Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 281148
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
648 AM EST Tue Feb 28 2017

.AVIATION...

Showers will continue to spread over the terminal corridor during
the morning producing borderline IFR/LIFR conditions. Low level
moisture will be on the increase ahead of the upstream warm front
with slight improvement into MVFR as showers end toward afternoon.
Showers will end for the afternoon and then return after 00z as low
pressure encroaches from the west. IFR/LIFR conditions can be
expected again as the associated warm front moves south to north
through Lower Michigan. These basic conditions will persist until
the low and attending cold front cross the region 09z-12z tonight.
Thunderstorms will be possible along the front overnight with some
timing and coverage refinement needed before addition to upcoming
forecasts.

For DTW... Mainly MVFR ceiling is expected during the morning with
intervals of IFR as rain showers move across the region. There will
be lull in showers this afternoon before coverage increases again
this evening. The best chance of thunderstorms will be from late
evening through overnight with timing and coverage refinement needed
before addition to the forecast.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling aob 5000 feet today through tonight.

* Low for thunderstorms impacting terminal tonight.

&&

.PREV DISCUSSION...
Issued at 340 AM EST Tue Feb 28 2017

DISCUSSION...

An active period of mid week weather remains on track to affect SE
Michigan today through Wednesday night. Several rounds of showers
and thunderstorms are expected today through tonight with a marginal
risk of severe weather later tonight. Rain is then expected to
change over to snow Wednesday afternoon and continue through evening
amounting to accumulation in the 2 to 4 inch range Tri Cities,
around 2 inches I-69 corridor, to an inch or less south of M-59
through metro Detroit to the Ohio border.

Satellite imagery, radar composite, and model analysis fields help
piece together the scenario for today. The exit region of the
leading upper level jet streak within broad southwest flow aloft is
providing enhanced support for upward motion over the Midwest and
southern Great Lakes. A respectable low level jet response is then
activating the first of a two stage pattern of isentropic lift and
moisture transport responsible for the inbound showers. There has
been some lightning strikes from central Illinois eastward through
Indiana that will be monitored as mid level lapse rates hold
generally less than 7 C/km in the 700-500 mb layer through this
stage of the event. The second stage will be driven by the main
upper jet moving downstream of the Rockies long wave trough this
afternoon. The strong and well defined exit region will further the
development of surface low pressure over the central Plains into the
Mississippi Valley with an eastward extending warm front. The
associated low level jet will force the next round of strong moisture
transport/theta-e advection that will spread showers over Lower
Michigan by early evening. Mid level lapse rates shown in hourly
mesoanalysis near 8 C/km in the 700-500 mb layer over the central
Plains will spread eastward with this forcing and bring a much
better chance of thunderstorms into our area compared to early
portions of the day.

The evening segment of showers/storms will generally be along and
ahead of the surface warm front that is projected with good model
agreement to follow into Lower Michigan. Consensus of high res model
depictions of non-zero surface based CAPE suggest the warm sector
will spread over most of SE Michigan by midnight and set the stage
for yet another round of showers and storms for the overnight hours.
This round will develop as the surface wave slides along the front
to our north followed by strong linear forcing under the upper jet
axis and along the trailing prefrontal trough and cold front. The
amount of instability that will be available for convection is the
greatest uncertainty during this segment of the event. As to be
expected during the night, high res models are very conservative
with surface based instability. They show generally less than 500
J/kg with surface dewpoint in the mid to upper 50s, and with 700-500
mb lapse rates generally less than 7 C/km. That being said, the
strong forcing supplied by the boundaries and associated ageostrophic
circulations will likely help to organize storms into a linear mode
that would make damaging wind the primary concern through sunrise
Wednesday. The latest day 1 outlook with marginal risk just over the
Ohio border reflects the highest probability area where instability
will be adequate for a greater degree of convective organization and
where wind gusts could possibly reach the ground through any surface
based stable layer. Along with severe weather potential is prospects
for excessive rainfall which could exceed 1 inch between midnight and
sunrise Wednesday morning on top of the quarter to half inch that is
likely with the prior activity expected today.

The very active period of weather for the Great Lakes will continue
Wednesday with secondary surface low development along the cold
front. This will be tied to the approach of the upper level trough
and associated vorticity maximum and will just reinforce the already
strong low level temperature gradient over the region. Critical
aspects of the forecast during this stage will be the timing of the
mid level dry slot during the morning and the eventual track of the
surface wave through Lower Michigan. Both will have influence on the
pace of rain changing to snow and snowfall totals through Wednesday
night associated with the strong and well-defined deformation zone
that will develop as the upper wave cross the area. A mid to late
afternoon rain/snow transition is maintained over the Tri Cities and
northern Thumb while nudging accumulation up into the 2 to 4 inch
range for the event total. There is some potential for
overachievement under the deformation forcing as mid level moisture
will be plentiful judging by 2-3 g/kg specific humidity in the 850-
700 mb layer with very low stability up to about 400 mb. The pattern
appears progressive enough to remain under advisory criteria for now
with all models indicating little to no additional accumulation
after midnight Wednesday night.

The end of the week will be quieter, at least relative to the mid
week period, as a textbook clipper is expected to move through Lower
Michigan Thursday night into Friday. This system could produce an
inch or two of snow as it moves through the region, mainly south of
I-69 on current projections. This will be followed by a warming
trend that remains on track for the weekend.

MARINE...

South to southeast flow will increase today in advance of an
approaching low pressure system. Relatively stable conditions
suggest that wind gusts should top out around 25 knots during the
day over Lake Huron. This primarily southerly direction will also
minimize wave concerns over the nearshore waters. The low pressure
will slowly track through the Central Great Lakes late tonight into
Wednesday.

Stronger north winds will develop as this system moves to the east,
and while a few gale force gusts will be possible over the open
waters of Lake Huron Wednesday into Wednesday night, it appears that
gusts to around 30 knots will be a more common occurrence. Given
this fact, will hold off on a Gale Watch with this forecast and
allow the model solutions to better define low pressure strength and
position. Winds will quickly diminish on Thursday as high pressure
begins to encroach on the area. This high will then build across the
area Friday and Friday night.

HYDROLOGY...

Deepening low pressure coming out of the Central Plains will spread
showers and thunderstorms over Lower Michigan today through
Wednesday. Total rainfall during this time is expected to range from
0.75 inches to 1.25 inches with the bulk of the heaviest rain
falling tonight. Bands of heavier showers and thunderstorms will
have potential to result in locally higher rates/amounts. Minor
flooding of lying areas is possible along with rises in area rivers
and streams. Much colder air will filter in by the end of the day on
Wednesday, changing any precipitation to snow. Snowfall Wednesday
evening will contribute minimally to the runoff.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....DG/BT
DISCUSSION...BT
MARINE.......DG
HYDROLOGY....BT


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