Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 281548
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1148 AM EDT Sun May 28 2017

.UPDATE...

Cosmetic update to refine timing of best convective chances
southwest to northeast through the forecast area late afternoon on
into the evening, namely delaying onset over Thumb several hours.

Otherwise, forecast rational remains unchanged as a warm front lifts
slowly into far southern lower Michigan as low pressure approaches
form the west. Scattered showers/storms should fire along this warm
front by 20z-21z, mainly I-94 south. There remains a slight chance of
severe storms which may produce wind gusts to 60 mph and/or modest
hail. Low pressure and cold front then cross area this evening and
expand thunderstorm coverage northward through the remainder of the
area.

&&

.PREV DISCUSSION...
Issued at 538 AM EDT Sun May 28 2017

AVIATION...

Increasing moisture and daytime heating will allow a ceilings to
develop between 4000-5000 feet during the late morning and very
early afternoon. Main concern today will center around thunderstorm
development. Stable conditions should keep the area dry through
about 18Z, before a surface warm front lifting into the area allows
conditions to destabilize. Although showers and thunderstorms may
develop during the very late afternoon, activity looks most likely
during the evening in the 21-03Z time frame. Some of these storms
may be strong to severe. Activity should end from west to east
between 00-03Z as a cold front sweeps through the area. MVFR
conditions look to prevail behind the front in a moist airmass left
behind by active convection. Skies and visibilities should then
improve late overnight.

For DTW...BKN cu field at or below 5000 feet is expected to develop
during the late morning. Best potential for thunderstorms at DTW
looks to occur between 21Z and 03Z.

//DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings at or below 5000 ft this afternoon and
  evening.

* Moderate in thunderstorms between 22-03Z.

PREV DISCUSSION...
Issued at 412 AM EDT Sun May 28 2017

DISCUSSION...

Holiday Weekend - Forecast Package Highlights.

1. Firmed up timing for PoPs today. A dry morning period with
   PoPs ramping up post 18Z. Shower and thunderstorm event
   (numerous PoPs) expected from southwest to northeast between
   21-03Z.

2. Isolated to scattered severe weather possible between 21-03Z
   for all of southeastern Michigan. Focus for the severe weather
   threat will be along and ahead of a warm front lifting through
   the area. Severe weather threats include: damaging wind gusts
   to 60 mph, large hail to 1 inch, and a low potential for
   tornado development.

3. On Monday, temperatures will reach the lower to middle 70s
   areawide but will feel cooler because of falling dewpoints.
   Warmest temperatures south and east of glacial terrain within
   the Detroit urban heat island. Cooler near the Lake Huron
   shoreline.

4. Isolated rain shower or thunderstorm chance during the very
   late afternoon or evening hours Monday, particularly north of
   the M 59 corridor. No severe weather potential Monday.

Quiet weather is forecasted this morning. The slab of high stability
in the 6 to 12 kft agl layer (see 28.00Z KDTX RAOB) is progged to
hang around into the afternoon while lowering a couple of thousand
feet. The back edge of the upper level ridge needs to exit the
region. Will likely see some echoes show up on radar, but expecting
this to remain virga through 18Z.

A significant lobe to the Canadian upper level PV anomaly will work
through the base of the trough this morning and continue to a
negative tilt over Wisconsin this afternoon. This action along with
left exist region dynamics overspreading the region will result in a
broad area of 1000-500mb geopotential height falls over the central
Great Lakes. This organized forcing will be strong enough to draw
richer low to midlevel theta e content from parts of far southern
IL/IN northward into southeastern Michigan. No question, the best
thing going for this setup will be the distinct, well developed low
to midlevel theta e ridge that will serve as the focal point in the
frontal wave that will roll across the state.

Not a whole lot has really changed in the latest iterations of the
NWP or from previous forecast thinking. The later timing that has
been bandied about for the past couple of days has held on. The
dominant forcing that is expected to govern shower and thunderstorm
development today will be the low to midlevel warm advection that
will occur along and ahead of a warm front. A slight shift in the
00Z suite was for a few solutions, most notably the operational NAM,
resolving a slightly weaker surface low reflection developing at the
triple point along with a flatter release of the low. Really
difficult to say definitely if this slightly southern solution will
end up verifying. The reason is that dynamical forcing today is not
high end or extremely crisp. Looking through the gradients of both
the mass and kinematic fields, the feeling is the SPC SWODY1 graphic
may be overly reliant on the NAM. Therefore, the belief is that
potential for isolated to scattered severe thunderstorms exists for
all of southeastern Michigan including the Tri Cities and the tip of
the thumb.  The biggest factor in this decision is the 700-500mb
warm front will lift northward through the cwa.

The largest negative factor for severe weather development today
will be very poor midlevel lapse rates. Will likely require solid
lower 60 dewpoints at more than skin level, in addition to, some
peaks at sun or higher transparency high cloud to result in MLCAPES
of 1000 J/KG. This is certainly doable given upstream dewpoint
observations in southern IL/IN. Cannot say for certain though with
the typical NAM already overdoing dewpoints locally this past
evening and MOS guidance some 3 to 5 degrees lower with dewpoints
than prior predictions. Now for the shear. Really kind of split as
deep layer shear numbers have improved regionwide with widespread 40
knots accessible, while in contrast, near surface shear both from a
directional turning perspective and 0-0.5km shear is notably less.
With that said, the heavy component of a flatter and weaker surface
low is playing into that. At this juncture, do not want to move much
from prior thinking. Bottomline, deep layer shear will be more than
adequate for mesocylone development. This supports an isolated to
scattered severe threat today for damaging wind gusts to 60mph,
large hail to 1 inch, along with a low potential for tornado
development. Peak of the event is still expected 20-03Z for all of
southeastern Michigan along and ahead of a warm front.

Weather and skies are expected to clear out rapidly overnight in the
wake of the event as very strong signal exists for 925-700mb dry
slot overspreading the area.

The main upper level low will be slow to dig into southeastern
Michigan Monday. The most obvious thing going on Monday is an
unfavorable coupling of upper level jet streaks over Michigan and
Ohio. Cyclonic vorticity advection will eventually arrive but it
will be later in the day. Lapse rates and depth of moisture are
shown to be weaker or less favorable for precipitation than earlier
progged. Isolated or scattered PoPs appears more than adequate.

MARINE...

Primary issue for mariners today will be thunderstorm development as
low pressure lifts through the area. Activity for the first half of
the day will be confined to far northern portions of Lake Huron.
Potential for storms will then increase through the late afternoon
for all areas, with showers and thunderstorms likely in the evening.
A few strong storms will be possible. West to southwest winds will
then develop behind a cold front tonight, then strengthen Monday and
Tuesday as low pressure becomes stalled over northern Ontario. Gusts
both days look to just top 20 knots for most marine areas, with
stronger speeds over Saginaw Bay possibly necessitating a Small
Craft Advisory Monday and Tuesday.

HYDROLOGY...

Chances for showers and thunderstorms will increase through the
second half of the afternoon, with activity becoming likely during
the evening. Basin average rainfall amounts are forecast to range
between one-quarter and one-half inch, but precipitation totals will
vary widely from location to location due to the hit and miss nature
of showers and thunderstorms. The more intense storms will have the
potential to produce isolated rainfall amounts of one inch or
greater, which could cause minor flooding in the low lying and poor
drainage areas. Showers and thunderstorms should end around
midnight, before scattered activity redevelops Monday afternoon.

&&

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

$$

UPDATE.......DG
AVIATION.....HLO
DISCUSSION...CB
MARINE.......HLO
HYDROLOGY....HLO


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