Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 151740
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1240 PM EST Thu Feb 15 2018


.AVIATION...

Mild air and increasing moisture will continue to lift northward
across the region through the remainder of the afternoon, as a
surface wave rides along a stalled warm front boundary draped along
the I-94 corridor. Moisture pooling along the boundary has resulted
in persistent IFR/LIFR fog and ceilings at KDTW, KDET, and KYIP,
with recent obs supporting VLIFR conditions. Further north,
visibility and ceilings are slightly higher, but still straggling
the IFR/MVFR line. The frontal boundary is expected to move little,
if any, to the north the rest of the afternoon, with continued
severe flight restrictions plaguing mainly KDTW, KDET, and KYIP.
Burst of showers transitioning to a short period of more steady rain
will affect the TAF sites south of the I-69 corridor roughly 19z-
03z, with thunder also possible particularly at KDTW and KYIP
between 19z-00z. Flight conditions begin to rapidly improve after
00z as the flow transitions from southwest to west/northwest and
allows drier boundary layer air to filter in. Low confidence as to
how long lingering MVFR ceilings take to erode through the night,
with current expectations that cloud cover may persist through most
of the night. Bigger push of drier air after 12z Friday leadings to
rapidly clearing skies for the remainder of the period.

For DTW...VLIFR/LIFR flight restrictions continue through much of
the remainder of the afternoon as fog remains pooled along a stalled
warm front boundary. Surface wave riding along front this afternoon
brings a period of showers/rain, with embedded thunder also
possible. Showers end 00z-03z, with light southwest flow
transitioning to northwest. MVFR ceilings may linger through much of
the night before drier air after 12z Friday brings rapidly clearing
skies.

/DTW THRESHOLD PROBABILITIES...

* High for visibility 1/2SM and ceiling 200ft through much of the
  afternoon.

* High for ceiling below 5000 ft through this evening, moderate
  tonight.

* High for rain as precipitation type this afternoon and evening.

* Low confidence in thunderstorm potential 19z-00z.

&&

.PREV DISCUSSION...
Issued at 1004 AM EST Thu Feb 15 2018

UPDATE...

The latest surface analysis shows a warm frontal feature extending
from a weak sfc low low over the MO/IA border across roughly the
southern tier Michigan counties. It is a spring-time airmass south of
this front where sfc dewpoints are in the 50s (and low 60s near the
Ohio River). The sfc low will ripple to the east-northeast along this
surface front today, tracking across metro Toledo around 21Z. An
intensification of the upper jet max across the upper Midwest/wrn
Great Lakes today will invoke an increase in the forcing along the
frontal slope across the nrn Ohio Valley/srn Mi. The corresponding
isentropic ascent will support increasing chances for rain during the
course of the afternoon. A region of showers/thunderstorms has
developed along the lead edge of a low-mid level moisture plume over
cntl IL/wrn IN, which will lift into metro Detroit between 18Z and
21Z. The combination of isentropic ascent within this moisture surge
will remain supportive of high probabilities of showers from roughly
8 mile south. Steepening mid level lapse rates also suggests weak
elevated instability, thus also warranting a chance of thunderstorms
in this region. The 12Z 3km NAM and 13Z HRRR actually show a axis of
convection impacting this region this afternoon and evening.

Observational trends and latest 12Z guidance is indicating the
convective instability is going to focus the bulk of the activity
along the lower tropospheric portions of the front (sfc to 925mb),
suggesting chances for showers will be much less from the nrn Detroit
suburbs northward. The degree of low level moisture advection atop a
very deep low level inversion should however ensure at least some
drizzle in these locals.

The high sfc dewpoints advecting into the sfc frontal boundary atop
the melting snow pack should also support periodic fog along/south
of the I-94 corridor. Temps near the MI/OH border (south of the
front) may actually rise well into the 50s. Farther north, most of
the warming will be above a deep low level inversion, thereby holding
temps in the 40s. A forecast update will be issued shortly.

PREV DISCUSSION...
Issued at 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.

&&

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

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

$$

AVIATION.....IRL
UPDATE.......SC
DISCUSSION...JVC
MARINE.......BT


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