914
FXUS63 KDTX 270355
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1155 PM EDT Thu Sep 26 2024

.KEY MESSAGES...

- Breezy east wind develops during Friday but with dry weather
through the day.

- Rain showers associated with the northern fringe of Helene move
into Lower Mi Friday night and Saturday (30-60 percent chance).

- Lower chances for rain persist through Sunday leading to dry
weather by Monday.

&&

.AVIATION...

A ridge of surface high pressure will remain in place over Southeast
Michigan tonight before breaking down as the deep upper level low
strengthens over the Tennessee River valley with tropical Helene
energy. Forecast soundings show deep tropospheric dry air in the
lowest 20.0 kft agl through 00Z 10/28 supporting VFR conditions.
There will be the potential for some transient scattered cumulus
development around 5.0 kft agl midday Friday. Most notable aspect of
the forecast is increasing easterly winds 15 to 30 knots Friday.
Deep moisture will increase by 03Z Friday evening.

For DTW/D21 Convection...Thunderstorms are not expected this period.

DTW THRESHOLD PROBABILITIES...

*  Low for ceiling at or below 5000 feet Friday

&&

.PREV DISCUSSION...
Issued at 357 PM EDT Thu Sep 26 2024

DISCUSSION...

Quite the unique larger scale upper air pattern governs conditions
east of the Rockies today and tonight. A north to south string of 3
mid level closed lows run from Quebec to the mid MS valley to
hurricane Helene approaching the FL panhandle coast. Lower Mi is
between these larger scale circulations where impressive mid to
upper level deformation produces a sharp cloud edge over surface
high pressure solidly in place. Temperatures managed to reach a few
degrees above normal for late September despite the plentiful cloud
cover this afternoon while the surface high ensures a lower level of
humidity. Low temperatures settling into the 50s are also on the
mild side as high clouds continue to thicken while spreading north
and west tonight into Friday morning.

An impressive merger of hurricane Helene with the current mid MS
valley closed low occurs late tonight and Friday. The resultant
system then spans the Ohio valley by afternoon bringing the northern
fringe of the rain pattern to near the Ohio border late in the day.
More noticeable for SE Mi will be the increase in easterly wind
across the region, especially toward the west end of Lake Erie and
the Ohio border where 30 mph gusts are likely from mid afternoon
into Friday night.

Timing and placement of the tropical moisture plume trends northward
for Friday night and Saturday. Today`s 12Z models all continue the
process of reconciling the dynamics of the Fujiwhara interaction
between the closed mid level circulations. The increased size and strength
of Helene helps lead the circulation center into western KY which
brings the mid level theta-e gradient into Lower Mi late Friday night
and Saturday morning. Mid level moisture transport is trending down
from 30-40 kt peak easterly flow as the wind field gradually weakens
but still sustains elevated instability while near 2 inch PW feeds
clusters of showers moving into Lower Mi.

The northern fringe of this hybrid tropical/mid latitude system
hangs around SE Mi during Sunday as the mid level circulation
elongates toward WV/VA. Improvement then occurs Monday as a short
wave ridge builds overhead while the remnant tropical system is on
track to open up toward the mid Atlantic coast.

MARINE...

High pressure over the region will keep light easterly flow into
tonight. Things change on Friday as the remnants of Hurricane Helene
lift north toward the Ohio Valley before stalling. The Great Lakes
will reside in between these two systems with compression helping to
tighten the gradient over the southern Great Lakes. The pressure
gradient will be tightest over Lake Erie, and gusts of 25 to 30
knots appear likely Friday morning into Saturday which will
necessitate a Small Craft Advisory. Winds will decrease heading
northward toward Lake Huron but look to still peak around 25 knots
over Lake St Clair so they will be included in the Advisory. Winds
decrease through the day Saturday as the low fills in and drifts
north over the region. With continued east/northeasterly flow over
Lake Huron this weekend, we will still need to watch for winds and
waves nearing Small Craft Advisory level with the onshore fetch.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...None.
Lake St Clair...Small Craft Advisory from 5 AM Friday to 10 AM EDT Saturday for
     LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 5 AM Friday to 10 AM EDT Saturday for
     LEZ444.

&&

$$

AVIATION.....CB
DISCUSSION...BT
MARINE.......DRK


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.