Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
602 am EDT Friday Sep 4 2015
Clouds have held firm over much of the area overnight reducing the
coverage/intensity of fog across the taf site. Winds around 5-7
knots have also proven enough to keep more of a low stratus threat
thus far. Will be a bit optimistic and favor prevailing VFR/MVFR
with some IFR not out of the question. The low stratus and fog will
lift this morning and attention then turns to precipitation chances
once again. Chances are lower today as the main forcing has pushed
east but there remains a chance of showers and thunderstorms through
tonight. At this point with no clear forcing to key off...coverage
may be too scattered to include precipitation in the taf. Best chance will
come this afternoon with a diurnal boost. Fog may again become an
issue late tonight and Saturday morning.
For dtw...low confidence forecast this morning as observations have
been either VFR or IFR. Conditions will improve through the morning
as mixing increases. Showers and thunderstorms will be possible
today with best coverage to the north of the terminal. Low
confidence on timing when precipitation would actually hit the site
so will hold off on including at the moment. Look for fog to develop
once again tonight.
//Dtw threshold probabilities...
* medium for ceiling at or below 5000 feet through the taf period.
* Low for thunderstorms through the taf period.
Previous discussion...issued 330 am EDT Friday Sep 4 2015
Short term...today and tonight
Another day with the stacked low pressure system wobbling about
lower Michigan. The main lobe of vorticity strung out north to S got a
nocturnal boost from the 850mb jet early in the overnight enhancing
the thunderstorm activity across the southern County Warning Area. The vorticity maximum has
since pushed east aligning along the Detroit and St Clair rivers.
This feature along with the region of deformation wrapping around
the low appear to be the main forcing mechanisms for precipitation chances
through tonight with the lack of any real flow and bulk of the
instability falling south of the area. The cyclonic flow will try to
pull some instability back northward along the lobe of vorticity
this afternoon adding to coverage. Precipitable waters are prognosticated to be slightly
less today than yesterday but still very healthy above 1.5 inches.
Main update for today is to realign the probability of precipitation to the
forcing...keeping likely probability of precipitation east by the forcing and north closer
to the center of the low and chance elsewhere for broad lift. Severe
weather is not anticipated due to lack of flow and forcing but
abundant moisture will result in locally heavy rainfall in any
thunderstorm that develops. The strongest storms will be capable of
wind gusts to 45 miles per hour primarily due to precipitation loading.
As the upper low lifts northeastward away from lower Michigan
tonight...the deformation band on the backside will slide through
the area. Will continue a chance pop until the extent of the precipitation
can be better defined but may be able to trim out some of the
Southeast Michigan will remain engulfed in the amplified subtropical
ridge through early next week. 850mb temperatures will hover between
17-19c through this time indicating high temperatures likely
confined to the middle 80s to low 90s largely contingent on cloud cover
and precipitation trends each day. Likewise, dewpoints will remain elevated
near 70 degrees which will effectively put a floor on overnight lows
and ensure muggy conditions until at least the middle of next week.
Moderate destabilization of 1500-200 j/kg MLCAPE is still on The
Table for Saturday with thermodynamic forecasts indicating unimpressive
capping. Middle-levels will remain cooler with the dampening closed
circulation still present in some respect over the central Great
Lakes, and this will be sufficient to move forward with the
inherited chance probability of precipitation - lack of stronger forcing precluding a more
aggressive pop. Ambient winds will be weak and poorly sheared. Any
convection will be Ordinary and the primary threat will once again
be heavy rain due to sluggish storm motion.
Essentially a modified persistence forecast then for Sunday to
account for continued decline of resident forcing in addition to
warming middle-levels, particularly above 700mb. Convectively enhanced
shortwave lifting across the Front Range during the predawn hours
this morning will be steered around the periphery of the ridge and
may be sufficient to trigger scattered convection around the edge of
the cap, which would favor northern areas on Sunday afternoon. For
now, will introduce a chance pop but will confine it to the waters of
Lake Huron for now.
Thunderstorms will essentially remain a daily threat across the
waters through at least the middle of next week. Otherwise, light to
moderate winds predominantly out of the south will be common into
next week. A more concerted moderate southwest wind will develop
in advance of an approaching low late Sunday into Monday but wind
gusts are expected to be no more than 20 knots at this time.
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).