Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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633
FXUS63 KDTX 140057
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
857 PM EDT Fri Oct 13 2017

.UPDATE...

Rain now advancing across the western Great Lakes is focussed along
the mid troposheric frontal boundary. This boundary is still expected
to settle across the northern half of the forecast area overnight.
Persistent frontogenetical forcing will strengthen the frontal
circulation, with the resulting isentropic ascent supporting fairly
widespread showers generally along/north of the I-69 corridor. While
an influx of mid level moisture may support some sporadic shower
activity across metro Detroit/Ann Arbor and points south, the better
forcing will remain north of this region. A forecast update will be
issued simply to graduate the precip chances a little more across the
forecast area (reduce pops to chance from Detroit city south and
nudge them upward slightly across the Tri Cities).

&&

.PREV DISCUSSION...
Issued at 707 PM EDT Fri Oct 13 2017

AVIATION...

A developing low to mid level frontal boundary across the western
Great Lakes is spreading rain into NW Lower Mi. This front will
advance into the Saginaw Valley tonight while intensifying. The
result will be fairly widespread showers expanding into MBS around
or shortly after midnight, some of which will push south into FNT
and PTK. This front will hold nearly stationary across Se Mi through
the day Saturday. The result of persistent rain/moisture advection
will lower ceilings during the course of the overnight into Sat
morning. With MBS remaining primarily north of the sfc front, IFR
and/or low end MVFR conditions are expected by Sat morning. Weak
elevated instability may support some thunderstorm activity, the
chances of which are too low to include in the terminals attm.

For DTW...The frontal forcing will largely remain north of metro
tonight through much of the day Saturday. This will hold most of the
showers and lower clouds north of the airport. Several model
solutions do however suggest a mid level moisture plume will advance
across metro Detroit late Sat morning, warranting a chance of
showers.

//DTW THRESHOLD PROBABILITIES...

* Low in ceilings below 5000 ft tonight. Medium on Saturday.

* Low in thunderstorms on Saturday.

PREV DISCUSSION...
Issued at 344 PM EDT Fri Oct 13 2017

DISCUSSION...

Digging shortwave energy into the Intermountain West and a 594dm
subtropical high pressure anchored over the Gulf Coast is
responsible for a very tight sw-ne oriented mass gradient over the
central CONUS and Great Lakes. Baroclinic zone over the Upper MS
Valley is undergoing a pronounced strengthening phase in response to
the rapidly strengthening 140-150kt upper jet. Showers have
developed within a corridor of enhanced 298/300k fgen. This activity
will propagate east through the evening and may expand southward as
moist isentropic ascent takes shape south of the front, though any
noteworthy QPF will remain tied to the narrow fgen band.

High uncertainty regarding placement and evolution of the resident
baroclinic zone through the day Saturday. Higher resolution models,
excepting the struggling NAM12, suggest southward displacement
perhaps in response to diabatic cooling associated with convection.
This is also noted in the GEM and 12Z ECMWF which have shifted the
surface over Ontario slightly south which each run. Given both the
synoptic scale signal by the GEM/ECMWF and the CAM runs, made a
decided shift south with the heavy rain axis on Saturday. Strong jet
dynamics and increasing isentropic ascent up the frontal slope will
ensure a strong fgen band capable of producing at least an inch of
rain 12-00z Sat. The forecast has been updated to shift this band to
the I-69 corridor. Cross-sections indicate a fair amount of
conditional instability south of the front, suggestive of a pattern
of showers/tstorms there as warm sector ascent increases through the
day.

As the dominant forcing regime shifts from frontal to synoptic
scale, non-NAM NWP are in strong agreement that a cyclone will
deepen to about 995mb as it lifts from the Quad Cities to Georgian
Bay Saturday night, a slightly flatter look than recent runs. Front
will lift north during this time as the bulk of activity becomes
focused on the surging low-level jet in advance of the front. Stout
southerly flow will keep temperatures elevated in the 60s, in spite
of any rain, through Saturday night. Strong cold fropa will take
place roughly 9-12z Saturday night. Cross-sections indicate a deep
tropospheric front with some density current features near the
surface, ideal for maximizing wind gust potential. Wind within the
unstable nose of the front approach 50 kts suggesting a brief pop of
40-50 kts certainly not out of the question early Sunday morning.

