Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
707 PM EDT sun may 3 2015
VFR conditions will hold through tonight with scattered-broken middle clouds
generally at or above 10kft ahead of approaching shortwave trough. This
trough will bring a cold front into the area late Monday and with
this feature...expect scattered rain showers and MVFR ceilings at times by the
end of the forecast period. In fact...initial activity will likely
work into kmbs...kfnt and perhaps kptk late tonight into Monday
morning before fading. Additional activity will then develop during
the coarse of Monday afternoon as instability in warm sector rises.
Wind gusts will end within an hour or two of the forecast start time
as daytime mixing is lost. However...southwest winds will gust to 20
knots or so by late morning to middle/late afternoon Monday in advance
of the cold front.
For dtw...cold front will move into area late in the forecast 00z
Monday or after...but prefrontal trough will bring scattered rain showers to the
terminal at least by the afternoon. Initial activity in the morning
will be dissipating as it moves south into area...so still expect
little more than sprinkles until after 18z-20z. While a thunderstorms and rain or two
are possible...confidence remains low given only weak instability.
//Dtw threshold probabilities...
* medium for ceilings at or below 5000ft Monday afternoon and evening.
* Low for thunderstorms and rain showers Monday afternoon.
Previous discussion...issued 341 PM EDT sun may 3 2015
Short term...through tonight
High pressure is holding on over Southeast Michigan while the approaching upper
level trough currently resides over the Minnesota/WI southwest into the
Central Plains. Cirrus debris has been a bit thinner than
anticipated while the SW winds have been solidly gusting to 20-25
knots which has lead to relative humidity values falling into the upper 20 percent
range across the area. This combined with the cirrus has helped make
it difficult for a more widespread cumulus field to develop.
Regardless...strong warm air advection caused temperatures to soar quickly into the upper
70s to low 80s over southern Michigan but increasing cloud cover will help
moderate that the rest of the evening.
The trough...extending southward out of the parent low in Ontario...
will continue to slide eastward while energy rides NE along it into
Ontario through the overnight. The cold front will not reach Southeast Michigan
until after 12z but prefrontal activity in a region of higher Theta-E
and a sheared ribbon of vorticity will reach the area between 06-12z
early Monday morning. This will warrant a chance of showers and
thunderstorms over the north and west portions of the County Warning Area. Models
advertise several middle level shortwaves traveling in the weak
westerly midlevel flow around the ridge centered to the southeast. The
first of which is currently producing a compact area of convection
over the Chicago area. It is expected to track east then start
turning southeast as the vorticity maximum weakens tonight thus have kept the
southern counties dry though there may be some thicker cloud debris
for a bit later this evening as a result of it.
Any thunderstorms that do develop within the area of showers late
tonight are not expected to be severe as shear will be
weak...generally less that 20 knots...forcing will be minimal. Cape
values will only be around 100 j/kg this far ahead of the front. The
upper level jet will be well to the north over Ontario and the ll
jet loses its organization while pivoting more west to east as it
drifts from WI over Lake Michigan through the night. So not a lot to work
with tonight ahead of the front.
Increasing cloud cover and an increased wind field tonight will help
keep low temperatures quite mild from about 55-60f.
Long term...Monday through Saturday
Moisture axis (850-700 mb Theta-E ridge) will be sliding through the
central Great Lakes tomorrow...but lack of upper level forcing (500
mb heights actually rising during the day) draws concern for
precipitation chances...as middle level flow becomes
westerly...becoming parallel with the surface cold front...which
still looks to be getting tied up around the southern Michigan
border Monday night. The 12z NAM is Adamant there will be showers
and thunderstorms tomorrow ahead of the cold front...as surface
based convective available potential energy flare up to around 1500 j/kg before convective scheme
kicks in. However...surface/low level moisture looks to be overdone
(forecasted lower 60 dew pts). In fact...even at the present
time...NAM appears to be a bit overdone over the Midwest with low
level moisture...and thus enough concern to hold probability of precipitation at inherited
value...likely probability of precipitation. Even so...majority of the day should be
dry...but timing difficulties preclude separating morning/afternoon
hours. Airmass potential is for lower 80s tomorrow based on 850 mb
temperatures in the lower teens...but clouds and convective activity will
likely be sufficient hold maxes in the middle 70s to around 80 degrees.
Stalled out frontal boundary along or just south of the Michigan
border Monday night through Tuesday night...leading to good low
level convergence as moisture axis wraps around the large Bermuda
high. Some upper level impulses ejecting from Four Corners region or
convective vorts to support a good chance for convective activity
during this time frame...riding along the front. Thus...likely probability of precipitation
toward the Ohio border seem in order...but exact timing and location
is uncertain...as nocturnal low level jet will provide
opportunities...along with peak heating/maximum instability of the
afternoon hours. Being on the north side of the front....low level
easterly flow will keep temperatures in check...holding in the low/middle 60s
on Tuesday...with the potential to be even bit cooler if good
coverage of shower activity does in fact materialize.
Frontal boundary will lift northward through the local area Wednesday/Wednesday
night as a warm front. A few showers cannot be ruled out but this
process looks to be mostly dry with no upper support for forcing.
Deeper low/midlevel southerly flow develops Thursday/Friday boosting
low-level temperatures and moisture again. No trigger for precipitation
evident until Friday when the upper ridge begins to break down.
Chance probability of precipitation for Friday into next weekend.
Mild air in place over central Great Lakes ahead of a cold
front...which will drop south through the region
tomorrow...triggering scattered to numerous showers and
thunderstorms. Winds will shift from the south-southwest to the
northwest...but generally remain under 20 knots. High pressure will
then build into the northern Great Lakes Monday night...leading to
light winds Tuesday and Wednesday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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