Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS63 KDTX 131741
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
141 PM EDT Fri Apr 13 2018


.AVIATION...

Deep moisture from an approaching low pressure system will advect
moisture and cloud cover from the west/southwest at roughly 6-9 kft,
with cloud bases quickly diminishing tonight and overnight as near-
surface and low-levels continue to saturate. Scattered showers will
develop tonight and into the early morning hours before coverage
increases to widespread rain as an elevated frontal boundary sets up
across Michigan. Temperatures are then expected to diminish below or
to the freezing mark across the FNT to MBS by late morning Saturday
which will then produce sleet and freezing rain across the terminals,
with freezing rain/sleet chances continuing into the day. Lower
confidence regarding freezing rain chances for the PTK terminal
through the TAF period as temperatures come close to the freezing
mark, however, opted to include a PROB30 group for the tail end of
the TAF. In addition to rain/freezing rain/sleet chances, an
increasing pressure gradient will produce ENE wind gusts between 25 -
35 knots, with isolated gusts up to 40 knots across terminals north
of the Metro region throughout the late morning and afternoon.

For DTW... Deep moisture will increase cloud cover across the Metro
terminals through the afternoon and into the evening. An elevated
frontal boundary will then produce scattered showers turning to
widespread showers this evening into the overnight period, with
embedded elevated thunderstorms possible, specifically in the 00z -
12z timeframe. At this time, conditions are too warm to introduce any
freezing rain chances into the TAF as temperatures reside above the
freezing mark through the TAF period, however, rain will persist
through Saturday afternoon and evening as ENE, with a strengthening
pressure gradient increasing ENE wind gusts up to 20-25 knots.

.

/DTW THRESHOLD PROBABILITIES...

* Low for ceiling 5000 ft or less early this morning. Moderate this
  afternoon and beyond.

* Low for CIGS below 200ft tonight.

* Low for thunderstorms to impact the terminal after 00Z.

* High in precip type as rain through TAF period.

&&

.PREV DISCUSSION...
Issued at 1147 AM EDT Fri Apr 13 2018

UPDATE...

Convection is ongoing across the far northern counties as the lead
edge of another mid-level moisture push interacts with the existing
conditionally unstable airmass. Satellite shows mostly clear skies
south of the I-69 corridor which is allowing temperatures to rise
into the upper 40s to upper 50s depending on location already this
morning. Adjusted high temps upward accordingly. The HRRR suggests a
healthy intrusion of 70s once again, but forecast soundings suggest a
very poor handling of the incredibly strong inversion noted on this
morning`s 12z DTX sounding. The more likely outcome is already
occuring just upstream, and that is an eastward-advancing boundary
layer stratus deck as moisture gets trapped beneath the inversion
during daytime heating. Expect these clouds will advance into the
area in time and help limit heating.

Lakeshore flood watches have also been issued for Bay & Tuscola and
Monroe & Wayne County. The latest 12z GLERL runs and NOS output
suggest warning-level flooding for Monroe County and possibly Bay
County while the NE fetch indicates Tuscola and Wayne are more likely
to fall into advisory territory.

PREV DISCUSSION...
Issued at 343 AM EDT Fri Apr 13 2018

DISCUSSION...

An ongoing deepening upper low over the central Rockies will slowly
lift across the Upper Midwest this weekend. This system will drive a
plume of deep moisture into the Great Lakes, supporting an extended
period of precipitation. Meanwhile, a polar low over Hudson Bay will
rotate across ern Canada today into Saturday. High pressure will
expand across nrn Ontario behind this wave on Saturday. The result
will be a strengthening e-ne low level wind field which will drive
shallow cold air across Lower Mi, resulting in freezing rain over
portions of the of the forecast area Sat morning through Sun
morning.

Entrance region dynamics within an intensifying upper jet to the
northeast of the Great Lakes today will enhance frontal forcing
along the baroclinic zone now draped across Lower Mi. Model
solutions suggest the 800-700mb portion of the frontal zone will be
more active this afternoon into this evening, targeting the Saginaw
Valley and thumb for more widespread showers. Low-mid level moisture
advection and decreasing mid level stability will however remain
supportive of at least sct-numerous showers (perhaps some elevated
thunder) farther south. The sfc front will hold along or just south
of the Mi state line, leading to much cooler temps today compared to
yesterday. While, afternoon highs may approach 60 near the state
line, strengthening northeast flow will sustain a shallow cold Lake
Huron modified airmass across Se Mi, leading to temps ranging from
the 50s in Detroit to 30s across the nrn Thumb and downwind of
Saginaw Bay.

