Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
344 am EDT Sat Sep 5 2015
Short term...today and tonight
The persistent low pressure system that has been wobbling about
lower Michigan for several days now can currently be seen over Southern
Lake Michigan on water vapor. Increasing SW flow through the central Continental U.S.
Around the strong ridge will provide a kick which will allow the low
to finally release out of the region later tonight. Until then...we
continue to deal with warmth...moisture...and diurnally driven
instability for yet another day. Main forcings will once again be
the 850mb ll jet...and broad lift due to the weakening low itself
and its deformation region. A main difference today will be a
shortwave embedded in the SW flow will nudge the stationary front
laid out across the Ohio Valley northward into lower Michigan. This will
lead to a boost in maximum temperatures reaching into the middle/upper 80s and a
boost in sb instability. All in all look for just a chance of
showers and thunderstorms today through the end of the diurnal
cycle. Strongest storms would be south of i94 with the better
instability but with weak forcing and no shear severe storms are not
expected. Locally heavy rainfall is the main concern with slow
moving cells and precipitable waters still over 1.5 inches but precipitation drag could
result in a few stronger wind gusts as well.
Heights will start increasing overnight as the low lifts NE out of
the Great Lakes. Dewpoints will remain high which will hold
temperatures in the upper 60s once again. Skies will try to clear
out through the night which may lead to some fog development by
Warm and muggy weather will persist in Southeast Michigan until
sufficient eastward progress of the West Coast trough is made to
shunt the resident airmass south and east. Attm, model forecasts in
general agreement that associated height falls will ease a cold
front through the region during the midweek period. Until then,
highs will remain in the 85 to 90 range with dewpoints around 70 and
heat indicies in the low 90s...perhaps middle 90s in the urban heat
Subtle height rises in response to the closed cyclone lifting into
central Canada will increase capping and reduce the probability for
any low amplitude shortwave forcing embedded in the flow to trigger
convection during peak heating on Sunday. Only exception will be
across the water and perhaps grazing the Saginaw Valley and thumb.
This potential will come as a convectively enhanced shortwave over
the Great Plains this morning is steered around the periphery of the
ridge. This may be sufficient to trigger scattered convection around
the edge of the cap within the fringe of enhanced southwesterly flow, which
could potentially bring a few showers or a thunderstorm to far
northern areas on Sunday afternoon. For now, will introduce a chance
pop but will confine it to the waters of Lake Huron and keep land
The warmest night of the present warm spell will be Sunday night as
height falls finally begin to push into the western and northern
Great Lakes and elicit subsequent enhanced southwesterly flow response.
Ongoing Theta-E advection from the surface upward will put a floor
of at least 70 degrees on low temperatures.
Solution spread increases by Monday owing to modeled strength of the
closed low over Manitoba. Though the daytime hours will largely be a
redux of Sunday except with a slightly better SW wind component,
the overnight period will be characterized by an uncertain degree of
strengthening fgen within the right entrance of a jet positioned
over the northern Great Lakes. Ggem solution offers a notably weaker
depiction of the closed low and subsequently weaker fgen/lack of
precipitation development. Preference at this time is to lean the forecast toward
the wetter European model (ecmwf)/GFS which are both stronger and in better
agreement. Accordingly, confidence is increased that showers will
begin to spread into the northwest half of the County Warning Area during Monday
night. Bumped Saginaw Valley/thumb areas to a high chance pop.
Surface and upper level high pressure will lead to predominately
quiet Holiday weekend marine weather over the region. Still looking
at a low chance for a shower or thunderstorm during the day on
Saturday...although the greatest potential appears to be for the
inland areas. Light southerly winds will be commonplace...before
southwest winds eventually increase into the 10 to 20 knot range
ahead of an approaching frontal boundary Sunday night and Monday.
Aviation...issued 140 am EDT Sat Sep 5 2015
Wide range of conditions across the taf sites with frequent
fluctuations between criteria as we reside under the wobbling upper
low. MVFR should prevail with temporary dips to IFR with the except
of mbs and yip which area favored regions of fog development. They
will likely be IFR with periodic drops to LIFR. The fog will once
again burn off this morning giving way to diurnally driven cumulus.
Shower/thunderstorm chances remain too low at any terminal to
include in the taf.
For dtw...area of clouds has exited the area allowing decreased
ceilings/visibilities to finally develop. Appears to be enough residual cloud
cover nearby to keep fog from becoming too dense by morning.
//Dtw threshold probabilities...
* medium for ceiling at or below 5000 feet through early morning and
again Saturday afternoon.
* Low for thunderstorms Saturday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).