Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
631 am EST Sat Dec 7 2013
Unstable regime over the Great Lakes has been allowing for
stratocumulus persistence over the Great Lakes with many locations
this morning under a healthy stratocu deck. Latest rap moisture
profiles supports trapping saturation and moisture under active
850-500mb subsidence. With a building surface anticyclone and a low
background relative humidity signal...suspect clouds will lessen significantly by
the afternoon hours. Conditions tonight will remain benign ahead of
increasing midlevel moisture tomorrow.
//Dtw threshold threats...
* low confidence in duration of ceilings below 5000 feet today.
Previous discussion...issued 406 am EST Sat Dec 7 2013
Short term...today and tonight
Localized surface ridging that has been sustained by low level cold
air advection will give way tonight to the center of a +1035 mb
surface anticyclone. Strong subsidence signal in the model soundings
as well as midlevel Omega plan views suggests no degree of dynamic
forcing supportive for precipitation. While models are having a
difficult time with moisture profiles as is always the
case...satellite trends on lake effect stratocumulus supports a
building scattered-overcast cloud deck early this morning. This cloud deck may
very well establish itself prior to the commencement of diurnal
mixing processes. Then the question is what sort of persistence the
SC deck could hold despite less than impressive model soundings and
background amount of moisture. The rap is a new and supportive
outlier suggesting that a good amount of saturation/cloud will exist
under the inversion base at 3.5 kft above ground level. Will take the pessimistic
cloud Route here for a meaningful chunk of today. The forecast will
read mostly cloudy transitioning to partly cloudy. Temperatures
today will only reach the lower to middle 20s...roughly 13 to 15
degrees below average.
The forecast question for tonight is what lows will drop to. It
appears the stage will be set for favorable radiative cooling with
the only missing condition...a fresh snowpack. Don't have a good
calibrated feel yet. Sided with a mix of MOS...which gives low teens
outside of the heat island...with middle teens in Detroit metropolitan.
Long term...Sunday through Friday
High amplitude middle level troughing currently centered from north
central Canada into the middle section of the country will remain the
predominant large scale feature well into next week as the main
height fall center slowly shifts eastward with time. This will
present a sustainable conduit for multiple reinforcing rounds of
Arctic air to infiltrate Southeast Michigan right through the upcoming
work week. Lead push of modified Arctic firmly in residence locally
to start Sunday lodged within a 1035mb low level anticyclone slowly
shifting through the region. Suppressed mixing potential within a
stable low level easterly flow will ensure highs again hold in the
Attention for the Sunday night period remains centered on the
downstream evolution of the lead shortwave currently anchoring the
base of the mean trough. Strong height falls barreling southward
out of central Canada this weekend will squeeze this wave
northeastward...the vorticity center shearing from northern Illinois/
southern WI through northern lower Michigan by Sunday night. Assessment
of latest 300-200mb potential vorticity fields suggest enough
separation will exist between these height fall cores to leave the
lead wave in a more weakened/less phased state as the attendant
surface wave shifts through the lower peninsula. While the better
moisture advection within the prevailing southwesterlies will get
pinched off to the south/east...solid depth and magnitude to the
inbound moist layer (2-4 g/kg of specific humidity within the
850-925 mb layer) will exist at the interface of the northward
arcing elevated warm frontal boundary. There may also be some lake
moisture contribution given easterly boundary layer flow. Given
the projected trajectory of this system...strongest forcing does
remain centered to the northwest...although the Tri-Cities will be
on the fringe on the main area of DCVA and better convergence.
With that said...certainly enough background ascent working into
a sufficiently moist environment to bring a widespread area of
light snow through Southeast Michigan Sunday night. The
combination of the added degree of forcing and likelihood of
slightly longer residence time will focus the greater accumulation
potential into the Tri-Cities...where some amounts in excess of an
inch still seem plausible. Elsewhere an inch or less with some
reduction in snow ratios expected with the low level profile
warming with time.
Cold air advection quickly ensues on the heels of this system
throughout Monday. Short lived dry slot for the morning period
before a lake enhanced moisture plume expands eastward within the
background of cold/cyclonic flow. This will support an extensive
stratus deck for much of the day...and allow for some light snow
shower/flurry production. The main Arctic blast will accompany a
clipper cruising through the northern Great Lakes on Tuesday. A
trailing wave may reinforce this airmass into Wednesday...the net
result being a stretch of temperatures a good 15 to 20 degrees
below normal for the 2nd week in December. A solid gradient
attendant to these northern stream waves will provide the
additive wind element...resulting in daytime wind chill readings
of single digits Tuesday and Wednesday.
A stronger westerly wind will remain in place today across Northern
Lake Huron as Arctic air continues to funnel across the waters.
Winds will decrease tonight and Sunday as strong high pressure moves
through the region. Southeast winds will strengthen again Sunday
night as low pressure tracks through lower Michigan. Winds will
shift to southwesterly in the wake of this system on Monday.
Increasing instability as Arctic air spills back into the region
will introduce the possibility for gales Monday into Monday night.
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).