Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
FXUS63 KDTX 220751
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
351 AM EDT THU SEP 22 2016
Trend in upstream convection and cloud cover overnight has been for
a thinning of clouds (including cirrus) as they approach stable
airmass over Southeast Michigan, while convection has been mostly
dying off by the time it reaches Lake Michigan. This trend should
continue through the early morning hours, however may see a few
showers or perhaps even a thunderstorm brush the Saginaw Valley or
Thumb. Global and hi-res models then take the shortwaves generated
from the overnight convection over IA/MN/WI into central and
northern Lower Michigan by this afternoon. This will focus chances
for precipitation over the northern portion of the state, and keep
pop mention confined to areas already mentioned.
Forecast soundings for today look stable and very warm, with H850
temps in the 14-16C degree range and H925 temps in the low 20s, as
we reside on the south side of the frontal boundary stalled over
northern Michigan and Lake Huron. Big story for today will end up
being well above normal temperatures as the calender officially
turns to fall. Cirrus may limit sunshine we see today making
temperature forecast a little more complex, especially since
coverage is harder to judge based on overnight trends of erosion.
Did not go with forecast temps quite as high as MET guidance with
potential for high clouds, but forecast temps are now in the low 80s
for the Thumb and Saginaw Valley ranging to the mid/upper 80s for
the rest of the area.
GFS/Euro still in line with a strong shortwave trough dropping
through Ontario late today and tonight as it pivots around the upper
low over Hudson Bay. This will push the front southward into Central
Lower Michigan tonight and southern Lower Michigan Friday. Coverage
of precipitation along the front will decrease as it drops into
Southeast Michigan as the boundary loses support from right entrance
region forcing and low-level jet, and surface trough ahead of the
upper wave slides east along the front late today and exits
overnight. Strong Canadian high pressure will become centered over
Ontario Friday and expand into the Central Great Lakes Friday
through Saturday, exerting a stabilizing influence which will only
help to decrease precipitation chances. Chances for thunderstorms
looks to end Friday evening. Cannot rule out a shower Friday night
and Saturday as 925mb front remains over Southeast Michigan, but
most of the weekend looks dry. Much cooler temperatures are expected
after today however, with max temps falling back into the upper 60s
and 70s Friday through Sunday.
Last few rounds of medium-range models (GEM/GFS/Euro) have shown
wide variations with how they handle the upper low dropping down the
west coast, and whether or not it becomes and then stays phased with
upper energy pushing through the Gulf of Alaska. This leads to
significant timing differences, both model to model and run to run,
with the timing of the next cold front and precipitation chances
over Southeast Michigan. Little change was therefore made to Monday
through Wednesday forecast.
Frontal boundary draped across northern Lake Huron today will sag
southward through early Friday. This frontal passage will bring a
transition to moderate northeast flow. The long fetch of onshore,
unstable flow over southern Lake Huron and into Saginaw Bay will
result in increasing wave heights. This will likely reach small
craft conditions across portions of the nearshore waters and Saginaw
Bay Friday and Friday night. High pressure building into the region
will lead to diminishing wind speeds and a corresponding reductin in
wave heights by Saturday.
Issued at 1148 PM EDT Wed SEP 21 2016
Remnant showers/thunderstorms are still expected to largely pass
north of the southeast Mi terminals overnight and this morning.
There is a small chance that a stray shower/thunderstorm may impact
MBS. Given the trends for convection to weaken as it approaches the
more stable mid level profile over Lower Mi, the chance of a
thunderstorm will continue to be left out of the MBS TAF.
Despite periodic high clouds traversing the area, boundary layer
moisture and radiational cooling have already led to some fog
development. This has been more extensive across the Saginaw Valley
where afternoon low level moisture advection was a little stronger.
Observational trends certainly support maintaining a mention of fog
in the terminals through daybreak. The likelihood of additional
cirrus debris advancing across the area during the early morning may
slow the pace of cooling and thus suggests refraining from going too
low with early morning visibilities just yet.
There was a weak southeasterly wind off Lake Erie this evening which
nudged sfc dewpoints up a little. The thicker cirrus should also
remain farther north and west. So radiational cooling will provide a
good chance for some MVFR type vsby in fog around daybreak.
//DTW Threshold Probabilities...
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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