Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
314 PM EDT Sat Sep 20 2014
Short term...this afternoon and tonight
Several outflow boundaries from overnight convection well to the
west are tracking across lower Michigan. However...18z DTX sounding
shows a rather robust cap in the 875-850mb area...which will likely
take quite some time to totally erode. Models...including primary
and parallel hrrr runs today...suggest convection will eventually
fill in along both outflow boundary sinking south/southeast through
the County Warning Area and also along vorticity spoke further west associated with
leftover weak mesoscale convective vortex. Given capping...and the lack of significant
instability...expect this activity to remain rather disorganized and
sub-severe for the most part.
Will still watch for the potential of a few severe storms as wind
fields are decent...increasing from 25-30 knots 850 mb-700 mb to 50 knots
at 500 mb. With additional heating...temperatures around 80f Detroit
south...over the southern County Warning Area and a lack of notable deep low level
moisture...inverted-v soundings may assist in gust potential given
the above atmospheric wind speeds.
This initial push of forcing...and whatever convection comes of
it...will shift east/southeast of the area late this afternoon/early
this evening. However...higher Theta-E air will continue to shift
over the County Warning Area...particularly M 59 south...within southwest flow in
advance of main shortwave trough axis. So...additional scattered
showers/embedded storms will build through area into this evening.
Once this activity ebbs late this evening...expect renewed shower
and storm development late tonight as main upper level shortwave
digs from the upper Mississippi Valley into the central Great Lakes.
With mild...southwest flow persisting through the night...minimum
temperatures will be rather mild...holding in the 60s in most
locations...especially as further clouds/precipitation provide
additional insolation and minimize radiational cooling further.
Latest water vapor imagery shows a strong but progressive upper
level system/circulation over southern Manitoba...with a stronger
embedded upper wave at the base of the trough over North Dakota. It
is this upper wave/cold pool (-18 to -19 c) which will arrive over
Southern Lower Michigan Sunday morning. With some leftover low level
moisture and more than adequate middle level lapse rate (6.5+ c/km from
700-500 mb)...additional showers and embedded thunderstorms appear
likely during Sunday. A secondary upper wave/cold pool will follow
on the backside through the western Great Lakes and into the Ohio
Valley. Deep northerly flow in this systems wake over the central
Great Lakes will allow for solid cold advection to take place...as
850 mb temperatures lower into the low single numbers...with slightly
negative values over Lake Huron...per 12z Euro/NAM. As such...fully
onboard with the 12z NAM 925 mb relative humidity/low clouds as northerly flow
comes off the relatively warm waters of Lake Huron Sunday night.
Have subsequently ramped up the clouds Monday morning...which will
likely take much of the day to mix out/erode...holding maxes close
to 60 degrees. Favorable radiating conditions Monday night expected
to allow mins to fall into the 40s.
Stubborn high pressure and relentless low relative humidity values through the
column will remain in place for the entire extended period. Flow
will continue to be weak under an Omega blocking high. Temperatures
will trend slightly above normal with no threat of precipitation.
Warmer airmass ahead of cold front...coupled with the weakening
pressure gradient as the front approaches is translating
to wind speeds below 25 knots...and will drop the small craft
advisories. However...showers and thunderstorms will likely
develop...and thunderstorms will be strong to possibly severe into
tonight. A secondary upper wave and cold front will lead to
additional showers tomorrow and possible thunderstorms. Brisk
northerly winds of 20 to 25 knots will follow this front...allowing
large wave to build over Southern Lake Huron Sunday night into
Monday...with small craft advisories likely need during that time
frame. High pressure will exert favorable influence on the waters
during the middle of next week.
Aviation...issued 1251 PM EDT Sat Sep 20 2014
Forcing along lead outflow boundary/weak mesoscale convective vortex center should bring scattered
convection to lower Michigan by 17z-19z...well ahead of the synoptic
trough axis west over Wisconsin. This activity will organize to some
degree as it moves into terminals...but given the earlier arrival of
activity...instability will remain limited as surface dew points
struggle towards 60f in advance of this initial activity.
After this push of rain showers/embedded thunder...higher Theta-E air will
continue to funnel into area in advance of main trough...leading to
a continuation of rather disorganized convection into the evening
generally south of M 59. Renewed activity can then be expected by
late tonight/Sunday morning as main upper shortwave digs into the
area...during which time ceilings will consistently drop to MVFR or
perhaps IFR at times.
//Dtw threshold threats...
* low confidence that a thunderstorm will impact kdtw beyond 22z
* Low confidence in ceilings below 5000 feet after 22z...with
progressively higher confidence from tonight into Sunday morning.
Small Craft Advisory...Saginaw Bay and the nearshore waters from
Port Austin to Harbor Beach...until 8 PM Saturday.
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).