Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
332 am EDT Tuesday Jul 7 2015
Short term...today and tonight
As of the 07z surface analysis...a cold front extends from Rogers City
Michigan to the northern Chicago suburbs. A rather good low level density
gradient for July exists across this boundary with a 10 to 20 degree
difference in temps/dewpts. This cold front will be driven across Southeast
Michigan today as a result of middle level height falls associated with a
middle/upper level trough axis traversing the northern Great Lakes.
Available observations are in support of model moisture profiles
showing an axis of deep layer moisture along this frontal boundary
/pwats around 2 in/. While the better middle level height falls will
occur to the north of the forecast area today...entrance region
upper jet dynamics will sustain good lower tropospheric
frontogenesis across the front as it works its way across Southeast Michigan. The
abundance of moisture in the prefrontal environment with the large
scale forcing will be sufficient to generate a healthy coverage of
showers. Given the narrow frontal circulation...this will likely
span a 3 to 4 hour time period along the surface front. This front
is expected to move across the Tri Cities between 12z and
15z...reaching Flint and The Thumb region around 16z and metropolitan
Detroit/Port Huron 19-20z. Isolated/scattered convection will likely continue
to precede the larger scale ascent along the lead edge of the middle
level moisture plume.
There has been some thunderstorms early this morning across SW lower
Michigan. Relatively warm air aloft is keeping middle level lapse rates
rather shallow...thus limiting the degree of elevated instability.
The better chances for thunderstorms will occur across the eastern half
of the forecast area where the later frontal passage will afford an
opportunity for the development of some surface based instability. Since
temperatures are already in the middle 70s ahead of the front...surface temperatures
around 80 across the far east seem attainable. This along with surface
dewpoints around 70 would generate around 1000 j/kg sb cape. Although
not spectacular...0-6km bulk approaching 30 knots this afternoon
combined with the potential instability may support some isolated
strong convection...generally east of a line from Sandusky to
Hillsdale. Given the narrow axis of tropical moisture along the
front...any convection will be capable of producing some very
Temperatures and dewpoints will rapidly fall with the surface frontal passage
today. Late day diurnal heating will at least support a rebound in
temperatures during the later half of the day in locals which experience an
earlier frontal passage. Post frontal strato cumulus should erode from north to
south during the afternoon and evening given the degree of dry air
advection. This expected clearing and upstream temperatures support MOS
guidance min temperatures down into the 50s tonight.
Upper level westerly confluent flow Wednesday morning will allow
large area of high pressure (1020 mb) to translate east into the
central Great Lakes with good subsidence and drying occurring...as
700 mb dew pt depressions rise to 40 to 50 c per 00z Euro/NAM. This
dry air will be important as lower amplitude wave comings out of
the Southern Plains and skirts through the Northern Ohio valley
Wednesday night into Thursday. Model consensus is for the surface
low to track south of the Michigan border...but the strength of this
low and subsequent moisture transport north still varying amongst
00z suite...see dry GFS vs wetter Euro/Canadian solutions. Seems
likely locations along and south of I-94 corridor will get clipped
with showers as 850 mb circulation tracks along/close to the
border...with diminishing chances farther north. Looking at highs in
middle 70s for Wednesday...as 850 mb temperatures rise to around 10 c.
Thursday's highs will be dependent how fast the clouds and showers
exit the area...with the potential for maxes to reach 80+ degrees as
850 mb temperatures end the day around 13 c.
Strong upper level ridge (594-595 dam at 500 mb) is forecasted to
build over the southeastern United States on Friday...continuing to
strengthen and retrograde to the west over the weekend.
Unfortunately...Southern Lower Michigan looks to be in the active
outer fringe of the ridge...with potential for showers and
thunderstorms over the weekend as ribbon of moisture (pw values of
close to 2 inches) rotates through.
a cold front passing through the central Great Lakes this morning
will produce showers and thunderstorms. Post-frontal winds this
afternoon into this evening look a bit stronger...in the 15 to 25
knot range...and have issued small craft advisories for
nearshore waters of Southern Lake Huron. Winds gradually diminish
tonight into Wednesday as high pressure builds across the region.
A period of modest easterly flow looks to develop over Lake Erie
as low pressure tracks through the Ohio Valley on Thursday.
Otherwise...light winds look to continue right into the weekend.
Aviation...issued 1157 PM EDT Monday Jul 6 2015
Moisture surging northward ahead of the cold front will continue to
support showers with the greatest coverage in the mbs area during
the pre-dawn hours...along with a few thunderstorms. Some IFR within
frontal zone will also be possible there as the front moves through
during the coolest part of the day. Daytime surface heating will
likely be able to prevent IFR development farther south/later in the
day but potential remains for solid MVFR ceiling with increasing
shower coverage and especially with the approach/passage of the cold
front. The Post frontal wind shift toward the north will bring a
rapid improvement in ceiling from middle Tuesday afternoon into Tuesday
For dtw... frontal timing at dtw in 15-18z time frame will be
subject to adjustments based on eventual coverage of showers during
the morning. Greater coverage will support faster timing...greater
intensity will support slower timing.
//Dtw threshold probabilities...
* high for ceiling 5000 feet or less late morning through afternoon
along and behind cold front.
* Low for thunderstorms along and ahead of cold front.
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).