Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 261514

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1114 AM EDT Sat May 26 2018


Motivation of the update is to affirm main messaging of summertime
weather this afternoon/evening with scattered thunderstorm
development and very low severe weather potential.

Messy near term forecast in front of us with noisy/dirty ridge amidst
little to no organized environmental wind profile. While difficult to
pin down on details a couple of features appear to govern the today
period. First item is the well organized midlevel theta e ridge that
is currently cresting through semich. Shower activity that has been
able to focus over portions of the eastern thumb is well aligned with
maximum of system relative isentropic ascent along the northern edge
to this theta e ridge. NWP suggests that this activity will continue
to backbuild in some coverage into the afternoon hours as models
show a positive increase in the 850-700mb shearing deformation
directly centering over the area. Forecast data does keep the
westward flank of the midlevel theta e ridge over the forecast area
through the early evening hours. Latest HRRR data keeps portions of
eastern CWA within an area of higher near surface laminar flow and
higher static stability. This will set an edge to a differential
heating boundary and provide a focus for shower and thunderstorm
development this afternoon. From this vantage point, probably looking
at areas west of US 23 to see relatively higher thunderstorm
potential during the mid afternoon timeframe. Consensus of output and
pattern recognition then shows a southward migration of the southern
edge to differential heating boundary (due to lake aggregate
mesohigh) down into Metro Detroit during the late afternoon and early
evening. Highest potential for scattered thunderstorm development
over portions of Metro Detroit is between 22-01Z.

Potential impacts appear categorically limited. Notable signal
exists within the HREF output that shows CAPE minimum over eastern
sections of Southeast Michigan tied to the residual higher stability
of early day clouds/precipitation. Lower confidence exists with
exactly what clouds will do today over eastern Lower Michigan. Models
are likely overdoing the clouds given clear sky satellite trends
over western Lower Michigan. With that stated, models show a lack of
steep midlevel lapse rates locally which will limit CAPE development
even if aggressive clearing occurs. Consensus now is for MLCAPE over
much of the cwa to limit out at less than 1500 J/kg. With no
environmental wind, there is very little concern for any severe
thunderstorm development today. Main thing to monitor will be
potential for locally heavy rainfall and potential for poor drainage
flooding concerns. There are three reasons for heavy rainfall
potential. 1. Very moist air mass, PWATS in excess of 1.50 inches. 2.
Storm motion today will be very slow from northwest to southeast at
approximately 20 mph. 3. Potential exists for training along
mesoscale/outflow boundaries.


Issued at 650 AM EDT Sat May 26 2018


VFR flying conditions will prevail through most of the period with
diurnal cumulus and mid/high cloud cover. Dewpoints rising into the
60s and daytime heating well into the 80s will yield plenty of
available instability for thunderstorm development later this morning
and through the afternoon hours. Tricky call on timing and placement
of convective initiation, which will likely largely be influenced by
early morning theta-e ridge induced showers that will affect most of
the TAF sites this morning. Differential heating boundary from this
shower activity will likely be at least one trigger source for
convection, but timing and placement remains uncertain. Secondary
initiation point may be near the Tri-Cities and closer to an incoming
piece of shortwave energy. Have gone with VCTS at all sites between
15z-00z, with highest coverage in TEMPO groups 19z-22z at KMBS, KFNT,
and KPTK. Will continue to monitor hi-res guidance and observational
trends this morning and adjust timing as needed. Outside of storms,
flow will be less than 10 knots out of the S/SW.

For DTW...Light showers early this morning will give way to
increasingly destabilizing airmass supportive of VCTS. Probabilistic
and latest hi-res guidance suggest 17z-22z may be the best window
for TS at the terminal, with this timing highly dependent on
differential daytime heating.


* Low confidence in thunderstorm occurrence and timing 15z-00z.

* Low in ceilings at or below 5 kft this afternoon.

