Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
750 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 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...
* medium confidence in ceilings at or below 5000 feet Saturday.
Previous discussion...issued 353 PM EDT Friday Aug 29 2014
Short term...this afternoon and tonight
An upper level ridge continues to dominate the weather across
Southeast Michigan...as evidenced by the extremely slow eastward
progress of upstream showers and thunderstorms across the western
part of the state. Although we have moistened up across the area
today...southeast winds continue to feed stable air into Southeast
Michigan with continued positive Li values. Although a pv anomaly is
forecast to track around the periphery of this ridge late this
afternoon and early this evening...the stable atmosphere will make
generating convection difficult. Therefore will continue to hold
onto Chancy probability of precipitation across the northern half of the County Warning Area during this
period...with the best chances across the Saginaw Valley closer to
the pv anomaly itself. Clouds are expected to stay in place for much
of the night...keeping low temperatures in the middle 60s.
Long term...Saturday through next Friday
Upper wave spinning over eastern Nebraska/Kansas will lift into
Wisconsin tonight then start to shear as it lifts across northern
Michigan Saturday and Saturday night. At the surface...low pressure
looks to track from Wisconsin to northern lower Michigan...dragging
a cold front across lower Michigan during the afternoon and early
evening. This should trigger a round of showers and thunderstorms in
the warm and humid airmass ahead of it.
Plume of deeper moisture will arrive from the Gulf of Mexico by
late Saturday as the wave draws closer...and has been depicted in
models as a surge of 2 inch precipitable water values and Theta-E ridge washing over
the area. This moisture can already be picked on on water vapor
imagery as it is starting to surge up into the southern Mississippi
River valley this afternoon. Showers and thunderstorms look likely
by afternoon as a weak cold front approaches. Severe potential looks
isolated with ml cape values only in the 1000-1500 j/kg range and
with fairly skinny cape profiles. Main threat will be damaging wind
gusts to 60 miles per hour given fairly healthy and unidirectional wind fields
and inverted v look to soundings through the first half of the
evening. Temperatures will be quite warm tomorrow as we reside in
the warm sector with h850 temperature rising to 16-17c. If we get enough
insolation during the day there is potential to overachieve on maximum
temperatures with a healthy mixed layer bringing some of this warmer air
down. The warm air and humidity will combine to produce heat index
values in the upper 80s to near 90. Winds will also be gusty ahead
of the front from the southwest...with gusts to 30 miles per hour expected to
mix down to the surface.
Frontal boundary should settle south and east of the area on
Sunday...although it may remain close enough to the Ohio border to
bring a chance for a shower/thunderstorm south of the M-59 corridor.
Could also see a storm or two over The Thumb where lake breezes
should advance inland during the afternoon. By late
afternoon...shortwave ridging will build into the area...bringing
subsidence and a capping inversion that should end any chances for
convection. The front that works through the area Saturday will be
weak...and temperatures on Sunday are still expected to remain in
the 80s...with heat index values in the middle 80s to near 90 as
surface dewpoints remain in the 60s.
Another low pressure system and cold front will approach the area on
Labor Day...bringing rain and thunderstorms to the forecast for the
late afternoon and evening. Much turbulence can be found at upper
levels ahead of 500mb height falls moving across the Midwest. As the
wave moves through by early Tuesday...surface high pressure and a
stabilizing upper air environment will bring tepid weather. Just
small chances for a quick shower are possible for the remainder of
the extended forecast as zonal flow sets up along and south of a
consistent latitudinal jet near the US/can border.
Low pressure will move from the Central Plains to the western Great
Lakes tonight...then lift up through western and northern lower
Michigan on Saturday. This system will produce moderate south to
southwest flow over all marine areas by Saturday afternoon...however
gusts look to remain at or below 23 knots. Winds and waves over the
nearshore areas are expected to be highest over Saginaw Bay where
the southwest flow will have a bit of a funneling effect. Waves
there are expected to remain below 4 feet however. The low will also
pull a weak cold front across the area...triggering showers and
thunderstorms Saturday into Saturday night. Quieter weather is then
expected on Sunday as high pressure briefly builds into the area.
Another low pressure system will approach for the end of the Holiday
weekend...bringing renewed chances for showers and thunderstorms on
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).