Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
544 am EST Sat Feb 28 2015
A clear sky and light wind field will exist overnight and into
Saturday under the influence of strong high pressure and a deep dry
layer. An increasing middle/high cloud deck Saturday evening will then
lead in the arrival of a deeper plume of moisture. This will result
in the development of accumulating light snowfall Sunday morning.
//Dtw threshold probabilities...
* high in ceiling falling below 5000 feet early Sunday morning.
Previous discussion...issued 336 am EST Sat Feb 28 2015
Short term...today and tonight
Surface high pressure building across portions of northern Indiana
and Ohio this morning has resulted in the persistence of a light
southwesterly wind. The lack of a complete surface layer decoupling
has led to a fairly noisy temperature distribution across all of
southeastern Michigan. Temperatures range from the single digits
below zero in The Thumb and Lenawee County...to the single digits
above for many areas and even 10 degrees above for sections of the
Detroit urban heat island. Low wind conditions and clear skies
supports a favorable cooling setup through sunrise. Have exercised
patience and will not deviate much from the inherited temperatures
for this morning.
Surface high pressure and a stable profile between 2.5 and 12 kft
above ground level will support quiet weather today. Sunny skies this morning
should then give way to at least some midlevel cloud/altostratus
from the south. Difficult to ascertain opacity and coverage of cloud
this morning on infrared imagery. The outgoing forecast will be light on
amount of clouds through the late afternoon.
The pinching off of a closed low height anomaly off of the coast of
California will lead to a merging of a northern stream jet branch
into the main westerly belt today over the Central Plains. The
downstream response of this is a rising of heights over eastern
North America and a concerted amplification of a long wavelength
ridge axis. What results is a wave pushing eastward tonight in the
midlevels and systematic warm air advection. Forecast isentropic
plan views along with sounding analyses supports best forcing for
ascent will reside up in the column...around 600mb. Saturation is
then prognosticated fairly quickly the latter half of the night with system
relative isentropic ascent prognosticated for the entire period from 06-
12z. Overall...stability and moisture profiles are expected to lead
to a period of light snow. Accumulations through 12z are expected to
remain at less than an inch for locations south of I 69...with
perhaps up to an inch possible near the Ohio border. Lows tonight
will occur early in the evening before temperatures rise late.
Light snow is still expected to impact the area on Sunday as a
steady stream of sheared shortwave energy ejects along the southern
edge of broad...low amplitude upper trough positioned over a large
portion of noam east of The Rockies. 00z model runs have tightened
the north to south precipitation gradient across the area and will
make minor adjustments to better define the expected snowfall with
lesser amounts I-69 north...around an inch...to 3 or 4 inches near
the Michigan and Ohio state line. Given the fact that these totals
come roughly over an 18 hour period...even 4 inches really does not
reach advisory levels as snowfall rates at any given time during
this event do not become particularly impressive.
The next system to affect the area on Tuesday will be a more well
defined storm as the southern and northern streams phase better as
jet energy in both emerge east from The Rockies early next week. The
track of the resultant low pressure remains roughly the same this
cycle...bringing the center of the low over to just northwest of the
County Warning Area. Initial precipitation within isentropic ascent in advance of
this system would fall as snow...but then transition quickly to a
wintry mix and eventually rain over at least the southern half of
the County Warning Area. Even over the northern County Warning Area...a primarily snow event will
most likely be punctuated by periods of sleet or perhaps freezing
Given the tight north to south gradient in low level temperature
profiles...any small adjustments to the track of this system will
lead to larger than normal fluctuations in precipitation type
forecasts. At this time...the transition from a mix changing over to
rain to a more or less all frozen precipitation scenario sits
approximately along the I-69 corridor...with uncertainty to great to
define the scenario any further.
Aside from the brief surge of relatively mild air over parts of the
area on Tuesday...the overall forecast through next week remains
below average...significantly so at times...for the first week of
March. That said...the extreme cold of the past several weeks will
transition to a less persistent as well as less intense period of
cold weather as high temperatures will range largely in the 20s and
lows in the single digits to teens.
High pressure will build through the Ohio Valley today with moderate
southwest return flow developing around this high over the central
Great Lakes. Central sections of Lake Huron will most likely see
winds increase into the 15 to 20 knot range. Otherwise...winds will
remain on the light side as this southwest pressure gradient weakens
late in the weekend as a weak low pressure system lifts into the
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).