Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
403 am EDT Friday Jul 25 2014
Short term...today and tonight
Surface high pressure center will shift from Western Lake Erie to
the middle-Atlantic coast by this evening. The proximity of the high
should still keep quiet weather over the area today...although
cirrus...diurnal cumulus...and debris off convective complexes over the
upper Midwest and northern plains will increasingly fill the sky.
Expect precipitation from storm complexes currently over Iowa and
Minnesota/South Dakota to dive southeast of the area for the most
part. Any remnants that do try to make it in will encounter dry air
(as evident on the 00z DTX sounding) and subsidence from the exiting
high. Temperatures today will continue to warm as thermal fields get
a boost by an increase in southwest flow late today. This should
push maximum temperatures to the middle 70s to 80 today...despite the
chilly start to the day.
Surface low pressure over the northern plains will gradually Glide
east tonight as the upper low over Alberta slides into Saskatchewan.
This will allow flow over Michigan to turn southwesterly during the
latter part of the day into tonight. Will initially see only a slow
increase in moisture until late tonight when the elevated portion of
a warm front and 30-40 knot low-level jet lift into Southern Lower
Michigan. Healthy Theta-E ridge will also begin to wash over the
area late...boosting precipitable water values to just over 1.5 inches. Expect
scattered showers and thunderstorms to develop as this works through
overnight. Model timing is still very divergent on when storms
should arrive from the west or develop overhead. Best timing looks
to coincide with the arrival of better forcing and moisture after
midnight. 0-6km bulk shear values will start to rise late in the
night...with values of 40-50 knots on the NAM/GFS by 12z Sat. This
will bear watching...but overall...think the limited elevated
instability will mitigate the threat for severe weather overnight.
Warm air advection...increasing dewpoints and cloud cover/rain will
bring much warmer temperatures overnight. Lows are only expected to
fall into the 60s.
The forecast for a potentially active convective pattern remains on
track for the weekend with prospects likely better Sunday compared
to Saturday. There are no big changes to expectations in the 00z
model data on the position and strength of the large scale weather
features through the period. By Saturday morning...the surface
warm front is shown to extend through northern lower Michigan and
Central Lake Huron from low pressure near the Minnesota arrowhead
which will move east and pull a weak cold front through lower
Michigan during Saturday afternoon. Confidence is high that
showers and storms will trail the exiting warm front and low level
jet pattern from Friday night into Saturday morning over Southeast
Michigan...but the character of that activity could range wide on
coverage and intensity either because of or despite the capping
influence of the warm sector as it settles into the region. The
source of the warm air in the low level temperature pattern seems
tied to larger scale subsidence along and north of the upper level
jet axis which favors convection tied more closely along and
North/East of the warm front. Assuming lower end coverage of
morning activity holds...then minimal mesoscale influence will not
interfere with development of instability for potential afternoon
development...although clouds may prove a different story. Soupy
boundary layer air and a slow start to surface heating will
continue to make the subsidence inversion a limiting factor.
Similar depictions are shown in the NAM...GFS...and sref mean
model soundings with readings as high as 13c at 750 mb difficult
to get around given the weak cold front indicated in both
temperature and pressure fields. Even if maximum surface temperatures can
push the upper 80s...a surface dewpoint near 70 will be difficult
to improve on for a more favorable parcel...but lower 80s/upper
60s still generates cape around 3000 j/kg with the core of the
upper jet offering 50 knots of mostly straight-line wind shear.
Considering these hefty doses of ingredients...careful monitoring
of severe weather potential remains a requirement considering
better large scale vertical motion support is so nearby to our
south late in the day and into Saturday evening.
Satellite depiction of the low coming out of the Canadian rockies
today fully supports the aggressive model solutions on the system
for Sunday. Short wave ridging out ahead of the system will keep
things on the quiet side late Saturday night through Sunday
morning...but a generous return flow of moisture ahead of the
surface low will set a better stage for convection Sunday afternoon
into Sunday evening. This will be a much more strongly forced
scenario compared to Saturday due to the upper circulation but with
similar moisture and instability parameters. The magnitude of the
wind field aloft will be weaker but adequate with the added
component of backed flow and strong turning in the low levels of the
hodograph supplied by the front returning northward and the surface
low sliding through lower Michigan. This increases the prospects for
severe convection during peak heating and the going likely pop
forecast looks solid from both a timing and areal coverage
perspective before the system sweeps the activity out of the area by
Thunderstorm activity will increase tonight and then last off and on
through the weekend. There will be potential for severe weather with
the storms both Saturday and Sunday which will hamper otherwise
favorable marine conditions in terms of wind and waves until Monday.
That is when a strong cold front will usher in another round of cool
air over the Great Lakes. Moderate to fresh north winds with gusts
to 25 knots or slightly higher will be common by Monday.
Aviation...issued 114 am EDT Friday Jul 25 2014
Conditions will remain VFR with only cirrus increasing overnight
into Friday. A further increase/lowering of ceilings at or above 15kft will
occur late in the forecast as high pressure settles to the south and
a wave of low pressure encroaches on the area. Any shower activity
from this system will hold off until at least after 06z. Light and
variable winds will become southwest...but remain 10 knots or less
for the most part...even during best mixing Friday afternoon.
//Dtw threshold threats...
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).