Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
347 PM EDT Wednesday Jul 30 2014
Short term...this evening and tonight
The dominant weather feature for the region remains the longwave
upper level trough and closed midlevel low pressure system that is
centered over James Bay. Lower Michigan is situated within the broad
cyclonic flow at the periphery...a location that has been at the
mercy of episodic shortwaves and varying low to midlevel saturation
pinwheeling around the low.
Two separate midlevel trough axes are providing some focus and
general organization to the precipitation in the Great Lakes region
this afternoon. The first trough axis is quickly centering into
portions of Lake Erie and Ohio. Radar and lightning observations
show the vast majority of the thunderstorm activity is occurring
ahead of this axis...within a ribbon of much deeper moisture. The
second of the two trough axes will soon enter the northern reaches
of far Northwestern Lake Superior. Satellite along with various
observational datasets have consistently shown a strong shortwave
tied to this -div zone along with a good surge of Atlantic
wraparound moisture. Shower and isolated thunderstorms have
blossomed within the overlap of these ingredients over portions of
Ontario and Lake Superior.
Across southeastern Michigan much of the activity this afternoon has
remained focused along the i69 corridor...roughly Lansing to Port
Huron. This focus is along a 700mb Theta-E gradient...a boundary
that was tied to the shortwave now entering Western Lake Erie. This
boundary will settle steadily southward through 00z carrying a
continued potential for scattered showers. A secondary location to
watch for will be along the Lake Huron shoreline 23-02z as some
enhanced moisture will swing down from northeastern lower Michigan.
Marginal convective instability and the overall low Theta-E content
of the overhead airmass limits the thunder and lightning potential.
Convective depths will increase this afternoon but not expecting
similar activity to yesterday.
For tonight...nwp advertises a contraction of the closed low due to
an impressive late July inner jet core digging straight southward.
This upper level jet axis now over Lake Superior will swing rapidly
to western PA/New York by 12z Thursday. This strong acceleration component
will make it difficult to keep organized forcing in any one location
for any length of time. For this reason...cut back on the probability of precipitation
distribution rather aggressively despite some elevated convective
instability persisting within the column. Some convection allowing
models suggest that some ragged showers may be advected within the
cyclonic flow to the Saginaw Bay region sometime/around 10-12z
Long term...Thursday through next Wednesday
Middle level heights will be on the rise across lower Michigan by Thursday
morning as short wave energy departs toward upstate New York and
drags the core of extremely cold air aloft /500mb temperatures down to
-20c/ east of the region. The anomalously deep upper low will
however remain just south of James Bay through the day Thursday and
will drive a surface trough into the northern Great Lakes. SW flow
in advance of this trough will lead to some moisture advection
through the day...proving an opportunity for weak daytime
destabilization. The highest instability should be over the Saginaw
Valley and thumb region given the closer proximity to the moisture
plume. Farther south...lower surface dewpoints and building middle level
heights will be a large limiting factor for afternoon
convection...supporting just a slight chance of late day
showers/thunderstorms. A plume of steeper middle level lapse rates is
forecast to expand into lower Michigan Thursday night under middle level west-SW
flow...while subtle moisture advection persists in the low levels.
This suggests a slight chance of some convection during the
night...with slightly higher chances closer to the remnant low level
convergence axis expected to be residing over central lower Michigan.
The upper low will be driven into eastern Canada Friday into
Saturday. There is good agreement among the model suite that short
wave energy rotating around the back side of this trough will
undergo some amplification as it tracks from the upper MS valley
Thursday into the Great Lakes and Ohio Valley Friday into Saturday. Subtle
cooling aloft and fairly good low level moisture quality will lead
to modest daytime destabilization both Friday and Sat....supportive of
the current chance of showers/thunderstorms. Most likely timing both
days will be late afternoon and evening. Low level warm air
advection on Thursday will boost temperatures back closer toward typical middle
Summer temperatures. Little overall change in the thermal profile
and expected mixing heights will sustain these near normal temperatures
into the weekend.
Weather pattern shows little change with a general ridge west/trough
east set up...precluding any Big Warm-ups. Upper trough axis/cold
pool will continue...so diurnally enhanced showers and thunderstorms
will be a possibility through much of next week. Temperatures are
expected to remain near to slightly below normal.
Winds will remain light through the night as a weak pressure
gradient resides over the area. The surface trough will push south
across the northern Great Lakes on Thursday. This will invoke
slightly stronger southwest winds on the lakes. The gradient will
then relax again by Friday...with light winds prevailing through the
weekend. The better chances for thunderstorms during the period will
be closer to the shorelines.
Aviation...issued 154 PM EDT Wednesday Jul 30 2014
A corridor of enhanced midlevel moisture over the southern half of
the County Warning Area will allow for shower and thunderstorm development this
afternoon. Models remain aggressive with the amount of subsidence
that will arrive after 21z. Will continue to Honor this output and
forecast the best time period for showers and possible thunder
between 18-22z. The weather pattern this evening will be influenced
by large scale subsidence...which will carry well into the nocturnal
heating minimum. From this Vantage Point...this limits the potential
for additional shower development late tonight and increases the
uncertainty in coverage and location. Will refrain from introducing
precipitation for tonight with this taf cycle.
For dtw... the most favorable time period for showers to impact the
terminal is 18-22z.
//Dtw threshold threats...
* high confidence in ceiling below 5000 feet this afternoon.
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).