Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210351
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1151 PM EDT Fri Jul 20 2018


.AVIATION...

The advection of low level moisture along a slow moving occluded
front will slowly allow lower clouds (MVFR and some IFR) to
gradually expand northward across Se Mi during the early morning.
Showers have thus far been struggling to hold together over Se Mi.
Rain chances will increase post daybreak as the upper low will
meander overhead during the day, providing occasional showers with
widely varying ceiling heights. Daytime destabilization will pose
the risk of a thunderstorm anytime on Saturday. The chances are
simply too low to include in the terminals at this time.

For DTW...Lower clouds will be on the doorstep at metro by TAF
issuance. While a stray thunderstorm can not be ruled out Saturday,
the chances are just too low to even place in the vicinity of metro.

.DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings at or below 5000 ft through the TAF period.

* Low in thunderstorms impacting the terminal Saturday.

&&

.PREV DISCUSSION...
Issued at 838 PM EDT Fri Jul 20 2018

UPDATE...

The high sub 900mb moist axis was pinched off well to the south of
the state this afternoon, which kept the higher instability reservoir
and strong/severe convection well to the south of the area. The loss
of diurnal heating has all but diminished what little surface based
cape was able to develop late this afternoon. The ongoing showers
and isolated thunderstorms now rotating across SW Lower MI/Nrn IN
will weaken a bit with the loss in diurnal heating. These showers
are however tied to a mid level vorticity axis rotating around the
upper low and narrow plume of higher mid level moisture. So at least
some scattered showers will lift into the area during the course of
the night. Cooling mid levels with the approach of the upper low will
still warrant a chance of a thunderstorm tonight, albeit a low one.
The lack of sufficient instability has ended any severe wx potential
for the night. An earlier forecast update was already issued to
remove the lakeshore flood advisory. No additional updates appear
warranted.

PREV DISCUSSION...
Issued at 418 PM EDT Fri Jul 20 2018

DISCUSSION...

Attention in the near term centers around the severe chances late
this afternoon and evening. Center of the surface low is now
positioned over SE WI on its way across southern Lake MI tonight.
Fairly wet forecast now through Saturday as the low cuts off and
parks overhead leading to scattered showers and thunderstorms.
Opportunity for severe weather will exist through this evening as a
line of convection on an occluding front and the nose of a low level
jet rotates around the low. As the events continues to evolve, it`s
looking more like the better severe chances will drop south of the
area as the jet directs the ongoing convection along or south of the
MI/OH border into the better instability. The north end of the line
and occluded front will pivot up through SE MI bringing our severe
chances as the convection encounters an airmass with some
instability and weak shear. A dry slot in the wake of the morning
showers was able to generate a wedge of instability on the order of
1000-1500 J/kg over SW MI. Some of this will bleed eastward into the
CWA as the front progresses. Shear will be weak, on the order of 20
knots or so, but could be enough to help a few cells gain strength.
Convergence along the front could also help a cell or two gain
strength. Overall we`re looking at a scattered thunderstorms through
tonight with best chance for severe weather between about 23-03Z.
Winds to 60 mph and 1 inch hail are the main threats and should be
mainly south of about M59. Localized flooding with potential for
training cells exists as well.

On Saturday, the stacked low pressure system will be stalled over
southern MI. Broad area of lift associated with the low in addition
to diurnal heating will create favorable conditions for numerous
showers through the day. Moist profile with PWATs of around 1.75
inches through the column will allow only skinny CAPE profile, but
instability none the less, thus will keep a thunder mention in
through the afternoon. As is the nature of these cut off lows, we
don`t expect a washout through the entire day, but a good coverage
of showers will be working across the area through the day. A
thermal trough in addition to excessive cloud cover and rain will
keep temperatures down in the upper 70s.

The low will work southward Saturday night bringing about northeast
flow by Sunday morning. Some showers may linger across far southern
MI but overall improving trend as dry air and subsidence fills in
from north to south through the day. Temperatures will only slightly
improve over Saturday as a bit more sunshine can be expected. Highs
will near 80 at many locations.

The start of next week will see the lingering long wave troughing
over the area stick around before another shortwave and additional
troughing move in later in the week.  Despite the upper level
troughing, southeast Michigan is expected to remain in weak ridging
through most of the week.  This will keep the area dry during this
time as temps climb back into the mid to upper 80s.

MARINE...

A large area of low pressure will slowly track southeast from
southern Lake Michigan to the Ohio Valley through the weekend. This
low will support unsettled conditions through the first half of the
weekend with widespread scattered showers and thunderstorms. Gusty
winds will gradually back from southeast tonight to easterly on
Saturday as the low moves south of the region. Small craft
advisories will remain in effect through tonight for the nearshore
waters of southern Lake Huron from Port Austin to Port Huron and
western Lake Erie as wind gusts approach 25 knots. The increased
winds will support significant wave heights ranging from 5 to 7 feet
tonight before gradually decreasing to 3 to 5 feet by late Saturday.
Conditions will improve on Sunday as the low tracks away from the
region allowing winds to decrease and back to the northeast as waves
fall below 3 feet.

HYDROLOGY...

A large, slow-moving area of low pressure tracking from southern
Lake Michigan to the Ohio Valley will support numerous scattered
showers and thunderstorms through the first half of the weekend. A
widespread one quarter to three quarters of an inch of rainfall is
expected across Southeast Michigan through Saturday night. Due to
the scattered nature of the precipitation, rainfall totals are
expected to vary greatly throughout the region. However, with
copious moisture present heavy rainfall with amounts exceeding one
inch will be possible in isolated spots. Minor flooding in poorly
drained areas will be possible in isolated spots that see heavy
rainfall. Drier conditions will then return on Sunday as the low
departs the region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 6 PM EDT Saturday for LHZ441>443.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....SC
UPDATE.......SC
DISCUSSION...DRK/SP
MARINE.......DRK
HYDROLOGY....DRK


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