Discussion of the forecast - this evening and Tonight
Broad area of return flow betwixt the exiting surface ridge over the Ohio Valley and weak low
rippling east across northern Ontario will continue to wash across southern Lower Michigan tonight.
The strengthening surface pressure gradient will support increasing winds the remainder of the
afternoon and this will continue through the evening.
Plenty of mid/high level cloud cover will be present throughout the next 18 hours or so. Meanwhile,
moisture return in the lower levels has been much less than advertised. Not expecting decoupling to
occur tonight as the combination of a robust pressure gradient /attendant geostrophic flow remains
in the 25Kt range/ and lack of negative radiational flux with the persistent cloud shield.
Consequently, not expecting temperatures to drift down too far this evening and should remain rather
steady through the night.
A weak impulse and moisture response around 700mb will rocket overhead late tonight. The window of
moist isentropic ascent is rather short and not enough to saturate surfaceward through evaporative
forcing. So not expecting much in the way of precipitation development, just an enhanced mid-cloud
deck and some virga.
Remaining Forecast interpretation -
Rather amplified upper level ridge over the Dakotas, with the upper level wave over Alberta breaking
the top of the ridge down as it comes over the top and tracks through the northern Great Lakes on
Monday. Still, a lot of mild air is forecasted to stream into the southern Great Lakes, as 925-850
mb temps rise solidly in the positive single numbers, with the increasing favorable low level
southwest flow. Do not see sufficient upstream dew pts to be terribly concerned with low clouds, and
I prefer to skew the forecast warmer, around 50 degrees, in line with the Euro, and factoring in
March 7th as a benchmark ( 925 mb temp of 4 C per 00z DTX sounding on March 8th ).
Tuesday is a challenging day as the surface cold front pretty much washes out, with substantial
baroclinic zone in place. Depth of the the cold air bleeding into southern Lower Michigan during the
day is in question, and areas south of M-59 look to be mild again ( 40s ), although 700 mb
fgen/clouds will likely prevent maxes from reaching 50 degrees again. In fact, increasing evidence
via UKMET/Canadian models precipitation from the mid level fgen will be streaking in by day's end.
If this does happen, wet bulbing effects should allow for changeover from rain to wet snow as
boundary layer cools, especially as we head into Tuesday evening.
The forecast for Tuesday night and Wednesday is then contingent on the phasing of northern stream
trough/upper wave coming out of central Canada and the trough/wave off the Pacific northwest coast.
12z models all converging on a rapidly deepening low tracking along or close to the Ohio River to
just south of pittsburgh, a favorable track for snow near the Michigan/Ohio border. The temp
gradient/baroclinic zone is impressive, with surge of arctic air under the low level north-northeast
flow during Wednesday, as 925/850 mb temps crash into the negative mid teens. 850-700 mb specific
humidity Tuesday night between 3-3.5 g/kg along and south of M-59 corridor, and with intense
lift/deformation/fgen for 6 hours, a swath of 6 inches of snow is possible near the Ohio border,
before the low races to the east coast late Wednesday. Another factor is the lead 500 mb low
currently over Mexico, and if and when it becomes absorbed within the developing storm system, as
this feature will likely take with it much of the Gulf of Mexico moisture. 12z Canadian/UKMET both
suggest bulk of activity will miss us just to the south. The 12z Euro came in with a track just
south of the Ohio River, and right know thinking around 1 inch between M-59/i-69 corridors, with
around 4 inches toward the Ohio border. This is a sensitive forecast due to the timing issues with
the dry arctic air funneling into the central Great Lakes, and with Lake Huron mostly iced over,
can't count on much lake effect contribution/moistening of the low levels, and thus confidence is
average at best this far out.
A very cold night is in store Wednesday night as the winter storm moves to the New England coast and
a surface ridge noses into Michigan. Details such as how quickly clouds clear and extent of
decoupling still need to be worked out, but current guidance depicts lows in the single digits.
Continued cold Thursday but low level warm air advection will ensue late Thursday into Friday /850
hpa temps increasing from -16C to +2C from 12z Thursday to 12z Friday/ causing a nice warmup for
Friday. The warmup looks to be short lived though as an extension of the Hudson Bay upper low works
its way into the upper midwest for the weekend, accompanied by a return to below normal
temperatures. Precipitation chances are slim to none for the latter half of the week into Saturday.
. . posted at 335 PM EDT Sunday March 9 2014