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afddtx

Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
550 PM EST Mon Feb 27 2017

Aviation...

Moisture/clouds (4-5 kft) look to be persisting this evening, but
then breaking up overnight, which could potentially allow for some
fog to develop. However, quickly increasing mid clouds overnight
expected to limit fog to the MVFR variety. Showers and low ceilings
still on track to arrive around or just after 12z Tuesday, with
IFR/borderline low MVFR ceilings expected through the day. There is
a low chance of thunderstorms during the day as well, but risk
appears to be too low to include in the tafs.

Light southeast winds (under 10 knots) through this evening,
increasing at or just above 10 knots toward sunrise Tuesday
out of the southeast-south.

For dtw...cloud bases around 5000 feet expected this evening
before dissipating overnight. Rain showers and low cigs to
arrive abruptly, at or just after 12z.

//Dtw threshold probabilities...

* moderate for ceiling 5000 feet late today into this evening, low
overnight, then high tomorrow morning.

* Low confidence in thunderstorms tomorrow.

&&

Previous discussion...
issued at 433 PM EST Mon Feb 27 2017

Discussion...

An active weather period will develop midweek over the center part
of the country in response to both a deep, midlevel trough advancing
into the contiguous United States tonight and a total phasing of the
northern and southern jet stream branches over the mid Mississippi
River valley on Wednesday. The narrative local to southeastern
Michigan is one of a very active, and long duration event that will
result in as many as 3 separate episodes of precipitation activity.
Will speak to each of these episodes in the discussion below.

Tonight...weak surface ridge axis is giving up position late this
afternoon with msas analysis placing the center over sections of
Lake Huron including Georgian Bay. This is playing out in the
moisture field on satellite with a return of southerly flow
trajectories and a gradual expansion of cumulus and stratus to the
north out of Ohio. Already seeing the ceilings over portions of the
northern Detroit Metro area, but will see increasing cloud trends
for all areas this evening with the moisture advection. Off of the
relatively warmer daytime temperatures today and with cloud cover,
expecting temperatures tonight to remain in the mid to upper 30s.

Precipitation episode number 1 - Tuesday morning...anticyclonically
curved upper level jet streak now over The Four Corners region will
emanate steadily to the northeast tonight in response to deep trough
pushing into the Pacific west. Incredible feature speed has it arced
in place over Wisconsin to lower Michigan by 12z Tuesday. It is
along the southern edge of this upper level jet streak that models
quickly generate a Wing of system relative isentropic ascent. Nice
little signature for slug of enhanced moisture advecting out of
Arkansas and southern Mississippi River valley tonight, then
streamlining out along an axis of 308-312k isentropic ascent. Best
potential appears set for the 12-15z window for much of the area
with pops of numerous or likely. Quantitative precipitation forecast of an .08 to .12 inch appears
solid. Removed thunder for early Tuesday with only moist adiabatic
lapse rates to work with. Deep dry air entrainment is then
forecasted to push into the area aggressively for the latter half of
Tuesday morning with an active subsidence signal supported in the
forecast soundings between 5000 to 15000 ft agl. Went aggressive
with no precipitation mention Tuesday afternoon.

Precipitation episode number 2 - Tuesday night...a very interesting
period appears to be setting up for Tuesday night as the main
southern stream jet streak lifts into mid Mississippi River valley.
Combination of left exit region dynamics off of aforementioned jet
streak, left quadrant nose of +65 knot low level jet will allow for
very strong 1000-500mb geopotential height falls to pass directly
over southeastern Michigan during the middle of Tuesday night. Just
an incredible amount of synoptic scale support for ascent, despite
less than ideal straight pv advection. There are a couple of items
that will be monitored moving forward for this period. 1. The
potential for organized thunderstorm activity pushing across
southeastern Michigan during the 05-08z time window. 2. The
potential for an axis of heavy rainfall at greater than 1.0 inch in
less than 6 hours to fall somewhere over lower Michigan. Both of
these items are intricately linked, each dependent on exactly which
level the warm frontal slope lights up. The NAM, arw, and a greater
number of the sref members have suggested a farther northern
solution with frontal activation which then supports thunderstorm
activity to push eastward within The Heart of the warm sector over
southeastern Michigan including the Detroit Metro area. In contrast,
the European model (ecmwf) and regional Gem are much more bullish with a more
southern solution to the heavy rainfall axis. This southern Camp
quickly collapses the bulk of the precipitation down along a hard
edge axis of 925-850mb fgen set along the western edge of the +65
knot low level jet. While uncertainty remains high with regards to
exactly where the heavy liquid axis will fall, did take the step and
forecast an axis of heavy qpf 6-12z Wednesday over the southern half
of the cwa. The last item to be addressed is the potential for
strong to severe thunderstorm activity Tuesday night. A fairly
consistent signal exists for MUCAPES of 250-500 j/kg developing on
the backs of steep 800-600 mb lapse rates. Shear magnitude will be
also be on the high side with 0-6km bulk shear in excess of 50
knots. The interesting development in the 12z suite was a model
derived cold pool in the arw solution for The Heart of the cwa. If
this were to develop then a fairly substantial mixed layer could be
supported which could lead to a strong to severe wind potential with
the tstorm activity 05-08z.

