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329
FXUS63 KDTX 202243
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
643 PM EDT Thu Jul 20 2017


.AVIATION...

Rapid reduction of any lingering diurnal cumulus with the loss of
heating within the next 1 to 2 hours.  Otherwise, extensive high
pressure will build into the region tonight and hold firm through
Friday.  This will yield a warm but stable environment, supporting
little in the way of cloud cover and generally light winds through
Friday evening.

//DTW THRESHOLD PROBABILITIES...

* None

&&

.PREV DISCUSSION...
Issued at 354 PM EDT Thu Jul 20 2017

DISCUSSION...

Aggressive stratocumulus development has occurred this afternoon and
is spreading rapidly across much of southeastern Michigan. Back loop
on satellite imagery suggests this cloud is resulting from higher
near surface moisture/outflow that emanated northeastward away from
MCS activity last night over portions of northern Illinois and
Indiana. Forcing for ascent lines up well ahead of the midlevel
700mb dry push that was highlighted in the update discussion.
Clearing behind this cloud and cumulus along the I 94 corridor back
from Jackson to Battle Creeks shows some potential for convective
development yet in the 20-22Z time window. Will continue a low
chance PoP south of I 69 late this afternoon. No severe weather
anticipated.

Rising 1000-500mb geopotential heights in tandem with possibly 2
seperate bouts of (negative) differential cyclonic vorticity
advection will support surface high pressure over the Great Lakes
regions tonight and Friday. A superb day of mid summer weather. It
will be very warm, increased temperatures to around 90 degrees.

Item of interest has now become what will the likelihood be for
numerous to widespread thunderstorm development late Friday night
and Saturday. The big difference with the pattern by then will be
the entrance region of the upper level jet axis will finally arrive
and push into the Straits by late Friday night. This will
effectively force the longwave to push eastward, movement that will
bring the main Theta e ridge/instability into a portion of the
southern Great Lakes. Some uncertainty does remain as to exactly how
far northward the instability gradient will get here in the cwa.
Models show the low level jet axis veering over into Lower Michigan
in response to the passing entrance region, which should allow for a
big warm air advection event centered around Saturday morning.
Certainly have no qualms with likely PoPs at this time. Depending on
trajectory of cells could even be looking at a heavy rainfall,
flooding potential. The big specter hanging over the certainty is
that there is no significant Potential Vorticity feature driving the
event, rather it will require some pre-existing convective
shortwave. Therefore, plenty of time exists for specifics. Some
potential does exist for severe weather Saturday, particularly if a
mature cold pool dominated MCS can track into the area (see SWODY3),
but uncertainty is high. Latest forecast soundings show plenty of
cloud and moisture which limits lapse rates and CAPE later on,
during the day Saturday.

Shower and thunderstorm chances will diminish by Sunday evening as a
low pressure system exits the area. High pressure will then build
into the Great Lakes region for Tuesday and Wednesday, bringing dry
conditions and sunshine. Shower and thunderstorm chances return
toward the end of the extended period as models indicate a weak cold
front moving through Southeast Michigan at some point between
Thursday and Friday. Daytime highs will be in the low 80s on Sunday
before a cold front will bring highs down into the mid to upper-70s
for Monday and Tuesday. A shift to southwesterly flow will then
bring temperatures back into the low-80s for Wednesday and Thursday.
Low temperatures will hover around the upper-50s to low-60s.

MARINE...

Outside of any thunderstorms, marine winds will be light and have
little impact on waves into the Weekend, as high pressure moves in
tonight. The next low pressure system is projected to arrive
Saturday morning, triggering numerous showers and thunderstorms.
Broad low pressure looks to be persisting over the Central Great
Lakes on Sunday as well, which could trigger additional scattered
activity with increasing northeast winds following Sunday night into
Monday. Wave heights at or above 4 feet across the southern Lake
Huron Basin will be possible on Monday.

HYDROLOGY...

Low pressure coming out of the Midwest Friday evening and tracking
into the Western Great Lakes Friday night is expected to produce
showers and thunderstorms late Friday night into Saturday. There is
a slight chance the storms will be severe. The high moisture content
of the airmass will lead to flood concerns, with widespread 1 to 2
inches possible within a couple hours time if the stronger storms
materialize, with localized higher amounts. However, it remains
possible the bulk of the complex and heavy rain tracks south of the
Michigan border.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....MR
DISCUSSION...CB/AM
MARINE.......CB
HYDROLOGY....CB

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