Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 150826
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
326 AM EST Thu Feb 15 2018

.DISCUSSION...

Near-surface moisture is making a final northward push this morning
in response to the mid-level disturbance lifting across the UP at
press time. This push is noted by northeastward advection of higher
dewpoints and dense fog out of the Chicago area across southwest
Lower Michigan and will continue through mid-morning. The dense fog
advisory has been expanded north by one more tier to include the I-
69 counties in anticipation of the momentary arrival of lower VSBY.
Passage of the upper wave and subsequent flattening of the height
field will allow drier boundary layer air to begin settling
southward through the area well in advance of the front itself. As
such, residence time, if any, of dense fog north of the advisory is
expected to be very short-lived. A gradual improvement in VSBY is
then anticipated in northern tier of the advisory. Uncertainty
increases for the remainder of the advisory after the scheduled
expiry time. Moisture axis will not make any progress through this
area until 21-00z period. However, proxies for boundary layer
stability such as Ri number and 0-0.5km theta-e lapse rate, do
indicate some reduction in near-surface stability suggesting
increasing mixing potential and subsequent vertical dispersion of
moisture late morning into the aftn. Will therefore leave headlines
in place and grant day shift the opportunity to extend forward in
time if necessary. Patchy drizzle mention was likewise extended
through the morning as some light returns are still evident on radar
and shear-induced mixing mixing due to elevated winds atop the
inversion continue for the next several hours at least.

Pops for this evening in good shape as energy originating over the
SW US shears across the area after about 21z this aftn. Moist
isentropic ascent will be modest and residence time of larger scale
forcing short. NAM12 cross-section reveals the vertical motion
response will be dominated by strengthening fgen focused near the
surface front over Ohio. The layer of low static stability above the
frontal surface is quite shallow and will greatly limit the
intensity of any response, except possibly south of the MI border
where some convective instability is noted. Highest pops south of 8
Mile Rd still looks reasonable. Higher end pops still expand
northward, particularly toward 00z as increasing forcing associated
with the impinging upper jet activates the higher elevations of the
frontal slope, albeit with diminishing efficiency as drier air
undercuts.

Progressive upper trough will usher in cooler and drier air tonight,
finally ending any fog threat for good and eventually scouring out
low clouds as confluent flow aloft forces surface high pressure into
the area late tonight into early Friday. No complaints with guidance
indicating near-normal temps for the remainder of the weekend.
Shortwave noted near the southern Alaska coast is progged by the NWP
to swing through the Great Lakes region late Sat/early Sun on a
northeasterly trajectory as it is steered along by background changes
in the flow field resulting from aggressive reamplification of the
western US trough. As such, a short-lived bout of snow on Sunday will
be quickly followed by 925mb temps surging into the teens through
early next week and correspondingly warm daytime temps no cooler than
the 40s. Several chances of rain through next week as strengthening
ridge over the SE US aids development of a quasi-stationary
baroclinic zone over the region Monday into Wednesday.

&&

.MARINE...

Southwest flow that has carried mild air over the cold Great Lakes
during the night will get cut off by a prefrontal trough during the
morning. The westerly wind shift will be capable of gusts possibly
reaching 30 knots over northern Lake Huron where some decrease in
surface stability will exist before the gradient weakens around mid
afternoon. This will precede the cold front that is still on
schedule to move through marine areas tonight. The latest model
guidance indicates more of a double barrel shot of cold advection
during the evening and overnight that prevents the wind field from
organizing more strongly. Deeper northwest flow will develop
overnight with gusts around 30 knots through Friday morning. A few
entry level gale force gusts are possible but duration remains too
brief for headline issuance. High pressure moving in Friday
afternoon will take a southward path and leave moderate westerly
flow on Lake Huron, however the backing tendency and just enough
decrease in gradient will help gusts settle below 30 knots into
Friday night.

&&

.PREV DISCUSSION...
Issued at 1153 PM EST Wed Feb 14 2018

AVIATION...

Extensive moistening of the near surface environment providing an
extremely favorable environment for dense fog and very low stratus
formation tonight. This will sustain a widespread coverage of
VLIFR/LIFR cigs and vsby restrictions through the early and mid
morning period. Sufficient depth to this moisture over the lowest
1000 ft may prove difficult to dislodge on Thursday, although some
drying above this layer and general diurnal turbulence may ease the
conditions somewhat. Currently favor a more pessimistic outlook for
Thursday, until near terms trends provide a more definitive signal.

For DTW...Current trends and recent model guidance point to a long
duration of 1/4 mile visibility restrictions in dense fog lasting
through at least mid morning. Confidence much lower in timing the
exit of this condition Thursday.

/DTW THRESHOLD PROBABILITIES...

* High for visibility below 1/2SM through daybreak. Diminishing
  confidence beginning late morning /after 13z/.

* High for ceiling below 5000 ft through Thursday morning, then
  medium Thursday afternoon.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Dense Fog Advisory until 9 AM EST this morning for MIZ060>063-
     068>070-075-076-082-083.

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

$$

DISCUSSION...JVC
MARINE.......BT
AVIATION.....MR


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