Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
707 am EDT Thursday Jul 31 2014
The broad upper low centered over James Bay will continue to
generate a mix of VFR cloud conditions today and tonight. Middle level
clouds with bases 8-12 kft will affect the terminals until cumulus from
daytime heating begins to form during the early afternoon. Showers
will be possible and a thunderstorm will be isolated...both of which
are expected to have less coverage than recent days and too low to
mention in the forecast in the absence of better observations.
A modest clearing trend tonight with the loss of surface heating
will give way to increasing clouds again overnight as an upper level
wave brings high based clouds back into the region. A few showers
will be possible as well but again too few to mention in the
forecast at this time.
//Dtw threshold threats...
* medium confidence in ceiling at or below 5000 feet today.
Previous discussion...issued 347 am EDT Thursday Jul 31 2014
Short term... today and tonight
The resident anomalous upper low will maintain influence on the
weather over the Great Lakes today. The primary difference compared
to the last two days will be that stronger small scale features
within the upper circulation will be absent...and even the upper low
itself will loosen its grip slightly. Early morning satellite
imagery illustrates the First Point as northwest flow aloft is
rather nondescript in the WV channel around the west flank of the
upper low from northwest Ontario into lower Michigan...but the cold
core aloft is maintaining instability over the low level moisture
axis. This is generating a few showers over Wisconsin and northern
lower which could drift over our area during the morning.
The 00z model package has captured the uniformity in the upper level
pattern and illustrate broad height rises taking place during the
day. The associated middle level warming will lift 500 mb temperatures from
near -20c Post midnight to about -13c by early afternoon. The
dramatic reduction in 700-300 mb lapse rates is shown in model
soundings...but there is some residual free convection as maximum temperatures
make a run into the upper 70s. The NAM is an outlier and looks
overdone in its depiction of an inversion centered around 650 mb.
Such an inversion would be a surprise given the broad cyclonic flow
still in place over the Great Lakes. Daytime heating should then
generate a healthy cumulus response capable of showers with low end
scattered coverage...perhaps a little more in The Thumb between the
Bay and Lake Huron. The large scale upper level warming and
otherwise nebulous surface pressure pattern do suggest that
convection capable of thunder will have no more than isolated
coverage and difficult to achieve.
The upper low will give way slightly to a short wave moving trough
the plains today which will induce some height falls over the
Midwest tonight. Early morning satellite imagery attests to model
analysis fields capturing the wave over the northern plains in the
00z run. Upper air solutions look reasonable in the development of
a trough over Iowa that will help back the middle level flow enough to
enhance moisture transport into lower Michigan overnight. Model
depictions of Theta-E advection in the 850-700 mb layer are on the
modest side and with the flow not quite strong enough for more than
a chance pop through sunrise Friday.
Long term...Friday through Wednesday
A few upper shortwaves can be seen this morning diving through the
upper Mississippi River valley as they are guided by the broad
upper trough centered just south of Hudson Bay. These waves will
amplify as they drop into the western Great Lakes
tomorrow...forming a weak upper trough that will slowly work
across the area through the first half of the weekend. Latest
round of models are in good agreement with a Theta-E plume working
up into the area ahead of the trough late tonight and Friday.
Cooler air will also be returning aloft as the upper trough
nears...working to destabilize the atmosphere as diurnal heating
ramps up. Expect to see an uptick in shower/tstorm coverage Friday
afternoon through the first half of the night (compared to today)
with the additional moisture working in and as the upper trough
approaches. The potential for showers/thunderstorms will then continue
into Saturday as the trough continues to work through the
area...with again an uptick in activity during peak diurnal
Both Friday and Saturday look similar with regard to thunderstorm
strength and threats. Forecast soundings show tall and skinny cape
profiles...with cape density highest near and above the freezing
level (not quite up through -20 c however). Maximum temperatures near 80 and
dewpoints rising back into the upper 50s to low 60s should
provide sb cape values between 1500-2000 j/kg. Wind field looks very
weak however...so would mostly expect pulse-type storms. Biggest
threat from any strong storms that develop would be hail of 0.5-1.0
inches...but a few strong wind gusts may also be possible given
inverted v look to soundings. Latest Storm Prediction Center outlook does include
Michigan in the 5% probability for severe...and this does look
appropriate as threat should stay isolated.
Upper ridging is no longer advertised by models to build into the
area on Sunday. Instead they are showing the upper trough just
starting to work into the eastern Great Lakes at the start of the
day...and did have to include a low chance for showers/tstorms.
On Monday a piece of energy that breaks off the upper low now
entering the northwest territories is advertised to track across
Ontario...guiding a weak cold front down into Michigan. Models vary
with timing the southward progress of this feature...not too big a
surprise this many days out...and also disagree with whether the
front drops far enough south to clear the area or whether it stays
quasi-stationary over the area into mid-week. The progress of this
feature...right now...is the main challenge for the forecast next
week as rounds of showers/thunderstorms look to form and track along the
frontal boundary...as another low pressure system drops into the
A weak trough of surface low pressure will track across northern
Michigan today. This will bring about an increase in southwesterly
winds today and tonight..however speeds are expected to remain near
or below 10 knots. Wind speeds will become light again by
Friday...with light winds then prevailing through the weekend. The
better chances for thunderstorms during the period will be closer to
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).