Strong cold advection will cause 850mb temps to plummet from around
15C Sunday morning to -1C Saturday evening. Steep low-level lapse
rates and a strong wind field in the wake of the departing cyclone
will cause temperatures to fall throughout the day, reaching the 40s
by evening. Strong mixing will generate several hours of strong
winds. Whether or not wind advisory criteria is reached depends
entirely on the strength of the low, which is notably flatter than
depicted on previous days. Bumped winds up to just shy of criteria
for most of Sunday. Despite the cold airmass, wind and the potential
for lake effect clouds make frost unlikely Sunday night.

Surface ridging moving in Monday will help keep skies mostly clear
and allow for a chilly Monday morning with lows falling to the upper
30s for most of SE Michigan. Highs will only recover to the mid 50s
for most areas as upper thermal trough moves through the region and
departs by Tuesday. Upper ridging then moves in from the west and
dominates the forecast through the rest of the week.

Warm air advection to move in beginning Tuesday as surface high
pressure sets up over the eastern U.S., leading to sunny skies and
highs rebounding to seasonable values in the mid 60s through the
week. A weak cold front is progged to pass through overnight
Wednesday into Thursday but impacts will likely be limited to just
an increase in cloud cover. Overnight lows through the extended
period will hover in the 40s and highs by Friday will approach 70
degrees.

MARINE...

A frontal boundary will settle southeast late today and tonight,
stalling near Saginaw Bay and central Lake Huron on Saturday. South
to southwest winds will persist south of the boundary, but veer to
the northwest and north over northern Lake Huron. Wind gusts will
remain 20 knots or less into Saturday evening, but increase late
Saturday night into Sunday as low pressure tracks across the area
along this frontal boundary. Showers and thunderstorms will increase
from Saturday into Saturday night as this crosses the central Great
Lakes.

While borderline gale conditions still appear possible within the
southwest flow over Saginaw Bay late Saturday night and early Sunday
morning, stronger northwest gales appear likely on Sunday once the
low passes east and colder air surges into the area. These stronger
winds will spread from northern Lake Huron Sunday morning south
through the remainder of the area Sunday afternoon. Peak wind gusts
of 45 knots appear possible over portions of the Lake Huron open
waters with at least low end gales possible elsewhere. A Gale Watch
will likely be issue in anticipation of these conditions. Wind
conditions will then improve markedly Sunday night as a ridge of
high pressure shifts east into the area.

HYDROLOGY...

A frontal boundary will sag south into the area tonight and stall
near the I-69 corridor on Saturday, becoming the focus for rain
overnight into Saturday morning. Rain will then become increasingly
widespread late Saturday into Saturday night as the front slowly
lifts back to the north with the approach of low pressure from the
west. During this time, embedded thunderstorms will also lead to
locally heavy rainfall.

While models differ somewhat in the north to south orientation of
strong FGEN forcing and the resultant band of heavy rainfall, it
appears increasingly likely that 1 to 2 inches of rain will occur
over parts of the area as precipitable water values increase to 1.50
inches or more. At this time, model consensus leans to areas along
and north of the I-69 corridor with decent rainfall of one half to
one inch expected as far south as M-59 as well. However, a slight
shift to the south is not out of the question given recent trends.
The heavier rainfall will end by early Sunday morning as the low
passes to the east with all rain ending during the day as a cold
front sweeps south of the area.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late Saturday night through Sunday evening for
     LHZ361>363-421-422-441>443-462>464.

Lake St Clair...Gale Watch from late Saturday night through Sunday evening for
     LCZ460.

Michigan waters of Lake Erie...Gale Watch from late Saturday night through Sunday evening for
     LEZ444.

&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...JVC/TF
MARINE.......DG
HYDROLOGY....DG


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