Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
313 am EDT Tuesday Oct 25 2016


A cold start to the day, as temperatures currently reside in the 30s
under mostly clear skies. Surface high/ridge axis over western Great
Lakes this morning to slide east today as upper level ridge builds
through tonight. A back door cold front attempts to sink south from
Northern Lake Huron this morning as strong high (1037 mb) builds
over northwest Ontario, and looking at more low clouds across
northern half of the cwa, underneath/closer to the cold pool (-4 to
-5 c at 850 mb), as just about every model (rap/NAM/GFS/hrrr)
indicating saturation at 850 mb sinking south during the day before
mixing out. 925 mb temps ranging from 1.5 c north to 4 c toward the
southern Michigan border suggestive of highs in the upper 40s to
lower 50s, in line with latest mav guidance.

Upper level energy working through The Rockies this morning, and
there has emerged some differences for Wednesday, as bulk of the
upper level energy/pv drops into the southern Mississippi River
valley. Subsequently, the surface low over the Midwest is not
forecasted to be as strong. Still plenty of trailing upper level
support and moisture advection to support developing rain Wednesday
evening/night, but 00z NAM draws into question the daytime hours of
Wednesday, as the 700 mb front becomes active just over the north
third of the cwa, with no precipitation associated with the 850 mb
fgen indicated for southern two thirds of the cwa. The NAM looks to
be an outlier compared to Canadian/GFS/Euro, as those solution also
show an active 700 mb zone with about 5.5 g/kg of specific humidity,
but slicing right through the center of Southeast Michigan,
supportive of widespread rainfall by the afternoon hours. Also, with
the weaker low, less of a surge of warm air, and a low track south
of the Michigan border, potential precipitation type issues still
lurking along/north of I-69. With that said, still will favor all
rain with easterly flow coming off the mild waters of Southern
Lake Huron, as raw surface temps (per colder Euro/GFS solutions)
forecasted to reside predominately in the upper 30s to lower 40s.
As the low passes east Wednesday night, more of a northerly
drainage wind (less influence from Saginaw bay) setting up across
Tri-Cities region could make for a close call with respect to wet
snow mixing in before rain tapers off. Even during the day on
Thursday, cold cyclonic flow with 850 mb temps of -4 c could
support convective showers of graupel with the steep low level
lapse rates as highs reside in the 45 to 50 degree range.

For the weekend, model consensus splits the upper level energy, and
looking at two weaker low pressures sliding through the Great Lakes
region, which is a significant deviation from last night's single
stronger low tracking through. With the energetic Pacific flow still
well offshore, still time for additional model adjustments, as
Canadian solution is much faster and weaker compared to Euro/GFS.
Also, Hurricane Seymour in Pacific could have a modest influence as
it interacts with the strong upper wave/trough approaching
California Thursday night.



Cold northwest flow will maintain unstable conditions across area
waterways today. Small Craft Advisory conditions will persist
during this time for all near shore waters of Lake Huron including
outer Saginaw Bay. Gusts of 25 to 30 knots will be common across
the open waters, but winds are expected to remain below gale force.
Winds and subsequently waves will gradually diminish late today and
tonight as high pressure builds into the region. Winds will
increase out of the east Wednesday and Wednesday night as low
pressure lifts toward the region. Strongest winds expected
Wednesday night and early Thursday, where gusts of 25 to 30 knots
are again expected. While the potential for a period of near gale
force is still possible over portions of the open waters, the
current forecast maintains gusts below gales at this time.



A low pressure system tracking from the Central Plains through the
Northern Ohio valley will result in a widespread rainfall event
locally Wednesday through Thursday. The heaviest rainfall will
occur late Wednesday into Wednesday night. Rainfall will taper off
in both intensity and coverage on Thursday. The current forecast
calls for a broad area of rainfall amounts around three quarters of
an inch.


Previous discussion...
issued at 1153 PM EDT Mon Oct 24 2016


A ridge of Canadian surface high pressure is now expanding into the
western and central Great Lakes. Northwesterly flow aloft remains
strong enough to keep a light, mixed northwesterly wind at the taf
sites. Stratocumulus continues to stream southeastward from northern
lower Michigan but is largely east of taf sites over Saginaw Bay and
Lake Huron. Otherwise, sky clear skies and flat diurnal cumulus at 3.5 kft
agl by late Tuesday morning.

//Dtw threshold probabilities...

* high confidence in scattered cigs aob 5kft by late Tuesday morning.


DTX watches/warnings/advisories...
Lake Huron...Small Craft Advisory until 8 PM EDT this evening for lhz421-441>443.

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.

National Weather Service Glossary of Abbreviations