Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1146 am EST Wednesday Nov 26 2014
Extensive high MVFR/low VFR stratus will persist. Ceilings will rise
to around 4kft and some potential for breaks in the cloud deck
will arise as light easterly winds winds back to the northwest
overnight and slightly drier air works into the low levels. A
broken coverage of snow showers may accompany a cold front as it
works across the area 20z-00z Thursday afternoon but confidence
does not yet warrant a mention in the official taf.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet through this afternoon.
Previous discussion...issued 308 am EST Wednesday Nov 26 2014
Short term... today and tonight
Weak surface ridging tucked beneath lower amplitude middle level
troughing will define conditions across Southeast Michigan today.
Extensive stratus canopy holding firm early this morning within the
lingering 925 mb thermal trough...a solid deck with a defined
clearing line just now exiting northeast Illinois. Recent trends provide
growing pessimism toward the cloud forecast heading into today.
Lingering thermal troughing will move little today...suggesting a
static thermodynamic profile carrying a solid inversion layer.
Recent model relative humidity fields retain a higher degree of moisture just
beneath the inversion...some reinforcement occurring by this
afternoon/evening as developing light east/southeast flow introduces
a weak Lake Erie/Huron component. A period of increasing
moisture depth may become adequate to support some flurries during
this time. Increasing convergence across Lake Huron as greater
lake aggregate troughing takes hold is forecast to drift westward
with time within this easterly gradient. Potential does exist for
this forcing to briefly work into the eastern thumb this
evening/tonight. Worthy of a lower probability mention of snow
showers at this stage...minor accumulation.
Cold resident airmass varies little from that noted yesterday...
highs this afternoon arriving in the lower and middle 30s. Lows
tonight middle 20s.
Cold (zero to -20 f at the surface) and dry air clearly on the move
over south central Canada...as strong upper wave drops southeast
through the Midwest early this morning. Really modest moisture
axis/850-700 mb Theta-ridge moving through Southeast Michigan on
Thanksgiving...with precipitable water values only around 0.25 inch...ahead of 500
mb shortwave trough axis/upper level pv filament. 925-850 mb average
temperature of -8 c through most of the day...with the colder air still
lagging through 21z...thus expect accumulating snow showers to
struggle to make it over Southeast Michigan. More favorable thermal
profiles arrive late in the day...but then the crashing inversion
heights and dry nature of Arctic airmass overwhelms the central
Great Lakes...and would expect any lingering light snow showers to
taper off to flurries or end...as good drying seen occurring at the
850 mb level. However...will have to watch for brief period of
northerly low level convergence off Lake Huron glancing northeast
thumb region early Thursday evening with light snow accumulations
before surface ridge arrives toward Friday morning. Airmass will be
very cold...as indicated by the negative lower teen temperatures at 925
mb...but lack of snow cover over the County Warning Area precludes going for single
numbers...and will hold mins in the teens.
Still looking for sheared out upper wave/warm advection pattern
for Friday...but there is barely a buckle in the 500 mb height
fields...with the upper wave weakening/dampening out.
So...confidence is not high we will be able to saturate the low
levels from the departing Arctic airmass coincident with the
middle level forcing...and will maintain just a chance of snow late
Friday into Friday night.
Good Post warm front southwest flow over the central Great Lakes
over the weekend...supporting solid warmup...as temperatures
approach at least 50 degrees on Sunday...with 00z Euro 925 mb temperatures
(7-10 c) indicating the potential to be even milder...dependent on
timing of next cold front and associated showers.
Very light winds around today as high pressure will be in control.
Tomorrow will be a much different story...as an Arctic front slides
through the central Great Lakes during the day. A brief period of
gale force winds appears likely for the central portions of Lake
Huron by 00z Friday...but the duration is in question...and not
sure if they will last 3 hours or more. None-the-less....elected to
issue a gale watch to cover this possibility. The cold airmass
streaming in Thursday night will allow for just a slow decrease in
the northwest winds...as the waning pressure gradient is offset by
the increasing boundary layer depth over the Open Lake. A warm front
lifting into the region Friday will flip the winds to the
southwest/south by Friday evening...with gusts of 25 to 30 knots
expected over much of Lake Huron before milder air and greater
stability arrives over the weekend.
Lake Huron...gale watch from Thursday afternoon through Thursday evening for
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).