Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1144 PM EST sun Mar 1 2015
Extensive MVFR stratus in place early this morning as a trough axis
swings across the region. This process will bring some pockets of
light snow showers...no accumulation expected. A modest increase in
west-southwest winds as lapse rates steepen...resulting in the
occasional gust into the 20 knot range. Drier air arrives on Monday
under increasing high pressure. This will lead to a decrease in
cloud coverage by late morning and early afternoon. Winds will ease
with time through the day.
//Dtw threshold probabilities...
* high in ceilings below 5000 feet this morning...low this afternoon.
Previous discussion...issued 305 PM EST sun Mar 1 2015
Short term...this evening and tonight
Upstream radar returns suggest the light snow which has impacted
metropolitan Detroit/Ann Arbor and points south this afternoon will
continue to diminish in intensity before coming to a complete end
between 20z and 23z. Some reintensification and a northward shift in
the middle level frontal dynamics led to a brief period of moderate
snow across Lenawee and Monroe counties...where total daytime
accumulations likely ranged from 1 to 2 inches. The dynamic forcing
for this snow occurred within the exit region of a 150kt upper jet
streak extending across far Southern Lower Michigan this afternoon. The frontal
circulation will slide east late this afternoon as an approaching
middle level wave focuses the upper jet support farther to the east.
Middle level dry air actually precedes this wave and will arrive into
Southeast Michigan this evening.
The 19z surface analysis does show a trough axis over Wisconsin. This
feature will track into lower Michigan tonight while a fast moving short
wave traverses lower Michigan in the 06z to 09z time frame. Despite rather
good middle level height falls and diff cyclonic vorticity advection...a
lack of middle level moisture will limit snow potential with this
system. There is however some low level moisture pooling along the
surface trough over Wisconsin and lingering lake moisture over western and
cntl lower Michigan. 850mb temperatures will drop along the surface trough late this
evening and overnight which will steepen low level lapse rates. This
combined with the low level moisture may support some brief light
snow showers and flurries. In fact...good supersaturation with
respect to ice within the cloud layer across cntl lower Michigan has been
quite efficient in generating some -sn/flurries this
afternoon...conditions which are expected to persist through the
night. The arrival of colder 850mb temperatures overnight will be
accompanied by an increase in the westerly gradient. With a deepening
mixed layer...some gustiness in the winds are expected /likely
pushing 25 miles per hour at times/. Surface temperatures may not drop into the teens
until very late in the night due to potential well mixed boundary
layer. The increase in winds will however drop wind chill readings
into the single digits overnight.
Models continue to disagree on dynamic interaction between today's
northwest territories/Nunavut trough and trailing low amplitude Arctic
energy leading to widening solution space (between ec/GFS/gem) aloft
despite decreasing lead time. The 12z GFS and 00z European model (ecmwf) seemingly
began to converge on a solution before the 12z European model (ecmwf) took a step
back toward yesterday's depiction. The GFS, meanwhile, has seemingly
leapfrogged the Gem from a consensus standpoint, leaving the latter
as an outlier as it appears to struggle with overall northern stream
evolution. A jump north from the UKMET at least offers some
confidence in the similar European model (ecmwf) of a weak low or open surface wave
translating from the quadrant-cities to the Tri-Cities. With that in
mind, a window of opportunity is opened for a more aggressive
forecast across the south in terms of higher temperatures and faster
transition to warmer ptypes.
By the time it reaches lower mi, main model differences are in
placement/amplitude but are generally similar dynamically which at
least provides some insight into overall governing dynamics. Primary
forcing will be strong 290-315k deep layer isentropic ascent
centered 15-21z per 12z European model (ecmwf). Ascent will be accompanied by
excellent moisture transport leading to a 6 to 9 hour period of 850 mb-
700 mb layer specific humidity over 4 g/kg even across the far north
where thermal profiles support all snow. Difficult to dissect timing
given that features are still outside of the nam's grid and causing
erratic solutions, but the event has the earmarks of a quick-hitter.
Heaviest precipitation rates should concentrate within strongest
warm advection which is largely expected to occur before lower
levels can rise above freezing. Thus, a quick 1-3", lower
south/higher north is expected. Not out of the question that a
focused area of higher intensity could produce locally higher
amounts somewhere, but that will be better addressed when the system
is within 48 hours.
700 mb-500 mb dry slot, aided by the strong middle-level wind field, will then
wrap beneath the jet maximum and make a strong push above the County Warning Area after
18z, calling into question the duration and intensity of any precipitation
that fills in behind the lead edge (e.G. Non-snow precip). Too early
to make any significant changes, but confidence in dry intrusion on
Tuesday night is sufficient to warrant downward adjustment to chance
probability of precipitation and introduction of drizzle 00-06z. Changeover to snow
pellets/ip/fzra could be rather abbreviated before fzdz/dz develops.
As noted yesterday, forecast highs are likely to occur beneath dry
slot, so many locations currently forecast to see low/middle 30s may
still see predominantly snow.
For the most part the extended period will remain quiet as high
pressure builds into the Great Lakes region. High temperatures on
Wednesday will briefly touch the middle 20s before colder air filters
in to the region. This colder air will allow low temperatures to
drop back into the low single digits for both Wednesday night and
Thursday night with a high on Thursday to only reach into the middle
teens. Temperatures slowly rise through the end of the week and into
the first part of the weekend. Dry conditions will also persist
through the remainder of the week and into the weekend.
A weak to moderate surface pressure gradient will maintain winds
below 20 knots through tonight. A cold front will graze the northern
portions of Lake Huron Monday and bring a fresh northwest breeze
with gusts to 30 knots possible late morning 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).