Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 180116
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
816 PM EST Fri Nov 17 2017

.UPDATE...

Rain showers have already advanced into the Tri Cities of 8 PM and
will advance into the remainder of the forecast area during the rest
of the evening. Recent satellite data have been showing cooling cloud
tops across Lake Mi and western Lower Mi within region of increasing
moist isentropic ascent. Significant strengthening of the low level
inflow into srn Lower Mi will occur tonight (850mb winds increasing
to 60-70 knots across nrn Indiana/far srn Lower Mi). This will
enhance the moisture transport along steepening isentropic surfaces,
resulting in expansion of rain across the forecast area. The peak
ascent looks to be focused around 06Z. The degree of elevated
instability looks weak. Taking into consideration the amount of
lightning that has occurred upstream and given the intensification of
the ascent over the next 3 to 6 hours, the chances for thunderstorms
will be increased tonight with the evening update. Otherwise, the
going forecast looks reasonable.

&&

.PREV DISCUSSION...
Issued at 614 PM EST Fri Nov 17 2017

AVIATION...

Increasing moisture transport within a strengthening low level
southerly wind field will result in widespread showers overspreading
the terminals in the 01 to 03Z time frame. Ceilings will initially
be VFR given the dry air overnight. The influx of low level moisture
will however result in rapid lowering of ceilings heights during the
overnight to IFR (perhaps LIFR). Elevated instability will also be
transported across Se Mi overnight. With thunderstorms already
spreading across northern Illinois, there appears a reasonably high
enough probability for thunder to carry a TEMPO group in the TAFs
(beginning at or shortly after 05Z). The strengthening low level
wind fields (40-50 knots) at 2k ft will also support a brief window
for low level wind shear late this evening into the early portion of
the overnight.

The region of heavier showers should actually exit east of the
terminals around or shortly after 08Z. Light rain or drizzle will
then affect the region through daybreak before another surge of
heavier showers arrives later in the morning. An additional influx
of low level moisture will likely support fairly extensive LIFR
and/or VLIFR cigs/vsby during a portion of the day Friday.

For DTW...Upstream radar and recent model solutions suggest the best
time period for thunder will around 06Z through roughly 08Z. With
these heavier showers will be a rapid lowering in ceilings heights.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less overnight through Saturday
  evening.

* Moderate in thunderstorms impacting the airspace tonight.

* Low in ceilings and/or visibilities below 200 ft and/or 1/2 miles
  Saturday morning.

* Moderate in crosswind thresholds being exceeded Saturday evening.

PREV DISCUSSION...
Issued at 402 PM EST Fri Nov 17 2017

DISCUSSION...

Main focus for this forecast will be the unsettled weather for the
weekend in the form of increased rainfall and gusty winds. An upper
trough exiting the Rockies and tracking across the central plains
this evening. This will begin advecting moist air into Lower
Michigan evidenced in the 700mb theta-e field as a cold front dips
south across the western Great Lakes. This front will be help drive
precipitation tonight as the LLJ kicks in and provides some deeper
forcing. The front will eventually stall out across Lower Michigan
as the surface low pressure system becomes more organized and gains
strength on Saturday while the aforementioned upper trough begins to
phase with another upper trough diving southeast across Ontario. The
low pressure system will move along this stalled boundary from
northern Illinois through southeast Michigan. All of Michigan will be
under the influence of the left exit region of the 140 knot southern
stream jet and the right entrance region of the departing upper jet.
Forcing and deep ascent with this is system will lead to moderate
rainfall through Saturday night with PWATs reaching over an inch
across far southeast Michigan. A chance for thunderstorms exists with
this system based on the track of the surface low. An area of
convective instability within the warm sector will be enough to
produce some thunderstorms. The track of the low has come into more
alignment with the NAM being the only outlier keeping it further
south. The others have it tracking a little north of the state line.
Believe chances are still good enough to leave a slight mention of
thunder in the forecast for the further southeast portions of the
CWA.

