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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
858 PM EST Sat Feb 28 2015


Burgeoning Wing of middle level warm air advection immediately in
advance of weak shortwave energy quickly filling east-northeast
into parts of Illinois/in late this evening. The resulting uptick in
moist isentropic ascent anchoring the expansion of light snow as
expected...radar/satellite presentation points toward the core of
this moisture targeting points south of Southeast Michigan. The northern
arc of this moisture plume does remain on target to sweep across a
portion of Southeast Michigan...ensuing onset of snowfall centered
05z-08z from west to east. A more tenuous northern periphery to
this snowfall...owing to the lack of greater ascent with northward
extent allowing the existing deep resident dry layer to hold
tough. Delineated south to north pop gradient for the early-middle
morning period remains appropriate...a higher end pop defined
roughly along and south of the M-59 corridor. 4-6 hour duration
window before the background forcing wanes 11z-13z per recent rap
guidance...enough to support accumulation of up to an inch through
this time...highest near the Ohio border.

Minor update forthcoming to realign probability of precipitation and snowfall expectations
slightly...particularly within the middle tier /I-69 to M-59/...where
the concerns on a limited duration appears more in play per
upstream trends.


Aviation...issued 656 PM EST Sat Feb 28 2015

A clear sky below 10k feet through midnight with simply an increase in
high level cloud expected as the middle levels begin to moisten. This
influx of moisture will allow an area of light snow to expand into
Southeast Michigan during the early and middle morning hours. Conditions will
deteriorate accordingly...steady saturation of the lower layers
resulting in a predominant period of MVFR/IFR restrictions /except
mbs/. Environment will remain favorable for some perpetuation of
light snowfall in MVFR through at least midday Sunday...particularly
across the Detroit corridor.

For lowering of ceilings after 04z-05z as upstream
area of snowfall approaches the metropolitan airspace. Onset of light
snowfall targeted for 06-07z with steady deterioration in conditions
through the early-middle morning hours...a period of IFR at the peak
10z-14z. Inch to inch and a half accumulation with the morning
snowfall...minor additional accums for the afternoon.

//Dtw threshold probabilities...

* high in ceilings below 5000 feet overnight into Sunday.


Previous discussion...issued 311 PM EST Sat Feb 28 2015

Short term...tonight and Sunday

The northern edge of the region of light snow now expanding across
MO and southern Iowa will advance into Southeast Michigan between 03z and 06z tonight.
This snow is occurring due to moisture advection within a broad
region of isentropic ascent ahead of middle level short wave impulse.
Model cross sections suggest specific humidities of 2 g/kg up through
700 mb advecting into the far southern portions of the area /Metro
Detroit south/ overnight...with up to 1 g/kg into Saginaw. The
ascent over most of the forecast area will be quite elevated through
most of the temperature regimes not supportive of good
dendrites. This and the initial wedge of departing low level dry air
will keep snowfall intensity rather light well into the night.

Strengthening deep layer frontogenesis will expand across the Northern
Ohio valley late tonight into Sun morning within the exit region of
an upper jet maximum nosing into the middle MS/Northern Ohio valleys. The low
level portions of the frontal slope /where deep layer static
stability will be lower and moisture more plentiful/ are expected to
be most active. This will largely occur south of the Michigan border.
Metropolitan Detroit/Ann Arbor and points south will however be close
enough to some enhanced forcing within the middle level portions of the
front to potentially see some periods of higher intensity snow /late
tonight through Sun morning/. There is expected to be some
contraction of the frontal circulation Sun morning...which may bring
some middle level drying and subsidence into the northern half of the
forecast area by late morning. This supports lowering probability of precipitation and snow
accums north of the M 59 corridor. The bulk of the 12z model suite
now show the better frontal dynamics sliding a bit farther south Sun
afternoon...suggesting the more widespread snow should come to an
end from north to south after 17z.

While liquid to snow ratios will be rather high at 15 to 18:1...the
lack of good dendrites will have an impact on accumulation rates.
Locations to the south of a Montana Clemens to Howell line are expected
to see a general 1 to 3 inches from tonight through Sun afternoon
/with the higher amounts confined largely to Lenawee...Monroe and
southern Wayne counties/. To the north...weaker forcing and less moisture
content will support just chance probability of precipitation with accumulations around a
half an inch or less.

