Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181929

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
329 PM EDT Tue Jul 18 2017


Temperatures solidly in the mid 80s at press time with a corridor of
mid 60s dewpoints gradually migrating east. Upper ridge axis
overhead is supporting fair weather with only boundary layer cumulus
to note. Stronger ridging at the surface has allowed dewpoints to
fall into the 50s and even mid 40s over Western Lower and northeast
Illinois. This pocket of drier air will shift across Southeast
Michigan tonight as troughing moves in from the east. The relatively
light gradient and falling dewpoints will allow temperatures to fall
into the low 60s in the northwest reaches of the CWA to the upper
60s the Metro Area where residence time of drier airmass will be

Richer moisture characterized by surface dewpoints in the low to mid
70s over the Upper MS Valley will spread across the CWA on Wednesday
as low and mid-level flow veers to westerly. The lead edge of this
moisture will carry a threat for a few showers or thunderstorms, but
prospects will be low tonight through much of Wednesday owing to
veering flow, diminishing convergence and marginal elevated
instability. Instability gradient may trigger a few pulse t-storms
south of I-69 along the instability gradient by Wednesday afternoon
as increasing mid-level moisture weakens the cap. 30 pop appears
more than adequate. Primary character of the day will be defined by
increasing moisture, particularly over the southern half of the CWA,
where dewpoints will rise into the 70s as high temps equal or
slightly exceed today`s.

Little change in synoptic scale pattern Wednesday night through
Friday night as the CWA lives on the edge of the instability
gradient. Warm high temperatures in the upper 80s and elevated
dewpoints will persist through this time while the primary forecast
issue revolves around diagnosis of MCS potential. First such episode
will take shape late Wednesday over the Great Plains as the LLJ
response to jet energy over the eastern Pacific snaking across the
northern tier of states. Timing of pattern evolution and pattern
recognition suggest a decaying MCS will impact the area on Thursday
morning. Some diurnal enhancement of any activity will be possible
depending on exact timing, but expectations remain low. Second
episode is modeled to initiate further east as monsoonal energy is
injected into the zonal westerlies. Convection is modeled to
translate into the western Great Lakes by late Friday afternoon.
This will at least open the door to the potential for an some
upscale organization before reaching the area late Friday evening.

On-and-off rain and thunderstorm chances will exist Friday through
early Sunday as the CWA sits in the northern fringe of a warm, moist
airmass. A series of short-waves/upper-level disturbances seen
mainly in the GFS and GEM models will also help aid the chances for
precipitation throughout the weekend. A cold front is then expected
to move through Michigan throughout the first half of next week,
however, timing discrepancies are noted across the GFS and ECMWF.
The ECMWF model swings colder air into the region Sunday into
Monday, which will bring rain chances to southern Michigan during
those times, with high pressure filling back in behind the front by
the mid-week. The GFS swings the cold front through Monday into
Tuesday, which would act to bring rain chances by Tuesday, before
high pressure moves in mid-week. The outlier of the three is the
GEM, as it brings in a cut-off low from the mid-levels starting
Monday. The cut-off low gets hung up across the Great Lakes through
Wednesday, before pushing off to the northeast. This solution would
bring rain chances through Wednesday, before high pressure builds in
behind the low. Overall, PoP values will remain in the 40 - 60
percent range Friday until Sunday morning as an unstable air mass
sits over the southern fringe of Michigan. High uncertainty exists
for the first half of next week, thus PoP values will remain
significantly lower at this time.



Light wind, generally from the south, will continue over all marine
areas through the evening as high pressure continues to drift into
the eastern Great Lakes. The next frontal system will move through
Lake Superior and reach northern Lake Huron tonight. This system
will be strong enough to generate thunderstorms but will only
produce a modest northwest wind shift as it moves through the region
and exits western Lake Erie Wednesday evening. Weak high pressure
then follows as the front stalls south of the Michigan border
Thursday and Friday.


Issued at 127 PM EDT Tue Jul 18 2017


High pressure will push towards the eastern Great Lakes region
through the early evening bringing a continuation of quiet aviation
conditions. Light southerly winds will continue across the area
before the next cold front approaches early Wednesday. VFR conditions
will continue to hold into early Wednesday before lower VFR ceilings
begin to move in over the northern TAF sites. Rain showers look to
hold off until after sunrise across the northern sites, with slightly
better chances for southern TAF sites by Wednesday afternoon.


* None.


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



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