Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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416
FXUS63 KDTX 291900
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 136 PM EDT Sat Apr 29 2017

AVIATION...

Existing dry northeasterly flow will gradually given way to an
increasing surge of moisture over the next 24 hours.  Remainder of
the daylight period today largely defined with this drier profile,
some lingering higher based VFR stratus tucked beneath thicker high
cloud.  Probability for showers and subsequent reduction in ceiling
height will increase tonight, particularly at FNT/MBS where MVFR
conditions will be more likely.  Periodic showers areawide into
Sunday, with the highest coverage/intensity still focused at
FNT/MBS.  Elevated thunderstorms will be possible tonight and
Sunday, but the ill-defined timing and limited potential will
preclude any mention at this stage.  Firm northeast wind through
tonight, turning intermittently gusty /20 kts/ late today into the
evening.

For DTW...Satellite and observational trends favor a ceiling height
holding at or just below 5000 ft through the afternoon.  Any loss of
this ceiling likely of short duration, as renewed expansion of this
stratus occurs this evening/early tonight.  Low probability for
elevated thunderstorms tonight, limited coverage and uncertain
timing precludes a mention.  Firm northeast winds continue through
tonight.

//DTW THRESHOLD PROBABILITIES...

* Medium confidence in ceilings 5000 ft this afternoon and evening.
  High confidence overnight and Sunday.

* Low chance for thunderstorms late tonight and Sunday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from midnight EDT tonight 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.

&&

$$

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


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at www.weather.gov/detroit.



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