Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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674
FXUS63 KDTX 010117
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
917 PM EDT Sun Apr 30 2017

.UPDATE...

Evening satellite imagery depicts a textbook mature mid latitude
cyclone affecting a broad swath of the central U.S. which will drive
the precipitation trend over southern Michigan during the rest of the
night. Smaller scale upper waves/MCVs and their interactions with
the warm/stationary front have been the main forcing over the last
24-36 hours, and will be for the evening pattern, followed by the
primary low pressure system becoming more involved overnight. The
good news here is that rainfall rate has been light overall during
the late afternoon and early evening, especially over the flood watch
area of the Saginaw Valley and northern Thumb, and storm totals
south of the watch area have averaged about 0.5 inch according to
surface obs and radar estimates. Any upgrades to or expansion of the
watch are therefore not anticipated at this point. Statements and
graphics will be issued to cover trends with the main short term
concern possibly being some urban ponding of water on roads in the
Detroit area.

The primary low pressure system and frontal occlusion will enter
Lower Michigan late tonight and bring another round of showers and
scattered thunderstorms to the region. This activity will move
quickly through SE Michigan while possibly disrupting the morning
peak travel period before being swept eastward into Ontario by late
morning.

&&

.PREV DISCUSSION...
Issued at 710 PM EDT Sun Apr 30 2017

AVIATION...

IFR will remain firmly in place tonight at MBS and FNT while
conditions will be much more variable from PTK south, especially
when factoring in restriction due to shower clusters and scattered
thunderstorms. The primary warm front remains south of the Ohio
border while the departing MCV is inducing a north/south trough from
about PHN to DTW which is producing some enhancement of dry
northeast flow from southern Ontario. The wind pattern will support
VFR in the DTW area through sunset after which fog concerns will
enter the picture for locations not already covered with IFR/LIFR
stratus.

Thunderstorms in Ohio will be replaced by a new round of development
over Illinois/Indiana that will move into southern Michigan during
the evening. Rainfall will be heavy enough for some IFR restriction
when mixed with fog near the surface frontal zone. The most favored
time windows for storms will be mid to late evening and again toward
sunrise as the cold front/occlusion approaches from the west. This
front will sweep the showers and low clouds out of the region by
late Monday morning and be followed by southwest wind gusting into
the 30-35 knot range during the afternoon.

For DTW... DTW will be on the edge of IFR stratus and fog during the
evening making it difficult to be optimistic with the forecast
outside of the first few hours this evening. Thunderstorm coverage
upstream supports a mention in the 01-04Z window.

//DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling 5000 ft or less through Monday morning.

* Moderate for thunderstorms through tonight.

PREV DISCUSSION...
Issued at 251 PM EDT Sun Apr 30 2017

DISCUSSION...

Rapid fire upper level disturbances/MCV`s embedded in moist airmass
with next weaker one seen on Goes R Ice Channel 5 already lifting
north through Western Ohio Valley. Blossoming area of showers on the
northeast flank has limited the destabilization over northern
Indiana, but still enough to maintain showers and possible
thunderstorms into southeast Michigan late today/this evening. With
the failure of warm front to lift into the State during daylight
hours and limited instability it remains highly doubtful we will see
any severe weather in our CWA. Still, 12z NAM/Regional Gem do
indicate 1000-850 mb capes nearing 1000 J/kg close to Lenawee county
at 00z, while RAP is displaced farther east with the pocket of
instability, which looks better based off the temps in the 80s over
Eastern Ohio. Much like the QPF bulls-eyes, models running a bit too
hot. None-the-less...one is always weary with such a strong low
level jet (50+ knots at 850 mb), even overnight as the jet
strengthens further, 65+ knots, but surface based inversion is
expected to hold.

Flood watch will remain in effect for Tri-Cities and adjacent
counties, as we received solid rainfall today pushing totals to
around 1 inch. Will get additional rain (half an inch to maybe 1
inch) late tonight with the cold front, as the large upper level low
over the Central Plains is finally booted off to the northeast from
the upper level energy working through the northern Rockies.
Moisture axis will slide east of southeast Michigan between 12-14Z
with good part of the day under influence of mid level dry slot.
However, good low level cold advection and cyclonic flow will
eventually spread scattered to numerous showers back into the area,
right into Tuesday as 850 mb temps of -4 C settle overhead.
Certainly plausible for graupel to fall with that amount of cold
air. Based of forecasted nam soundings and local probabilistic wind
forecast, wind gusts of 35 to 40 mph look to be common late in the
day Monday, with gusts at above 30 mph continuing right into Tuesday,
as 992 mb low slowly tracks through the northern Great Lakes. Highs
around 50 degrees on Tuesday will likely be the best we can do.

High pressure building in across the Ohio Valley will keep
conditions dry throughout Wednesday as temperatures remain moderate,
with daytime highs capped in the upper-50s. The next chance for rain
showers will enter Thursday morning into Friday as a low pressure
system pushes in from TN into PA. PoP values will continue to remain
sub-30 across the Metro Detroit area for this potential system as
only the GFS and to some extent the ECMWF solution bring the low
north enough, allowing the precipitation shield to clip southern
Michigan. Breezy conditions and a slightly better chances for rain
showers look to take place throughout Friday as moist-laden
conditions wrap around the low pressure system to its northwest.
Lastly, an upper-level disturbance pushing in from southern Ontario
into the Great Lakes throughout Saturday will bring additional
chances for rain showers. High pressure is then expected to build
over Michigan Sunday into Monday, bringing some relief from the
rain. Otherwise, temperature trends will remain fairly consistent
through the middle and end of the week, with daytime highs remaining
in the upper 50s to lower 60s and lows in the 40s through Sunday.

MARINE...

Moderate easterly winds over northern and central lake Huron will
begin to ease this evening as the gradient relaxes. A secondary
increase in easterly flow remains possible across northern Lake
Huron tonight and early Monday.  Winds will turn to southerly and
increase on Monday as low pressure lifts west of Lake Michigan.
Gusts as high as 30 knots look to develop by afternoon and continue
through the overnight hours.  The combination of higher waves under
the easterly flow through tonight, transitioning to gusty conditions
on Monday will maintain an extended period of small craft advisory
conditions for Saginaw Bay and portions of the nearshore waters.
Winds are expected to shift to the west behind a cold front on
Tuesday, with windy conditions persisting through Tuesday night.

HYDROLOGY...

Widespread rainfall is expected through Monday morning, with
periodic heavy rain possible. There will also be a chance for
thunderstorms during this time. This additional rainfall in
combination with rain that has fallen already this weekend will
result in total rainfall amounts up to 2 inches.  The highest
amounts will be focused along and north of a line from Flint to
Harbor Beach. Locally higher totals are possible for areas that
experience thunderstorms, as storms will bring brief periods of
torrential rainfall. Rivers and streams will see significant rises
through the middle of the week, and ponding of water may develop in
low-lying areas and roadways. Flooding is possible, particularly
over the Saginaw Valley.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Flood Watch through Monday morning for MIZ047>049-053-054-060-061.

Lake Huron...Small Craft Advisory from noon Monday to 4 AM EDT Wednesday for
     LHZ442-443.

     Small Craft Advisory until 4 AM EDT Wednesday for LHZ421-422-441.

Lake St Clair...Small Craft Advisory from noon Monday to 4 AM EDT Wednesday for
     LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from noon Monday to 4 AM EDT Wednesday for
     LEZ444.

&&

$$

UPDATE.......BT
AVIATION.....BT
DISCUSSION...SF/AM
MARINE.......MR
HYDROLOGY....MR


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