Precipitation episode number 3 - Wednesday...a northern stream upper
level jet streak is forecasted to make the turn through the base of
the deep trough axis on Wednesday completing a phasing of the
northern and southern streams. Overall, magnitude and structure of
main potential vorticity anomalies will become increasingly
organized with this change in jet forcing. Arrival of this pv
anomaly into Southern Lower Michigan late Wednesday afternoon will
then kickstart another period of enhanced synoptic scale support. It
is becoming increasingly likely this forcing will manifest itself in
a strengthening layer of midlevel deformation and widespread
precipitation changeover to snow. Still some noise on timing of the
changeover, but forecaster preference is actually on the quicker
side particularly north of the I 69 corridor. In fact, given degree
of cold air advection, do not see the daytime being that much of an
inhibiting factor for northern areas. Widespread accumulating
snowfall then likely for late Wednesday afternoon through Wednesday
evening. Latest thoughts on amounts for event are 1 to 3 inches
north of M 59 (widespread 2 to 3 inches north of I 69) with less
than an inch across the south including Detroit.

A weak Alberta clipper will push southeast over the Great Lakes/Ohio
River valley Thursday into Friday morning, bringing the chance for
light accumulating snow. While many long range models converge on
low pressure developing over Alberta by 12z weds, track development
and strength of the low varies between models. Both the GFS and NAM
have the low clipping the Michigan/Ohio border by around 00z Fri, while the
European model (ecmwf) and Gem models take a more southerly track, keeping it and the
brunt of the quantitative precipitation forecast over central in/OH, as it travels east. As a
result, pop values will shy on the lower side until better agreement
can be seen.

A thermal ridge is expected to move into the Great Lakes region
Saturday through Monday, which will return temperatures back into
the 40s and 50s for a daytime high. The initial push of warm air advection Friday
into Saturday will bring the chance for precipitation, the onset of
which will start as snow as surface temperatures and the air above
are expected to remain below freezing. Lingering precipitation seen
throughout the late morning/early afternoon may seen a transition to
a wintry mix/rain before diminishing altogether. A second chance for
rain will be possible throughout Sunday as a shortwave quickly
pushes east across central Michigan. Otherwise, long-range models
are starting to pick up on a potential low pressure system
developing over the Central Plains and pushing northeast into our
County Warning Area Monday into Tuesday, which will bring additional rain chances to
the region.

Marine...

Southerly winds developing tonight ahead of a low pressure system,
with winds topping out around 25 knots tomorrow over Northern Lake
Huron. The south direction will minimize wave concerns on nearshore
waters while reinforcing another round of mild air and rain showers
into Tuesday night. Low pressure will slowly track through the
central Great Lakes Tuesday night into Wednesday. Stronger north
wind will develop as this system exits with gusts of 30 knots or
greater projected to bring colder air back into the region Wednesday
afternoon into Wednesday evening. Low end gales are possible, but
confidence was not high enough to issue a gale watch at this time,
as duration look to be short lived as well. The wind will quickly
diminish by Thursday while the colder air lasts through Friday.

Hydrology...

Deepening low pressure coming out of the Central Plains tonight,
with rain arriving tomorrow morning as the low slowly moves through
the central Great Lakes tomorrow night into Wednesday. Total rainfall
in the Tuesday-Wednesday time frame is expected to range from 0.75
inches to 1.25 inches, with the bulk of the heaviest rain falling
Tuesday evening/night. Embedded thunderstorms will also bring the
potentially of locally higher rates/amounts. Minor low lying flooding
is possible, with river and streams also rising. Much colder air
will filter in by the end of the day on Wednesday, changing any
precipitation to snow. Snowfall Wednesday evening should contribute
minimally to the runoff.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&

$$

Aviation.....Sf
discussion...cb/am
marine.......sf

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