Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 191720
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
120 PM EDT Wed Apr 19 2017

.AVIATION...

The cold front passing through the region will be the focus of
aviation weather during the afternoon into early evening, mainly
through the DTW corridor where thunderstorms are most likely during
the 20-23Z time window. Farther north, a less humid post frontal
boundary layer will produce VFR conditions during the afternoon
through much of tonight that will spread south over the entire
region as the front exits into Ontario/Ohio. Low pressure developing
over Iowa will then turn the boundary into a warm front overnight
and pull it back into Lower Michigan through Thursday morning.
Showers will increase coverage as a first sign of this process
followed by IFR ceiling spreading south to north over the region.
The warm front is projected to move north of DTW with VFR to follow
mid to late Thursday morning and then north of MBS during the
afternoon.

For DTW... Thunderstorms remain likely in the 20-23Z time window,
both for activity arriving from upstream and for new development
over/near the terminal. Early evening exit of the activity and the
front will then lead to VFR. Northeast surface wind post front will
veer to southeast by sunrise as IFR develops prior to the boundary
moving back into the region as a warm front.

//DTW THRESHOLD PROBABILITIES...

* Medium for ceiling 5000 ft or less.

* Medium for thunderstorms this afternoon and evening.

&&

.PREV DISCUSSION...
Issued at 338 AM EDT Wed Apr 19 2017

DISCUSSION...

Well-defined baroclinic zone stretches from the Straits into
southwestern Nebraska at press time. Weakening low-level jet
associated with the system departing to our north and east is
supporting a scattered coverage of showers early this morning. As
the wind field and attendant convergence continues to weaken,
coverage is expected to gradually wane over the next few hours.
Quiet day expected across most of the area with fropa occuring early
and dewpoints falling into the 40s across the north. However,
ongoing southwest flow in advance of the front will support an
increasing chance of convection south of M59 this afternoon. Progged
soundings are only weakly capped and suspect the frontal circulation
may be sufficient to generate convection on its own, particularly as
T/Tds increase toward 75/60+F near the MI/OH border. Low density
cape profiles of around 1000 j/kg and sub-optimal shear pose an
isolated risk of sub-severe hail and wind at best, the greatest risk
near the border. One item worth watching is any convectively induced
shortwave emerging from ongoing activity along the front in eastern
Nebraska early this morning. Any such wave would likely lift toward
the southern CWA within the prevailing flow and could yield an
uptick in coverage by late afternoon.

The next shortwave in the sequence is in the process of emerging
over the front range, cyclogenesis taking place on the western
reaches of the aforementioned baroclinic zone. The resultant
nocturnal LLJ currently supporting the eastern Nebraska convection.
As the cyclone deepens and the surface low lifts toward southern
Wisconsin early Thursday, today`s front will assume a warm front
character as it lifts back to the north. Weak isentropic ascent will
likely limit shower coverage for most of the night, but the arrival
of a 40+kt LLJ warrants a categorical mention for the Tri-Cities
region and the Thumb. 00 NWP generally depicting less impressive
lapse rates than 24 hours ago, suggesting a very limited to nil
severe hail threat with any elevated convection through tonight.

Model consensus that warm frontal boundary will likely be lingering
over the CWA on Thursday morning as the upper wave approaches from
the west. Arrival of steeper lapse rates and T/Td south of the front
potentially exceeding 75F/60F supports MLCAPE of 1000-1500 j/kg
while elevated instability exceeds 500 j/kg to the north. Strong
forcing along the prefrontal trough and deep unidirectional wind
field oriented roughly perpendicular to the convective line will be
strongly favorable for a severe wind threat posed by bowing
segments. Strongly veered deep layer wind field north of the front
may compensate for lower instability meaning a hail threat is in
play for all locations. Aside from solid bulk convective parameters,
there is particular concern regarding the position of the surface
warm front Thursday afternoon. Should it fail to clear the CWA, as
suggested by the 00z ARW for example, a corridor of strong low-level
instability and greatly enhanced SRH immediately along the front
will exist for the production of low-level mesocyclones/tornadoes.
Greatest tornado threat at this time appears most likely to reside
somewhere north of M-59.

High temps fall into the 50s for the remainder of the week as
strongly confluent flow aloft builds into the great lakes and low-
levels come under increasing influence of surface high pressure.

MARINE...

Moderate southwest wind this morning will ease as a cold front lifts
through the region.  A period of northerly winds immediately behind
this front will remain modest during the day.   The approach of
strong low pressure will then bring an increase in northeast winds
tonight and Thursday across portions of lake Huron.  The potential
still exists for gusts to reach near gales late tonight and early
Thursday across the northern third of Lake Huron.  A gale watch
remains in effect for this corridor.  In addition, this will result
in small craft conditions for higher waves along the nearshore
waters and Saginaw Bay beginning late tonight and likely lasting.
through Friday. An unsettled period will showers and thunderstorms
possible, particularly tonight and Thursday.

HYDROLOGY...

A cold front sliding through the region will bring a chance for
showers and thunderstorms today, particularly south of the M-59
corridor.  Unsettled conditions will persist areawide tonight and
Thursday, as this front lifts back northward and a strong low
pressure system tracks into the region.  The potential will exist
for localized rainfall amounts in excess of 1 inch, particularly
should a concentrated region of thunderstorms track across the same
areas.  The Tri-Cities will be more favorably positioned to witness
heavy rainfall tonight through Thursday.  The potential for some
minor flooding will exist, particularly given the wetter than
average conditions noted in recent weeks.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.

Lake Huron...Small Craft Advisory from 4 AM Thursday to 10 AM EDT Friday for
     LHZ421-441.

     Small Craft Advisory from 8 AM to 10 PM EDT Thursday for LHZ442-443.

     Gale Warning from 8 AM to 8 PM EDT Thursday for LHZ361-362.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....BT
DISCUSSION...JVC
MARINE.......MR
HYDROLOGY....MR


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