Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 290207
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1007 PM EDT WED SEP 28 2016

.UPDATE...

The large cut-off low pressure system over Indiana and Ohio is
producing a classic pattern of isentropic ascent into mid level
deformation over lower Michigan tonight. This is an efficient
rainfall producer on its own but even more so when the moisture
supply is so rich and with a perfect trajectory for parcels to
borrow some instability from Lake Erie. The 00Z DTX sounding
indicated PW at 0.92 inch which matches up pretty well with model
soundings at 00Z and are forecast to rise near 1.25 inch by 12Z.
Hourly mesoanalysis also indicates Lake Erie producing CAPE around
1000 J/kg and giving rise to bands of showers and thunderstorms.
The storms have not held together very far inland over southern
Ontario, instead broadening into bands of moderate showers.
Synoptic scale models show average mid level lapse rates and just
about no MU/MLCAPE inland from Lake Erie through the night. That
will produce minimal thunderstorm potential but which will not
really be needed for locally heavy rainfall. The persistence of
isentropic ascent, occlusion dynamics, and increasing moisture
will all combine to maintain bands of showers mainly south of M-59
to the Ohio border tonight through Thursday morning. It is
possible that 12 hour rainfall, midnight to noon Thursday, in the
1-2 inch range will be within reach in this pattern. This will be
closely monitored for urban and small stream headline needs,
especially if multiple bands set up to affect the Detroit metro
area through the morning commute.

&&

.PREV DISCUSSION...
Issued at 713 PM EDT Wed SEP 28 2016

AVIATION...

A large area of low pressure will remain over central Indiana and
Ohio tonight through Thursday. The system will continue an abundant
supply of showers over SE Michigan, mainly over the DTW terminal
corridor. In addition, increasing easterly flow off Lake Erie will
combine with the rain to produce a downward trend through MVFR into
IFR during the night. This will also be supported by the proximity
of the low near the Ohio border for an extended period of time. That
also being the case, FNT to MBS stand the best chance for low end
VFR to return during Wednesday being farther north into drier air.
For now, plan to bring ceiling back into MVFR from PTK to DTW during
Wednesday afternoon assuming modest daytime heating will help with
some improvement.

For DTW... Early evening radar trends suggest brief moderate showers
producing MVFR restriction but little or no coverage of
thunderstorms over the D21 air space. The main concern will be
increasing easterly wind and coverage of moderate showers leading to
IFR ceiling overnight through Thursday morning.

//DTW Threshold Probabilities...

* Medium for ceiling at or below 5000 ft during evening. High
  overnight through Thursday.

* Low for thunderstorms tonight.

PREV DISCUSSION...
Issued at 303 PM EDT Wed SEP 28 2016

First major vorticity spoke has pivoted north to northwest into
Southeast Lower Michigan around parent upper level low pressure
system now dropping gradually south through the Ohio Valley. Rain
shield associated with this wave has featured light to occasionally
moderate rain for the most part with isolated thunderstorms also
noted during the day along the northeast periphery of the main area
of rain where solar insolation from the southern tip of Lake Huron
on into Thumb region has led to a pocket of better instability.

While the higher resolution model solutions vary considerably in
the mesoscale details of the anticipated bands of moderate to
locally heavy rainfall, there is a strong consensus of maintaining a
rather persistent arc of rain from western Lake Erie/far southeast
lower Michigan northwestward through a bulk of the 1/2 to 2/3 of the
forecast area. Will increase precipitation chances into the 80-100
percent range along this axis of most persistent rain and taper off
from that, particularly over the northern forecast area. That said,
even locations from parts of the Saginaw Valley back to the Thumb
will have a good chance of showers along with a few thunderstorms,
especially into evening before daytime heating and instability
subside.

Forecast thinking has not changed much through the rest of the week
as the upper low settles south to Kentucky and the feed of moisture
back west to northwest into the area remains steady. In fact, as an
increasing levels of moisture from the Atlantic is drawn into the
area within this flow and precipitable water values climb to 1.50
inches or so, periods of rainfall should become more prolific from
tonight into Friday.

The most persistent rain will focused roughly along/south of M-59
during this time frame, but a gradual expansion north through the
rest of the area should also occur as the overall abundance of
moisture expands. While not all locations will receive heavy
rainfall amounts, several inches of rain can be expected where
the this most persistent banding becomes established.

With the increase in moisture and a more or less persistent shield
of clouds over the area, temperatures will remain rather steady with
highs in the mid/upper 60s for the rest of the week and lows in the
middle 50s.

Upper low projected to drift back northward across Lower Michigan
this weekend with unsettled weather continuing. Upper ridging builds
in early next week ahead of deep troughing over the western CONUS,
leaving Southeast Michigan dry with a modest warming trend into
midweek.

MARINE...

Strong northeast winds will develop over the Central Great Lakes
tonight as high pressure settles in over Ontario and as low pressure
drops into the Ohio Valley and then Kentucky. This pattern will
remain fairly stagnant into Friday, providing a prolonged period of
northeast winds with sustained speeds in the 15 to 25 knot range.
The persistent northeast wind and large fetch over both Lake Huron
and Western Lake Erie will allow large waves to build from tonight
into Friday night. Small craft advisories remain in effect for all
marine areas into late Friday night due to large waves and gusty
winds. The area of low pressure will lift back northward into
Michigan late Friday night into Saturday. This will decrease the
surface pressure gradient and turn winds more to the east, allowing
wind speeds and wave heights to decrease overnight.

Moisture will meanwhile continue to wrap around the area of low
pressure into the weekend, providing numerous showers but only a low
chance for thunderstorms. The cool airmass will provide a chance for
waterspouts tonight and Thursday as the colder air resides over the
warm lake waters.

HYDROLOGY...

A large area of upper level low pressure will remain positioned
between Kentucky and Michigan into the weekend, keeping a cool and
showery pattern in place through Saturday. Total rainfall this
evening through Friday night is expected to generally range between
1 and 2 inches, although some locations near the Ohio border may
receive higher amounts. Periods of steadier and slightly heavier
rain are expected to occur over the next few days. The first is
expected to impact areas along and south of M-59 late tonight and
Thursday, and another is expected to impact just about all of
Southeast Michigan late Thursday night through Friday. The rain is
expected to cause rises on area creeks and streams, and may allow
some ponding to develop on roadways and low-lying areas.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.

Lake Huron...Small Craft Advisory until midnight EDT Friday night FOR LHZ421-422-
     441>443.

Lake St Clair...Small Craft Advisory from 8 AM Thursday to midnight EDT Friday
     night FOR LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 8 AM Thursday to midnight EDT Friday
     night FOR LEZ444.

&&

$$

UPDATE.......BT
AVIATION.....BT
DISCUSSION...DG/DT
MARINE.......HLO
HYDROLOGY....HLO


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at www.weather.gov/detroit.



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