Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 192310
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
710 PM EDT Sat May 19 2018

.AVIATION...

Low pressure exiting Lower Michigan will pull a cold front east of
the region at the start of the evening. Upstream observations
indicate borderline IFR/MVFR ceiling extending back across Lake
Michigan and up through northern Lower Michigan. These clouds fill
in across SE Michigan tonight in the post frontal flow shifting from
west to north during the night. Continued the trend of the previous
forecast calling for IFR ceiling and visibility to become more
dominant overnight with a gradual improvement from north to south
during the late morning and early afternoon Sunday. Plan to monitor
upstream trends before adding showers to the forecast Sunday morning
as there remains potential for decaying thunderstorms to slide south
of the Ohio border.

For DTW... Borderline IFR/MVFR ceiling and visibility will become
more firmly IFR during the night. Post frontal northwest wind
becomes light north to northeast through Sunday morning potentially
requiring northeast traffic flow operations. Monitoring upstream
trends for potential decaying storms Sunday morning as ceiling and
visibility improve into MVFR toward early afternoon.

.DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less through Sunday morning, medium
  confidence by midday Sunday.

&&

.PREV DISCUSSION...
Issued at 303 PM EDT Sat May 19 2018

DISCUSSION...

NEAR TERM / REST OF TODAY THROUGH TONIGHT

As of 300 PM EDT...Thunderstorm activity continues to ramp up across
portions of southeast Michigan this afternoon, especially south of
the I-96 corridor. Here, storms are forming at the nose of the
instability gradient, with SBCAPES punching above 1000 J/kg north of
the Ohio border thanks in part to anomalously moist air advecting
northward into the region. Dewpoints have surged into the low and
mid 60s across southeast Michigan, with even a few upper 60s noted
south of the M-59 corridor. This moist airmass is also evident in
PWATs exceeding 1.60 inches, basically a record for this date.

Luckily, overall shear profiles are on the weaker side to prevent
any substantial storm-scale organization despite broad-scale lift
from an area of low pressure traversing northeast across the region
this afternoon. With such abundant moisture in place, have added
heavy rainfall to the thunderstorm wording for this afternoon, as
localized torrential rainfall will be possible that may lead to poor
drainage flooding concerns. Brief multicell clusters and quasi
broken lines may also lead to enhanced rainfall rates, and cannot
rule out a quick 1+ inches where the heaviest rain falls. Across
most areas though, a quarter to half an inch of rain can be expected
across most areas as the showers and thunderstorms become more
widespread as the afternoon progresses. An isolated wind gust up to
40 mph is also possible especially if a big enough cold pool can be
generated in a multicell cluster.

Hi-res guidance is in good agreement that the instability axis
shifts eastward with time heading into this evening, focusing the
activity over the Detroit metro and south along the I-75 corridor
towards Toledo. Midlevel shortwave energy moving in the southerly
flow regime aloft will likely act to increase convective coverage
this evening as well. Will need to watch rainfall rates closely in
urban areas of the Detroit metro, especially if multiple rounds of
thunderstorms move through.

Despite abundant cloud cover for much of the day, the warm air
advection regime and southerly low-level flow has allowed
temperatures to warm into the 70s for most areas, with upper 60s
north of the M-46 corridor. Convection rapidly wanes late this
evening from west to east as the system`s cold front swings through.
Despite cooler air filtering in as the winds switch to the
northwest, low-level moisture will be slow to retreat southwards,
and expecting overcast low stratus for much of the night. Cannot
rule out a period of drizzle overnight across the
Flint/Howell/Pontiac/Lapeer area as cool and moist upslope northwest
flow meets the Irish Hills. Towards sunrise, the winds will continue
to veer to northerly, and possibly decouple south of the I-69
corridor. With overnight dewpoints still well into the 50s, patchy
to areas of fog will be possible towards sunrise across most areas.

SHORT TERM / SUNDAY THROUGH MONDAY NIGHT

Weak surface high pressure will attempt to ridge southward into
lower Michigan Sunday as the flow aloft becomes increasingly
confluent in nature. With the west/northwest flow aloft setting up,
the main concern during the short term period will be tracking
upstream convection in the form of remnant MCS`s, and whether or not
they can make it far enough east into southeast Michigan before
dissipating. Short term model guidance is highly variable trying to
capture these convective trends, with the placement and origin
region of these features critical to the potential weather we may
experience Sunday into Monday. For now, will go with chance PoPs
during much of the time period, although confidence in seeing any
additional showers and/or storms will be closer to the Ohio border
where the better instability will reside. Temperatures Sunday and
Monday will only reach into the 60s, with 50s possible along the
Lake Huron shoreline Sunday in northeast flow. Easterly flow setting
up on Monday will also act to help keep temperatures on the cooler
side to start the work week.

LONG TERM / TUESDAY THROUGH FRIDAY

A quieter period of weather returns for the long term period as
stable high pressure and building heights aloft stay in vicinity of
the region. Dry weather looks to prevail Tuesday and through at
least Thursday with moderating temperatures each day as thicknesses
slowly increase. By Thursday, temperatures may reach the 80 degree
mark across most inland areas. Unsettled weather then returns to
round out the work week and heading into next weekend as a series of
disturbances moves east across the northern Plains.

MARINE...

Low pressure will bring scattered showers and thunderstorms to areas
from southern Lake Huron southward into western Lake Erie into early
this evening. Northwest to north winds will increase somewhat in the
wake of this low late tonight with gusts into the 20 to 25 knot
range over northern Lake Huron. However, light winds are expected on
Sunday as high pressure builds into the area. The next chance of
rain will hold off until late Sunday into Sunday night.

HYDROLOGY...

Scattered showers and thunderstorms will bring locally heavy rain to
far southeastern portions of the forecast area into early this
evening. While average rainfall will range between one quarter to
one half an inch, local amounts of an inch or more will be possible
with this activity. This will lead to local ponding of water on
roadways and minor flooding in poorly drained areas. Most of this
activity will miss the Huron River basin near Hamburg, so no
additional rises area expected as the river slowly edges back down
after heavy rainfall last week.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
DISCUSSION...IRL
MARINE.......DG
HYDROLOGY....DG


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