Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210357
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1157 PM EDT WED JUL 20 2016

.AVIATION...

Light south wind during the night will increase from the southwest
by afternoon. The deeper and stronger southwest flow will carry
increased moisture that model guidance projects will be capable of
added coverage of VFR cu/altocu during afternoon, especially north
of PTK. Convection is expected to remain north of the region through
the day although some high cloud debris will be overhead by Thursday
evening. Storms are then increasingly possible toward midnight
through Friday morning.

For DTW... No operational concerns during the night through morning
as light south wind continues under mostly clear sky. Southwest wind
increasing to gusts near 20 knots will bring increased moisture for
scattered coverage of cumulus during afternoon into Thursday
evening. Plan to hold off on the mention of storms in the outlook
period of the forecast in favor of more refined timing in upcoming
issuances.

//DTW Threshold Probabilities...

* Low for ceiling 5000 ft or less during afternoon.

&&

.PREV DISCUSSION...
Issued at 331 PM EDT Wed Jul 20 2016

DISCUSSION...

The two main concerns through the remainder of the week will be the
building heat and humidity and also severe thunderstorm potential as
this increasingly hot and unstable airmass overspreads the region.

The leading edge of the heat will actually begin building into the
area today (minus the higher humidity levels) as temperatures top
out between 85 and 90 late in most locations late today. The airmass
will then gradually become more humid tonight into Thursday with dew
point temperatures reaching the upper 60s to around 70. This will
initially lead to milder overnight low temperatures tonight between
65 and 70, and then allow heat indices to build into mid/upper 90s
Thursday afternoon as temperatures climb into the lower 90s in most
locations.

There is a small concern that scattered thunderstorms (or at least
convective debris) may temper these hot/humid conditions. While it
seems more likely that the area will remain dry into afternoon,
elevated convection has been firing consistently throughout the day
over the upper midwest on the leading edge of the highest theta-e
air. SW flow forcing this activity will expand into lower Michigan
on Thursday, so cannot discount the possibility.

Given the fact that heat indices will approach 100 degrees during
the afternoon Thursday with a rather abrupt increase in humidity
levels, will go ahead and issue a heat advisory for all but the
Thumb region. Conditions only get worse heading into Friday as the
hot/humid air continues to build into area. Although thunderstorms
may temporarily truncate warm conditions overnight Thursday, will
run the advisory from early Thursday afternoon into late Friday
evening as heat indices climb above 100 most locations by midday
Friday with southern parts of the forecast area actually approaching
excessive heat warning conditions, heat indices of 105 degrees, for
a few hours.

As eluded to above, with this building heat/humidity, there appears
to be an increasing likelihood of severe thunderstorms Thursday
night as a rather decent shortwave for mid summer tops the upper
ridge along the US/Canadian border and drops ESE/SE into the
northern Great Lakes. Between the 2000-3000 J/kg of MLCAPE feeding
into the area Thursday evening and 0-6km bulk shear of 40 kts as H85
flow increases to 35-40 kts and H5 flow to 45-50 kts, an MCS should
develop over the U.P. or northern Lower Michigan and then become
well organized as it dives quickly south into area by late
evening/midnight and crosses the forecast area overnight.

Depending on how well developed the cold pool with this system
becomes as it enters the forecast area, there could be a pretty
significant threat of wind damage across the area. For now, the
region remains under a slight risk of severe thunderstorms. This
will have to be monitored closely late tomorrow as the convective
activity initiates/evolves as it encroaches on the forecast area
from the north.

Would expect main convection to clear area before Friday, and with
warming mid-level temperatures spreading in from the west as the
core of the hot/humid airmass enters lower Michigan, capping should
be rather significant and limit any leftover or additional activity
during the day to the isolated/widely scattered variety.

SATURDAY THROUGH THURSDAY:

High pressure builds over southeast Michigan on Saturday.
Northwest flow will allow dew points to drop...providing some
relief from the high Heat Index values endured on previous days.
The relief is likely to be short-lived. On Sunday, an elongated
area of low pressure centered over the Montana/North Dakota border
pushes northward into Manitoba. Warm, moist air will be pulled
northward once again, generating another round of hot and muggy
with a potential for showers/thunderstorms, for both Sunday and
Monday. The good news, at least for the moment, is that once we
clear Monday, high pressure resumes and normal July temps follow
for the remainder of the extended period.

MARINE...

Light southerly winds will become moderate southwesterly late
tonight into Thursday night when there will be potential for a
severe thunderstorm complex to impact area waters. A weak pressure
gradient will sustain light winds through the remainder of the
weekend. However, there will still be a slight chance for
thunderstorms Friday and Saturday. More organized thunderstorm
activity will be possible again on Sunday.

HYDROLOGY...

A complex of thunderstorms is forecast to develop over the Northern
Great Lakes on Thursday afternoon. The storms will then dive south
during the evening and potentially impact Southeast Michigan late
Thursday night into early Friday morning. The potential exists for
basin wide averages of .50 inches or greater.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Heat Advisory from 1 PM Thursday to 10 PM EDT Friday FOR MIZ047-048-
     053-060>063-068>070-075-076-082-083.

Lake Huron...NONE.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....BT
DISCUSSION...DG/DE
MARINE.......JVC
HYDROLOGY....JVC


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