Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
358 am EDT Sat Aug 30 2014
Short term...today and tonight
Upper level trough over the Midwest early this morning...with main
shortwave tracking through Wisconsin this morning and into the u.P.
Even so...substantial height falls over Southeast Michigan today
coupled with another surge of moisture (secondary 850-700 mb Theta-E
ridge)...pushing precipitable water values to near 2 inches...which should be
sufficient to generate numerous showers and thunderstorms as daytime
instability sets in...with MLCAPES of at least 1000 j/kg. Bulk wind
shear is notable...0-6 km shear of 40 knots...with even 20 knots
down low...20 knots in the 0-1 km layer. So...if instability is a
bit more robust than indicated...than severe storms will be a
concern. Even though freezing levels are high (14+ kft)...maximum cape
density is located around that level...so hail and especially wind
are a threat (dynamic Bow echoes/mini supercells)...along with
the slight chance of tornadogenesis...per local severe checklist
using 00z NAM. However...middle level lapse rates (700-500 mb) are
pathetic and probably the saving Grace/deterrent. Regardless...heavy
rainfall will occur with any thunderstorm...as 850 mb dew points reach
There is a second strong shortwave coming out of the eastern
Dakotas...which is expected to move through northern lower Michigan
tonight. This feature will likely sustain/support additional showers
and thunderstorms overnight with large scale forcing and still
sufficiently moist low levels with waning/modest instability.
Maxes today will be dependent on initiation time of convection...but
middle 80s appear likely with strengthening low level southwest flow
and observed maxes near the southern Michigan border yesterday.
Elevated mins in the upper 60s tonight as surface cold front
mostly washes out over Southern Lower Michigan.
Long term... Sunday through next Friday
Low pressure moving through the u.P. Today will pull a weak cold
front into lower Michigan Sunday. The forecast challenge will be in
how far south the front will be able to travel before it moves back
north as a warm front Sunday night. There is also the question of
shower and thunderstorm potential along the front and in the warm
and humid air mass lingering over the rest of Southeast Michigan.
The best chance of rain will be early in the morning near the rivers
and Ohio border before the main low level moisture axis exits to the
east. After that...the latest model runs are in general agreement
with the position of the cold front stalling west to east through
the Saginaw Valley and northern thumb by afternoon. There is also a
noticeable difference in the amount of middle level dry air this run
compared to the last few model cycles. This appears to be a product
of increased subsidence under the short wave ridge aloft which has
greater amplitude in the latest 00z solutions due to the shortening
wavelength between the eastern Great Lakes wave and today's West
Coast trough moving into the plains by then. Considering the models
likely have a better handle on the strength of the upper jet driving
the Pacific trough...the trend on its eastward timing seems reliable
enough to expect it will hold in upcoming forecast cycles. That
being the case...the large scale subsidence is expected to be
adequate to minimize convective potential both along the front and
within the warm and humid air mass to the south through Sunday
afternoon and evening. No more than an isolated storm is expected
during peak heating as convective inhibition builds through the day.
Surprisingly...NAM model soundings show little to no capping
inversion compared to what would otherwise be expected based on the
character of the 500 mb height field and mean relative humidity profile. The GFS
looks better with a deeper layer of warm air between 800 mb and 600
mb but its surface parcel looks too dry. The GFS sounding modified
to 83/68 barely sneaks by the inversion with cape around 1200 j/kg
by late in the day. On the other hand...both the NAM and GFS may be
underdone in the amount of middle level subsidence warming judging by
the most recent sref mean and the majority of its members that show
a solid cap in place through the day. This supports shying away from
the NAM solution of generous convective development but is not good
enough to completely remove probability of precipitation from the forecast.
What is left of the cold front Sunday will move quickly back north
as a warm front Sunday night as the next low pressure system moves
into the Midwest. Dry air lingering over the region will ensure a
break in the precipitation action as low to middle level flow gradually backs
toward the south. Satellite imagery today already supports model
depictions of a larger and more dynamic wave compared to the weekend
system. The added strength will allow it to quickly retap the deep
Gulf moisture plume that was shunted into the Ohio Valley and
Southern Plains. The return of this moisture...combined with the
more impressive dynamics of the wave...will make showers and storms
likely by Monday afternoon through Monday evening. The European model (ecmwf) and GFS
have come into better agreement on the timing of the trailing cold
front during Tuesday and show it exiting lower Michigan by Tuesday
night. A dry start to the second half of the week looks solid in the
global model solutions given the breadth of the larger scale upper
ridge and surface high...but predictability tails off toward the end
of the week with the next low pressure system organizing anywhere
from the Central Plains to central Canada.
Low pressure will move from central Wisconsin today through northern
lower Michigan by tonight. This system will produce moderate south
to southwest flow over all marine areas...but with gusts generally
below 25 knots. Saginaw Bay will be most prone to the elevated wind
conditions as the flow channels down the Bay. A solid 2 to 4 foot
wave pattern will result. The low will also pull a weak cold front
across the area...triggering showers and thunderstorms today through
tonight. Quieter weather is then expected Sunday as the weak front
stalls through Saginaw Bay and returns north as a warm front Sunday
night. This will occur ahead of another low pressure system that
will approach for the end of the Holiday weekend...bringing renewed
chances for showers and thunderstorms Monday into Tuesday.
Aviation...issued 1152 PM EDT Friday Aug 29 2014
Large amplitude upper level ridging will keep stable conditions in
place throughout the overnight. A persistence of southeasterly flow
across the region will continue to make it difficult for moisture to
advect into place. A secondary warm frontal feature is forecasted to
lift through southeastern Michigan during the late morning hours on
Saturday. Precipitation chances are expected to increase
significantly once this feature moves through. With the surface
pressure gradient increasing and the boundary layer mixing
out...southwesterly surface winds are expected to increase to 12
knots and above by Saturday afternoon.
//Dtw threshold threats...
* low confidence in ceilings at or below 5000 feet Saturday.
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).