Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
631 PM EST Monday Mar 2 2015
Clear skies and VFR will persist through tonight beneath the center
of high pressure. Northwest winds around 10 knots will slacken this
evening and become light to variable before turning more decidedly
southeast toward morning. Plume of deeper moisture will then expand
across Southeast Michigan from late morning through the afternoon
period. A period accumulating snow in MVFR/IFR all locations 14z-
18z...before a south to north transition in ptype during the
afternoon. Snow accumulation of 1 to 3 inches...highest amounts ptk
northward. Will go ahead with a defined 3-4 hour afternoon window
for freezing rain all locations except mbs. Eventual move toward
rain/drizzle in IFR as surface temperatures climb above freezing
late afternoon into the evening.
For dtw...dry air will initially impede progress of snow. Ceilings not
expected to fall to 5kft until around 12z. Approx start time for
snow is 14-15z. A period of IFR visibility in moderate snow is possible
shortly after onset but duration will be short. Precipitation will
gradually become less organized as rising temperatures force a
change to rain. In the interim, there will be the potential for a
period of freezing rain and/or sleet 17z-20z. Eventual transition to
drizzle/br appears likely late Tuesday afternoon as dewpoints rise
well into the 30s.
//Dtw threshold probabilities...
* high in ceilings below 5kft after 12z Tuesday
* medium in visibility below 1/2 mile in fog after 21z Tuesday
* high in snow as dominant ptype through 17z / freezing rain - sleet 17z-20z
/ rain or dz after 20z
Previous discussion...issued 320 PM EST Monday Mar 2 2015
High clouds embedded within the strengthening upper wind field are
advecting across the upstream low amplitude shortwave ridge axis and
are racing east across the upper Midwest. These clouds will provide
the only inhibition to radiating for the first half of the night as
surface high pressure slowly departs from the region and maintains a
light gradient through 06z. During this time, decent radiating
conditions should allow temperatures to fall into the teens
areawide. Increasing southeast flow and thickening cloud cover will bounce
surface temperatures back up toward 20 degrees by sunrise.
Long term...Tuesday through Sunday
Main foucs is on the mixed precipitation event expected Tuesday. In
summary...Winter Weather Advisory is in effect beginning 13z. 1-2
inches of snow/sleet for I-94 counties and south...with 2-4 inches
north. Additionally...up to 0.1 inch ice is possible from the I-69
corrdior southward. Brunt of the event is expected between 15-21z.
Good intermodel and run-to-run consistency with the general
evolution of the large scale pattern and timing of local impacts.
Shortwave energy will eject from the Southern California upper low and zip
across the plains into the Great Lakes Tuesday. No real phasing
between this energy and the northern stream trough dropping out of
the prairie provinces...so expecting this event to be a relatively
quick-hitter. Moisture transport is expected to be excellent
between the departing surface high and a broad area of surface low
pressure attendant to the large-scale upper trough...with NAM
forecasts showing precipitable water near 1 inch along the Ohio border for a brief
period around 21z...and cross sections showing 850 mb-700 mb specific
humidity of 4-6 g/kg from 15-24z. Main forcing mechanism will be
isentropic lift along the low/middle tropospheric temperature
Despite model agreement with the large scale features and
evolution...considerable uncertaintly remains with respect to
thermal fields and resulting p type. General progression will be
snow/sleet to freezing rain/rain. Model soundings indicate a
considerable dry layer between h9 and 700 mb must be overcome...so lift
will initially go to saturating the column. The good moisture
transport will be able to overcome the dry layer though with precipitation
breaking out roughly 13-15z and quickly spreading across the region.
Precipitation should be snow initially before the warm nose centered around
800 mb pokes in from south to north. This will cause precipitation to mix with
and change to sleet and freezing rain. Surface temperatures are a tough
call...with warm air advection being offset by wet bulbing. There does exist a
brief window for freezing rain along/south of I-69 roughly 17-21z as
current forecast calls for surface temperatures to remain at or below freezing
until 18-21z. However...this does not appear to be a classic setup
for a good icing event given the strong warm air advection /lack of low-level cold
air resupply/ and marginal magnitude of the warm nose. Back roads
will likely see greater impacts though given frozen ground and may
see liquid freezing on surfaces even if air temperatures are above
freezing. Moderate/locally heavy precipitation is possible 15-21z given the
moisture quality and reduced static stability above the low-level
temperature gradient as indicated by cross sections...possibly
leading to a more convective/showery looking precipitation shield.
Model quantitative precipitation forecast consensus is not great with European model (ecmwf) spitting out a broad
0.25 aross the area while NAM appears to be more agressive with the
dry slot drying things out over the north and showing enhanced
convection to the south of the dry slot leading to locally up to
0.50". Opted for a compromise with 0.25" across the north to 0.35"
across the south...though given convective nature of precipitation it will
likely not end up being a smooth distribution. A combination of the
dry slot arrival and main moisture axis peeling off to the east will
cause precipitation to come to an end 21-00z. Some potential for dz/fzdz as
this occurs. Cold air advection ensues after 00z as winds veer westerly...with lows
in the low/middle 20s by 12z Wednesday.
Long wave trough amplification across the western Great Lakes on
Wednesday will usher in another round of Arctic air to the region
/850mb temperatures are forecast to drop to -22c by 06z thurs/. This will
result in Wednesday high temperatures occurring in the morning and slowly falling
during the afternoon and evening. A strong baroclinic zone across
the eastern US and long wave trough amplification will intensify the
upper jet streak over the Great Lakes /190+ kts/ on Wednesday. While
the frontal dynamics and deep moisture will reside well south of the
state...a fair amount of middle/high level clouds are likely to reside
over Southeast Michigan. This along with some low level moisture along a
secondary surface trough positioned across the northern Great Lakes may
sustain a good deal of clouds well into Wednesday night...supporting a
conservative approach to Wednesday night mins /around zero/.
Thursday will be the coldest day of the forecast period as highs may
struggle to reach 20 with a deep pool of Arctic air over lower Michigan.
Surface high pressure will then slide into the middle Atlantic on
Friday as middle level heights rebound across the Great Lakes region.
With the departure of the deep cold pool to the east and
strengthening southerly flow...a modest warming trend will occur Friday
into Saturday. The overall long wave pattern this weekend into early
next week will continue to feature a long wave ridge over the West
Coast and trough across the Great Lakes/eastern Canada. The wavelength
will just not be as amplified as the last month. This and several
shorter wavelength perturbations rotating into the northern Great Lakes
will hold temperatures a little colder than early March means during the
later half of the forecast period.
The departure of high pressure to the east and approach of a low
pressure system will cause southerly winds to develop tonight. The
surface low is forecast to track across The Straits on Tuesday.
Southerly winds will increase as this low approaches. Winds will
then veer to the west Tuesday night as the low departs to the east.
There will be an increase in the westerly winds on Wednesday as another
surge of Arctic air overspreads the lakes. There is a potential for
gusts to approach gale force over far Northern Lake Huron on
Wednesday. The chances for any gale force gusts to exceed two hours
in duration does not appear high enough to justify a gale watch
at this time.
Michigan...Winter Weather Advisory from 8 am to 7 PM EST Tuesday for
Winter Weather Advisory from 8 am to 4 PM EST Tuesday for miz075-
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).