Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 211920
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
320 PM EDT Wed Jun 21 2017

.DISCUSSION...

Fair weather conditions today maintained by diffuse surface high
pressure beneath subtle ridging aloft. Temperatures will top out on
target in the mid to upper 70s late this afternoon with dewpoints in
the mid 50s for now. A weak low-level jet is currently analyzed over
eastern KS and SW IA providing weak inflow into disorganized showers
propagating southeast over IA/IL. A few showers noted on the
national mosaic along the nose of the LLJ but but satellite and
radar trends suggest any activity in this region will be fleeting
for the next several hours.

Strong left exit dynamics over northwest Ontario in association with
the rapidly approaching 130kt jet over the northern Great Lakes will
elicit a much stronger response from the gently ascending nocturnal
LLJ overnight. Strong mid-level convergence along its lead edge will
manifest as a deep warm front, forcing a band of showers to lift
through the CWA roughly 7-14z tonight into Thursday morning.
Instability will be marginal, but unstable parcels launching off the
elevated warm nose will support a few rumbles of thunder. Magnitude
of forcing alone yields confidence to increase central and NW
portions of the CWA to categorical pops. Duration of rain at any one
location should be no more than a couple of hours with no severe
convection anticipated.

New airmass flooding into the area on gusty southwest winds will
send high temperatures well into the upper 80s as dewpoints rise
through the 60s and perhaps touch 70 degrees by Thursday evening.
Strong mid-level capping will be present through the day suggesting
the bulk of the day will be characterized by quiet/muggy wx.
Forecast becomes quite muddied by late Thursday evening. Each member
of the NWP suite offers its own flavor of convective development 21-
06z Thurs night, including many that maintain capping through this
period. The depiction of 925mb theta-e on the NAM12 and 12z ECMWF
indicate that enhanced moisture around the periphery of the tropical
plume to the south will be drawn northward well in advance of the
surface cold front and coincident with developing isentropic
ascent/strengthening mid-level westerlies in the warm sector. The
increase in moisture is reflected in PWATS nearing 1.75" and also in
the associated late-day deterioration of the cap. The NMM/GFS offer
no such moisture influence and simply maintain a strong cap through
the evening. Organized severe convection is certainly plausible
given bulk shear in excess of 30kts and MLCAPE around 1000 j/kg.
There is, at least, confidence that the 4km NAM convection is
overdone owing to its usual oversimulation of instability.

As convective uncertainty plays out in the warm sector, strong right
entrance dynamics will be sharpening and activating the front from
Dubuque, IA to northern Lower. Deep tropospheric frontal circulation
will promote convection on the warm side while developing
deformation in the presence of high PWATS on the cold side favor
widespread moderate to heavy rain. At this time, it appears that the
most significant heavy rain threat for the area is diminished as the
deformation zone remains progressive, but up to an inch or so is not
out of the question in the Saginaw Valley and Thumb by 12z Friday
morning. Deformation rapidly peels away early Friday with a quick
early-day fropa for remaining locations, likely prior to the
development of any notable instability. Skies will clear and
dewpoints will fall back into the 50s by late Friday.

Through the weekend spotty rain chances will exist. On Saturday, an
upper-level disturbance will move through the area, however, there
is quite a bit of uncertainty between the models. Sunday looks to be
the better chance for rain and isolated thunderstorms as an upper-
level trough swings to the east. Weakly unstable showalter indices
in addition to a weak cap eroding in the afternoon could bring rise
to isolated thunderstorms for both Sunday and Monday as troughing
continues across the Great Lakes. GFS shows amplified troughing,
while the ECMWF and Canadian models are much less pronounced,
bringing uncertainty once again for rain chances on Tuesday. An
upper level ridge in conjunction with a surface high pressure will
then begin to move into the Great Lakes on Wednesday diminishing
rain chances for the end of the extended period.

&&

.MARINE...
Light southerly flow will gradually increase over night into
Thursday, then veer southwesterly Thursday night ahead of an
approaching cold front.  Showers and thunderstorms are possible late
tonight/Thursday morning with warm front passage with scattered
activity after and until the cold front sweeps through Friday
morning. Winds will back to the west northwest and remain that
general direction through the weekend.  Unseasonably cold airmass
this weekend will bring the chance for light showers across most of
Lake Huron.

&&

.HYDROLOGY...

Showers and thunderstorms will be possible late Wednesday night into
Thursday as a warm front lifts north into lower Michigan. The main
focus for this activity appears to set up along and north of I-69 at
this time. With precipitable water levels climbing to around 1.50",
heavy rainfall will be possible, especially in area where frontal
forcing is maximized.

This warm front stalls over lower Michigan late in the day with a
low pressure then tracking east into the area Thursday night. As the
front slowly sinks south with the passage of this low, showers and
thunderstorm chances will increase significantly southward through
the remainder of the forecast area. Precipitable water levels will
remain high, 1.50"-1.75", so the potential for relatively widespread
heavy rainfall will persist until the front drops south of Southeast
Lower Michigan Friday morning.

All told, some bands of 1 to 2 inches of rainfall can be expected
with the best chance of 2 inch amounts along and north of I-69.

&&

.PREV DISCUSSION...
Issued at 1242 PM EDT Wed Jun 21 2017

AVIATION...

High pressure will continue vfr conditions through the first part of
tonight with mainly light westerly winds going variable around
sunset along with a sct-bkn cu around 5kft. A warm front will lift
northeast through the area from about 08-12z producing a broken band
of showers and thunderstorms along with an increasing southeasterly
flow. Expecting brief mvfr ceilings and visibilities in heaviest of
convection. After warm front passes mid level clouds will begin to
clear out. Southwesterly winds will increase quickly behind the warm
front in the 10-15kt range with some gusts to around 20kts towards
end of forecast.

For DTW...VFR conditions through at least first half of night with
Light variable winds with a westerly bias. Lake breeze may effect
terminal after 20z shifting winds to light southeast but with
afternoon highs on the lower side have kept the shift out as
expecting front to stop short. Warm front will lift northeast through
DTW airspace in the 09-13z time frame with showers and thunderstorm
activity. Ceilings and visibilities may go mvfr briefly in heaviest
of convection. Winds will go light southeasterly ahead of the front
then shift to the ssw after frontal passage and increase rapidly to
around 15 knots during the morning. As skies clear, diurnal mixing
will increase into low level winds of 30-40kts during the afternoon
translating to gusts to around 30ktsfrom the southwest at the
surface.

//DTW THRESHOLD PROBABILITIES...

* Low for thunderstorms impacting terminals after 08z tonight.
* Low for ceilings aob 5kft


&&

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

$$

DISCUSSION...JVC/AM/MV
MARINE.......DRC
HYDROLOGY....DRC
AVIATION.....DRC


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