Scientific Forecaster Discussion
fxus63 kdtx 231950
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
350 PM EDT Tue may 23 2017
The mid week period will be best described as unsettled due to a
large low pressure system over the Midwest moving into and lingering
over the southern Great Lakes through Thursday. The abundance of
clouds and showers will also keep temperatures in check, especially
Wednesday night into Thursday as stronger northeast wind develops
over the region. Expect readings to average in the 60s to around 70
each day, a few degrees below the normal values that are in the
lower 70s for this point in late may.
Today's late afternoon activity continues to be driven mainly by
broad and weak lift within the entrance region of the upper jet Max
over the Great Lakes. This forcing is getting a boost by instability
due to daytime heating both of which are acting on the low level
Theta-E ridge within the warm sector of the low pressure system. The
resulting convection is an Ordinary pulse Mode given the Low Cape
and weak wind field/low shear environment. Afternoon mesoanalysis
leading to press time has shown 100 mb ml cape under 500 j/kg and
mu/surface cape under 1000 j/kg, just enough for some lightning strikes
before diminishing during the evening.
The afternoon and evening showers/storms will be followed by a break
for much of the late evening and overnight hours before the chance
of showers and elevated storms returns toward sunrise. The break in
the action will develop as the current upper jet segment exits
northward and as deep southwest flow backs toward the southeast
ahead of the primary upper trough. The backed flow will initially
draw in some dry air from the Ohio Valley before the low pressure
system renews moisture transport from both the Gulf and Atlantic
coasts. This will fuel some elevated instability through Wednesday
morning based on 850 mb Li running from 0 to -2c which high
resolution models show converted to surfaced based cape early in the
diurnal cycle. The day 2 convective outlook appears to also include
the appropriate shadow in the Lee of easterly flow off Lake Huron.
Thunderstorm coverage will be on the scattered side while showers
associated with the steady/moderate Theta-E advection will be
numerous to widespread while spreading from the Ohio border
northward through the day.
The incoming stream of both Gulf and Atlantic moisture will set up
the wettest period of this event from late Wednesday through
Wednesday night. There is reasonable model agreement with the slow
evolution of the low pressure system over the Ohio Valley during
this time while the system deepens to about 995 mb. This is shown to
form a strong occlusion/trowal over lower Michigan Wednesday night
that will be an efficient rainfall producer. Model solutions also
show the occlusion progressing steadily across the region as a
better defined dry slot pushes in from the south. This is reflected
in some more reasonable model quantitative precipitation forecast that is generally less than 1 inch
for the 24 hours ending Thursday morning.
Pop chances remain low throughout Friday as upper-level ridging sets
up across the Great Lakes, although a few embedded shortwaves could
provide the slight chance for a brief shower. The biggest change for
the weekend thus far is the pop chances Saturday morning into the
afternoon. Both the Gem and European model (ecmwf) 12z model suites are starting to
converge on a weak surface low that's poised to push from northern MO
into the Ohio Valley Friday into Saturday. During prior runs, the
low kept most to all precipitation just south of Michigan, however, a
more northerly track now brings precipitation chances to Southern
Lower Michigan. The GFS is a little slower with the surface low and
pushes it across southern Michigan late Saturday into Sunday, but
would not be too surprised to see it play catch-up with the European model (ecmwf) and
Gem run within the next day.
Continued rain chances will exist Sunday into early next week an
upper-level cut-off low moves in from North Dakota into the Great
Lakes late Saturday into Tuesday. This will act to produce on and
off rain showers with mostly cloudy conditions. Thunderstorm chances
remain low during this time, however, did put thunder in the
forecast throughout Sunday as a weak cold front pushes through.
Otherwise, daytime high temperatures are expected to consistently
peak in the 70s through the weekend and into early next week.
A large area of low pressure over the Midwest will evolve into the
southern Great Lakes through mid week. This will keep marine
conditions unsettled from a weather standpoint as a steady increase
in northeast wind builds an onshore waves pattern through Thursday.
The forecast maintains moderate speed, generally under 20 knots, as
thermal profiles appear very stable over Lake Huron while Saginaw
Bay and Western Lake Erie will behave much like the surrounding land
area. This long fetch of moderate onshore flow and lower stability
over nearshore areas could result in waves exceeding Small Craft
Advisory threshold late Wednesday, Wednesday night, and Thursday.
The low pressure system will then begin an eastward exit by Friday
which will produce backing and weakening flow heading into the
Low pressure developing over the Ohio Valley Wednesday will lift
north to over Southeast Michigan Wednesday night then stall for a
while before slowly moving east by Thursday night. Periods of
showers and thunderstorms from Wednesday through Thursday will
produce rainfall averages around a half inch across the Saginaw
Valley and thumb to near an inch towards the Ohio border. There may
be locally higher amounts in areas that experience thunderstorms.
Ponding of water on area roads and in other prone areas is likely,
especially considering the widespread activity over the weekend that
left the ground saturated in most areas.
issued at 107 PM EDT Tue may 23 2017
Upper level closed low over Minnesota will rotate down to southern
Illinois by Wed afternoon. A weak frontal zone and mid level forcing
extends from Chicago area northeast to The Thumb. This will be
focus of light showers from mid level cloud deck through the
remainder of the afternoon and effect mainly ptk-mbs. The southern
sites could receive a brief light shower or sprinkle also. Mid and
high level clouds tonight then light rain showers, MVFR ceilings and
visibilities move in from the southeast as developing surface low
over Ohio lifts northward. Winds tonight will be mostly light
southeasterly then increasing slightly and backing more to east
northeast Wednesday. Elevated instability will also increase
slightly Wednesday morning leading to the possibility of a
thunderstorm but probability at this time too low to include in tafs.
//Dtw threshold probabilities...
* moderate chance for ceilings aob 5kft Wednesday.
* Slight chance for thunderstorms Wednesday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.