Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1153 PM EST Tuesday Nov 24 2015
Patchy MVFR fog will continue to slowly expand overnight...but an
increasing pressure gradient will help maintain 3-5 knot southeast winds
and limit the possibility of dense fog. Any fog that does develop
will lift in the 13z-15z time frame. VFR conditions will prevail
thereafter as mainly clear skies give way to increase clouds at or above
10kft late in the forecast period.
For dtw...MVFR fog...mainly of the shall variety...will be the rule
overnight. While there remains a small chance of patchy dense fog by
dawn...generally expect visibilities to hold at or above 2sm.
//Dtw threshold probabilities...
* low in ceilings and/or visibilities below 1/2sm and/or 200 feet
Previous discussion...issued 341 PM EST Tuesday Nov 24 2015
Short term...through tonight
Upper level height rises continue locally both due to the departure
of the northeastern US trough and in advance of a weak shortwave
over the Central Plains. The accompanying low/mid-level warm air advection is
impressive...with 850 mb temperatures forecast to increase from -6c at 18z
today to +4c at 12z Wednesday. The inbound airmass is dry...and with the
warm advection expect the remaining clouds around 5kft /which were
trapped under an inversion/ to continue to dissipate. This will
leave only broken/scattered middle/high clouds for the rest of the afternoon
and evening. Models suggest these clouds will diminish overnight as
At the surface...high pressure is nosing into lower Michigan. This
high will tend to expand into the northeastern US overnight...while
pressures fall over much of the Great Plains and upper Midwest in
response to a deep upper trough over the Great Basin area. Expect
the high pressure to hang on over lower Michigan for at least the
first part of the overnight hours before the gradient tends to
increase late. A number of factors support fog formation overnight
including the expected lack of cloud cover...the initial weak
gradient...the existing snowpack and associated lingering boundary
layer moisture /evidenced by reduced visibility hanging on well into
the afternoon hours today/...and observation from earlier this morning
across northern Indiana and southern Michigan. This fog was quite
dense...and the potential for dense fog locally is there...though
there is still uncertainty with the limiting factor of the increased
gradient late tonight. Actually may see fog develop fairly quickly
after sunset and tend to dissipate toward morning as the gradient
kicks in. Expect lows to range from the middle-20s in the typically
colder spots of the interior thumb...to the lower 30s in the
metropolitan Detroit heat island.
Southeast Michigan will settle within a deepening middle level southwest flow
through the middle week period...positioned between upper ridging
draped across the southeast and an expanding region of height falls
encompassing the western Continental U.S.. warm air advection will strengthen
in earnest Wednesday and Wednesday night...netting roughly a 8c
increase in temperature through the 850-925 mb layer during this
time. Near surface rise slowed initially by retention of a less
responsive low level southeasterly gradient and the existing
snowpack...with a corresponding limitation to mixing potential.
Drying of the boundary layer as this advective process takes hold
will favor a high degree of sun...once any lingering pockets of fog
mix out. This will support highs reaching into the middle and upper
Southeast Michigan will remain along the immediate downstream side of the
emerging frontal zone marking the edge of the stronger height falls
Wednesday night and Thursday. Northward low-middle level moisture
transport will increase through this time under continued/
strengthening deep layer south-southwest flow. This will translate
into an increasingly mild and moist environment for the upcoming
Holiday. Strongest ascent tied to frontal dynamics will hold off
until Thursday night and Friday...leaving precipitation chances
largely dictated by the prospects of less defined lower amplitude
shortwaves lifting through the southwest flow augmenting a
background of weak but persistent moist isentropic ascent.
Positioning of the primary moisture transport vectors will focus
greater precipitation potential toward the northwest on Thursday.
Northern piece of energy shearing through the upper Great Lakes will
drive the cold front through Southeast Michigan on Friday. Solid
period of low-middle level frontal forcing beneath favorable upper jet
support and a pre-conditioned existing moist profile will favor a
solid widespread rainfall event during the daylight period. Ensuing
late day cold air advection will contribute to a muted diurnal
temperature response...readings in the 40s. Main moisture axis
appears to largely outpace the cooling process...leaving a limited
window for freezing levels to lower enough to support a brief
mix/change over to snow Friday evening before drier air takes hold.
Saturday through Wednesday. High pressure builds in behind fridays
weather system and temperatures drop with highs Sat/sun in the middle 30s
north to low 40s south. Overnight lows will dip back into the 20's
for early Sunday morning. On Sunday afternoon/evening another system
barrels out Texas and heads north. At present time...looks like
Sunday/Monday will carry just some slight chances of rain/snow
showers...with the low pressure centering itself over Ohio. The
front reorganizes Tuesday and pulls in better chances for
precipitation for the beginning of December.
Moderate southerly winds will develop on Wednesday. Warmer air
accompanying this process will lead to increasing Overlake
stability. This will limit the gust potential to 30 knots or less
despite an increasing sustained wind. This will result in a brief
period of marginal small craft conditions for elevated waves along
the near shores. Given the limited duration and marginal
conditions...will refrain from the issuance of a headline at this
stage. A modest pre-frontal southwest flow remains in place on
Thursday. Cold frontal passage on Friday brings a wind shift to
northwesterly. Expect a long duration rain event late Thursday into
Friday as the front slowly tracks east.
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).