Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1147 PM EST Sat Nov 28 2015
VFR conditions will prevail as a ridge of high pressure remains in
place across the area. With this ridge...winds will also remain on
the light side...average 5 knots or less throughout the forecast.
//Dtw threshold probabilities...
Previous discussion...issued 345 PM EST Sat Nov 28 2015
Short term...this evening and tonight
The southern branch of the main tropospheric westerlies remains
draped over southeastern Michigan this afternoon. A lack of
equaterward progression to the jet axis has put the brakes on the
southward drift of the midlevel frontal boundary and frontal cloud.
Nwp supports one jet impulse aloft to work directly through semich
during the next 6-9 hours with the shortwave ridge axis and
associated supergeostrophic region of westerlies pushing eastward.
Looking at right entrance quadrant dynamics for a brief period this
evening...before modest height rise bubble allows a narrowing
surface ridge axis to expand southward through semich and the
southern Great Lakes. A mention is made of the right entrance
dynamics because of a sudden expansion of midlevel cloud over
portions of IA/WI/IL. There is some low potential for cloud
maintenance or a cloud increasing trend this evening south of I 94.
The overwhelming narrative for tonight is one of decreasing clouds
for almost all areas with clear skies developing overnight. The
surface layer is expected to become decoupled supportive of
favorable radiative cooling. Somewhat uncertain with exactly how low
temperatures can go tonight. Generally took a degree or two off of
the forecast...but just enough of a continued Lake Huron
influence...and higher ambient soil moisture conditions from recent
snow melt and rainfall suggests dewpoints will not be able to free
fall too much from current values. Current dew point observations
range in the 20s across much of the County Warning Area...with some teens spilling
into areas northwest of the Tri Cities. Forecasted low temperatures are
similar with near 20 readings north and west of the terrain...middle
20s to the south and east within the Detroit urban heat island.
Surface high pressure will build over Ontario and Quebec tomorrow
behind the passage of an upper trough. This will strengthen the
surface ridge aligned through Michigan. Models continue to advertise
a back-door cold front trailing the Canadian trough dropping across
Lake Huron and into The Thumb and possibly south-central lower
Michigan tomorrow afternoon and evening. Models differ with how
agressive they are with this feature...with the NAM and arw looking
to progress the front furthest inland. While not a strong front...it
may have impacts on the forecast as reinforcing shot of shallow low-
level cold air will steepen low-level lapse rates and strengthen an
inversion near 900mb. The front itself looks to only provide a
small push of low-level moisture...but will turn the winds northeast
and then east off the lakes. May see a layer of stratus develop late
Sunday into Sunday night as moisture locks in under the subsidence
inversion...particularly north and east of a line from Saginaw to
Port Huron. Nam12 forecast soundings are the most aggressive with
the low-level moisture (typical of the nam) but even the gfs20 hints
an increase in clouds is warranted. Will be interesting to see if we
can get some drizzle to form if the lowest 150mb saturates...as
models show a good layer of dry air and directional wind shear
immediately above inversion level. NAM quantitative precipitation forecast output is indeed spotty
like it typically is with drizzle...but this is the only model
showing this level of aggression at this time. For the
forecast...given set-up...will favor more clouds across the northern
thumb and south-central lower Michigan but not go so far as to add a
drizzle or freezing drizzle mention as confidence level is low.
Would like to monitor upstream trends and see additional higher-res
model output before any confidence can be gained in this.
Other issue Sunday night/Monday morning will be potential for fog to
form over Southeast Michigan as winds turn off the lake under calm
conditions and high pressure. Cool flow off the lakes and weak cold
air advection will keep cool (near-normal) temperatures in place
Sunday...with maximum temperatures in the low/middle 40s. Temperatures Sunday night will
be heavily dependent on any stratus that can form...and went towards
the higher end or slightly above MOS for the northern portion of the
forecast area. Min temperatures in the 20s to near 30 in Detroit are
Next feature to watch will be the slow advancement of the large
upper low spinning west of The Rockies. Models have overall trended
slower with the eastward progression of the low over the past few
days (often seen with cut-offs)...but this trend seems to have
leveled off since 00z model runs last night...boosting forecast
confidence slightly with timing. GFS/NAM/Euro lift a weak elevated
warm front up through lower Michigan Monday as a broad area of
surface low pressure begins to eject out of the plains. Aside from
dry air in place as the front lifts across...isentropic ascent looks
very weak and forecast remains dry through the early evening.
Upper/surface low pressure will then strengthen as they lift from
the Central Plains into the western Great Lakes Monday night into
Tuesday as the system becomes positioned with the left exit region
of a strengthening upper jet streak clipping through the Desert
Southwest and Southern Plains. Surface warm front/occlusion look to
lift across Monday night and early Tuesday. Stronger isentropic
ascent and low-level Theta-E surge will combine with large-scale
lift to provide widespread rain to Southeast Michigan. Probability of precipitation remain
in the high likely range for Monday night...however this is more a
factor of inherent uncertainty surrounding the timing of cut-off
lows and still being a few days out than with the
confidence/coverage of precipitation. Dry slot should work through
on Tuesday...changing rain over to drizzle. Colder air wrapping into
the back of the system looks to hold off until Tuesday night...and
all precipitation is expected to remain rain through early Tuesday
By the overnight hours Tuesday into Wednesday...the low will be in
the process of pushing off towards the East Coast. Colder air will
be filtered into the area as winds shifts to the northwest.
Continued slight chance for scattered rain/snow showers through much
of Wednesday with the departing low and colder air behind the
departing low. High pressure then builds in over the Ohio Valley
for the remainder of the week into next weekend as drier conditions
return to Southeast Michigan.
A building ridge of high pressure will keep quiet conditions over
the central Great Lakes through Sunday night. Easterly winds will
increase Monday and Monday night in response to low pressure lifting
into the western Great Lakes. Persistent east to southeast winds
will allow for a gradual increase in wave heights Monday and Monday
night...and could necessitate low-end small craft advisories for the
southern portion of Lake Huron.
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).