Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 300358
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1158 PM EDT Sat Apr 29 2017

.AVIATION...

Late evening observations are helping answer the thunderstorm
question for the early morning hours. The mature convective complex
moving out of Indiana will move through most of the terminals by 08-
09Z with brief MVFR/IFR visibility restriction in heavy rainfall, and
then leave an unorganized pattern of weaker showers around through
sunrise.

Low pressure over Oklahoma will be moving northward and bring the
warm front closer to the Ohio border during the morning resulting in
MVFR ceiling with IFR becoming more likely along the DTW corridor.
Northward progress of the warm front will be disrupted by easterly
flow over the colder Great Lakes, especially while the primary
surface low is to the southwest. It may take until the occluded
frontal passage Sunday night before MVFR/IFR is swept out of SE
Michigan. In the meantime, low ceiling and reduced visibility in
showers/storms will be widespread over the region through Sunday.

For DTW... Thunderstorm timing and coverage is now pinned to the 05-
08Z period considering the storms nearby over Ohio and the line with
greater organization over Indiana. Visibility restriction with this
activity will be replaced by ceiling reduction during the morning.
Light to moderate northeast to east wind will persist through the
forecast.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less tonight through Sunday.

* Moderate for thunderstorms tonight. Low Sunday afternoon.

&&

.PREV DISCUSSION...
Issued at 925 PM EDT Sat Apr 29 2017

UPDATE...

An increasing trend looks solid in both coverage and intensity of
showers and scattered storms during the evening and overnight hours,
matching up nicely with the flood watch valid time. Model analysis
fields indicate a strong warm front over southern Lower Michigan in
the 850-700 mb layer by measures of theta-e and thickness, supported
by the 00Z DTX sounding that shows the top of a sharp inversion
around 750 mb. Deep and strong southwest flow under the broad
160 kt anticyclonic upper jet is driving strong moisture
transport/theta-e advection and frontogenesis through the elevated
frontal zone. Recent radar composite also indicates an MCV organizing
within the mature convection over Illinois. This will only enhance
the low level jet that is already projected to be in the 40-50 kt
range at 850 mb through the night. Coverage in SE Michigan will be
more widespread toward the Tri Cities and a bit less in the Detroit
area. The widespread/categorical POPs in the north to likely/numerous
toward the Ohio border in the going forecast look good with no
adjustments needed. Plan to just add a few short term forecast
products to advertise coverage and strength trends during the night
that will likely include heavy downpours and possibly some small
hail.

PREV DISCUSSION...
Issued at 300 PM EDT Sat Apr 29 2017

DISCUSSION...

Cool/dry low level northeast flow mostly in control today as high
pressure translates through the northern Great Lakes, with a more
dominate center (1032-1033 MB) taking over Tonight over just south
of James Bay.

Deep surface based convection fired up along the Ohio River this
morning and was the focus of heavy rainfall during the morning
hours, cutting off moisture transport into the southern Great Lakes.
However, transport vectors quickly ramping up this afternoon/evening,
as wave ejects out of northern Missouri this afternoon, and and
still looking at ribbon of up to 5 C dew pts at 700 MB lifting
through southern Lower Michigan Tonight. Depending on the strength of
this wave/ripple, with have to watch locations close to the southern
Michigan border as showalter index reaches between -2 to -5 C,
depending on your model of choice, with HRRR showing stronger
convective reflectivity tracking along the border as well.
Fortunately, no concerns for surface based instability, and would
expect a sufficient stable surface-925 mb layer to prevent any
stronger wind gusts. Small hail will be possible however.

Rapid development/quicker spin up of the upper level low/circulation
over the Four Corners region earlier Today. Thus with the deeper
system and low level jet displaced a bit farther to the west, likely
assuring the better/prolonged 850-700 MB FGEN will be farther
northwest Tonight into Sunday, but still likely clipping Tri-Cities
region, where we have elected to issue a long duration flood watch
(added a few surrounding counties as a buffer as well). Local
Probabilistic SREF weighted guidance still projecting around 2
inches of total rainfall up that way by Monday Morning, with
potential for 3+ inches. Good chance of Flooding along Saginaw River
based on those amounts. PW values of 1.5 to 1.75 inches in place
Tomorrow will allow promote localized heavy rainfall farther south
with any vigorous convection/thunderstorms, but confidence is lower
in coverage/duration/locations. Still thinking the surface warm front
will not get much past the Michigan border, but could clear Lenawee
county, setting up very tight temp gradient along I-94 corridor and
points south, with high degree of 0-1 km bulk shear to work with if
any thunderstorm is able to latch onto this boundary. Outside of this
area, MUcapes of 500-1000 J/kg would just support small hail.

