Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 132258
AFDDTX

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

.AVIATION...

Rain showers will be most concentrated north of PTK during the
evening with short duration of an isolated thunderstorm. This while
increasing northeast wind in the MBS area to northern Thumb helps
drive IFR ceiling southward across SE Michigan to very near DTW by
midnight. Once in place, IFR is expected to hold firm as rain
showers become widespread late tonight and last through much of
Saturday. There could be an occasional short break into MVFR during
the afternoon just due to the strong wind and incoming cold air. At
the same time, the cold air will introduce freezing rain and sleet
first at MBS and then spreading southward to FNT by late morning.
The onset time at PTK remains a little more uncertain with a
potential start during early afternoon in mid April, however the
strong northeast wind will be a force in driving the cold air
southward. Sustained wind is expected to increase to around 25 knots
with gusts around 35 knots during the afternoon over the entire
area.

For DTW... Upstream thunderstorms diminished to weaker showers
during late afternoon and now storms appear unlikely at DTW until
later tonight. In the meantime, IFR ceiling moving south from MBS to
FNT will reach DTW shortly after midnight and then hold through
Saturday. Rain showers will become widespread late tonight with a
chance of thunderstorms and continue through much of Saturday.
Northeast wind increasing to gusts around 25 knots will be able to
carry colder air into the area, but at this time temperatures are
expected to remain above freezing until possibly after sunset
Saturday evening.

/DTW THRESHOLD PROBABILITIES...

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

* Low for ceiling 200 ft or less late tonight and Saturday.

* Low for thunderstorms to impact the terminal after 08Z.

* High in precip type as rain through TAF period.

&&

.PREV DISCUSSION...
Issued at 408 PM EDT Fri Apr 13 2018

DISCUSSION...

Relatively little change in overall thinking for this weekend. This
afternoon`s forecast package features mainly cosmetic changes to the
going forecast and an upgrade of the existing watch to a winter
storm warning for freezing rain.

Showers and a few thunderstorms, including a half inch hail report
at Harbor Beach this afternoon, are ongoing over the far northern
CWA and northern lower Michigan. Temperatures warmed up nicely
today, largely owing to clear skies for the first half of the day
south of the mid-level front. Even so, progression of the Canadian
high over Ontario and incoming rain with the next round of mid-level
forcing will be sufficient to push the baroclinic zone well to the
south by morning. General consensus on a half inch to an inch in the
06-12z period late tonight with high res models keying in more on
the conditionally unstable airmass and producing localized bands in
excess of one inch. This is all reasonable. In addition, some
thunder can be expected and the possibility for localized pea sized
hail. Most of this is expected to fall prior to the influx of cold
northeast flow forcing temperatures below freezing at the surface.
Thus, the WSW does not start until 12z Sat morning. Little change in
the synoptic and frontal scale environment through the first half of
Saturday as fgen intensifies and moisture gathers in the
strengthening mid-level deformation axis. Moderate to locally heavy
rain is expected to persist through 18z Saturday as sub-freezing
temps gradually sag south. With the 12z EC more supportive of recent
aggressive NAM trends, but not terribly different from previous
runs, confidence is sufficiently increased to expect ice accretion
to begin Sat morning. Despite high QPF, high rates and antecedent
wet conditions will serve as an initial mitigating factor. As
forcing and the deformation axis shift east Saturday afternoon, the
formidable inversion and shearwise forcing in the boundary layer
will take over as the dominant forcing mechanism and will continue
to contribute to light freezing rain or freezing drizzle for the
remainder of the afternoon and into the evening in the warning area.
This will likely be no less hazardous given the nature of fzra is
that diminished rates are countered by more efficient accretion. The
greatest uncertainty through Saturday will be the depth of cold air
across the far north. It remains possible that locations north of
M46 see a prolonged period of sleet which could result in a
reduction in the total ice forecast there.

As the upper low approaches and takes on a negative tilt, the warm
conveyor will surge northward Saturday night against background of
strong synoptic scale dynamic support. The warm nose will blow away
any mid-level cold air, ensuring that sleet does not complicate
matters the second time around. Conditional instability and PWATs
around 1.2" will support widespread showers, heavy at times,
yielding freezing rain in the warning area. It is within the realm
of possibility that an advisory for freezing rain will be needed for
the M59 counties, or at least for locations along/west of the
glacial ridge. Increased ice accretion totals to a few hundredths
along an Ann Arbor to Warren line and to over a tenth along/north of
M59.

