Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
603 am EST Thursday Jan 29 2015
Lead forcing in advance of shortwave will push most precipitation
east of the terminals as the forecast begins with just scattered
-frzapl south of ptk and -snpl north. Middle level dry slot will then
end precipitation until the actual vorticity center crosses the area
in the 16z-23z time frame. At that point...scattered -shsn will be common
ptk north with scattered -shra mixed with and changing to -shsn along the
I-94 terminals. VFR ceilings will be replaced by MVFR to occasional IFR
ceilings with this system with ceilings rising again tonight as drier northwest
flow sets up in its wake. These winds will be a bit gusty tonight as
downward momentum will be enhanced within area of strong cold air
For dtw...a few patches of -frzapl will end early in the forecast
and then be replaced by midday by -shra...which will turn back to
light snow showers by late in the day. Borderline MVFR/IFR ceilings
should spread into the area in the 16z-18z time frame with the
lowest ceilings occurring 19z-23z or so.
//Dtw threshold probabilities...
* high in ceilings below 5000 feet after 16z.
* Medium in precipitation type as -fzraip mix this morning and -shra
changing to -shsn this afternoon.
Previous discussion...issued 242 am EST Thursday Jan 29 2015
Short term...today and tonight
A shortwave will track from the middle Mississippi Valley into the
central Great Lakes today...spreading moisture into the area early
this morning. 00z kdtx sounding shows a very dry layer in place over
lower Michigan from 750 mb down to the surface...which has kept a
band of moisture within region of isentropic ascent in advance of
this the main system elevated overnight.
Forcing along the lead edge of the vorticity center will progress
east to northeast into the area from its currently position from
southwest lower Michigan into northern Indiana. However...between
the dry low levels still in place in advance of this band of rain
and snow...and a middle level dry slot trailing this feature into the
area...precipitation intensity and duration will be limited early
this morning over much of the County Warning Area. Only the northern third to half
of the County Warning Area may receive 3 or 4 hours of precipitation...mainly sleet
or snow...along the northern edge of the dry slot.
Further south into metropolitan Detroit and points south...a brief one or
two hour period of light showers or sleet will cross the area. With
temperatures in the middle 20s...any liquid that does fall will most
likely freeze on surfaces. At this time...only patchy slick spots
are expected given the lack of coverage of the the precipitation.
No headlines are anticipated at this time.
As the shortwave crosses the area today...additional snow showers
will occur as colder air associated with the wave overspreads the
area. Temperatures at the surface will remain at or above freezing for much
of the area from late morning into the afternoon as stronger cold
air advection holds off until this evening and tonight. Total snow
accumulations of an inch or two will be possible during the entire
period over the northern County Warning Area with much lesser amounts southward from
metropolitan Detroit to the Michigan and Ohio state line.
Cold air will filter back south into the area tonight with the low
temperatures ranging from the single digits to around 10 degree over
the north to the low/middle teens I-94 south. Any remaining snow
showers will be insignificant.
Arctic high pressure butted against the Front Range of The Rockies
in Alberta this morning is intensifying as shortwave ridging
develops downstream of the Aleutian low. Collapse of upper support
due to increasing confluence over northwest Canada will force the
high to weaken and quickly translate southeast into the upper
Midwest this morning before settling over the Ohio Valley tonight.
This will drive a persistent northwest gradient across the County Warning Area through the
first half of Friday. Cold advection will ease as the broad surface
ridge axis traverses the area, but with 925mb temperatures hovering
near -17c, highs will be stuck in the teens on Friday. Rapid
eastward progress of the high and dry northwest flow, supported by
relatively dry moisture fields in the 00z NAM, suggest that this
will be the case even in the presence of mostly sunny skies over
most of the area. The potential exception may be across the far northwest
where Lake Superior-Michigan plume will potentially contribute to a
more persistent stratocu deck. For this same reason, lows should
easily fall into the single digits Friday night, but have a higher
likelihood of remaining in the 10-15 degrees across the far west.
A moisture starved wave embedded in northwest flow will race across the
Great Lakes early Friday bringing nothing more than a few snow
showers across the far north with no accumulation expected.
Developing return flow will push highs into at least the upper 20s.
High uncertainty for the second half of the weekend. A shortwave,
which has yet to actually develop, is forecast by the suite of
guidance to emerge out of the Aleutian low late tonight and come
onshore in British Columbia on Friday evening. This wave will
quickly shear to southeast and interact to some degree with a closed
low over the Baja California.
What is known is that there is a reasonable likelihood for a
prolonged period of light to moderate accumulating snow to develop
over a wide area given given the broad area of isentropic ascent and
northward transport of 4-5 g/kg 700 mb specific humidity courtesy of
southern stream involvement. The key player in the local forecast
will be just how aggressively the northern branch digs over Montana,
Wyoming, and Colorado Saturday into Saturday night. The greater the
interaction with the closed low, the greater snow prospects become.
As it stands, the NAM and Gem are by far the most aggressive. The
European model (ecmwf) and GFS are considerably less so and confine accumulating snow
south of the Ohio border. However, the latter two still made
noteworthy moves toward more aggressive solutions at 00z in
comparison to their 12z counterparts. 00z GFS ensembles are split
between north/south solutions with the operational GFS and two
control runs remaining right in between and centering the action
along/south of I-80. Given that the short wave has yet to emerge, typical
nwp problems with closed lows, and general low medium range
predictability this winter, will simply allow the blended extended
grids to speak for themselves at this point.
Fresh to strong southerly winds will persist for the next few hours
as a weak low approaches from the west. Gusts will be limited to
around 30 knots given the stability of warm south flow. Winds will
veer to the northwest and increase considerably this evening into
tonight. Widespread gales are expected over Lake Huron with gusts to
strong gales expected over central and Southern Lake Huron where
significant wave heights will exceed 12 feet with higher maximum
wave heights. The gale watch has therefore been upgraded to a
warning. Considerable heavy freezing spray will accompany these
conditions and a warning has been issued accordingly. Winds and
waves will subside Friday afternoon into Friday night.
Lake Huron...heavy freezing spray warning from 7 PM this evening to 10 am EST
Friday for lhz362-363-421-441-462.
Gale Warning from 7 PM this evening to 10 am EST Friday for lhz362-
Heavy freezing spray warning from 10 PM this evening to 10 am EST
Friday for lhz442-443-463-464.
Gale Warning from 10 PM this evening to 10 am EST Friday for lhz442-
Heavy freezing spray warning from 7 PM this evening to 4 am EST
Friday for lhz361.
Gale Warning from 7 PM this evening to 4 am EST Friday for lhz361.
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).