Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 150703
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
303 AM EDT Fri Sep 15 2017

.DISCUSSION...

Thus far tonight, fog development has been on a slower pace than
last night. Regional satellite data show mid level moisture holding
over Se Mi. Even though this moisture has only been able to sustain
few to scattered mid clouds, it appears sufficient enough to have
some impact on the radiational cooling in the boundary layer. The
gradual departure of this moisture during the morning does however
suggest some continued fog development through daybreak.

Mid level heights will rise across the Great Lakes today into
Saturday as mid level ridging amplifies across the region. This will
support persistent sfc high pressure across the eastern Great Lakes.
A similar thermal profile and mixing depths as yesterday will lead
to highs from the upper 70s to low 80s. Light ESE sfc winds today
will advect some of the marine layer inland, supporting a potential
for some fog development again tonight. The sfc gradient is forecast
to increase a bit, leading to strengthening winds out of the SSW
atop the shallow stable layer. This may be enough to inhibit
widespread fog development. The SSW flow will persist through
Saturday, leading to slight warm air advection and supporting Sat
highs a degree or two higher.

An upper wave is forecast to lift from the northern plains Sat night
into nrn Ontario by Sunday night, driving a low to mid level frontal
feature into Lower Mi. The better mid level height falls will remain
well north of Se Mi, while mid level ridging holds across the ern
Great Lakes. This will result in a weakening/fracturing of the
frontal feature as it traverses Lower Mi Sunday afternoon into
Monday. Therefore, very low chances for convection will be supported
with this feature. An amplification in the long wave pattern is
forecast by the medium range model suite during the course of next
week. Lower Mi is forecast to remain under the influence of mid level
ridging. This will support mainly dry forecast with daytime highs
on the warm side of mid September averages.

&&

.MARINE...

High pressure remains over the region heading into the weekend
resulting in favorable boating conditions. Winds will be out of the
south through the weekend, largely remaining below 15 knots which
will keep waves heights in check. A weak cold front will drop
through Sunday evening, possible triggering a few showers. Winds
will shift northerly behind the front early next week.

&&

.PREV DISCUSSION...
Issued at 1153 PM EDT Thu Sep 14 2017

AVIATION...

Timing and coverage of fog will be the primary aviation weather
concern during the night through mid to late morning. Observations at
midnight are very similar to last night which supports an early
onset of fog at the terminal sites. The usual fog prone areas
elsewhere around lower Michigan were already reporting at least IFR
restriction during the late evening. Expect FNT, PTK, and YIP to
begin a downward trend shortly after midnight with less than 1 hour
of MVFR before quickly transitioning to IFR and LIFR. The potential
for dense fog at 1/4SM or less remains in play for all areas subject
to timing and coverage refinements in later updates. An exception is
likely the urban area around DET and possibly the MBS area where
some mid clouds and a few puffs of wind are lingering which could
prevent visibility from bottoming out there during the night. Model
soundings then indicate surface based moisture will be shallow enough
to follow a standard trend for dissipation mid to late morning. This
will be followed by VFR under scattered cumulus and light south wind
through afternoon into Friday evening.

For DTW... Expect fog development to get underway shortly after
midnight and last through mid morning. The lack of higher cloud
cover compared to last night will allow quick IFR/LIFR development.
At this point, 3/4SM looks most likely but 1/4SM or less is possible
toward sunrise through early morning before rapid improvement mid to
late morning.

//DTW THRESHOLD PROBABILITIES...

* Moderate for visibility 1/2SM or less late tonight through early
  morning.

PREV DISCUSSION...
Issued at 329 PM EDT Thu Sep 14 2017

Discussion...

A ridge of high pressure will remain in place through the upcoming
weekend. This will lead to a continue mild (to warm) stretch of
weather with highs climbing into the lower 80s in many locations
both Friday and Saturday while low temperatures build from 55-60
early Friday morning to the lower 60s in many area by early Sunday
morning.

There will be a continue chance of late night fog, especially
tonight, as low level moisture values remain rather high as the
remnants of Irma only gradually shifts to the east late today and
tonight. Patchy fog may even be possible late Friday and Saturday
nights as flow remain very weak under the aforementioned ridge of
high pressure. With the longer nights of early fall, these ideal
radiative cooling conditions may be enough to produce additional
bouts of minor fog formation.

An upper level shortwave exiting the northern plains toward Hudson
Bay will drag a cold front through SE Michigan Sunday night into
early Monday, bringing with it a chance for showers for the entire
region. The front is progged to slow and weaken just to our south,
which could lead to a slight chance of more showers into Tuesday for
areas near the Ohio border. Temperatures Sunday will be above
average in the lower to mid 80s, while post-front temperatures
Monday and Tuesday will be slightly cooler in the mid to upper 70s.

Model guidance then shows an upper ridge building into the area mid
next week. However, the models diverge on the timing and placement
of the ridge axis. The ECMWF solution places a strong ridge axis
directly over the Great Lakes from Wednesday through Friday which
would result in dry conditions for Michigan. On the other hand, the
GFS solution places this ridge axis to our east with a possible
disturbance and chance of rain on Thursday. For now, we will keep
low chance of precipitation mid-late week until we see more
agreement between model solutions.

MARINE...

Light and variable winds tonight will become increasingly southerly,
reaching into the 10 to 20 knot range by Saturday ahead of a weak
cold front, which looks to be moving through Sunday evening,
potentially triggering a few showers. Weak northerly flow to follow
the passage of the front early next week.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...SC
MARINE.......DRK
AVIATION.....BT


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