Scientific Forecaster Discussion
fxus63 kdtx 180450
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1150 PM EST sun Nov 17 2019
There was only one area of isolated light rain/ice pellet mix that
lifted northward this evening. The precipitation was able to trigger
surface observation equipment across Metro Detroit before
evaporating/dissipating. It remains a very difficult pattern to pin
details down and provide timing to sensible weather changes as deep
moisture is lacking and tangible features are very muddled. Initial
surge of equivalent potential temperature is now underway with a
signficant rise in surface dewpoints the past 6 hours. Model data
shows a neutral temperature advection pattern now during much of the
forecast period with virtually no system relative isentropic ascent.
As a result, favor a dry forecast. Kept with prevailing MVFR
conditions on Monday with moisture now over the state.
Dtw... no more precipitation anticipated through tonight.
Dtw threshold probabilities...
* moderate for ceiling 5000 ft or less tonight. High Monday.
issued at 400 PM EST sun Nov 17 2019
20z surface analysis depicts weakening surface high over the
northeast while some kinematic influence is retained until early
tonight. Surface flow will remain east/southeast this evening with limited
variability in surface dewpoint temperatures, but denser mid-high
cloud will continue to stream in overhead from the SW. High degree
of uncertainty with tonight's precip forecast given the wide spread
in geographic placement of quantitative precipitation forecast. Hires solutions have been slightly
more consistent in producing some early precip through the central
portion of the County Warning Area as an upper level trough pivots east and tilts
negatively overnight. Forecast sounding profiles generally suggest
the maintenance of top-down saturation through at least the 750 mb
level suggesting plenty of ice nucleation potential aloft as in-
cloud hydrometeors initialize as snow. Looking at the lower
tropospheric depth, models have trended toward a fairly shallow sub-
freezing near-surface layer as cross sections indicate ample
moisture advection just above in the 800-650 mb layer. While local
probabilistic guidance holds onto a low-end chance for fzdz as well
as blended model guidance, hedged forecast toward light rain
overnight with the afternoon update. Higher cloud cover and
temperature obs out-pacing the forecast should help limit the
spatial coverage of surface/pavement temps falling below freezing
prior to the dissipation of any light precip that does occur. Light
winds transition to nearly calm and variable after midnight. Lows
bottom out a degree or two below freezing prior to sunrise which is
more in-line with normal readings.
Primary circulation aloft with attend front will begin exiting lower
Michigan Monday morning. Airmass modification will not be overly
impressive as flow becomes more zonal through much of the column
promoting gradual drying, but near surface layer winds will be weak
limiting thermodynamic changes to cold-season diurnal cycle. Given
weak isentropically driven forcing (mainly confined north of I-69),
pulled pops down keeping Monday mainly dry and cloudy for much of
the day. Deep layer moisture quality is largely absent with variable
degree of dry air in sub-850 mb layer. Given a similar thermal
profile from Sunday to Monday, highs will within a few degrees
Monday afternoon near 40 degrees.
Low-end precip chances extend into Tuesday as shortwave energy
ripples through spinning up some mid-level vorticity, especially in
the late afternoon-early overnight timeframe. Nothing more than slgt
chc pops at this time, mainly in liquid form. Flow begins to back
southwesterly on Wednesday which will initiate a period of lighter
warm air advection in conjunction with height rises and impinging ridge axis.
Plenty of dry air aloft will keep conditions dry and capped,
although there will be some trapping of incidental near-surface
moisture before an semi-active subsidence regime works lower
highlighted by a positive Omega field. Temps will continue to trend
a few degrees warmer during this time with weak winds. Upper level
moisture will keep high cloud socked in throughout the day and
overnight helping maintain overnight lows above freezing.
Next well organized system arrives on Thursday as a warm front lifts
into the Great Lakes nudging 850 mb temps close to 10 c. Widespread
and longer duration rainfall is expected with the warm front and the
subsequent cold frontal passage as low pressure deepens and races through
northern lower Michigan Thursday evening driven aloft by a sharp
trough folding into the southern branch of a split jet. Changeover
to snow is possible late Thursday night as stronger cold air advection from
northwest promotes freezing temperatures return. Mainly dry
conditions through next weekend with cool temps.
Diminishing winds and waves this evening followed by favorable
marine conditions to close out the weekend as high pressure advances
into the Canadian Maritimes. Approaching surface trough tonight into
Monday morning will veer winds towards the south-southwest. Winds will remain
light and waves low through the first half of the week.
Northwesterly gusts to gales possible Thursday night as deepening
low pressure lifts through Michigan.
Lake St Clair...none.
Michigan waters of Lake Erie...none.