Long term...

Monday and Monday night...shortwave ridging and expansive surface
high pressure beneath nothing more than a partial coverage of high
clouds will support very warm temperatures in the upper 20s on Monday.
Aided by increasing cloud cover and developing light southerly flow
Monday night, lows will be likewise be relatively mild, ranging from
5 in The Thumb to 15 from the metropolitan area south.

Tuesday...strong argument for a 12z European model (ecmwf)/Gem blend across the board
for the Tuesday storm. Analysis of model forecasts reveals no shortage
of moving parts to complicate matters, so exercised simplicity in
approaching the grids, particularly given this winter's
characteristic poor predictability.

Key components are the wave that crossed the Bering Strait this
morning, a compact low just north over the Arctic Ocean and out of
view of the GOES satellites, along with energy cutting off over
California today. Suny Stony Brook ncep+cmc ensemble eof sensitivity
analysis confirms suspicions that the most sensitive part of the
Tuesday forecast, and potential Tipping Point, resides in the
evolution of the northern trough, in particular that portion which
stems from the evolution of the alaskan shortwave trough with the
compact Arctic circulation. Notable differences remain between the
European model (ecmwf)/Gem, but they are in considerably better agreement than the
GFS and are better generally better trusted over the data-sparse
Arctic. Best illustration is hour 00z Wednesday, when the GFS is
considerably more aggressive digging this energy, helping to amplify
the longwave and access a bit more of the pv Reservoir over the
southwest. The vortmax over North Dakota is held over
Alberta/Saskatchewan by the Gem/ec. The subsequently more amplified
solution offered by the is an outlier among the guidance.

This provides a good segue into today's dominant 12z trend, which
was toward a decidedly flatter evolution. Less connection with the
southwest low that will be closing off west of Southern California
between now and 12z Tuesday is shown. This is predominantly due to
slightly less amplified northern stream. Quicker trough progression
allows existing southwest energy to kick out considerably faster,
helping to suppress the downstream ridge over the Great Lakes. Even
the GFS trended in this direction to a certain degree. In all cases,
substantial isentropic ascent and large scale forcing will produce
widespread precipitation across the area on Tuesday. However, the
weaker/more open surface circulation depicted by the ec/Gem suggests
holding the rain/snow line somewhere south of M59. At the present
time, looks like a quick hit of moderate to heavy snow followed by a
transition to snow pellets/ip/fzra and dry slot, which is still
poorly depicted in model quantitative precipitation forecast fields. If current trends hold, have a
feeling drizzle will be the theme Tuesday evening. Ramped up pops,
but only made very minor changes to ptype distribution at this early

The 12z forecast package sharpens also the high temperature gradient
across the area Tuesday, ranging from 25 in the north to near 40 in
the south. Even areas with highs in the low/middle 30s could feasibly
see predominantly snow given expected temperature rise after dry slot
intrusion. As alluded to above, subfreezing surface dewpoints and
best chance for all snow are maintained M59 north. Broad brushed
introductory snow accums...1-3"...less toward Ohio and more toward
The Thumb.

As the low pressure system moves off to the east...colder air
returns to the Great Lakes region. Highs are forecasted to be in the
middle to upper 20s on Wednesday with lows dropping back down into the
single digits on Wednesday night. There will also be a chance for
some lake effect snow on Wednesday as northwest winds increase
throughout the day. High pressure then returns for the remainder of
the week bringing dry conditions and plenty of sunshine to Southeast
Michigan. High temperatures will gradually increase from the upper
teens on Thursday into the middle 30s by Saturday.


Weak to moderate surface pressure gradient will maintain winds below
20 knots through Sunday night. Approaching high pressure from the
west will allow a fresh northwest breeze to develop over the
northern half of Lake Huron during the day Monday with cold and
marginally unstable conditions supporting a potential borderline
gale scenario. This period will be monitored for watch issuance.
Nearly calm conditions will take hold Monday evening into Monday
night before moderate southerly winds emerge ahead of another storm


DTX watches/warnings/advisories...
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.

short term...SC
long term....jvc/rk

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