For high temperatures on Sunday, looks like a 30-35 degree
temperature difference from north to south, with highs in the upper
40s across northern Bay/Midland counties, to around 80 degrees late
in the day along the southern Michigan border.

The highest confidence in the most widespread rain showers is late
Sunday Night with the main upper level forcing/moisture axis and
cold front finally moving through, still looking to clear southeast
Michigan by Noon Monday. A very wet Monday morning commute, and
normally flood prone roads in Detroit Metro area could experience
problems with the high rainfall rates. Mid level dry slot taking
hold, with good low level cold advection from the southwest, as 850
mb temps tumble toward zero by day`s end.

Steep low level lapse rates and increased mixing depths, as well as
local probabilistic guidance suggest wind gusts of 40+ MPH by Sunset
Monday into Monday evening. Wind potential will obviously hinder on
how fast the low fills/weakens Monday afternoon-night as it lifts
through the Western Great Lakes.

Lingering isolated light rain showers will be possible throughout
Tuesday morning and afternoon as low pressure pushes northeast from
the Upper Peninsula into southern Quebec. As the low moves out of
the Great Lakes, a surface high pressure system is expected to build
in from the west, bringing relief from the rain late Tuesday into
Wednesday, as temperature daytime highs cap in the mid-50s for both
Tuesday and Wednesday.

Low to moderate confidence regarding rain potential starting
Thursday morning and continuing into the weekend. The latest 12Z
ECMWF model suite shows a low pressure system strengthening over
Kentucky/Indiana early Thursday, and pushes the low northeast over
Ohio by Friday morning, bringing the chance for rain showers across
Southeast Michigan as it moves northeast. The GEM model tries to
derive a similar low, however, the low pressure system remains weak
and keeps all precipitation just south of Michigan, similar to what
the GFS is resolving, albeit some slight timing and track
differences. As a result, PoP values will remain sub-30 until
convergences is seen exhibited future runs.

For the weekend, the GFS solution shows a nice omega blocking
pattern in the mid-levels which would be mild and dry conditions,
however, if the GEM and ECMWF solutions pan out, the potential for
dry conditions could be spoiled with isolated showers as both models
keep the 500 mb low over PA/NY, well northeast of the GFS run.

MARINE...

Unsettled conditions through early next week as a strong low
pressure system slowly tracks through the area.  The approach of
this system will establish a long duration of moderate northeasterly
winds lasting through the latter half of the weekend.  The
combination of gusty conditions and corresponding increase in wave
heights across the western lake Huron basin will lead to extended
small craft advisory conditions.   Winds turn southerly and
strengthen throughout Monday in advance of a cold front.  Potential
for gusts up to 30 knots from western Lake Erie to southern Lake
Huron during this time.  Winds then become westerly with the frontal
passage on Tuesday.  A wet and active pattern tonight through early
Monday, with periods of heavy rain and thunderstorms possible.

HYDROLOGY...

Several rounds of rain and possible thunderstorms are forecast to
lift across southeast Michigan between tonight and early Monday.
This rainfall could become heavy at times.  Total rainfall through
Monday afternoon is expected to range between 1 and 3 inches, with
the highest amounts focused across the Saginaw Valley. Locally
higher totals will be possible for areas that experience
thunderstorms Sunday into Monday, 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 could become a
concern by Sunday and Monday, particularly over the Saginaw Valley.

&&

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

Lake Huron...Small Craft Advisory until 4 PM EDT Sunday for LHZ443.

     Small Craft Advisory until 6 AM EDT Monday for LHZ421-422-441.

     Small Craft Advisory from 4 AM to 4 PM EDT Sunday for LHZ442.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 4 PM EDT Sunday for LEZ444.

&&

$$

AVIATION.....BT
UPDATE.......BT
DISCUSSION...SF/AM
MARINE.......MR
HYDROLOGY....MR


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