Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
101 PM EDT Wednesday Aug 20 2014
IFR stratus has mostly lifted this afternoon to MVFR levels. This
trend will continue for a few hours into this taf period. Visible
satellite shows many areas to the west have cleared the stratus and
developed MVFR cumulus clouds. Bases should lift above 3000 feet
with time into this evening...but there will also be a chance for
showers and isolated thunderstorms to briefly lower visibilities. Have timed out
best chances for precipitation in the tafs...generally 18-01z. Flow is
rather weak...so storm motion will be slow /west to east at 15kt/.
Patchy fog should develop again tonight under light flow and high relative humidity.
//Dtw threshold threats...
* high confidence in ceiling below 5000 feet this afternoon.
* Low confidence in thunderstorms occurring.
Previous discussion...issued 357 am EDT Wednesday Aug 20 2014
Short term...today and tonight
Showers and thunderstorms have tracked east as a cold front has
swept through the central Great Lakes. However this cold front will
not bring much relief with regards to moisture as dew points are
still in the lower to middle 60s. The lack of dry air behind the
front and fairly calm winds brings with it the potential for fog
across the area. Lingering clouds should delay the onset of fog
formation...with areas to the east probably not going down until
around sunrise. Although there are expected to be areas of dense fog
this morning...it is not expected to be widespread enough to warrant
a dense fog advisory at this point. However if visibilities drop
significantly will issue the appropriate headlines as needed. The
lingering moisture may still spark off some light shower activity
early this morning as a ribbon of high low level Theta-E air remains
over Southeast Michigan.
Daytime heating will once again cause an unstable atmosphere as it
combines with a still moist boundary layer to produce MUCAPES of
1500 to 2000 j/kg. This should once again spark off thunderstorms
across the area as the middle level low pressure system tracks through
the Soo today. However unlike yesterday 0-6km bulk shear values will
be much weaker at around 20 knots...making the threat for severe
weather much lower. Storms today may produce wind gusts up of 40 to
50 miles per hour with hail around one half of an inch up to dime size with the
Although the middle level low will remain over Lake Huron
overnight...rain chances will drop off with the loss of daytime
heating. Rain should be confined to the areas near the low which
will be The Thumb region...with chances creeping in across the
extreme southern County Warning Area late tonight as a quick moving shortwave tracks
through the southern Great Lakes.
A longwave amplified ridge axis will build aggressively into
northeastern North America during the next 48 hours. It GOES without
saying...but an extremely atypical event for this Summer. What this
will do is essentially throw a curve Ball to the local weather
pattern...likely bringing in the largest run of Summer warmth this
region has seen all season long. Before this can really become
established during the latter half of the weekend...southeastern
Michigan will have to contend with an internal ridge structure and
secondary jet core. It is this secondary jet overhead that will
direct shortwave energy and potential ridge rider type mesoscale convective system into the
area likely anytime through Saturday. An active period is in store
through the end of the week.
Thursday...a lower geopotential height anomaly will build off of
Lake Huron and into the northeastern United States during the day on
Friday. It is the combination of this action and an upper level jet
core cresting the ridge axis over the plains/northern Mississippi
River valley that that will freeze the height pattern over lower
Michigan. Quite the gradient in temperatures is expected with a
stationary frontal boundary draped from portions of Ohio
northwestward to central lower Michigan northwest to portions of the u.P.
The frontal boundary is not expected to move much keeping the heat
to the southwest. Models keep the bulk of the County Warning Area dry...although the
frontal boundary will bring the potetnial for showers and
thunderstorms particulalry south and west of M 59. Temperature
should tick above the 80 degree mark for many areas.
The aformentioned 300mb jetlet is forecasted to crest the ridge
Thursday evening and slide directly into southeastern Michigan
Thursday night/Friday morning. Models are extremely noisy...but
overall there is a very good signal that an mesoscale convective system could build across
the northern misssippi River Valley Thursday and release towards the
area Thursday night. Alot of time to figure this out...but degree of
instability immediately to the south due to strong lapse rates aloft
could lead to severe weather risk Friday morning. One thing to
monior is the mesoscale convective system will likely organized/intensify in an area of
strong left exit region dynamics. The overall system is then shown
to outrun the upper level jet support by diving to the south.
Friday...Friday is a difficult forecast period. Will likely need to
combat subsidence and midlevel latent heating leftover from the mesoscale convective system
impacting the region. Upper level jet core will maintain enough
integrity to bring pesky shortwave energy into play. Models support
advecting high Theta-E into the state enough for what could end up
to be a very high cape setup.
A weak area of low pressure will slide across lower Michigan today
bringing a weak southeasterly wind to many areas. A second weak area
of low pressure will push into the northern Great Lakes on
Friday...maintaining a rather sizable surface trough structure over
the Great Lakes region. Outside of continual shower and thunderstorm
chances...an extended stretch of marine weather is anticipated.
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).