The low pressure system will continue to deepen as it moves
northeastward into the eastern Great Lakes by tomorrow evening. The
timing of how quickly the moisture exits and the cold air arrives in
the wake of this system will cause a transition from rain to a
period of a rain snow mix tomorrow night for areas mainly north of M-
59. Expect the coldest air aloft to lag behind just enough that it
will be difficult to see any snowfall accumulate from this system.
Temperatures near the surface look to remain warm enough as the bulk
of the precipitation exits. The deepening low will also increase the
pressure gradient resulting in strong wind gusts late Saturday
afternoon through Sunday morning. Wind gusts around 30-40 mph
will be possible as strong winds above the surface should be able
to mix down.

Temperatures at 850 mb will drop to around 10-11 degrees C by Sunday
afternoon. High temperatures will only come up a few degrees from
the morning lows and top out in the low to mid 30s under mostly
cloudy conditions. The cold air advection under stout northwest flow
will result in the potential for scattered snow showers throughout
the day on Sunday aided by the instability across the warmer
Great Lake waters.

Ridging translating east across the region early next week will
yield dry, milder conditions as highs top out in the mid 40s on
Monday. Low pressure moving eastward across northern Ontario on
Tuesday will pull a cold front through the region late Tuesday.
Ahead of the front, increasing southerly winds will boost temps
close to 50 Tuesday afternoon. A few showers will be possible across
the Thumb with the frontal passage, but overall a dry frontal
passage is expected with the bulk of forcing staying well north of
the region. A colder air mass with 850-mb temps falling below -10 C
will then overspread the region Tuesday night into Wednesday as
upper troughing sets up through late next week. Highs will be
noticeably colder with readings in the 30s for both Wednesday and
Thursday. However high pressure building in from the west will keep
dry, calm conditions in place through Thursday.

MARINE...

Wind will be the primary marine weather concern tonight through the
weekend highlighted by high end gales on Lake Huron Saturday night.
Moderate southerly wind will continue tonight ahead of the
responsible low pressure system organizing over the Plains. Small
craft advisories continue for the Lake Huron shoreline where waves
will be elevated due to an onshore wind component there. The gales
are expected to develop as the Plains low pressure moves into Lower
Michigan Saturday on a track near the west end of Lake Erie. It will
then strengthen considerably over Lake Ontario Saturday night and
produce strong northerly gales over Lake Huron where gusts near 45
knots will be possible producing significant waves of 10-14 feet and
max waves around 20 feet over the south third of the lake. Gales
will be lower end intensity over marine areas closer to land such as
inner Saginaw Bay, Lake St Clair, and the Michigan waters of Lake
Erie. Gale warnings are now in effect for all marine areas through
Sunday. The system will otherwise produce widespread rain showers
tonight through Saturday and possibly a rain/snow mix before ending
Saturday night.

HYDROLOGY...

Moderate rain tonight through Saturday will occur in two rounds late
tonight and Saturday afternoon. Widespread rainfall in the 0.25 to
0.5 inch range tonight will be focused mainly in the 10 pm to 4 am
time period. Totals could approach 0.75 inch from the Detroit area
to the Ohio border where a thunderstorm will also be possible. The
second round will again produce widespread 0.25 to 0.5 inch totals
Saturday afternoon and totals around 1 inch from the Detroit area to
the Ohio border before diminishing Saturday evening. Storm total
rainfall in the 1 to 2 inch range will be capable of minor urban
flooding and moderate rises in area rivers and streams, especially
in the Metro Area and surrounding suburbs.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning from 3 PM Saturday to 7 PM EST Sunday for LHZ361>363-
     462>464.

     Gale Warning from 7 PM Saturday to 10 AM EST Sunday for LHZ421-
     441>443.

     Small Craft Advisory until 4 AM EST Saturday for LHZ441>443.

     Gale Warning from 7 PM Saturday to 7 AM EST Sunday for LHZ422.

Lake St Clair...Gale Warning from 7 PM Saturday to 10 AM EST Sunday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 7 PM Saturday to 10 AM EST Sunday for LEZ444.

&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...AA/JD
MARINE.......BT
HYDROLOGY....BT


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at www.weather.gov/detroit.



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