Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210109

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
909 PM EDT Sat May 20 2017


A robust thunderstorm response this afternoon and early evening
across portions of northwestern Indiana. This activity is being
driven in the center of a mesoscale circulation and small height
fall bullseye that is being kicked out ahead of the main vorticity
anomaly going negative tilt over the mid Mississippi River Valley. It
really is sort of messy tracing back but it appears the shortwave
energy has a history back to previous convection yesterday over MO
and AR. All things lining up for this response as NAM suggests this
is on the northern edge of what should be strengthening 925-850mb
frontogenesis and deformation. It is from this northern edge of the
thunderstorm activity that an elevated triple point is forecasted to
emerge over southwestern lower Michigan after 06Z tonight. Would not
characterize this setup as one with a tremendous amount of dynamical
forcing: detached well away from upper level Jet streak, removed from
the most direct of cva, and no organized convergence maximum to the
low level jet. Rather, it appears to be a good mix of prior
convective vort max, subtle low level limiting streamline and north
to south orientated convergence zone, and a surface warm front.

Surface analysis places this surface cold front from portions of
northeastern Illinois down into central Illinois. Middle 70s for
temperatures and mid-upper 60s dewpoints has contributed to +1000
MLCAPE and approximately 1500 SBCAPE to the south of the boundary.
SPC RAP based mesoanalysis depicts a very sharp instability gradient
with little to no instability in place at this time across the
northern couple of tier of counties in northern IN. Radar trends from
KIWX support this initialization analysis as the most northern
activity has been weakening or glaciating.

The rest of tonight...Given the aggressive nature of the convective
response thus far, feel the nam solutions with a deeper cyclonic
circulation is the direction to hedge. Latest iterations of the HRRR
are in agreement with steady northward development of showers and
thunderstorms throughout the remainder of the evening hours. Best
consensus would bring showers and thunderstorms to the doorstep of
much of the western cwa (including Tri Cities) sometime around 5Z or
soon after midnight. Strong easterly flow here over southeastern
Michigan has a very cool and stable airmass in place (dewpoints are
overwhelmingly in the 40s). Pouring through forecast soundings
suggests little to no potential for surface based convection even as
the warm advection kicks into overdrive. The reason for the high
confidence in no surface based convection is that strong static
stability will exist in the lowest 1800 ft agl through 12z. The RAP
does suggest some potential for the existence of a steep midlevel
lapse rate pocket despite a lack of one in the Mesoanalysis. Just
enough cooling with height in the midlevels to potentially result in
upwards of 750 J/kg of MUCAPE, particularly across the far southern
cwa and Detroit. Farther north, all forecast soundings point to
straight moist adiabatic. With the amount of moisture that is
expected to converge over lower Michigan and the look of the forecast
soundings there will be a heavy rainfall potential tonight. Would
not be surprised at all if some locations were able to get a quick
+1.00 of rainfall.

The forecast message...showers and thundestorms becoming likely
after midnight...primarily in the 5-9Z timeslot. Some potential
exists for the activity to miss metro Detroit by scraping past to the
NW. However, confidence in completely missing is low. The threats
tonight for the strongest of storms will be small hail, heavy
rainfall, and lightning.


Issued at 740 PM EDT Sat May 20 2017


Numerous to widespread coverage of showers associated with yet
another synoptic scale stretching deformation axis is now pushing
through northern portions of the cwa at press time. Progressive
nature of the band of showers has it well removed from any of the
area terminals here to start the taf forecast. Main potential
vorticity anomaly is shown to pivot aggressively to a negative tilt
over the central Mississippi River Valley between 03-12Z tonight.
This strongest of forcing is shown to miss the area to the west.
However, rich lower tropospheric theta e content is shown to draw
due northward out of the Ohio River Valley and eclipse or surge
northward through western and central sections of lower Michigan. It
is in this general vicinity that a secondary height fall maximum is
shown to lift through as a triple point develops over southwestern
Michigan under a boatload of 925-850mb frontogenesis.

Midlevel lapse rates will be unimpressive at moist adiabatic or
slightly better. Not expecting any strong thunderstorms tonight or
explosive development. Latest timing has a ragged line of multicells
developing over southwestern Michigan and pushing directly through
semich in the 05-10Z timeframe. Some lingering light showers or
drizzle is then expected through mid morning. One other timeframe to
monitor will be at approximately 21Z Sunday when activity could re-
initiate along the cold front slicing across the area. Attm, belief
is this development will occur over far eastern lower Michigan east
of the terminals.


