Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
FXUS63 KDTX 300717
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
317 AM EDT FRI SEP 30 2016
The regional radar show another round of showers advancing across
much of the forecast area this morning. Recent radar trends indicate
that the intensity and thus rainfall amounts through the morning
rush hour will be much less than what occurred yesterday. This next
round of rain is still associated with the quasi stationary upper
low centered over Kentucky. The water vapor loop indicates good
upper difluent flow across the upper Ohio Valley and eastern Great
Lakes. There is also strong transport of Atlantic moisture extending
from the Mid Atlantic region into the eastern Great Lakes. These
factors are supporting the extensive swath of showers from Se Mi all
the way to wrn Virginia.
During the course of the day, the center of the upper low is
forecast to only inch up toward the Indiana/Kentucky border. A
strengthening of the cyclonically curved upper jet max to the east
of the mid level circulation (across the Upper Ohio Valley) is
forecast to increase upper divergence across Se Mi. This will then
trigger an increase in mid level deformation across Srn Lower Mi.
Model cross sections do show a rather deep layer of weak elevated
convective instability with respect to a saturated air parcel. This
does suggest the potential for the development of a rather deep mid
level frontal circulation. The main forecast uncertainty at this
point is the location of the more persistent mid level frontal
forcing and thus heaviest rainfall today. The NAM and GFS continue
to suggest this mid level frontal circulation focusing the more
persistent rainfall from the northern Detroit suburbs up through the
I-69 corridor into southern Saginaw county this afternoon into the
evening. The HRRR and especially the ARW suggest the frontal
dynamics will become established a little farther south (Metro
Detroit/Ann Arbor south to the Ohio border). Given the compact
nature of the upper low, it certainly seems possible for any mid
level frontal forcing to set up farther south closer to the mid
level low center.
So it seems possible for locations (generally south of a Saginaw to
Bad Axe line) to pick up another half inch to inch of rain, with
a few locals receiving up to an additional two inches of rain.
For this reason and and with some uncertainty where any nearly
stationary bands of more moderate rain will set up later today, the
current flood watch will remain intact.
The upper low will inch its way northward across Indian tonight. The
upper level divergence and better mid level frontal dynamics will
slide to the west of the forecast area, while the feed of the
Atlantic moisture fractures. This will lead to decreasing chances
for showers during the night. While there is of course some slight
differences amoung the 00Z model suite in the exact track of the
upper low, they all suggest it will lift into srn Lower Mi on
Saturday, then slowly depart to the east of the region Sunday into
Monday. The associated cold pool aloft and resulting steep mid level
lapse rates will reside over the forecast area Sat into at least
early Sunday. This will support some redevelopment of showers and a
few thunderstorms. The potential for deep convection within close
proximity to the slow moving upper low could certainly support some
additional localized heavy rainfall Sat into Sun. Ample cloud cover
and showers will maintain a low diurnal spread in temps over the
next three days (highs in the 60s and lows in the 50s). The
departure of the upper low will bring drier and warmer conditions to
the region for the first half of next week.
Stalled low pressure system south of the Great Lakes will maintain
persistent fresh onshore flow into Saturday morning. This will
warrant continuation of existing Small Craft Advisories for all
nearshore zones due to both winds and elevated waves. Easterly winds
will diminish in intensity and back to the northeast by the end of
the weekend as the low begins to lift out of the area.
The pattern today will remain largely unchanged from yesterday.
Rounds of showers will continue to pivot around a stalled low
pressure system with additional rainfall totals of 2 inches possible
on a local basis, including in the Detroit Metro area. The heaviest
rain is forecast to fall between noon and 6pm today, mainly along
and south of the I-69 corridor. Urban and small stream flooding will
once again be possible along with minor flooding on area rivers.
Issued at 1158 PM EDT Thu SEP 29 2016
IFR ceiling will be widespread over SE Michigan during the night and
through the morning. This will be mixed with mostly MVFR restriction
in rain showers as they increase in coverage from Ohio. Nightfall
and infusions of moisture into the boundary layer from rain will
promote persistence of the low ceiling. MBS will have the best
chance to waver between IFR and MVFR within the farther north
reaches of the low pressure system and where the frequency of rain
will be lower until later in the day. IFR visibility will be more
dependent on coverage and duration of heavy rainfall as the surface
wind will remain moderate from the east and prevent much fog
component. Determination of coverage and duration of heavy rain
remains a challenge to pin down outside of the 0-3 hr time range in
this type of low pressure system. This is also the case for
otherwise low thunderstorm potential which will remain out of the
initial forecast and defer to later updates as needed. Ceiling and
visibility conditions are expected to progress upward into MVFR
assuming modest daytime heating during the afternoon will be
sufficient to lift ceiling outside of heavier showers.
For DTW... Ceiling on the cusp of IFR during the evening will settle
below 1000 ft during the night and remain there through the morning.
Visibility restriction remain mostly above 3 miles in moderate
showers. The main concern in the absence of thunderstorms will be
continued moderate northeast wind that will force traffic operations
to the northeast through Friday evening.
//DTW Threshold Probabilities...
* High for ceiling 5000 ft or less through Friday evening.
* Low for thunderstorms through Friday evening.
MI...Flood Watch through this evening FOR MIZ060>063-068>070-075-076.
Lake Huron...Small Craft Advisory until midnight EDT tonight FOR LHZ422.
Small Craft Advisory until 5 AM EDT Saturday FOR LHZ421-441>443.
Lake St Clair...Small Craft Advisory until midnight EDT tonight FOR LCZ460.
Michigan waters of Lake Erie...Small Craft Advisory until midnight EDT tonight FOR LEZ444.
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