Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 150403
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1203 AM EDT Tue Aug 15 2017

.AVIATION...

Surface pressure analysis over the Midwest and western Great Lakes
indicates the next wave of low pressure on the way into Lower
Michigan during the night through late morning. Expect coverage of
showers to increase over the MBS, FNT, and PTK terminals during the
late night through early morning possibly reaching DTW toward
sunrise through mid morning. Thunderstorms are possible but coverage
will be low compared to the showers. The surface low is then
projected to move through the area near the Ohio border presenting
the best chance of lower stratus coverage up to about PTK during the
morning, which is currently expected to be borderline VFR/MVFR
ceiling. A decreasing cloud trend and increasing northeast surface
wind will follow the exit of the system during the afternoon into
Tuesday evening.

For DTW... Showers are expected to redevelop over western Lower
Michigan later in the night. Some of this activity is expected to
brush the terminal toward sunrise through mid morning. VFR ceiling
less than 5000 ft is then likely as low pressure slides by just
south of DTW during the morning through early afternoon.

//DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling below 5000 ft toward sunrise through
  afternoon.

* Low for thunderstorms through late morning.

&&

.PREV DISCUSSION...
Issued at 953 PM EDT Mon Aug 14 2017

UPDATE...

The evening forecast update will boost POPs between the Tri Cities
and Pontiac where showers appear likely late tonight through early
Tuesday morning. As the scattered evening activity exits the Thumb
region, there will be a break until later in the night when the next
upper wave arrives from the west. This system looks plenty strong in
evening water vapor satellite imagery and is projected to move
quickly into Lower Michigan during the night. Model data indicates a
well organized system coming together over the area before sunrise in
a combination of DCVA colocated with the entrance region of the
upper jet and a surface reflection of low pressure near the southern
Michigan border. This will set up a region of focused ascent through
both mid level theta-e advection and dynamic forcing centered on the
I-69 corridor. The moisture supply has already proven to be more than
adequate as demonstrated by the evening activity, and will only
increase with the next system. Mid level lapse rates will remain weak
and limit convective potential but there will be a chance of
ordinary thunderstorms as the system passes through and exits SE
Michigan toward noon Tuesday.

PREV DISCUSSION...
Issued at 338 PM EDT Mon Aug 14 2017

DISCUSSION...

Well-defined surface trough has taken shape over western Lower this
afternoon in advance of a shortwave crossing Lake Michigan at press
time. Convergent southerly flow within the trough has begun to yield
a partial coverage of showers that will likely exhibit a modest
increase in coverage over the next few hours. Forcing will sweep
east of the forecast area by 03z providing several hours of
downtime, although a few isolated showers are not out of the
question within the lingering moisture axis during the overnight
hours. Trailing wave over eastern MN/western WI this afternoon is
supporting enhanced cloud cover and light rain showers over the NW
Great Lakes region. This wave will track directly overhead during
the first half of Tuesday, providing adequate uplift for a healthy
coverage of stratocu and another round of showers along the lead
edge of the attendant theta-e gradient. Instability will be greatly
limited by stable mid-levels limiting concerns for thunder. With
virtually no change in resident airmass characteristics other than
ongoing modest advection of slightly higher dewpoints, highs will
fall in the low 80s once again.

Jet digging into the base of the West Coast trough will serve to
eject energy from the 4-corners region Tuesday night into the
northern great Lakes by Wednesday night. Downstream shortwave
ridging is modeled to be on the stronger side. Associated surface
high pressure will be augmented by the lake aggregate yielding high
confidence in dry conditions through Wednesday. Northeast flow will
limit mixing heights in at least the NE third of the CWA limiting
highs to the low to mid 70s while locales along and west of a
Saginaw/Flint/Warren line will all have a chance at 80s as 850mb
temps hold steady in the mid teens.

Low pressure lifting through northern Michigan on Thursday will
bring the next chance for widespread rain and thunderstorms as it
pulls a cold front through SE Michigan. Ahead of the cold front,
southerly flow will advect a very moist airmass northward with dew
points near 70 F and PWATs approaching 2 in. This will bring the
potential for heavy rainfall in any storms that develop on Thursday.
An upper level trough then looks to translate eastward through the
region over the weekend bringing cooler conditions. Surface high
pressure building into the region next weekend will bring mainly dry
conditions. However, isolated showers cannot be ruled out during the
late week period as weak disturbances drop southeastward through the
trough. Highs look to remain in the 70s on Friday and Saturday
before moderating to the lower 80s on Sunday. Lows will range from
the upper 50s to mid 60s Friday through Sunday.

MARINE...

Weak frontal boundary easing across the region will sustain a low
potential for thunderstorms over Lake Huron and Saginaw Bay through
tonight. The overall gradient will remain weak as this boundary
exits to the south by early Tuesday, replaced by weak high pressure
building back into the upper great lakes. This will sustain
relatively light and variable winds through Tuesday. A modest
increase in easterly winds Tuesday night and Wednesday as the
gradient strengthens. The arrival of a stronger area of low will
then result in a period of more unsettled conditions Wednesday night
and Thursday. South-southeast winds are forecast to strengthen in
advance of an associated warm front during this time, with gusts
potential exceeding 20 knots at times. This system will also bring
an increasing potential for thunderstorms. The passage of this
system Thursday night will result in a veering of the winds to the
west by Friday.

HYDROLOGY...

An increasingly moist and unstable environment will accompany a warm
front into the region Wednesday night and Thursday.  This will lead
to a good chance for showers and thunderstorms during this time.
Widespread rainfall totals of a quarter to three quarters of an inch
are highly probable. Locally higher totals are expected in
thunderstorms considering the high level of moisture that will be
present over a considerable depth of the atmosphere.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
UPDATE.......BT
DISCUSSION...JVC/JD
MARINE.......MR
HYDROLOGY....MR


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