Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
327 PM EDT Tuesday Jul 22 2014
The cold front associated with a compact shortwave tracking north of
Lake Superior is currently draped along a line from east-central
Wisconsin across far Eastern Lake Superior. 12z soundings revealed
strong capping in place across the Great Lakes this morning which
has helped keep things in check so far today. Nonetheless,
convergence along the front is quite healthy as evidenced by fine
lines in regional radar reflectivity and convergent boundary layer
cloud signatures in visible imagery over Wisconsin. Along and ahead
of the front, a narrow ribbon of low-level moisture advection is
progressing east. Thunderstorms in association with the lead edge of
this moisture plume are ongoing across the northern Great Lakes
where forcing is much stronger and capping is weaker.
The primary threat for thunderstorms tonight will come as this
moisture plume folds east across Southeast Michigan between 00z and
09z. Forecast soundings indicate that capping will remain rather
strong, but will weaken with time as weak height falls associated
with the Ontario trough work into the area and low-level moisture
quality improves. Even with the weakening cap, forcing will not
really be comparable to what has been observed over Lake Superior so
far today today. The focus for middle-level forcing will instead be
driven by the low amplitude circulation evident in 1730z WV imagery
this afternoon. Confidence is reasonably high in at least some
broken coverage of showers as the lead edge of the Theta-E gradient
works into the area. However, thunderstorm potential will remain
tied to just how quickly the cap erodes. The high resolution
guidance have generally trended toward more aggressive solutions,
though none necessarily impressive. The hrrr, once today's most
bullish model, has backed off considerably. Thus, 40 probability of precipitation remain in
place for the County Warning Area with a bump up toward 50 percent across the
Saginaw Valley and thumb where forcing will be stronger/more
organized. An outside shot at isolated severe hail or wind remains
in play, largely due to steep 700 mb-500 mb lapse rates.
Lows will fall into the 60s across the northwestern 2/3 of the County Warning Area
behind the surface front while the metropolitan area will likely still be
in the upper 60s to low 70s by daybreak.
Long term...Wednesday through Monday
Early Wednesday...strong isentropic decent will be ongoing at the
330k surface behind the cold front. Strong winds will be ongoing
through the day as pressure rises on the order of 3-5 mb/6 hours
will continue through the morning. An ongoing decrease of Theta-E
will also characterize the Post frontal airmass. All this spells a
cool and breezy day that will foreshadow the remainder of the week.
The upper level pattern indicates a strong ridge over the Rocky
Mountains and troughing over northern Ontario for the forecast
period. The upper trough affecting Michigan does not appear to
contain many waves or vorticity maximum lobes...which should keep the period
dry under high pressure. The upper level pattern will flatten out
slightly Friday which will allow temperatures to gradually climb closer to
normal values. The aforementioned upper trough appears to take
control of the eastern US for the extended forecast.
A weak shortwave trough will slide just south of Southeast Michigan
on Friday night. The extreme southwest counties /Lenawee and Monroe/
could see some rain showers with this system on Friday night.
Otherwise...the rest of the County Warning Area will hold off to see rain until
Saturday as a low pressure system develops over the northern plains
and tracks...eventually stalling over the Great Lakes sending a series
of shortwaves across the region. These shortwaves will result in
rain showers through Monday. The timing is still uncertain with this
system though and therefore have included only chance probability of precipitation in the
forecast from Saturday through Monday morning. Temperature
wise...Saturday will be the warmest day with highs in the low 80s.
High temperatures will then drop into the middle 70s through the
beginning of next week. Low temperatures however will hover in the
low to middle 60s through Sunday night before dropping slightly into
the upper 50s on Monday and Tuesday nights.
A cold front set to cross the Great Lakes tonight will provide a
boost to wind speeds overnight and through Wednesday. Winds will
become north at 20kt with gusts near 30kt. The onshore flow and long
fetch across Lake Huron will amplify wave heights through the day.
Open waters can expect sig waves over 6 feet with nearshore areas in
a smallcraft advisory with sig waves in the 3 to 5 foot range. High
pressure will return to the area Thursday and provide more ideal
marine conditions for the following few days.
Aviation...issued 104 PM EDT Tuesday Jul 22 2014
Deepening mixed layer will contribute to southwest wind gusts around
20 knots this afternoon. Warm air aloft will cap thunderstorm
potential through the early evening. However, an increase in
middle-level moisture will contribute to an increasing probability of
thunderstorms between about 03z and 09z. For now, confidence
continues to warrant nothing more than a prob30. Cold advection and
lingering low-level moisture in the wake of the front is expected to
contribute to an MVFR layer of stratus by daybreak Wednesday...with
ceilings then forecast to mix up to VFR during the morning.
For dtw...the best chance for thunderstorms will be after 05z. The
cold front will exit the area by 09z with northerly winds filling in
//Dtw threshold threats...
* low confidence that a thunderstorm will impact kdtw
* medium confidence in ceilings below 5kft Wednesday morning
Small Craft Advisory...nearshore waters from Port Austin to Port
Huron including outer Saginaw Bay...from 5 am Wednesday to 10
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).