Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 220246

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
946 PM EST Sat Jan 21 2017


Minor update again this evening to better define fog/stratus
trends. Much like last evening, dense fog is slowly expanding to
encompass more and more of the forecast area. Currently focused
along and northeast of the I-75 corridor, trendrils continue to
migrate south/southwest through northern portions of Metro
Detroit. Within the areas of dense fog, visibilities are quite
poor, reduced to near zero in some instances. Expect current
trends to continue into the overnight.


Issued at 606 PM EST Sat Jan 21 2017


Leading edge of low level drying has stalled from near KMBS
southeast to just northeast of KFNT/KPTK and 11u-3.9u imagery
suggests lower stratus is just beginning to build back to the
south/southwest. Additional stratus will most likely form as this
mild/moist airmass rides over ice over Lake Erie as light winds back
to the southeast and east. Timing may be a bit difficult this
evening, but end result will be another long period of generally
LIFR/VLIFR conditions late this evening into Sunday morning. Did
adjust forecast for a quicker return of this conditions given HRRRX
fields and recent trends, but still "stair step" visibilities down
during the course of the evening.

For DTW...VLFIR cigs/vsbys should build back over area with time
this evening as light southeast flow off of Lake Erie provides an
excellent source for saturating low levels, especially once the
boundary layer fully decouples. These conditions, in general, should
hold most of the forecast through 15z or so Sunday morning.

//DTW Threshold Probabilities...

* High for cigs aob 5kft after 02z.

* High for cig/vsbys aob 200ft and 1/4SM tonight 02z-15z.

Issued at 415 PM EST Sat Jan 21 2017


Today really was a page taken from the early Spring chapter as a
stationary front hung up along the terrain. This led to a stable
lake aggregate effect/persistent dense fog over the northern cwa,
and a legitimate southwesterly flow warm sector over the southern
cwa. As it stands here at press time, temperatures have made a run
at 60 degrees in Detroit, with low 50s at Flint. These readings are
only 5 degrees off of record values. The warmth has been a direct
result of an equivalent potential temperature ridge that was able to
opportunistically bubble into the central Great Lakes region with
the rapid demise of the the broader wavelength midlevel
trough/Potential Vorticity anomaly that had been in place. Really is
surprising this warmth happened with the time of year, clouds, and
preceding lowered geopotential height anomaly. This added theta e
advection has only added to the moisture in the lower troposphere.

Tonight...Lack of upper level Jet overhead and slight drifting
northward of the WI/MN PV anomaly will allow for slight geopotential
height rebound over Lower Michigan. This will allow for a weak
positive surface pressure tendency preserving a lack of gradient
flow over southeastern Michigan throughout the overnight. The lack
of any true dry air advection and no pattern turnover screams once
again for a persistence forecast of higher impact fog development.
There is a big difference today which has been the development of
some better near surface mixing across the southern cwa with
downright warm temperatures and actual surface T/Td depressions of 5
to 8C. However, consensus of the model data including suite of 4km
hi resolution output and MET/MAV MOS is again very bullish on fog.
Forecast soundings are again supporting a strong signal of near
surface saturation below a very low subsidence inversion. With the
lack of improvement across the north, the easy call was to issue a
dense fog advisory beginning immediately over the Tri Cities and
Thumb. More difficult across the south with the clearing, but did
begin the dense fog advisory at 00Z tied to sunset.

Sunday...Extremely dynamic upper level jet structure and Potential
Vorticity Anomaly to scream across the southern US tonight. The
anomaly will track east of the Mississippi River Sunday afternoon.
This will allow a sprawling, initial surface reflection/low pressure
center to lift into the Tennessee River.  An impressive midlatitude
cyclone that far south, with result in an increased easterly
pressure gradient to develop over the central Great Lakes region.
Models may have backed off on timing of better east winds a smidge,
but expecting the east gradient winds increasingly notably Sunday
afternoon. Not a whole lot of impact on the tangible weather with
mainly dry conditions expected. The one exception may be for areas
immediately downwind of Lake Erie where enhanced near surface
lakeshore convergence may yield some heavy drizzle or light rain

Sunday night and Monday...Precipitation chances will then
increase significantly beginning 03Z or 06Z Sunday, depending on
which model runs one favors. Forcing for much of this event will
come in the form of midlevel deformation that will shed off and
emanate northward away from the upper level low. It is also
suggest that few bands of near surface convergence may develop
under increasingly cyclonic flow. The big negative early on until
Monday morning will likely be a high amount of dry air that need
to be overcome or saturate. Forecast models show this to be a
tangible struggle. However, a northward drift of the main upper
level low is forecasted on Monday which should allow some
contraction of midlevel theta e axis. As a result, rainfall is
likely for Monday afternoon. Conditions will be noticeably cooler
Monday afternoon with wind chill values in the upper 30s.

Unsettled weather pattern continues next week, as a series of
surface and upper level waves migrate through the region. Chances of
rain or rain/snow mixed will generally persist, with a potential for
a brief break in conditions Tuesday. Temperatures will remain above
average into Thursday, with the warmest temps occurring during the
overnight hours. A decent cold front begins pushing through the area
during the day on Thursday, which will start the downward trend on
temperatures. Temperatures, more in line with the seasonal norm, are
expected by the weekend and will persist into the following


Light east to southeasterly winds will remain in place through the
overnight hours... with the exception of a more southerly component
predominant over wesern lake erie south of a slow northward moving
warm front. Widespread fog should also continue over most marine
areas of Lake Huron overnight.  Interestingly, not much fog has
occurred over Lake Erie, Lake St. Clair or even Lake Micigan despite
dewpoints climbing into the 50s. Likely there is an extremely
shallow cold marine layer keeping moisture off the surface.

Weak to moderate surface low expected to pass south of the Great
Lakes Sunday will gradually back winds to a more easterly direction
and perhaps briefly northeast. Wind speeds likely to increase to
around 25-30 knots or so by the end of the day Sunday and continue
through Monday afternoon.

Next low progged to cross Lower Micigan and Lake Huron between 12z
and 18z Monday turning wind westerly in the southern lakes Wednesday
and northwesterly in the northen lakes by Thursday morning.  Speeds
should remain just sub-gale for the most part.


MI...Dense Fog Advisory until 10 AM EST Sunday FOR MIZ047>049-053>055-

Lake Huron...Dense Fog Advisory until 10 AM EST Sunday FOR LHZ361>363-421-422-

Lake St Clair...Dense Fog Advisory until 10 AM EST Sunday FOR LCZ460.

Michigan waters of Lake Erie...NONE.



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