Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
705 am EDT Wednesday Jul 23 2014
Residual Post frontal moisture and a deepening low level inversion
courtesy of shallow cold air advection led to the development of an
extensive MVFR stratus deck this morning. The cold air will deepen
during the morning and afternoon. This along with daytime heating
will lift the inversion base which in turn will lift the bases of
the clouds. Dry air advection during the course of the afternoon
will also begin eroding the strato cumulus deck late in the afternoon
into early evening...leading to a clearing trend.
For dtw...northwest winds in the wake of the front will quickly veer
toward the north over the next one to two hours. Wind speeds should
remain around 12 knots as cold air advection and daytime heating
keep the low levels well mixed.
//Dtw threshold threats...
* high confidence ceilings will be below 5000 feet this morning
through middle afternoon.
Previous discussion...issued 333 am EDT Wednesday Jul 23 2014
Short term...today and tonight
The surface cold front...which as of 06z was nearly bisecting the
forecast area...will be exiting metropolitan Detroit to the southeast
shortly after 09z. The instability axis along and ahead of this
front will in turn move out of the area as well...ending the risk of
additional convection before 7 am this morning. High pressure will
then build across the western Great Lakes/upper MS valley today
amidst a building middle level ridge over the northern plains.
The magnitude of cold air advection behind this front is actually
quite good for July. 925mb temperatures will in fact drop from +25c last
evening to +10c this morning. Post frontal surface temperatures will rapidly
fall into the 60s this morning...only to recover into the low 70s
this afternoon. Thus maximum 24-hour temperatures for Jul 23 will be the
overnight readings. The cold air advection is rather shallow...
which will lead to a deepening Post frontal inversion this morning.
Regional surface observation and infrared imagery is in support of model soundings
which suggest an abundance of low stratus this morning residing
under this inversion. Diurnal heating during the late morning and
afternoon will lift the stratus to a strato cumulus field. A north-NE
trajectory off Lake Huron may provide just enough added low level
moisture to boost the diurnal contributions to the cumulus field...
possibly leading to broken to overcast skies over much of the
forecast area into late afternoon or evening.
Middle level heights will continue to fall across the region tonight as
a short wave /now pushing south across Manitoba/ moves across lower
Michigan. This wave and some enhanced upper jet support will only result
in some high clouds...given the very dry air in the middle levels. The
center of the surface high will however be suppressed to the west of the
area which will maintain a north-NE gradient and light winds through the
night. This will hinder the degree of decoupling in the boundary
layer and will support leaning on the higher side of guidance min
temperatures tonight /mainly in the 50s/.
Long term...Thursday through Tuesday
Upper level confluent flow and associated very strong subsidence
from 10 to 30 kft above ground level will force a surface anticyclone to migrate
from central Canada southeastward through the central Great Lakes
and down into the Delaware-Maryland-Virginia. The stable surface anticyclone is
forecasted to pass overhead during the late afternoon Thursday and
into the evening hours. Expect picture perfect weather for all of
semich as full insolation will allow temperatures to slowly warm
into the middle to upper 70s...low 70s near the Lake Huron
shoreline. There will be a cool feel to the air...factoring in fresh
surface dewpoints for late July of around 50 degrees. The confluent
flow will persist again on Friday as the region is located in a
pinch point between the SW US ridge structure and the amplified upper
level trough over NE Canada. Therefore...a surface ridge axis is
forecasted to linger back to the northwest. Highs Friday should
moderate a couple of degrees into the upper 70s...dewpoints will
remain in the lower 50s.
Uncertainty in the forecast then ramps up over the weekend...owing
to differences in the timing of the breakdown of the lower
tropospheric ridge. NCEP models suggest this will occur right away
on Saturday as collected moisture and lead shortwave energy washes
over the state from the southwest. On the other hand... the European model (ecmwf)
has its own idea suggesting the arrival of deep moisture and
precipitation chances won't arrive until Sunday afternoon. The offered
guidance and the forecast is skewed heavily towards the NCEP Camp.
The feeling is this will be modified toward the European model (ecmwf) solution
(slower) in subsequent forecast packages.
Post frontal cold air advection this morning will rapidly allow
unstable conditions to develop over the lakes. Observations across
the northern Great Lakes are already supporting this. The northerly
gradient will support of few gusts of 25 to 30 knots over Lake Huron
today. This will lead to rough boating conditions...particularly on
the Southern Lake Huron basin. Winds will become rather gusty on
both Lake St Clair and Erie as well...leading to some choppy seas.
At this point...conditions are looking as though they will fall just
shy of Small Craft Advisory conditions on lakes St Clair and the Michigan
waters of Lake Erie. The wind component over Saginaw Bay is expected
to be more northeasterly which has prompted the expansion of the
Small Craft Advisory to cover the entire Bay. The gradient will
slowly weaken tonight...leading to a steady decrease in the winds
/mainly after midnight/. High pressure will be centered over the
region Thursday and Friday...leading to favorable boating
Michigan...Beach hazards statement...miz048-miz049-miz055-miz063...until 10 PM
Small Craft Advisory...nearshore waters from Port Austin to Port
Huron including Saginaw Bay...until 10 PM Wednesday.
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).