Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1200 am EDT Tuesday Jul 29 2014
Stratus/strato-cu within unsettled cyclonic flow has eroded enough
to denote as few-scattered overnight into likely Tuesday morning. Expect
ceilings to fill back in as next major vorticity spoke pivots through
the area and daytime heating aids in overall instability. Will also
time scattered rain showers through area from northwest to southeast during the
afternoon as this feature crosses the area. Ceilings should then break
up again by 00z Tuesday evening as subsidence in the wake of the
wave combines will loss of daytime heating/instability. Northwest
winds may gust to 20 knots or so with best mixing Tuesday afternoon
and especially in conjunction with a few of the rain showers...but overall
expect relatively benign winds much of the forecast period.
//Dtw threshold threats...
* medium confidence in ceilings at or below 5000 feet 18z-24z Tuesday.
* Low confidence in an isolated thunderstorm affecting the terminal
Previous discussion...issued 306 PM EDT Monday Jul 28 2014
High impact weather potential: none.
Pattern summary/implications: all too familiar ridge-trough pattern
in place across North America this afternoon with yesterday/S severe
weather maker now moving through the northeastern states. In it/S
wake much drier middle level air has arrived from the north with ample
moisture trapped beneath a significant subsidence inversion /seen to
the north this morning in the 12z apx raob/. The llevels have
gradually been drying...with morning MVFR stratus gradually lifting
into a broken cumulus deck. Temperatures have been held in check
given this cloud cover and ongoing cold advection /t8s near 6c/ with
most spots still in the 60s /even 59 at kbax/. This general pattern
will remain in place through the short term forecast period tonight
with our eyes then turning to next upstream shortwave in the
northwest flow seen as enhanced clouds north of Lake Superior that
will sink south and re-moisten the middle levels after midnight
Through early this evening...mostly cloudy skies will be tough to
give way beneath the inversion...but clearing trend now entering
northern lower will slowly work it/S way south with skies going
partly cloudy towards sunset. This yields Little Hope for late
afternoon temperatures reaching above the 65-70 range ongoing at this time.
Tonight...clearing trend from northwest to southeast will
continue...but will be brief as aforementioned wave north of Lake
Superior will quickly spread middle clouds back into the region after
midnight...with broken middle clouds /along with a few clouds below this/
likely towards daybreak. Otherwise...no sensible weather to speak
of other than the cool temperatures.
Fog: should have no issues with increasing cloud cover after
midnight...along with some remaining llevel gradient /see below/ and
llevel dry advection.
Temperatures: llevel drying...some evening clearing...and the the
fact that temperatures Don/T have to fall far argue for lows having no
trouble dropping into the 50s...right in line with guidance
Winds: the gusty northwest winds of today will subside rapidly this
evening with the loss of daytime mixing and a weakening gradient.
Expect most spots to maintain northwest winds around 5 miles per hour for the
Long term...Tuesday through next Monday
Prolonged period of cooler than normal temperatures with
afternoon bouts of showers/storms...
High impact weather potential: minimal through the period...with
sub-severe thunderstorm activity each afternoon through the period
outside of Thursday.
Pattern summary/forecast: longwave pattern across the western
hemisphere taking the form of an Omega block early this
afternoon...with persistent troughing over the Gulf of Alaska and
just east of our longitude. This pattern changes very little
through the long term forecast period with the aforementioned
troughs retrograding slightly as the pattern actually amplifies a
little more towards middle week. There is some relaxation to the flow
as we move towards the coming weekend/early next week...but with the
impressive nature of the initial block...gut feeling is to lean more
with the European model (ecmwf)/gefs mean which would keep the pattern more
amplified/less progressive right through the end of the forecast
Pattern implications: troughing certainly spells a below normal
period of temperatures through the long term forecast...with a very
slow trend towards climatology as we move towards the end of
this forecast period as uncertainty in amplification of the pattern
increases. As for precipitation chances...trough aloft with it/S
associated cool temperatures suggests diurnally driven
shower/thunderstorm chances that will be modulated by shortwaves
rotating around the parent longwave trough. The first of these
comes Tuesday with lower confidence of another shortwave on
Friday-Saturday. Thunder chances not great particularly early with
little heat for cold pool aloft to work with...but with gradual
moderation of the pattern by late week and into the
weekend...thunder chances will increase.
Model agreement/confidence: agreement is quite good through the
coming weekend as would be expected given well defined large scale
features. Biggest disagreement comes by early next week when the
GFS wants to lift eastern troughing faster than the European model (ecmwf)/gefs
mean. Sided with the slower European model (ecmwf)/gefs mean given blocked nature of
Middle Range /Tuesday through Thursday/:
Cool temperatures aloft and the arrival of shortwave energy late
Tuesday spells a good likelihood of rain showers activity Tuesday afternoon
and evening. Instability is very thin...with perhaps a rumble of
thunder...but little more than this. Given cloud cover and t8 only
around 9c...highs will struggle to reach much above the lower 70s.
This wave passes south and east of the region Wednesday...with
additional shortwave energy brushing US to the north and east.
Can/T rule out additional afternoon rain showers chances...but coverage
looks less than on Tuesday. Similar temperatures aloft for the day
Wednesday...but with a little more in the way of sunshine...highs
should reach the 70-75 range.
Similar pattern in place on Thursday...although the bulk of
shortwave energy will be placed north and east of the forecast
area...so this is our best chance of a dry day and will go for this
in this afternoon/S forecast. Pretty much persistence in terms of
temperatures aloft /9c at t8/...with highs perhaps a degree or two
warmer than Wednesday...generally in the middle 70s.
Long range /Friday through Monday/:
After making it/S closest approach on Thursday...core of eastern
longwave trough lifts north and east slowly...and allows another
shortwave to arrive in the northwest flow aloft. This suggests
re-invigoration of diurnal convection for Friday-Saturday with
llevel temperatures warm enough to support enough MUCAPE for thunder
to be included in the forecast. An early look at the Sunday-Monday
timeframe suggests some potential for additional shortwave energy in
the northwest flow aloft to allow for at least one more day of
potential afternoon shower/thunderstorm activity...but will avoid
painting both days with probability of precipitation as the fast moving nature of any
feature should keep the window of shower/storm activity fairly
T8s remain 10-12c through the period suggest highs remaining in the
upper 70s to around 80 through this period...still shy of seasonal
norms...suggesting that August will begin much like July ends.
Today/S strong northerly winds will diminish as low pressure over
the eastern Great Lakes continues to lift north and east into Quebec
and allows the gradient to slacken. Northwest flow will continue
through midweek as high pressure remains west of the Great Lakes
region with weak troughing overhead...but winds/waves will remain
below headline thresholds. There is a threat for thunderstorms
during the afternoon Tuesday...which may bring locally gusty winds.
However...severe storms are not expected.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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