Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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081
FXUS63 KDTX 301720
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
120 PM EDT Sun Apr 30 2017

.AVIATION...

Deepening low level moisture north of a warm front will maintain
extensive MVFR/IFR conditions in low stratus and periodic/numerous
showers through tonight.  Tougher call on coverage of thunderstorms
given limited instability, but will maintain a mention for this
evening across the Detroit corridor given closer proximity to the
warm front and greater instability over Ohio.  Window for additional
thunderstorms will exist overnight as a cold front sweeps through.
The warm front will struggle to lift into the southeast Michigan
airspace, favoring retention of moderate flow with an easterly
component during this time.  Gusty post-frontal southwest winds
will emerge Monday. Gusts 30 to 35 knots will be attainable.

For DTW...Low stratus holds under cold easterly flow through the
evening as a warm front struggles to reach the airspace.
Thunderstorm potential still hinging on attaining some level
instability north of this front.  Still some potential, particularly
this evening and again overnight. A defined mention will continue
to focus on the evening window.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less through Monday morning.

* Low for thunderstorms through tonight.

&&

.PREV DISCUSSION...
Issued at 940 AM EDT Sun Apr 30 2017

UPDATE...

12z DTX sounding checked in with 3 C dew pt at 700 MB, with an 850 MB
dew pt of 12 C and PW Value a little below 1.5 inches.

Updated zones to increase pops to categorical across southern areas
as MCV along northern Indiana/Illinois border works north, and
isentropic ascent and further moisture advection north of the warm
front allows for shower development over southeast Michigan through
much of the day.

Speaking of the warm front, right now comfortably south of
the state line, but late day push northward expected, reaching the
Michigan border toward sunset. Could potentially be enough
instability for severe storms, with any storm along the front
capable of spinning up Tornado, but expectations at the present time
is severe threat will be along and south of Michigan border. Lenawee
probably having the best chance farther removed from Lake Erie
influence.

Flood watch continues for Tri-Cities region, and adjacent counties,
as rainfall amounts overnight ranged from a quarter to half an inch.
Certainly reasonable for another 1 inch to potentially another 2+
inches to fall Today and Tonight, and probably going to have to pull
the trigger as some point today and go with Flood warning for
Saginaw River, going above Flood stage late Tomorrow.

PREV DISCUSSION...
Issued at 324 AM EDT Sun Apr 30 2017

DISCUSSION...

The region of convection which has moved across the forecast area
overnight has been forced largely by lift ahead of a mid level short
wave. This feature will exit east by daybreak, leading to an overall
diminishing trend in coverage of convection. Rain may be a little
more persistent across the Saginaw Valley and thumb at least through
the early morning where good frontal forcing exists in the mid
levels. An increase in the east-northeast gradient is supporting
some modest winds. With temps in the low-mid 40s, this will lead to
relatively chilly morning conditions.

In terms of the large scale pattern, a deep upper low now churning
over the Texas panhandle will lift into ern Kansas by evening. The
stationary front (now over the srn Ohio Valley) will lift north as a
warm front during the day and will attempt to cross the Mi/Oh state
line late this afternoon or evening. The 00Z model suite generally
suggest the 925-850mb portion of this front (which will extend
across the Saginaw Valley and nrn thumb) will remain more active
today through tonight, supporting the higher fcst rainfall totals in
this region. The large MCS across srn MO and Arkansas is not being
handled particularly well by the model solutions, leading to some
uncertainty as to how the large scale forcing will evolve today. The
remnant MCV should lift into srn Mi this afternoon. This will lead
to an increase in coverage/intensity of convection during the day.

The warm front will have a very tight thermal gradient along it
today. In fact, temps across Ohio this afternoon will be in the 70s
and low 80s while most of Se Mi will remain in the 40s and low 50s.
The nearly vertically stacked upper low will lift into the mid Miss
Valley tonight. This will drive the warm front northward across Se
Mi with the approach of the occlusion, leading to rising temps
during the night over most of Se Mi. SPC has placed locals south of
the I-94 corridor in a slight risk today. This is predicated upon
the warm front lifting north of the state line before diurnal
instability wanes. The wind fields certainly suggest a potential
severe threat assuming the instability can get drawn this far
northward.

The warm conveyor is still expected to lift into Se Mi tonight with
the approach of the upper low. Additional moisture transport is
forecast to send precipitable water values up toward 1.8 inches.
This along with decent elevated instability and a deep warm cloud
layer continues to argue for intense rainfall rates with any
convection. The expected quasi stationary nature of the mid level
front across the Saginaw Valley still raises concerns for some more
persistent rainfall in this region tonight. Thus 1 to 3 inches of
total rainfall from overnight through early Mon morning remains
possible in the flood watch area.

The mid level dry slot will move overhead Monday morning. Increased
mixing depths during the day will support temps well into the 60s
(if not low 70s). The diurnal mixing will also lead to gusty
southerly winds, likely topping 30 MPH. The upper low is forecast to
lift across the straits region Monday night, driving colder air
across srn Mi. This and an influx of moisture from late Monday into
Mon night will support increased chances for showers. Cool and
cloudy conditions with some diurnal enhanced showers will then
persist through Tuesday as the upper low makes a slow departure to
the northeast of the region. Although there remains differences with
respect to timing and amplitude, long wave trough amplification over
the ern US late in the week is forecast by the medium rang models.
This will sustain relatively cool temps through the rest of the work
week.

MARINE...

Unsettled conditions will continue through the early part of this
week as a strong low pressure system lifts out of the Central Plains
and tracks through the western and northern Great Lakes. A tight
surface gradient between the approaching low and a large dome of
high pressure over Ontario and Quebec will maintain strong easterly
winds today. These winds will gradually ease from south to north
during the late afternoon and evening as the low draws closer and
the gradient weakens. Large waves generated by these winds will be
slower to subside over Saginaw Bay and off the tip of the northern
Thumb, with large waves lasting through late tonight.

Winds will turn southerly and increase once again on Monday as low
pressure lifts west of Lake Michigan. Gusts as high as 30 knots look
to develop by afternoon and continue through the overnight hours.
Winds are expected to shift to the west behind a cold front on
Tuesday, with windy conditions persisting through Tuesday night.

In addition to the extended period of gusty winds and large waves,
this low pressure system will keep a wet and active pattern over the
Great Lakes through the early part of the week. Thunderstorms and
periods of heavy rain will be possible through late Monday morning,
with showers lasting into Tuesday.

HYDROLOGY...

Rain and thunderstorms will increase in coverage and intensity by
afternoon, with the best coverage expected tonight into Monday
morning. Total rainfall between Saturday afternoon and Monday
afternoon is expected to range between 1 and 3 inches. The highest
amounts will be focused along and north of a line from Flint to
Harbor Beach. Locally higher totals are possible for areas that
experience thunderstorms, as storms will bring brief periods of
torrential rainfall. Rivers and streams will see significant rises
through the middle of the week, and ponding of water may develop in
low-lying areas and roadways. Flooding is possible, particularly
over the Saginaw Valley.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Monday morning for MIZ047>049-053-054-060-061.

     Coastal Flood Advisory until 10 PM EDT this evening for MIZ048-054-
     076-083.

Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ442-443.

     Small Craft Advisory until 10 PM EDT Tuesday for LHZ421-422-441.

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

$$

AVIATION.....MR
UPDATE.......SF
DISCUSSION...SC
MARINE.......HLO
HYDROLOGY....HLO


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at www.weather.gov/detroit.



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