Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 152307
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
607 PM EST Thu Feb 15 2018

.AVIATION...

Lingering frontal boundary sustaining a narrow strip of dense fog
stretching directly across the DTW/YIP terminals early this evening.
Recent observational trends and latest model guidance suggest a 2-3
hour window yet for 1/4 to 1/2 mile visibility restrictions, before
drier air makes greater inroads after 03z. Diminishing coverage and
magnitude of fog north of this corridor now easing conditions in
VFR. Window of VFR for all terminals overnight, with a watchful eye
on the inbound area of stratus just now dropping south into northern
lower MI. This moisture tied to an increase in cold air advection
during the early-mid morning hours, potentially translating into a
period of MVFR stratus for all locations. Much drier air trailing
this cloud deck then favors a mostly clear sky for much of the
Friday period.

For DTW...2-3 hour window yet for 1/4 to 1/2 mile visibility
restrictions, before drier air makes greater inroads and visibility
improves after 03z. Coincident removal of LIFR stratus deck during
this time, potentially leaving a period of VFR for a portion of the
overnight period. Window for some renewed low stratus centered 09z-
14z Friday morning.

/DTW THRESHOLD PROBABILITIES...

* High for visibility at or below 1/2SM and ceiling 200ft through
  02z, improving conditions thereafter.

* High for ceiling below 5000 ft prior to 04z, the low overnight
  through Friday morning.

&&

.PREV DISCUSSION...
Issued at 316 PM EST Thu Feb 15 2018

DISCUSSION...

Through much of the day, there has been a persistent region of dense
fog along/north of a sfc warm frontal feature. This front has held
nearly stationary just north of the state line all day. An abundance
of low level moisture transport into this front and atop a melting
snow pack, with inbound sfc dewpoints across nrn Indiana in the 50s,
has managed to sustain dense from from the state line up through the
I-94 corridor. In fact, visibilities have remained below a quarter
mile through much of this region. A surface reflection now over
Illinois will lift across metro Toledo early this evening. The
additional convergence will likely cause fog to expand in coverage,
possibly necessitating the expansion of the dense fog advisory. The
departure of the sfc wave and frontal feature to the east after 00Z
will bring a little drier air into the boundary layer, thus eroding
the fog.

The ageostrophic response within a strengthening upper jet core has
resulted in a increase in forcing along the lower tropospheric
portion of the front. This along with a surge in moisture in the 900-
800mb layer has led to an expansion of showers, primarily from Metro
Detroit and Ann Arbor and points south. An axis of weak mid level
instability south of the I-94 corridor will support some higher
intensity showers (some possible thunder) near the Ohio border.
Given the lower portions of the frontal zone will remain most
active, rain chances north of roughly the M 59 corridor will remain
low through the evening. The frontal forcing will slowly push south
and east of the region during the course of the evening, allowing a
push of cooler and drier post frontal air. Mid level subsidence
within the descending branch of the frontal circulation will support
a dry forecast through the overnight, although some stratus may
develop within the post frontal cold air advection.

Mid level short wave ridging will expand across Lower Mi Fri into
Fri night, with associated sfc high pressure building from the Upper
Midwest across the Ohio Valley. Ample dry air advection and diurnal
mixing should erode any morning stratus, supporting afternoon sun.
Despite 925mb temps dropping toward -10C Friday, diurnal mixing will
boost afternoon highs into the 30s. There is some semblance of
agreement in the 12Z model suite driving a mid level short wave from
the mid Mississippi Valley into the Ohio Valley Sat/Sat evening,
providing a chance of snow showers. A plume of deep moisture is
shown to be shunted off across the Upper Ohio Valley as the wave
approaches srn Mi, so at this point, accumulating snow does not
appear likely.

High pressure departing the Ohio Valley on Sunday will maintain dry
conditions through the day as increasing southerly flow allows highs
to reach the lower 40s. The first of a series of low pressure
systems originating from the central Plains is then progged to eject
northeastward on Monday along a tightening baroclinic zone to the
west of Lower Michigan. The track of these lows will likely keep SE
Michigan in the warm sector Monday through Tuesday as deep southerly
flow advects warm, moist air across the region. Repeated periods of
showers can be expected starting early Monday as moisture
overspreads the region before continuing through late Tuesday. Some
rumbles of thunder will also be possible late Monday into Tuesday as
elevated instability pushes north across the region ahead of the
frontal boundary. The final low pressure system lifting through the
region late Tuesday will then pull a cold front eastward across SE
Michigan bringing an end to precipitation by midweek. Temperatures
behind this front will fall closer to average with highs in the
upper 30s as high pressure builds back into the region.

MARINE...

Mild southwest flow will transition to two surges in northwesterly
flow in the wake of a passing surface wave of low pressure across
the Michigan/Ohio border this afternoon and into this evening. The
first surge in northwesterly flow will occur ahead of a cold front
passage early tonight, with gusts to 30 knots possible over the ice
free waters of northern and central Lake Huron. The second surge
will occur behind the frontal passage late tonight into early Friday
morning, with another round of gusts to near gale force. Lack of a
consistent momentum field accompanying the frontal passage will
limit the wind gust potential both in magnitude and duration for a
Gale Warning. High pressure will build across the region Friday
leading to decreased winds. A low pressure system and attendant
frontal boundaries will pass by across the northern Great Lakes
Saturday leading to a tighter pressure gradient and gusts
approaching 30 knots.

HYDROLOGY...

An area of low pressure will continue to ride along a stalled warm
front this afternoon, bringing a round of rain and possible
thunderstorms to areas south of the I-69 corridor. Rainfall amounts
around a quarter of an inch will be possible near the Ohio border
through this evening. Combined with milder air lifting north into
the region aiding in snowmelt, some ponding of water on roadways
will be possible in the heavier showers.

Another surge of warm air early next week will accompany the
approach of a slow-moving frontal boundary and multiple waves of low
pressure. Depending on the eventual placement of the frontal
boundary, heavy rainfall is possible next week which may lead to
rapid rises on area rivers and poor drainage flooding.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Dense Fog Advisory until 7 PM EST this evening for MIZ075-076-082-
     083.

Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....MR
DISCUSSION...SC/JD
MARINE.......IRL
HYDROLOGY....IRL


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