Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
343 PM EST sun Jan 25 2015
Short term...this afternoon and evening
Erosion of cloud and precipitation in lower Michigan evolved largely
as expected and outlined in the earlier update. Deeper easterly flow
became established and shut down the isentropic ascent that was
happening before 18z. Regional radar mosaic shows reflectivity
fairly ragged and sparse now into a good portion of northern
Indiana/Ohio. This is increasing confidence that light snow will not
impact the Ann Arbor and Detroit vicinities. Z echoes are marginally
increasing south of Toledo and pushing northward toward the Michigan
and Ohio state line as of 20z. Visibile satellite imagery aligns
well with more opaque cloud. Given timing...this matches up as the
start of the clipper system dynamics approaching the far south. Hi
resolution/explicit convection allowing models and determinitic
solutions have advertised the 21-02z time window as the one to watch
for accumulating light snow. Out of respect for the lighter returns
early this afternoon...did cut probability of precipitation back to a high chance. Snowfall
accumulations of a half inch to inch remain possible for southern
Lenawee and Monroe counties.
Other item of note is thin lake effect banding that is streaming
into The Thumb and Tri Cities off of Lake Michigan and Saginaw Bay.
Ragged and thin appearance along with surface observations that are
largely void of visibility reductions supports jus a chance pop with
perhaps a minor accumulation in a location or two. Persistent
easterly wind tonight should impact the temperature falls...to the
single digits to around 10 degrees in many locations. Wind chills by
Monday morning are expected to range from 0 to 5 degrees below zero.
Long term...Monday through next Sunday
A middle level short wave impulse will Cascade down the Lee of the western
US long wave ridge on the nose of a 150kt upper jet streak and track
across the western Great Lakes and lower Ohio Valley on Monday. This
wave will phase with the longer wavelength trough rotating across
the East Coast...leading to rapid deepening off the coast of New
England on Tuesday. There is strong agreement among the 12z model
suite that this shorter wavelength impulse will remain highly
sheared as it tracks across the western Great Lakes on Monday. This will
hold the deep layer moisture and surface reflection across
Wisconsin...forcing a surface ridge axis to expand from NE Ontario into
lower Michigan. Surface high will then further build across the Great Lakes and
Ohio Valley on Tuesday as the New England storm system intensifies and
middle/upper level confluent flow envelops the region. This surface high
will actually persist across the region into Wednesday.
North-NE flow early in the week associated with the building surface high
will sustain subtle low level cold and dry air advection into Tuesday.
Model soundings suggest some degree of low clouds will persist
underneath the low level inversion. The degree of dry air now
upstream of Lake Huron raises some concerns that model solutions
/particularly the NAM/ may be too bullish with low cloud development.
While there still should be some middle/high clouds from time to
time...the dry air argues for a partly cloudy forecast. Anticipating
some degree of diurnal response will support forecast daytime highs in
the 20s and nighttime lows mainly in the single digits through middle
Main story starting Thursday into the upcoming weekend will be
another clipper system with colder air following behind the system.
The clipper system that looks to move through on Thursday will bring
the next chance of snow to Southeast Michigan before quickly exiting
by Friday morning. This will be dependent on the exact track and
timing of the system and currently some of the models do bring the
center of the low across northern Indiana and Ohio. Cold
temperatures then take over for the weekend as the area along with
the possibility for snow showers as shortwaves progress through the
longwave trough that will remain set up over the area.
Low pressure tracking across the southern Ohio Valley this evening
will sustain some degree of gustiness to the northeasterly winds
tonight...mainly across Southern Lake Huron and Lake Erie. Winds
will diminish a little overnight into Monday as high pressure nudges
into the region from the north. Deepening low pressure off the coast
of the New England will lead to some light increase in the northeast
winds on Tuesday. High pressure will then expand back across the
central and eastern lakes region by Wednesday...causing the winds to
become light and variable.
Aviation...issued 121 PM EST sun Jan 25 2015
MVFR stratus held this morning in the face of increasing dry air
advection. Coverage of MVFR ceiling observation at 17z and some Shallow Lake
Huron moisture makes it difficult to totally move away from broken MVFR
ceilings but the trend is definitely drier. Will amend this afternoon
as needed. Clipper system is now pushing into Indiana. Will
continue to monitor the northern edge of the precipitation
shield...but all indications are the light snow will remain south
of the dtw terminal. The time period of particular interest for
passing synoptic scale forcing will be between 21-02z
//dtw threshold probabilities...
* high in ceilings below 5000 feet through this afternoon.
* High confidence in precipitation type snow. Low probability
this will affect dtw.
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).