Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
700 am EDT Monday Jul 6 2015
Lingering ground fog across the area will lift between 12z and 13z
as daytime heating gets underway. Otherwise...very dry air across
the area will sustain clear skies below 20k feet through the evening.
Light winds this morning will slowly give way to increasing
southerly winds during the course of the afternoon in advance of a
slow moving cold front moving across Wisconsin.
//Dtw threshold probabilities...
Previous discussion...issued 322 am EDT Monday Jul 6 2015
Short term...today and tonight
Southeast Michigan will lie between a weak upper low meandering across the
upper Ohio Valley/cntl Appalachians and an amplifying northern stream
trough/frontal system sliding from the upper Midwest into the western
Great Lakes today through this evening. Enhanced large scale
subsidence to the northwest of the upper Ohio Valley low has led to
an arc of warm middle level temperatures across Southeast Michigan. This cap is shown
nicely on the 00z DTX sounding. These warm middle level temperatures will hold
across Southeast Michigan today through this evening...supporting a dry forecast
and likely suppressing diurnal cumulus development. So outside of a few
cirrus...ample sunshine can be expected today. The speed of the
upstream frontal system has slowed somewhat compared to previous
model solutions. This will result in a slower departure of the
surface ridge extending across the eastern Great Lakes today and
slower arrival of the better SW flow/low level warm air advection.
This warrants lowering forecast afternoon highs down a couple
degrees...with low to middle 80s across much of Southeast Michigan /upper 80s
still possible across the Saginaw Valley where low level SW flow
strengthens late in the afternoon/.
The approach of the frontal system will increase the low level S/SW
flow tonight /925mb wind forecast to increase to 30-40 knots overnight/.
This will inhibit the degree of decoupling in the boundary layer.
This combined with low level warm air advection and rising surface
dewpoints overnight will support mild min temperatures /around 70 in most
places/. Although the surface cold front is not forecast to move into
cntl lower Michigan until 12z Tuesday...the lead edge of the deep layer moist
axis will move into Southeast Michigan late tonight. This will support a chance
of some showers /possible thunderstorms/ prior to daybreak.
Relatively shallow middle level lapse rates and lingering warm middle
level temperatures will however be big limiting factors to convection
tonight...especially across the eastern half of the forecast area.
Strong upper wave tracking through far northern Montana early this
morning...slipping through Lake Superior and into Ontario during
Tuesday. Bulk of upper level support will be north of Southeast
Michigan...but strong middle/upper level westerlies to keep the front
progressing east through the central Great Lakes during
Tuesday. Moisture axis/850-700 mb Theta-E axis prognosticated to be
along or east of US-23 by 18z Tuesday...with very impressive
drying then occuring toward 00z...as precipitable water values fall from 2 inches to
around 0.5 inch. The 00z NAM looks way overdone with the
instability. The surface cold front is displaced from the middle level
cold air...as 500 mb temperatures look to be running around -5 c.
Thus...middle level lapse rates (700-500 mb) will be weak...around 5
c/km. NAM is surging surface dew points into the middle 70s...which is
probably a good 5 degrees too high...along with the surface temperatures
which are being advertised to top out around 90 degrees. At the
other extreme...the 00z GFS is showing barely any instability...with
sbcapes only rising to around 500 j/kg. This does not give a lot of
confidence in showers/thunderstorms...but low level convergence
looks good and due to the high amount of moisture and favorable
timing (though not ideal)...high likely probability of precipitation will be
forecasted...with a northwest-southeast high temperature gradient
of middle 70s to middle 80s.
High pressure will pass through the central Great Lakes on
Wednesday...with 850 mb temperatures in the high single numbers (per 00z
euro) pointing to highs in the low to middle 70s.
Upper wave in Four Corners region and associated moisture will
quickly be racing off to the northeast...with associated surface
low/reflection forecasted to skirt along the southern Michigan
border on Thursday...with the 00z Canadian even faster...supporting
warm frontal precipitation late Wednesday. As tough as it is to
believe with the amount of dry air which moves into lower Michigan
Tuesday night/Wednesday...an increasing chance of showers seems
warranted Wednesday night into Thursday with the big increase in
Warm southerly flow today between 10 and 20 knots ahead of a cold
front...which will move through tomorrow...triggering showers and
thunderstorms. Northwest to north winds behind the front look to top
out around 20 knots Tuesday evening...but then diminishing Tuesday
night into Wednesday as high pressure returns. Low pressure will
lift up from the south as we head toward Thursday...but the strength
of this low is in question.
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).