* High for ceiling aob 5000 feet tonight.

* Moderate for thunderstorms between 06-08Z tonight.

Issued at 353 PM EDT Sat May 20 2017


MVC from earlier convection over the Plains is now lifting from
southwest Michigan into Central Michigan, and should track through
the Saginaw Valley during the very late afternoon and early evening.
Showers and thunderstorms associated with this feature will continue
to lift across the area, and should continue to exhibit a gradual
weakening trend they encounter higher stability over Michigan.
Overall, expecting high coverage of measurable rainfall.

Large upper low over the Central Plains will slowly lift northeast
through the upper Midwest late tonight and Sunday, and into Ontario
by Monday. This system will provide a good chance for precipitation
as it lifts to our west, with the main area of precipitation still
expected to push across the area late tonight into Sunday. This will
occur as the better moisture axis works through the area and a
shortwave trough axis pivots through the Western and Central Great
Lakes late tonight and Sunday. There looks to be a break in the
forcing during the late morning to early afternoon, with some drier
air also filling into the area, before the cold front sweeps across
the area later in the day. Extent and strength of any second round
of convection will depend, per usual in Michigan, on how much we can
destabilize before the frontal passage and perhaps even on how much
dry air becomes entrained into the boundary layer. NAM/GFS show
activity becoming surface-based after some daytime heating, with
about 500-1200 J/kg of SBCAPE able to build from roughly Flint east
by the time the cold front works across late in the day. (This is
assuming we reach forecast highs.) Skinny CAPE profiles and not too
impressive wind field should limit the threat for severe storms.
Temperatures will be a little tricky given muddled start to day, but
southerly flow ushering in warmer air ahead of the front and mild
start to the day should translate to max temps in the 70s.

Another strong shortwave looks to pivot through Central/Northern
Michigan as it rotates around the upper low over Ontario on Monday.
Forcing and moisture from this feature looks to remain primarily
north of the area however, and expect that is where precipitation
will remain. More noteworthy item for Monday will be the development
of breezy conditions once again as a westerly gradient tightens
between surface low pressure over Ontario and high pressure
strengthening over the Plains and Ohio Valley. Temperatures will
cool in the post-frontal airmass on Monday, reaching into the 60s,
which is just slightly below seasonal normals.

Mid-level troughing in association with an cut-off low will bring
rain chances for much of the extended period. A surface low pushing
east from IL/WI into lower Michigan will start off by bringing rain
chances Tuesday into Wednesday. The cut-off low is then expected to
be stationed over or near the Ohio Valley, and will spin-up a series
of upper-level disturbances which will bring additional chances for
pop-up showers Wednesday into Thursday. While a rumble of thunder
may be possible embedded in a shower or two, stability parameters
remain not all that impressive, thus general thunder chances have
been left out of the forecast. As the cut-off low moves east into
New England, upper-level ridging is then expected to settle across
the North Central Plains and Great Lakes through Friday and
Saturday, which will act to diminish rain chances.

A warming trend is also looking more probable for Memorial Day
weekend. Expect daytime highs peaking in the 60s during the middle
of the week, with a warm-up into the 70s for both Friday and


High pressure over the eastern Great Lakes and low pressure over the
Midwest will combine to support continued moderate easterly wind
early tonight. Small craft advisories remain in effect through the
evening before a weakening and veering wind trend develops
overnight. This will shift stronger wind over north sections of Lake
Huron through Sunday as a weak cold front moves through the region.
Southwest flow will then strengthen during Monday as low pressure
stalls and deepens north of Lake Superior. This will likely produce
wind exceeding small craft advisory levels on Saginaw Bay before
diminishing Monday night. The middle of the week looks unsettled in
terms of rain as another low pressure system develops over Missouri
Tuesday and moves into Lower Michigan Wednesday.


A weakening pattern of showers with isolated thunderstorms will move
through SE Michigan during late afternoon. Locally heavy downpours
with this activity will be capable of a quick 0.25 inch of rainfall.
This will be followed by a break in coverage through most of the
evening before another round of showers and storms develops toward
midnight and lasts through mid Sunday morning. There will be a
better chance of thunderstorms and heavy rainfall during this time.
Additional amounts of 0.25 to 0.75 will be possible producing totals
up to 1 inch in spots. The break between the late afternoon and
overnight activity will minimize potential for flooding, although
some ponding on area roads and in prone areas will be possible.


Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ421-422-

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for LEZ444.




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