A plume of deep moisture will be driven into Lower Mi tonight as the
upper low meanders toward the Upper Midwest. This moist plume will
interact with a rather robust deep layer frontal circulation over
Lower Mi, resulting in widespread precipitation. There will also be
a convective component as mid level convective instability develops
along the frontal slope, supporting at least scattered coverage of
thunderstorms. This forcing is expected to persist into the first
half of the day Saturday. The 00Z model solutions are all in
reasonably good agreement in showing a region of mid level dry air
overspreading Se Mi late Sat/Sat evening with the departure of a
lead mid level short wave feature. Ample low level moisture may
however lend itself to a drizzle/freezing drizzle situation. A
second surge in deep layer moisture is then forecast to lift into Se
Mi within the warm conveyor of the aforementioned upper low
overnight Sat and persist well into Sun. The upper low is forecast
to develop a negative tilt Sunday, supporting an increasing region
of upper diffluence across Lower Mi, thus sustaining widespread
precip.

Thermal profiles support all rain today well into tonight. Cold air
advection near the surface will then occur overnight through the day
Saturday as the northeast gradient increases. The airmass associated
with the sfc high building across nrn Ontario is quite cold and dry.
This will force sfc temps to drop below freezing across the Saginaw
Valley/Thumb region late tonight and then continue to slowly drop
farther south during the day Saturday and Saturday night. Model
trends have taken the colder air a little farther south. The GFS is
on the warmer side of model guidance with respect to sfc temps,
however it is not handling the degree of dry air to the northwest of
the region well attm. So in light of the trends, the winter storm
watch for freezing rain will be expanded across the I-69 counties.
There is concern that subfreezing air may advance to the Irish
Hills, suggesting that Livingston and portions of Oakland Counties
(perhaps even farther south) may experience a period of freezing
rain, especially Sun morning when the next surge of moisture is set
to arrive. On the other hand, with model solutions trending farther
south, there is the potential that the cold air depth may be enough
to support more sleet across Midland/Bay/Huron Counties, thus
cutting down on icing. So given these uncertainties, confidence is
not high enough to go with a warning for any one location. Therefore
a winter storm watch will remain in effect for potential icing
accums of a quarter to around one half inch. Given that much of the
precip will be falling during the daytime Saturday, probabilities in
receiving ice amounts to an inch are still rather low.

In addition to the freezing rain/sleet concerns, total precipitation
over the next three days is expected to range from 2 to 3 inches. So
rises on area rivers are possible. Gusty northeast winds are also
expected, raising concerns for shoreline flooding in Bay county.

The upper low is forecast to finally lift across the Great Lakes
region Sun night through Monday. The depth of the cold air will be
deepening as northwest flow ensues with the departure of the system.
This will support cold mid April temps to start the work week.

MARINE...

A stalled front across the southern Michigan border will become a
warm front today as a low pressure system over the plains slowly
tracks eastward along it. Meanwhile, a strengthening high pressure
system will build across James Bay increasing the gradient across
the Great Lakes leading to a period of strong northeasterly flow
Saturday and Sunday. Gusts may reach gale force during this period
thus a Gale Watch is in effect. Best chances for gales will be
across central and northern Lake Huron where gusts could increase to
around 40 knots. The persistent onshore fetch will lead to rapidly
building significant wave heights of 5-10 feet across the nearshore
zones of Lake Huron Friday night and Saturday, and up to 20 feet at
times over the open central Lake Huron waters.

HYDROLOGY...

A frontal boundary over the Michigan/Ohio border will lift north as
a warm front today, bringing scattered rain showers across Mid
Michigan. This warm front will precede the arrival of a powerful and
moisture laden late season winter storm system that will slowly
track across southern Michigan through the weekend. Periods of rain,
heavy at times within embedded thunderstorms, will occur late Friday
and through the weekend. Further north, freezing rain will be
possible which could lead to significant icing especially along and
north of the I-69 corridor. Total rainfall of 2-3+ inches of rain
will be possible with this system over the course of several days.
Because the rain will fall over a period of multiple days, rivers
are forecast to remain below flood stage at this time.

Additionally, strong onshore northeasterly winds will lead to a
heightened potential of lakeshore flooding across Saginaw Bay and
portions of the Lake Huron shoreline.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Watch from Saturday morning through Sunday morning for
     MIZ060>063.

     Lakeshore Flood Watch from Saturday morning through Sunday
     afternoon for MIZ076-083.

     Lakeshore Flood Watch from Saturday morning through Sunday morning
     for MIZ048-054.

     Winter Storm Watch from late tonight through Sunday morning for
     MIZ047>049-053>055.

Lake Huron...Gale Watch from Saturday morning through Sunday afternoon for
     LHZ421-422-441>443-462>464.

     Small Craft Advisory until 8 AM EDT Saturday for LHZ421-422-441.

     Small Craft Advisory from 4 PM this afternoon to 8 AM EDT Saturday
     for LHZ442-443.

     Gale Watch from Saturday morning through Sunday evening for
     LHZ361>363.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory from 8 AM Saturday to 4 PM EDT Sunday for
     LEZ444.

&&

$$

AVIATION.....AM
UPDATE.......JVC
DISCUSSION...SC
MARINE.......DRK
HYDROLOGY....DRK


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.