Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 211550
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1050 AM EST Sat Jan 21 2017

.UPDATE...

A quick update discussion to provide a few thoughts and tentative
plan on dense fog advisory that was extended until noon. Surface
observations, since sunrise, have been painting a much more dire
picture with surface visibilities at many locations of 1/4SM or
M1/4SM. Those observations led to the decision to extend the dense
fog advisory for all counties through the remainder of the morning.
Traffic camera video pinpoints the dense fog at a majority of the
area, primarily over and adjacent to the spine of the glacial
ridge. In other words, from southwest to northeast across the
CWA. Interestingly, the Tri Cities and downtown Detroit/Monroe are
two areas that are not experiencing higher impact reduced
visibilities. Southeasterly surface wind trajectories suggest some
minor upsloping component to this fog.

Will be monitoring observations the next 1.5 hours to make a final
determination on the end time of the ongoing dense fog advisory.
However, there are a couple of items to suggest the fog will lift
enough in the near term to allow the advisory to expire at noon.
The HRRR output has been consistent in retreating the dense fog
to the Lakes and Ontario by 18Z this afternoon, and aforementioned
traffic cam video has shown some small improvements recently.

Will address fog potential for tonight in this afternoon`s
forecast package.

&&

.PREV DISCUSSION...
Issued at 436 AM EST Sat Jan 21 2017

AVIATION...

VLIFR vsbys firmly in place north of a warm front positioned
immediately to the south. High confidence in improving vsbys as the
front lifts north. However, warm sector inversion combined with the
potential for some added moisture flux off of Lakes Erie and Saint
Clair casts doubt on cigs improving beyond IFR during peak heating,
especially in the Detroit area. Allowed for some brief improvement
in cigs to low MVFR in the wake of the warm front for KMBS/KFNT.
Similar conditions redevelop tonight, thus the inclusion of 1/4SM
FG at all locations. Stratus/FG encroaching off the lakes will
warrant watching through the evening for potential timing
adjustments.

For DTW...Window for potential breakout in the afternoon is
diminishing as confidence increases in SE flow off the water. IFR
could easily plague the area through the day even if other
locations clear out. Elected to retain BKN006 as a result.

//DTW Threshold Probabilities...

* High for cigs aob 5kft

* High for cig/vsbys aob 200ft and 1/4SM tonight.

PREV DISCUSSION...
Issued at 353 AM EST Sat Jan 21 2017

DISCUSSION...

Conditions remain favorable for widespread dense fog due to the
exceptionally mild and moist air mass in place over the region.
These conditions will only be reinforced during the morning as a
warm from moves in with a mixture of drizzle and rain showers. The
dense fog advisory remains in effect through 10 AM before some
improvement is expected south to north through the region, possibly
as early as mid morning near the Ohio border. This will be aided by
temperatures making a run into the lower and mid 50s most locations
during the afternoon, but given this mild and moist air mass, fog
will likely develop and thicken again tonight.

Early morning satellite imagery indicates an upper level circulation
moving northward through the Midwest with a weak reflection in the
broad surface trough through the Plains. This system is
strengthening the warm front that extends eastward through northern
Indiana and Ohio before sunrise and will pull it northward into
Lower Michigan during the morning. The result will be a modest
increase in the pressure gradient that will produce light southeast
wind over the region and introduce an advective component to fog and
drizzle maintenance through the diurnal minimum. Surface dewpoint
ranges from the lower 30s in the Saginaw Valley to mid 40s near the
Ohio border at press time which will develop northward with the
front. Corresponding low level theta-e advection will also support
scattered rain showers which will be just a cosmetic concern.

The surface low is projected in model solutions to reach SE
Minnesota by mid afternoon, far enough north to pull the warm front
into central Lower Michigan. This may not be far enough north to
scour the fog from the Tri Cities and northern Thumb and an
extension of the advisory may be required there. The warm sector of
the system should make it to the I-69 corridor by late in the day
with enough of a mixed layer to produce broken low cloud coverage
from there south and bring in the lower to mid 50s surface temps.
The low pressure system is then shown to move north of Lake Superior
tonight and leave a 1000-850 mb thickness gradient over Lower
Michigan as the surface pressure gradient flattens out again between
it and the next low pressure system moving through the southern
Plains. Rich boundary layer moisture will be left behind judging by
surface dewpoint in the upper 40s to possibly lower 50s indicated in
some of the mesoscale model output. Modest coverage of mid and high
clouds will expose the boundary layer to radiational cooling and
open the door for redevelopment of fog during the evening through
Sunday morning.

That southern Plains system will be the remains of the current
Pacific coast upper trough. It will be driven inland by a new round
of long wave trough development over the eastern Pacific and will
become take more of center stage in the forecast for the Great Lakes
during Sunday. The 00Z model package still shows some spread in the
position of the system but depicts a general northward trend Sunday
into Sunday night, at least in terms of the northern fringe of the
precipitation pattern. This appears reasonable as there will be an
upper level trough connection to the northern stream that has
potential to set up an energetic deformation pattern over the Great
Lakes. The good news here is that the system will interact with the
still mild thermal pattern over the region. It won`t be as warm as
today with readings limited to the 40s by increasing easterly
surface wind but will be warm enough to maintain an all liquid
precipitation type through Sunday night and even into Monday. Not
until Monday night does enough cold air get drawn in from eastern
Canada for a rain/snow mix before the system exits eastward by
Tuesday.

The break in the action will be short as the next low pressure
system is set to arrive in the Great Lakes by Wednesday. Global
model solutions project a track through the central Great Lakes and
another round of mixed precipitation for SE Michigan. Above normal
temperatures through mid week then take a turn down to readings more
typical for late January for the end of the week into next weekend.

MARINE...

Dense fog will impact the waters through at least Sunday morning,
though clearing is possible for Lakes Saint Clair and the Michigan
waters of Lake Erie during the daytime hours today. Dense fog
advisories are now in effect for all marine zones. Winds and waves
will remain quiet until late Sunday when strengthening low pressure
well to the south will bring a period of fresh easterlies that will
persist well into Monday. Onshore flow will build waves toward small
craft advisory criteria during this time...conditional on ice
cover as usual.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Dense Fog Advisory until noon EST today FOR MIZ047>049-053>055-
     060>063-068>070-075-076-082-083.

Lake Huron...Dense Fog Advisory until 10 AM EST Sunday FOR LHZ421-422-441>443-
     462>464.

     Dense Fog Advisory until 10 AM EST Sunday FOR LHZ361>363.

Lake St Clair...Dense Fog Advisory until 10 AM EST Sunday FOR LCZ460.

Michigan waters of Lake Erie...NONE.
&&

$$

UPDATE.......CB
AVIATION.....JVC
DISCUSSION...BT
MARINE.......JVC


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