Forecast thoughts - through Tonight
Resurgence in lake effect has occurred over the past hour, with snow bands off Lake Michigan having
re-expanded north of the I-94 corridor after only a few hour break. This is likely due to colder air
pushing across Lake Michigan and into southeast Michigan behind a cold front. This has effectively
increased lapse rates over the land and sent lake to 850mb temperature differentials over the lake
above 20 C. Prior to the colder air's arrival, drier air had been making inroads into the lower
levels and limiting ice nuclei. As the colder air has settled in however, the moisture has become
placed solidly within the dendritic growth zone, despite depth of saturation shrinking. This change
is apparent when looking at the change that has occurred in the snowflakes over the past hour,
looking more like dendrites here at dtx. Pops were increased to likely/categorical for favored areas
north of the I-94 corridor this evening, and did carry at least some chances all the way down to the
Ohio border as winds shift slightly from southwest to west and potential for a few bands to focus
south of M-59 increases. Light accumulations are possible, up to one half inch under the more
Tricky forecast then for the remainder of this evening as cold air continues to stream over the
warmer lake and drier air continues to work in behind the front. Forecast soundings do show some
drying in the lower levels overnight. This is a little uncertain considering moisture continuing to
stream in from off the lake under an inversion. Satellite imagery from over Lake Michigan the past
few hours has shown a pronounced erosion of the lake effect bands from west to east over the lake as
the drier arrives, and did keep pops overnight only in the chance category.
Temperatures will become quite cold overnight as dewpoints plummet into the single digits and gusty
winds subside. Areas that have several inches of snow on the ground from the past few days will
especially see temps fall, as the fresh snow cover helps the ground radiate and winds decouple.
Looking for only single digit min temps in many locations across the Thumb and tri-cities, with
low/mid teens elsewhere.
Remaining Forecast considerations - Tuesday through Saturday
The broad longwave trough over the Great Lakes does not look to go anywhere soon. Both GFS and ECMWF
show this trough holding for at least the next week. Several shortwaves will drop into the trough
bringing some chances of snow and reinforcing shots of arctic air in their wake.
On Tuesday, after a brief period of sfc ridging in the wake of Mondays low pressure system, a second
low will track across Lake Superior dragging a cold front through lower Michigan in the afternoon.
Gusty sw flow ahead of the front will veer westerly behind the front. Wind speeds will be solidly
around 30 knots in the lowest few thousand feet. The inversion looks to wash out allowing decent
mixing through the day. This should allow gusts to reach 30 mph or so at the sfc for most of se
Michigan. Models show a narrow band of moisture with the front across the Saginaw valley but the
cold air advection behind the front will promote lake effect. Question as always is how far east
will the lake effect make it? models show dry air moving in at 850mb with the best lake convergence
signature not arriving til its too late.
Better chance of snow will come on Wednesday although still not overly impressed. A compact mid
level vort max will round the base of the trough and pass through the northern Ohio Valley. The best
forcing and lift appears to pass to our south but still weak troughing and frontogenesis should
provide some lift over the area. In addition it could receive a boost from the convergent band
already located over sw lower Michigan. Overall a marginal event as moisture cannot really be found
in any model. An afternoon total of only a few hundredths is the common theme which would amount to
less than an inch. Some lake effect will linger through the evening hours. This system will usher in
some of the coldest air we have seen so far with 850 mb temps possibly cooler than -20C entering the
cwa. Wind chills will rarely exceed 10F through mid week with Wednesday night falling to near -10F.
High pressure builds to the Ohio Valley and southern Great Lakes on Wednesday night and into
Thursday while another clipper system tracks across the northern Great Lakes. This system will bring
yet another chance for some snow showers on Thursday through Friday to mainly the northern forecast
area /saginaw valley and northern Thumb/. West-northwest winds will prevail though the extended
timeframe keeping high temperatures in the low to mid 20s through the end of the week. The next
major system to impact southeast Michigan will be on Saturday and Saturday evening as a low pressure
system tracks across the Ohio Valley. At this time, southeast Michigan will be north of this system
keeping precipitation as all snow. However, being nearly a week out did not feel confident putting
much more than chance pops in the forecast for Friday night through Saturday night.
. . posted at 359 PM EST Monday December 9 2013