Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
621 am EST Monday Dec 29 2014
A broken coverage of lower VFR stratus will exist across the mbs/fnt
through the half of the day as colder air works into the region.
Coverage will remain more limited toward the south...a dry northwest
flow tending to restrict further southward expansion. Low
confidence does exist on the prospects for additional stratus to
fill back in tonight given this drier air. Lighter northwest winds
through the taf period.
For dtw...probability for low stratus development yet this morning
continues to diminish per upstream trends. There remains a low
potential for a lower VFR stratocumulus deck to develop with daytime
heating. However drier air will remain a definitive limitation and
will lean more optimistic with limited coverage at this time.
//Dtw threshold probabilities...
* low confidence in ceilings falling below 5000 feet through tonight.
Previous discussion...issued 351 am EST Monday Dec 29 2014
Short term... today and tonight
Northern stream upper troughing will take an increasing hold across
Southeast Michigan early this week. The corresponding downward
trend of the low level thermal profile already underway early this
morning...the process providing another round of extensive low
stratus. A favorably drier northwest gradient effectively confining
a modest moisture flux off the upstream Great Lakes over western
portions of the state. This environment will remain relatively
static throughout the day...the blanket of stratus now in place
slowly diminishing in coverage both from the northwest and south as
supportive moisture depth diminishes and contracts to the west.
Noting the inbound middle level cloud just upstream...satellite trends
will support a mostly cloudy forecast for at least central/northern
areas through at least the early afternoon period. A few stray
flurries not out of the question...but limited coverage/duration
will preclude a mention except across The Thumb. Highs today
ranging from upper 20s north to lower 30s south.
Secondary push of cold air will ramp up starting tonight...likely
translating into some form of southeastward stratus expansion.
Acknowledging this process will commence within a persistent drier
northwest flow...will simply lean toward partial coverage at this
stage. 925 mb temperatures fall below -10c by 12z Tuesday. Overall
strength of this airmass and allowing for a period of clear sky
overnight supports a slight nudge downward in lows. Readings
primarily middle to upper teens.
Long term... Tuesday through Sunday
Models through middle week are in good agreement with the large scale
pattern change taking shape. A digging trough over the West Coast
looks to get pinched off by a strong jet surging into western
Canada. This newly developing southern stream cutoff low will then
slowly track across the southern states in weak upper level flow
while further east the northern and southern streams attempt some
degree of phasing over the Great Lakes region. This is where models
start to vary by wide margins. More on this below.
For the middle week period we will be dealing mostly with a large
surface high pressure system settling south and east out of
Canada...eventually covering much of the central and eastern Continental U.S.
By Thursday. This occurring under a region of confluent flow of the
phasing jets. Models are advertising a very dry airmass above 850mb
through this period as Arctic air settles in. 850mb temperatures will be on
a downward trend as well. Currently coming in around -10c...they
will dip to near -15c by Tuesday and almost -20c by Wednesday. This
will result in a period of below normal temperatures with highs only
in the middle 20s Tuesday and near 20f on Wednesday. These are roughly
10 degrees below normal. With temperatures this cold aloft...some
lake effect snow will be generated off the lakes. Kept Southeast Michigan dry
through this stretch as inversion heights are coming in around 4kft
and most likely will not survive the trek east. Would not be
surprised to see some flurries...especially for
Shiawassee/Livingston/Washtenaw...but will hedge toward the dry side
Low confidence in how the rest of the extended forecast will play
out as the main suite of extended models remain very far apart. This
is a problem considered the potential development of a notable
surface low as the cutoff low lifts out of the Gulf Coast states
trying to phase with a northern stream wave this weekend. The European model (ecmwf)
has been trending slower and further south the last couple runs
while the GFS still has the low tracking right up through southern
Michigan which would be quite wet and potentially snowy on the backside as
very cold Arctic air filters in behind it. The GFS parallel is
faster than the GFS keeping the two streams separate and not
maturing the surface low until the East Coast. This line up better with
the ec but with a wide spread...little confidence can be given.
Trying to give credence to the southern shift the last couple runs
and hedged probability of precipitation down slightly to chance/slight chance this
weekend...but expect further changes down the Road.
An Arctic cold front will drop through the Great Lakes today
allowing much colder air to funnel into the region. This will
produce moderate northwest winds through Tuesday. A tightening
gradient and passing trough on Wednesday will lead to an uptick in
winds through Wednesday night. This may result in west or southwest
gales over central portions of Lake Huron during this time.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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