Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
725 am EDT Sat Oct 10 2015
High cloud cirrostratus associated with weak shortwave energy is
advancing rapidly eastward this morning. Latest timing on satellite
supports the back edge of the cirrostratus canopy to exit
southeastern Michigan by 15z. Dry air and stable midlevels will
support sunshine this afternoon before the next shortwave will bring
a period of high cloud again this evening. Surface winds are
expected to increase modestly this afternoon particularly over
central lower Michigan including mbs. A sustained southwesterly wind
is expected to persist tonight.
For dtw...VFR conditions are expected throughout the period. High
cloud cirrostratus will exit within 2 to 5 hours after the start of
//Dtw threshold probabilities...
Previous discussion...issued 350 am EDT Sat Oct 10 2015
Short term...today and tonight
Shortwave energy is advancing progressively eastward through the
western and central Great Lakes this morning. There is very little
going for the feature with respect to large scale dynamical forcing
for ascent. With that said...it continues to be a prolific high
cloud producer with a cirrostratus canopy over portions of Ontario
north of Lake Superior...clear down into sections of north central
Illinois. While difficult to gauge what the current opacity
is...should be something close to a 1:1 from 10/9 late day visible
imagery. There has been some indication for opaque cloud in the
surface observations...with surface temperatures climbing a couple
of degrees as the cloud builds overhead. Did go pessimistic with
cloud cover to start the day and removed the patchy fog mention as a
result. Per a linear extrapolation of timing the back edge...did
transition skies to partly cloud by the middle to late morning...mostly
sunny for the afternoon. High temperatures today will be a degree or
two within average as the remaining cool midlevels offsets afternoon
The Heart of the surface ridging will get squeezed to the southeast
of lower Michigan tonight as the large wavelength upper level ridge
advances into the nations middle section and the pressure gradient
increases over the central Great Lakes. There is expected to be a
southwesterly breeze over the land areas tonight...remaining in the
sustained 5 to 10 miles per hour range. Lows tonight are expected to fall into
the lower to middle 40s with the lowest values over the far southern
County Warning Area...south of the Detroit urban heat island.
Water vapor shows a strong elongated shortwave centered near 145w/
40n. This wave is modeled by the suite of nwp to break apart from
the parent goa low as ridging aggressively builds in behind it in
advance of another rapidly deepening npac wave. As it races from the
West Coast to the western Great Lakes between tonight and Monday
morning, the southwesterly gradient will increase markedly and local
ridging will undergo some additional amplification. 850 mb temperatures
will surge by 10 to 12c into the middle teens on Sunday, although the
subsequent inversion will cap mixing heights. Even so, plenty of
insolation courtesy of a dry atmosphere and persistent deep layer
subsidence will support highs in the low to middle 70s within this
environment. Moderate wind field will support a component of
gustiness on Sunday but it should be limited in magnitude given the
lack of downward momentum transport aside from Ordinary mixing.
The wave will also entrain a Canadian airmass as it emerges from The
Rockies. The resultant cold front will be characterized by an
increasingly sharp temperature gradient and will also be well-
supported by strong shortwave dynamics, especially further north,
and modest right entrance jet support. However, the progressive and
higher latitude nature of the wave will inhibit respectable
northward moisture transport. Indeed, cross-sections across
Southeast Michigan on Monday indicate very poor moisture depth,
which is consequentially reflected as a lack of quantitative precipitation forecast by the nwp
during frontal passage. Thus, expecting a predominantly dry day on Monday with
a frontal passage that will be mainly characterized by dropping temperatures
in the evening and developing gusty west-northwest winds.
Further, moderate lake instability is shown to develop in the wake
of the front on Monday night as 0-1km Theta-E lapse rates fall to -7
to -10c. A healthy lake effect response can be expected, at least,
in terms of boundary layer clouds. However, cold advection will
steepen lapse rates across the lower peninsula and it is plausible
that a Few Lake effect showers work into portions of the central or
northern County Warning Area toward Tuesday morning. Confidence in this potential
does not yet warrant a mention of weather in the grids.
Waves have subsided in the wake of diminishing onshore winds. Fresh
southwest breeze will develop by this evening with the strongest
winds expected from Saginaw Bay into Central Lake Huron. Relatively
warm air will limit wind gusts but another round of small craft
advisories has been issued from outer Saginaw Bay to Harbor Beach.
In addition to winds up to 25 knots...the persistent fetch will
build maximum wave heights up to 7 feet in these areas overnight.
Moderate to fresh SW breeze will then persist through Monday until
winds turn to the west-northwest and become gusty by Monday evening behind a
passing cold front.
Lake Huron...Small Craft Advisory from 8 PM this evening to 4 PM EDT Sunday for
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).