunder have developed across northern lower Michigan...with a
secondary area of enhancement over central lower Michigan...where
isolated showers are attempting to sprout. Will advertise a tempo
shower with broken conditions at kmbs and kfnt. Farther south...the
forcing is less impressive...so will hold off on precipitation mention at this time.
Equally as important is the gusty westerly winds over the region
this afternoon. Expect peak values to run around 25kts as we reach
maximum boundary layer mixing. Similar setup tomorrow with the
onset of gusty conditions around noon.
//Dtw threshold probabilities...
* low for ceilings at or below 5kft
* low for tstorm impacting airspace
Lake Huron...Small Craft Advisory until 8 PM EDT this evening for lhz421-422-
Lake St Clair...Small Craft Advisory until 8 PM EDT this evening for lcz460.
Michigan waters of Lake Erie...Small Craft Advisory until 8 PM EDT this evening for lez444.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).