Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
602 PM EST Thursday Nov 26 2015
Shield of rain within Theta-E ridge along/in advance of trough will
edge northward a bit more this evening...likely exiting kmbs for a
time. Thereafter...expect this area to shift back south through all
terminals later tonight on through Friday as said trough progresses
through the central Great Lakes. Expect MVFR to possible IFR ceilings
withing best pooled moisture in advance of trough/cold front late
tonight into the first part of Friday. A strengthening gradient will
maintain S/south-southwest winds at 10-12 knots with some higher gusts...even
overnight. Winds will swing to northwest and then north-northeast upon frontal passage Friday.
For dtw...VFR conditions will hold overnight as surface ridging
remains in place. Cold front will then bring increasing showers and
MVFR to possible IFR ceilings early Friday morning. While ceilings may come
up somewhat by afternoon...several ripples of jet energy will delay
deeper frontal passage until early evening...so -shra will likely persist for
much of the forecast period.
//Dtw threshold probabilities...
* high in ceilings at or below 5kft after 10z tonight into much of Friday.
* High in precipitation falling as rain on Friday.
Previous discussion...issued 335 PM EST Thursday Nov 26 2015
Short term...through tonight
Trends in water vapor imagery show the upper trough stretching from
Ontario into the northern plains sharpening as it advances westward.
The approach of this feature has allowed heights to rise over
Michigan this afternoon...and with increased subsidence and some
drier air...we have have seen a temporary respite from precipitation
over much of the area. This will be short-lived however...especially
for the northern portion of the forecast area as regional radar
shows the next batch of precipitation making its way across middle-
Michigan. Arrival into the Tri-Cities is anticipated after 4
PM...with an eastward expansion expected into the northern thumb
through the evening.
The showers are filling in along an elevated frontal boundary...in
an area of middle-level fgen and where a 50-60 knot low-level jet is
beginning to nose in. Precipitation through the late evening should
stay mostly north of the I-69 corridor along this frontal boundary.
Coverage will really increase north of Saginaw after 8 PM...and then
north of M-59 after midnight as an axis of deeper moisture and lower
stability slides in ahead of a cold front now working through
Wisconsin and Iowa. Models show a pocket of cooler air sliding in
just ahead of the low-level trough axis this evening. Enhanced lift
ahead of the trough combined with steeper middle-level lapse rates (7-8
c/km) may be sufficient for a few thunderstorms along/north of I-69.
Observational evidence of lightning this afternoon over northern
Illinois is supportive of this idea...and did add a slight chance
mention to the forecast. Otherwise...warmer air...moisture...and
winds remaining out of the south at 10 to 20 miles per hour will really limit
cooling and make for a mild night. Temperatures are expected to cool
into the low/middle 50s this evening...but should not cool much after
that. The exception will be towards the Saginaw Valley and northern
thumb where the front will start to work in just before
sunrise...allowing for a quick drop of temperatures into the 40s.
Positively tilted longwave trough anchored by a closed low over the
southwest United States has contributed to several days' Worth of
height rises across the eastern Continental U.S. And west Atlantic. A long
duration of northwest flow into The Heart of the US and southerly return
flow around record-breaking East Coast high pressure has formed an
airmass boundary across the central US. The arrival of upper air
support from the more progressive northern stream has shored up this
boundary and deepened the effective fgen layer to encompass the depth
of the troposphere. This forcing will usher the boundary through the
area late tonight through Friday in the form of a sharp, slowly
moving cold front. Temperatures will fall precipitously behind the
front. After early morning highs in the low to middle 50s, the
northwest 2/3 of the County Warning Area will be in the middle to upper 30s by 4pm.
Rain will spread into the area from northwest to southeast between
06z and 15z Friday. The heaviest rain rates will be colocated with
the front where lower-level fgen will be most intense and convergent
forcing will be strong. Rain will then linger for several hours
behind the front as surface temperatures fall and forcing
transitions to the upper levels. Multiple waves of low pressure
propagating along the frontal boundary will provide some temporal
variability to the event. The first wave will exit to the northeast
late Friday morning or early afternoon with the departure of the original
strong right entrance dyanmics. 2 or 3 more waves are then likely to
track across the area as height gradient/jet core intensifications
develop in response to upstream convection. This will have the net
effect of slowing down the front and prolonging rainfall across the
far southeast of the County Warning Area. In accordance with the 12z arw/WRF/ECMWF,
likely probability of precipitation have been extended through all of Friday night from
Detroit city south. Rainfall amounts of a half inch are likely for
all locations, although additional ripples of low pressure and
slower frontal movement could tack on additional amounts of a
quarter inch or more for aforementioned southerly areas heading into
Guidance appears rather cold for Friday night, given the strong
northeast gradient off of Lake Huron. Considering there will also be
middle-level clouds and likely lake effect clouds, raised low
temperatures considerably into the low to middle 30s. Little rebound in
temperatures will be observed Saturday afternoon with 850mb
temperatures of 0 to -2c...highs near 40. Strong, sprawling high
pressure build during the day Saturday, leading to high confidence
of the erosion of any lingering boundary layer/lake effect clouds.
Clear skies, a column modeled to be extremely dry, and a weak
gradient within the ridge axis of a 1035-1040mb upstream high offer
high confidence in ideal radiating conditions for Saturday night.
Widespread low 20s to upper teens appears a good bet at this time. A
blend of the 12z mav/ggem yields a reasonable first pass at low
Fresh northerly flow will build southward across Lake Huron in the
wake of the cold front on Friday. Winds will gust to 25 kts and
waves will easily remain elevated enough to warrant a Small Craft
Advisory for outer Saginaw Bay and the Lake Huron nearshore zones
into late Friday night. Light air will build in with high pressure
on Saturday with light to moderate winds persisting through Monday.
East wind will ramp up in advance of the next low pressure system
Monday night into Tuesday.
Lake Huron...Small Craft Advisory from 7 am Friday to 4 am EST Saturday for
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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