Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
554 am EDT Friday Aug 22 2014
Variable conditions across the area this morning as we try to scour
away the IFR fog from the overnight. The moist airmass will remain
in place through the taf cycle as we still reside on the north side
of a slowly retreating warm front. This will result in a chance of
showers and thunderstorms through the day and more fog during the
overnight. Low confidence in coverage of precipitation as there is
no real forcing mechanism to key in on so will leave out of the tafs
for now though better chances will be over the Detroit tafs as
opposed to fnt and mbs. Skies will attempt to clear tonight which
will aide in IFR/MVFR fog formation.
//Dtw threshold threats...
* moderate confidence on ceiling below 5000 feet through this
* Low confidence in coverage and timing of any thunderstorms today.
* Low confidence on visibility dropping below a 1/2sm tonight.
Previous discussion...issued 332 am EDT Friday Aug 22 2014
Short term...today and tonight
Easterly flow will persist through the forecast period as we will
remain under the elevated warm front which is rooted at the surface
along Michigan/Ohio border. The surface warm front will not return to the area
before it begins to retreat southward later today thus we will be
dealing with elevated convection periodically through the day.
Current radar mosaic shows the next convective complex developing
over the Chicago area. It...as the previous mesoscale convective system Thursday evening
did...will track to the south of the area as the shortwave over Ohio
continues to reorientate the 850mb jet more west to east across the
Ohio Valley. Even without the forcing of the jet...the atmosphere
remains very ripe for scattered showers as ribbon of very rich Theta
east resides across southern Michigan. Showers have persisted through the
night on the northern edge of this rich airmass...precipitable waters around 1.75
inches. Weak flow into the middle levels...30 knots at 500 mb per the
DTX 22.00z sounding has led to a slow forward speed of the
showers/storms making flooding a concern for any area that gets a
healthy storm or two.
The overall pattern does not change much from what we see currently
into the afternoon today. With a lack of notable forcing mechanism
or shortwave and model forecast cape values only around 200
j/kg...find it hard to continue the likely probability of precipitation thus lowered into
middle/high chance for the southern border and lowering as one heads
north. A general chance pop will handle the scenario well until a
more organized complex shows itself. Hires models all agree with
this change by having more organized complexes tracking to the south
with sporadic showers/thunderstorms over the area on the elevated
The tonight period is a little tricky. The steepening of the middle
level ridge over the upper Mississippi Valley will pull the deep
moisture axis back westward but leaves some residual pockets over
the area before high pressure from the east can take over. Overall
the showers/thunderstorms look to shut off around sunset although
would not be surprised to see a linger shower between 00-03z. With
the resident bl airmass changing little over the next day...elevated
dewpoints in the middle 60s will keep min temperatures up but also produce
areas of fog once again as skies try to clear toward morning.
Long term... Saturday through Thursday
Ongoing deepening of the long wave trough over the Pacific northwest
will lead to downstream amplification of the subtropical ridge over
the eastern US this weekend. The latest model suite continue to support
fairly good middle level height rises across Southeast Michigan this weekend as the
ridge axis extends across lower Michigan. The upper level anticyclone now
parked over northern Quebec will build south before being absorbed into
the expanding subtropical ridge. This will force the large surface
high over eastern Canada to expand into the eastern Great Lakes this
weekend...leading to slightly stronger east-southeast flow across the forecast
area. Advection of slightly cooler and less humid air will result.
Model soundings suggest daytime insolation will still boost Sat and
sun highs into the low 80s and dewpoints should still remain in the
60s. The more stable airmass building in from the east and warming
temperatures in the middle levels will support a dry forecast for the entire
weekend. All of the 00z model solutions do however suggest some
higher low level moisture advancing from the middle Atlantic into the
eastern Great Lakes Sat night within slightly enhanced southeast flow. This
combined with the Warm Lake Erie waters may lead to considerable low
clouds and/or fog Sat night into Sun morning.
Energy ejecting out of the western US long wave trough will track into
cntl and eastern Canada early next week. This will dampen the middle level
ridge axis over the Great Lakes region. The low level flow will
however turn toward the S-SW and will open the door for heat and
humidity to filter into Southeast Michigan. High temperatures Monday and possibly into Tuesday
may top 90 degrees. Model soundings are suggesting the atmosphere
will be capped on Monday. The most recent European model (ecmwf) is actually
suggesting a strong cap persisting into Tuesday...so convective
chances may actually be quite low. A trend toward a more progressive
flow next week has been shown by the medium range model suite...
featuring middle level troughing advancing into the Great Lakes by the
middle of next week /tues night - Wednesday night/. Despite poor run to
run continuity with respect to timing and amplitude of this
energy...the middle level height falls across the moderate instability
over Southeast Michigan will support a good chance of convection during this time
period. Cooler and drier air then looks to funnel in by the end of
the forecast period.
A fairly large region of high pressure will expand across the
eastern Great Lakes over the next couple of days. This will lead to
light east-southeast winds across the lakes. Some slightly gustier
NE winds may occur within the warmer waters of Saginaw Bay from time
to time as the wind gets funneled into the Bay. Otherwise...winds
and waves will provide favorable boating conditions.
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).