Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
345 PM EDT Tuesday Oct 6 2015

Short term...this evening and tonight

The extremely sluggish response in improving sky fraction came to
fruition today with deeper column ridging in control. The scouring
of higher ambient moisture was non existent early on with a lack of
any tangible advections. A couple of factors will align tonight
making a clearing trend more likely. The some advertised
modest dry air advection. A clearing line that matches up with the
drier midlevel moisture content has been steadily progressing to the
southeast across western lower Michigan today. A linear
extrapolation of the clearing...times out across much of
southeastern Michigan at/around 22/23z. The second factor is the
loss of an upper level jet streak and the associated right entrance
region dynamics. This forcing is expected to vacate the region over
the next 6 hours...prompting some larger scale subsidence. Throw the
loss of a diurnal heating contribution into the mix and conditions
tonight appear favorable for the substantial loss of lower
tropospheric cloud.

The exit of the aforementioned jetstreak will allow a very weak and
subtle frontal boundary to slip into the northern Great Lakes. The
front will be more of an artifact of convergence or the buckle of
the geopotential height field than any active frontal circulation.
It is along this weak structure/gradient that a low chance of a
shower will exist with the help from a lake effect component. The
chance of a shower will remain out over Lake Huron.

As for the potential redevelopment of cloud tonight. The surface
wind field is expected to change from what has been recently
calm/none to northwesterly trajectories with boundary layer
magnitudes greater than 10 knots across the board. In addition
.Looking at some inbound middle cloud potential as a shallow
convectively unstable mixed layer is forecasted to push into semich
very late tonight. Amount of midcloud development upstream over
portions of Minnesota and WI supports this potential. Cannot definitively
rule out some development of light fog particularly south of the
Detroit metropolitan for late tonight. At this appears visibility
restriction would remain down in the 3-5sm range...for areas south
of Detroit.


Long term...

Good drying occurring tomorrow behind a cold front...with precipitable water values
slipping down to around 0.5 inches. Solid low level cold advection
during the day...with 850 mb temperatures ranging from 4 c north to 8 c
south near Ohio border by day's end. With good mixing under
northwest flow and at least partly sunny skies...looking at highs in
the middle 60s to around 70 degrees. Surface ridge axis/high in place
Wednesday night will provide favorable radiating night...and mins
falling to normal values into the 40s seems like a given...with
normally colder locations possibly slipping into the upper 30s...but
there may be just enough influence from relatively mild lakes. Also
return flow/warm advection begins to kick in by 12z Thursday...and
some cloud development is possible...especially if one is to believe
the GFS 850 mb relative humidity.

Upper level energy crashing ashore the southern British Columbia
will tap into some of the moisture the upper level low over The Four
Corners region has pumped along the eastern foothills of The
Rockies...with good agreement on the timing of the surface low
tracking through the central Great Lakes Thursday evening/night.
Good warm advection out ahead of the low during 850 mb
temperatures push to 12 c (per gfs)...with moderate low level jet of 30 to
40 knots at 850 mb. Showalter index does drop to around zero...with
adequate middle level lapse rates (6 c from 700-500 mb) to support
isolated thunderstorms. Positively tilted upper level trough will
make for progressive system...drying out on Friday...with precipitable water values
dropping at or below half an inch again. Looks like enough low level
moisture (925-850 mb) to hold on to clouds through much of the day
with low level cold advection...especially with some wind component
off Lake Huron...which is expected to hold temperatures in the upper 50s to
lower 60s.

Dry and near normal temperatures will be the story for the upcoming
weekend as surface high pressure and upper level ridging works in
overhead. Highs on Saturday will start out off around 60 and
gradually warm into the upper 60s on Sunday. Low temperatures during this
time will generally be hovering in the upper 40s to lower 50s.



A weak cold front will slip south through Lake Huron tonight. This
will bring an increase to northwest winds...with sustained speeds
expected to remain between 10 and 20 knots tomorrow. The
approach of a low pressure system will then allow stronger southerly
winds to develop on Thursday...and spread rain and perhaps a few
thunderstorms into the area late Thursday and Thursday night. Winds
will flip to the northwest late Thursday night behind the passage of
a cold front...and looking for gusts around 25 knots over the open
waters of Lake Huron on Friday.


Aviation...issued 150 PM EDT Tuesday Oct 6 2015

Low stratus remains locked in place over southeastern Michigan early
this afternoon. Deep column ridging is maintaining a very sluggish
wind field and slow responding environment. Opacity of cloud has
been improving marginally over the last hour or so...and model data
continues to suggest some modest low-midlevel dry air advection from
the northwest during the late afternoon. Trajectory of inbound
clearing suggests its going to be real close for areas south of I 94
before nightfall. Loss of diurnal contribution than will lead to VFR
conditions for the majority of the night. A better component of
gradient flow will discourage low cloud/fog formation late tonight
over much of the area. Cannot definitively rule out nocturnal
development of stratus...with some MVFR type fog for areas south of

For dtw...low stratus will continue to be very slow in lifting this
afternoon. Still expecting some modest dry air advection and loss in
diurnal heating component to allow for a concerted clearing trend
very late this afternoon/early evening. Some signal exists for
reformation of stratus overnight with some possible MVFR reductions
to visibility...particularly to the south of dtw.

//Dtw threshold probabilities...

* high for ceilings at or below 5kft through this afternoon. Low to medium
confidence in ceilings below 5kft late tonight.


DTX watches/warnings/advisories...
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.


Short term...cumulonimbus
long term....sf/ss

You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).

National Weather Service Glossary of Abbreviations