Scientific Forecaster Discussion
fxus63 kdtx 221138
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
738 am EDT Tue Oct 22 2019
Clear skies and relatively light winds prevail in the wake of last
night's cold front. These conditions will be short lived as upstream
obs indicate widespread clouds and gusty southwest winds poised to
fill in during the mid to late morning hours. Low Lake enhanced VFR
stratocu then expected to be accompanied by SW winds gusting 25 to
30 kts during peak heating. Lake response will support increasing
rain shower coverage into the Saginaw Valley area where further
deterioration to MVFR will be possible this evening. Uncertainty
with respect to coverage warrants inclusion of vcsh for kfnt/ptk
while toeing the line with the dry forecast in the immediate Detroit
area. Clouds begin to erode from south to north late in the period
as the moisture axis lifts north.
Dtw threshold probabilities...
* moderate in ceilings at or below 5000 feet.
issued at 317 am EDT Tue Oct 22 2019
Regional reflectivity mosaic and msas analysis early this morning
shows the center of vertically stacked low pressure passing east of
the Mississippi River into far western Wisconsin. Satellite imagery
shows an expansive dry slot sliding over Lake Michigan and a greater
portion of lower Michigan. An exception has been an extremely narrow
convective band of cumulus cloud, roughly 5 to 10 miles wide along
the cold front and abrupt wind shift. The wind shift has really been
a non-issue overnight with some evidence of winds in the 25 to 35
mph range immediately along the feature. Very interesting to note
in the model data that system relative isentropic descent is
virtually non-existent. Forecast soundings also support this with
little subsidence inversion development in the near term. As a
result, not much of a wind response during the remainder of the
morning in Tow of the front. In fact, surface observations upstream
are showing nothing more than winds of 10 to 20 knots. Relatively
quiet and cloud free conditions this morning.
Internal pv structure of barotropic low pressure system will cause
the center of surface low pressure to wobble within and north of
Lake Superior today. This will place Southeast Michigan in
prevailing southwest flow today. Forecast soundings show rapid
development of surface mixed layer with advent of daytime heating.
Mixing heights greater than 3.0 kft agl will allow for environmental
winds of 30-35 knots to be mixed down to the surface. The going
forecast had this in hand and no changes were required for this
issuance. The precipitation forecast is not a trivial one as model
data is showing a lack of low to midlevel wrap-around moisture
making it back into lower Michigan today. Despite a lack of synoptic
moisture, forecast soundings are showing diurnal based shower
potential with saturated conditions between 2.0 and 7.0 kft agl. No
question there will be lake effect component to the precipitation
primarily west of the cwa. Main support for ascent today will likely
come along/behind lobe of pv that will pivot into central lower
Michigan 18-00z. There is a signal that best boundary layer support
may occur along and behind a wind shift that will cause flow
trajectories to become more due westerly. This would open the door
for some of the lake effect activity to survive into the forecast
area. Did use a general outline of enhanced 925 -(div) and 1000-
850mb upward vertical velocities to include likely pops 21-03z north of I 69. The NAM is
bearish, but the hrrr runs suggest a potential for areas south of I
69. Included slight to low chance pops south of Detroit.
Loss of daytime heating will allow for mixing depths to decrease
with time tonight. Additionally, modest increase in midlevel
subsidence will erode moisture depths from the top-down due to
increasing dry air entrainment. Did go dry the latter half of the
night. Continued mixed flow will keep low temperatures somewhat
For Wednesday, the question is what will high temperatures be with
some conflicting information in the guidance. NAM is cooler while
the European model (ecmwf) is definitely on the warmer side. The issue boils down to
the timing of the warm advection particularly in the afternoon in
advance of the frontal wave. European model (ecmwf) is quicker and more aggressive
with the warm advection. Light precipitation chances return
Wednesday night with right exit region dynamics and frontogenesis.
Fairly diffuse baroclinic zone, cold resident air mass, and
underwhelming absolute vorticity maximum suggests nothing more than
light amounts. Tendency for longwave troughing through the Great
Lakes ensures little warm up through the end of the week with a
below normal period through at least the first half of upcoming
The deep low responsible for yesterday's strong winds will slowly
lift into northwest Ontario today. The gradient will once again ramp
up, this time from the southwest, as the cold side of the system
envelopes the central Great Lakes. Influx of cold air will sustain
well-mixed conditions over the water supportive of wind gusts to 40
kts, especially over the central axis of the lake including Saginaw
Bay. Fresh southwest flow will continue through Wednesday as high
pressure building over the Ohio Valley maintains an elevated
Michigan...Lakeshore Flood Warning until midnight EDT tonight for miz063-070.
Lake Huron...Gale Warning until midnight EDT tonight for lhz362-363-421-422-
Gale Warning until 4 PM EDT this afternoon for lhz441>443.
Lake St Clair...Small Craft Advisory until midnight EDT tonight for lcz460.
Michigan waters of Lake Erie...Small Craft Advisory until midnight EDT tonight for lez444.