Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
607 PM EST Tuesday Dec 10 2013
The subtle increase in surface dewpoints over the last couple of hours
are suggestive of some better moisture flux off Lake Michigan. The
westerly flow should thus sustain periodic flurries and MVFR clouds
at fnt and ptk into the night. Larger scale subsidence will
overspread southern Michigan this evening into the early part of the overnight
and should diminish the response off Lake Michigan...possibly leading to
some clearing at the terminals. An upper level wave will then pass
through the region on Wednesday. This will invoke a substantial
response off Lake Michigan and will lead to higher intensity snow
showers. These snow showers should be at or in the vicinity of fnt
and ptk during the morning /ariving around 11z/...then shift into
Detroit during the afternoon as the flow veer to the west-northwest.
Accumulations around an inch are certainly possible if the snow
showers remain persistent enough at the terminals.
For dtw...the low level flow has backed slightly toward the south-southwest the
last couple of hours. This will likely hold MVFR conditions and
flurries/light snow showers north of metropolitan through the evening and
possibly well into the morning. Ceilings are also expected to be
highly variable through the night.
//Dtw threshold threats...
* medium confidence in ceilings below 5000 feet tonight. High
confidence on Wednesday.
* High confidence precipitation type will be all snow.
Previous discussion...issued 405 PM EST Tuesday Dec 10 2013
Short term...this evening and tonight
Shortwave energy seen on WV imagery has now moved to the east of the
County Warning Area...but the broad 500 mb trough and corresponding cold air aloft
remains in place. 850 mb temperatures remain in the negative teens,
but lapse rates are not exceptionally steep on the east half of the
State. Lake Michigan provides plenty of moisture and warmth for our
western areas to see light lake effect snow...and that chance will
last through the night. The thermal trough emitted from Lake
Michigan will produce enough of an Omega field to produce some light
snow northwest of the glacial ridge early this evening as well.
The lake effect setup will get a synoptic boost from a cold front
looking to Cross Lake Michigan overnight. Ahead of the front, some
surface moisture convergence will help funnel an Omega maximum from
M-59 to I-94. A half-inch of snow will be possible in that corridor
before 7am...and will be the favored location for snow bands to
setup for the remainder of the day tomorrow.
Low temperatures will be held up (closer to MOS guidance) with the
strong wind field and broken/overcast cloud deck in place. Despite upper air
values well below normal, our min t's should stay within 10 degrees
of normal in the low to middle teens. Wind chill values tonight,
however, will feel like 5 below to zero in a 10 to 20 miles per hour wind.
Long term... Wednesday and beyond
Afternoon satellite imagery offers evidence of the next short wave
over the northern rockies and plains on track to affect the Great
Lakes Wednesday. Corresponding radar composite and surface
observations indicate respectable pockets of snow given the moisture
supply limited to middle level Pacific sources. A boost from Lake
Michigan is then expected to enhance the pattern of snow as the
system moves through lower Michigan during Wednesday morning through
about middle afternoon. The surface reflection of low pressure is
expected to merge with lake induced convergent flow off the south
end of Lake Michigan to form a hybrid band/Arctic frontal feature
that will extend through Southeast Michigan roughly along the Interstate 69
to M 59 corridor and then settle southward and eastward with time.
Cross sections in the NAM indicate a distinct but shallow
ageostrophic circulation within the low level Theta-E gradient
setting up within the larger scale surface trough. Model vertical
motion is also confined to the low levels but within favorable
temperature around -15c with good saturation. That along with steep
lapse rates below 850 mb adds confidence to the potential for lake
bands to survive the trip from Lake Michigan within the surface
trough/developing Arctic front. There is some uncertainty on
location and duration, but a quick burst of snow is likely to
produce up to an inch of accumulation as the larger scale weather
features move quickly through the region. There is some potential
for extra achievement based on duration which will have to be
evaluated in the short term.
The passage of the system will bring in a reinforcing surge of
Arctic air Wednesday night into Thursday. Min temperatures in the single
digits look easily reachable by Thursday morning based on below zero
afternoon readings today over northern Minnesota. Some minor
modification from the lakes will soften the air mass slightly,
along with some pockets of cloud cover, but lingering gradient wind
will maintain wind chill near advisory values of -15f.
The next major system to impact Southeast Michigan will be on Friday
night and Saturday as a low pressure system tracks across the Ohio
Valley. Southeast Michigan still appears to be north of this system
keeping precipitation as all snow. Snow will begin on Friday night
across the southern counties and eventually impact all of Southeast
Michigan by Saturday morning. However...there is still quite a bit of
uncertainty regarding the track of the system...quantitative precipitation forecast amounts along
with resulting snow accumulation amounts and length of lingering
snow after the system departs. Even with this uncertainty...feeling
fairly confident that it will snow during the Saturday timeframe and
have bumped up precipitation chances to likely probability of precipitation across all of
Southeast Michigan. Have included a chance of snow showers for the
area for Saturday night through Sunday to account for lingering
showers. High pressure returns along with very cold Arctic air for
Sunday night /lows in the single digits/ lasting through Monday
before the next clipper system moves across the Great Lakes on
Tuesday bringing another chance of snow.
The combination of very cold temperatures with a tightened pressure
gradient will bring westerly gales for the open waters of Lake Huron
tonight through Wednesday morning. A heavy freezing spray warning
also remains in effect to accompany the Gale Warning. A weak surface
ridge will try to push through the Great Lakes on Thursday...
however...this high is expected to be squeezed southward which will
keep relatively strong gradient flow over the Great Lakes well into
heavy freezing spray warning...lhz361...from 10 PM Tuesday to 10 am
Gale Warning...lhz361...from 10 PM Tuesday to 10 am Wednesday.
Heavy freezing spray warning...lhz362-lhz363-lhz421-lhz441-lhz462-
lhz463-lhz464...until 10 am Wednesday.
lhz464...until 10 am Wednesday.
Small Craft Advisory...lhz422-lhz442-lhz443...until 10 am Wednesday.
Lake St Clair...
Small Craft Advisory...until 4 am Wednesday.
Michigan waters of Lake Erie...
Small Craft Advisory...until 4 am Wednesday.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).