Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1156 am EST sun Mar 1 2015
The widespread light snow will remain south of ptk through the
duration of the afternoon. Some enhanced subsidence and daytime
mixing has actually cleared the low clouds from ptk northward. There
are however a few areas of MVFR strato cumulus near Lake Michigan and across
portions of Wisconsin. A surface trough axis now over Wisconsin will be
driven across Southeast Michigan overnight in response to the passage of an
upper level short wave. This should allow these lower clouds
upstream to filter back into Southeast Michigan this evening. Some low level cold
air advection tonight along the surface trough will steepen low level
lapse rates and will support some gustiness /around 20 knots/ with
the westerly winds overnight.
For dtw...the northern edge to the light snow has lifted back across
metropolitan late this morning. Trends in radar...surface observation and latest model
data suggest the snow will persist through roughly 22z before
ending. Afternoon accumulations of a half inch to an inch of dry
powdery snow seem possible.
//Dtw threshold probabilities...
* high in ceilings below 5000 feet this afternoon and tonight.
Previous discussion...issued 907 am EST sun Mar 1 2015
The northern edge of the frontogenetically forced light snow now
extends from north-central Illinois into southern Monroe County. The
latest rap and nam12 suggest a slight northward shift and
reintensification of the middle level fgen between 18z and 21z. This
supports the current forecast which holds fairly high probability of precipitation up
through the Ann Arbor to Detroit corridor and points south. The
expected brevity of the stronger forcing remains supportive of
additional snowfall accums of an inch or less.
Southwest flow off the remaining open waters of Lake Michigan has been
enough to generate some light snow across west central lower Michigan
this morning. A veering of the low flow toward the west has
brought some of this low level moisture into the Tri Cities
region. Conditions appear to be supersaturated with respect to
ice within these clouds which is likely causing some decent
Snowflake size /also suggested by some of the visibility reductions/.
The latest radar trends show a weakening of returns. Diurnal
mixing into some drier air may also erode some of this low level
moisture. In light of current observation...an update will have to be
issued to at least carry some flurries across the Saginaw Valley
Previous discussion...issued 357 am EST sun Mar 1 2015
Short term...today and tonight
Synoptic scale ascent anchored under the left exit quadrant of a
powerful zonal jet axis and weak system relative isentropic ascent
are the mechanisms responsible for the broad footprint of light snow
across the region. The lead edge of the precipitation was able to
lift northward through the Detroit metropolitan area prior to 6z...timing
well with a compact 850mb temperature gradient. Nwp data supports
this steepened frontal structure to continue to lift northeastward
away from the County Warning Area...making it as far northward as Central Lake Huron
and southern Georgian Bay by 12z. An additional steepend frontal
structure resides to the south of the state...now over central
sections of Indiana and Ohio. This feature is responsible for the
higher z returns and lower visibilities in surface observations. Models
support this baroclinic zone to remain south of the area through the
duration of the event. With this in mind...expectations are for a
status quo to snowfall rates through 12z with very light snow
accumulations this morning.
With the upper level jet forcing and lead moisture advection
vacating the premises...forcing will then distill down to weak
isentropic ascent. While persistent...there is serious question on
whether or not this upglide will be sufficent in continuing to
generate measurable precipation. Current radar mosaic does show a
definitive back edge of the precipation shield...with dry air
already advancing through chicagoland and into far southwestern
lower Michigan. Various solutions then suggest it will require the
ageostrophic response to a positive d(mag)/dt increase of the upper
level jet maximum to allow precipation to fill back in across far
southern sections of the County Warning Area for the afternoon. This is expected to
occur between 19-00z. Given the trends in numerical output in
addition to the radar data...was fairly aggressive in lowering probability of precipitation
and amounts for areas north of Detroit. In sum total...snowfall
accumulations of 1 to 2 inches will be possible from Detroit
southward with an inch possible between the M 59 and I 94 corridors.
The main midlevel trough axis and northern stream wave will catch up
to lower Michigan tonight passing through the County Warning Area after 6z.
Shallow...lower tropospheric cold air advection is forecasted to
steepen lapse rates in the 1.5 to 5.5 kft above ground level layer. The unstable
profile in combination with prognosticated saturation with respect to ice and cyclonic
vorticity advection supports hydrometeor production the latter half
of the night. Total lack of quantitative precipitation forecast supports nothing more than a mention
of flurries at this time.
Model trends explored during the 12z cycle largely hold true for the
00z runs with a bit less disparity in solutions. Essentially...the
broad and somewhat flat southern edge of the existing noam upper
trough will remain in place over the Continental U.S. As a potent northern
stream shortwave now exiting Alaska takes a track southeastward
through the prairie provinces and then east along the southern edge
of Hudson Bay during the first part of this week. This will steer
southern stream energy into the area on a rather flat trajectory. In
the meantime...while the Arctic airmass in place to our north as
eroded to some extent during the past day or two...it still remains
expansive enough to maintain a tight temperature gradient across the
Great Lakes. Between these two factors...chances are that cold air
will be difficult to displace over the area. This should lead to a
fair amount of the precipitation from this system falling as snow.
Even nam12/GFS now support a basically all snow event for the far
northern County Warning Area.
The main question will be the timing and placement of a changeover
to sleet/freezing rain and then rain in some locations by afternoon
as the surface low reflection of this system crosses the area. Will
only make minor adjustments to the forecast as models do not support
a strong shift either north of south in the precipitation type
forecast. Generally...areas from I-69 north are expected to remain
largely snow or sleet throughout the event. For the rest of the
forecast area...precipitation will start as snow Tuesday morning
with a gradual changeover to sleet/freezing rain from south to north
as some milder air works into lower Michigan and then a change to
rain through the southern third to half of the area as surface and
boundary layer temperatures climb into the middle/upper 30s. Overall
precipitation intensity forecasts from the models suggest only light
to moderate periods over a 8-10 hour period during the day Tuesday
so while it will most likely become rather messy in terms of travel
this is not a particularly powerful winter storm.
As for temperatures...relatively mild readings as compared to the
past few weeks can be expected Monday into Tuesday. Still with
average highs in the upper 30s/lower 40s for early March the area
will remain at or below average even during this period. Much colder air
will then dump into the area behind this system during the last part
of the week as another Arctic high pressure system builds southeast
through the eastern Continental U.S. Within amplifying upper troughing pattern
that develops as additional northern stream shortwave energy digs
into area behind the exiting storm system. As this trough progresses
eastward by the end of the forecast period...a moderation is
expected into next weekend with temperatures at least climbing back
into the 30s.
A weak to moderate surface pressure gradient will maintain winds
below 20 knots through Sunday night. A cold front will graze the
northern portions of Lake Huron Monday and bring a fresh northwest
breeze with gusts to 30 knots possible as cold and marginally
unstable conditions develop. High pressure will then build into the
area from the west with nearly calm conditions taking hold Monday
night before moderate southerly winds expand into the region in
advance of the next storm system due into the area by Tuesday.
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).