Persistent strong northeast winds will be ongoing throughout this
time. The 12z GLERL and NOS runs are strongly supportive of
lakeshore flooding issues in Bay County and Monroe County, where the
water level alone will likely meet lakeshore flood warning criteria,
 the additional inland penetration due to high wave action.
Storm surge is also a threat create problems on the Saginaw River.
Tuscola and Wayne Counties, oriented slightly more flow-parallel,
appear more likely to only achieve advisory criteria at this time.
In addition, gusty winds coming off Lake Huron will approach or
exceed wind advisory criteria during the day Saturday along the
Huron County shoreline prompting the issuance of a wind advisory
beginning 8am Saturday.

The expansive low pressure system will lift away from the region on
Monday as colder air aloft on the backside of the low transitions
the ptype for any lingering showers to snow/rain. Temperatures will
remain well below average through the early part of next week with
highs in the upper 30s to low 40s on Monday before slowly moderating
on Tuesday as weak riding brings drier conditions. The next low
pressure system is then progged to lift across the region midweek
bringing another round of precipitation to the region Wednesday.
Will need to monitor the track of this low but current indications
favor a more northerly track allowing warmer air to surge north
keeping highs in the mid 40s north to mid 50s south and rain as the
predominant ptype.

MARINE...

An approaching low pressure system and strengthening pressure
gradient will increase wind and wind gusts across the lakes and bay,
specifically across Saginaw Bay into all of Lake Huron. ENE gusts
between 30 - 40 knots will persist early Saturday morning into all
the way through the bulk of Sunday. This will produce wave heights
ranging between 10 - 14 feet across nearshore Lake Huron through
this timeframe, with waves up to 5-6 feet possible in the inner
Saginaw bay. Small craft advisories will remain in effect until at
least 8 AM EDT Saturday, in addition to a gale watch that will be in
effect from Saturday morning through Sunday afternoon.

HYDROLOGY...

An elevated frontal boundary will produce scattered rain showers
across SE MI tonight, with coverage increasing to widespread showers
overnight and throughout Saturday as low pressure approaches the
state. This dynamic system is expected to have the bulk of
precipitation fall as sleet, freezing rain, or a combination of the
two across I69 north, while the Metro region is expected to remain
on the warmer side, producing rain through at least Saturday
afternoon. Uncertainty does remain regarding how far south the
freezing line will extend late Saturday into Sunday, which could
help introduce sleet/freezing rain chances to the northern Metro
region. 2 - 3 inches of liquid precipitation is expected to fall
over the next 48 hours, between Friday afternoon into Sunday
afternoon. In addition to precipitation, strong ENE flow will bring
the chance to see lakeshore flooding across Bay and Tuscola county,
and down into Wayne and Monroe county. As a result, a lakeshore
flood watch is in effect for the four counties.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Winter Storm Warning from 11 AM Saturday to noon EDT Sunday for
     MIZ060>063.

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

     Winter Storm Warning from 8 AM Saturday to noon EDT Sunday for
     MIZ047>049-053>055.

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

     Wind Advisory from 5 AM to 4 PM EDT Saturday for MIZ049.

Lake Huron...Gale Warning from 2 AM Saturday to 10 PM EDT Sunday for LHZ361>363-
     462.

     Gale Warning from 5 AM Saturday to 11 AM EDT Sunday for LHZ463.

     Small Craft Advisory until 2 AM EDT Saturday for LHZ421-422-441.

     Gale Warning from 2 AM Saturday to 4 PM EDT Sunday for LHZ421-422-
     441.

     Small Craft Advisory until 5 AM EDT Monday for LHZ442-443.

Lake St Clair...NONE.

Michigan waters of Lake Erie...Small Craft Advisory from 8 AM Saturday to 4 PM EDT Sunday for
     LEZ444.

&&

$$

AVIATION.....BT
DISCUSSION...JVC/JD
MARINE.......AM
HYDROLOGY....AM


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