Issued at 358 AM EDT Sat May 26 2018


The mid summer weather pattern underway across the Great Lakes will
feature an increase in humidity today compared to the last couple of
days. Early morning surface analyses indicate an axis of lower to
mid 60s dewpoint moving from the Midwest and Mississippi Valley into
Lower Michigan. It is being directed eastward by weak southwest low
level flow occuring under an equally weak but effective short wave
aloft. These features supported development of showers and
thunderstorms upstream yesterday, some of which are ongoing at
forecast issuance. The questions of maintenance or dissipation of
this activity are important to the forecast going forward through

Recent satellite and 00Z model analysis fields indicate the upper
wave just west of Lake Michigan and moving slowly eastward since
midnight. It is just strong enough to back the mid level flow and
create a minimal mid level theta-e ridge marked by the showers over
SW Lower Michigan as of 4 AM. The weak dynamic support and weak mid
level moisture transport are expected to maintain at least scattered
light showers into SE Michigan during the morning, mainly south of
I-69, as the larger scale features continue to move slowly eastward.
Early morning convection over the northern Great Lakes is associated
with another wave moving through Ontario and remains to our north.

The morning cloud and shower trends then loosely set the stage for
afternoon development. Initiation timing is complicated by cloud
debris impacts on surface heating of the moist and uncapped boundary
layer shown in model soundings. Location is unclear due to the lack
of a well defined surface focusing mechanism in the wind/pressure
field. The best estimate would be to call for the first storms in the
Tri Cities and northern Thumb where there will be the chance for a
cloud cover induced differential heating boundary. Storms could also
move in from northern and western Lower Michigan where CAPE maxima
occurs first during early afternoon. MLCAPE around 1500 J/kg and
MUCAPE near 2000 J/kg are respectable values assuming high temps in
the mid to upper 80s are reached after morning clouds. Convection
will be limited to pulse mode within an even weaker wind profile than
advertised in earlier forecasts. Model soundings show wind speed 20
knots or less below 500 mb. The deep moisture profile of PW near 1.5
inches and K-index in the upper 30s, combined with slow storm motion
and a tendency for activity to be locked on outflow boundaries,
indicates the main hazards are locally heavy rainfall and possibly
some downburst wind gusts.

After any evening storms dissipate, warm and humid conditions
continue tonight through Sunday. A chance of thunderstorms will
linger during the afternoon but without the additional support from
larger scale features. Model soundings indicate a similarly uncapped
thermal profile and weak wind profile but the upper air pattern is
more neutral northwest to weakly anticyclonic. Expect more isolated
convective potential linked to lake breeze convergence zones during
afternoon peak heating. Cloud cover will be less problematic for
temperatures, especially in the morning, which allows high temps to
make a run at 90 over a greater portion of the region. See below for
the temperature records compiled by the forecast crew yesterday.

The potential for record highs continues on Memorial Day subject to
some MCS cloud remnants from the northern Great Lakes. However,
model projections indicate the upper level ridge building over the
region and bringing an increasingly capped environment over SE
Michigan. A weak backdoor front is still shown settling in from
Ontario and Lake Huron, at least in terms of 1000-850 mb thickness,
but is very diffuse in the pressure field. This suggests another run
at 90 to lower 90s Monday before guidance temperatures indicate some
response to onshore/northeast wind and lower to mid 80s by Tuesday.
Dry conditions are shown during this time until some potential for
Alberto remnants arrive for the second half of the week.


Light southwest flow will back southerly today as a weak area of low
pressure approaches the northern Great Lakes. A weak cold front will
accompany this area of low pressure/surface trough, gradually
washing out with time as it attempts to slowly drift southward over
the Straits this weekend. The light flow will promote waves 2 feet
or less and favorable boating conditions for the holiday weekend.
The only exception will be in and around scattered thunderstorms,
which will be possible throughout the weekend mainly during the
afternoon and evening hours with an increasingly moist, warm, and
unstable airmass over the region. Higher pressure and drier
conditions will then build across the region early next week.


Here is a list of record high temperatures for
this holiday weekend.

Saturday, May 26th:

Detroit                92 (set in 1944)
Flint                  92 (set in 1933)
Saginaw/Tri-Cities     89 (set in 2010)

Sunday, May 27th:

Detroit                91 (set in 1941)
Flint                  90 (set in 1987)
Saginaw/Tri-Cities     94 (set in 1911)

Monday, May 28th:

Detroit                95 (set in 2012)
Flint                  93 (set in 2012)
Saginaw/Tri-Cities     92 (set in 1977)


Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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