Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 181718
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1218 PM EST Thu Jan 18 2018

.AVIATION...

Southwest flow (10 to 15  knots) to continue over the TAF period,
potentially decreasing slightly below 10 knots during the overnight
period. Aggressive dry and warm advection pattern has and will
continue to dissipate any Lake Michigan MVFR stratus, with mostly
clear skies for the majority of the TAF sits. MBS is the most
uncertain of the sites, as once low clouds dissipate this afternoon,
redevelopment will be possible late tonight, but especially tomorrow
morning.

/DTW THRESHOLD PROBABILITIES...

* None.

&&

.PREV DISCUSSION...
Issued at 325 AM EST Thu Jan 18 2018

DISCUSSION...

NEAR TERM / TODAY THROUGH TONIGHT

As of 325 AM EST...Quiet weather pattern continues today and tonight
with anticyclonic flow aloft. Initial upper-level ridging will be
dampened late today and into tonight by the approach of a moisture-
starved northern stream shortwave impulse. This feature will pass by
to the north of southeast Michigan across northern Lake Huron and
will only bring an uptick in mid/high cloud cover with its passage.
Dry weather will prevail today, although periods of stratocu will be
possible as Lake Michigan moisture plume gets going, mainly north of
the Detroit metro. Southwest flow will continue to be breezy at
times, with gusts of 20-25 mph. High temperatures will continue their
moderating trend with most areas reaching near the freezing mark
today. Low temperatures tonight will range from the upper teens to
lower 20s, coldest near the Ohio border.

SHORT TERM / FRIDAY THROUGH SATURDAY NIGHT

Quiet and dry weather continues through the short term period with
flat upper-level ridging. Lake aggregate moisture in the low-levels
will continue to lead to periods of stratocu, especially north of
the Detroit metro. Surface high pressure across the southeast states
will continue the southwesterly breezy return flow across the
region, advecting in a moderating airmass with time as temperatures
rise into the mid and upper 30s Friday and near 40 on Saturday.

Upstream, vigorous Pacific energy moving into the Intermountain West
will carve out a deepening longwave trough that will induce lee
cyclogenesis over the central Plains Friday into Saturday. This low
pressure system will deepen as it tracks northeastward towards the
upper Midwest for the second half of the weekend, discussed further
below.

LONG TERM / SUNDAY THROUGH WEDNESDAY

Active period of weather will occur for the second half of the
weekend and into early next week as aforementioned low pressure
system tracks northwest of southeast Michigan. Long range guidance
continues to be in generally good agreement that the track of the
low will be to the northwest across the upper Midwest and eventually
into Lake Superior. Guidance continues to diverge with regards to
timing, which is noted in PV evolution differences likely a result
of differences in the model data assimilation schemes resolving
inbound Pacific energy. The GFS, as typically on par in the 4-6 day
time period, is faster than the ECMWF by roughly 12-18 hours, again
a likely result of internal model diagnostics which will be ironed
out in the next 24 hours as the Pacific energy moves onshore and is
better sampled. Also of note is increasing spread in how deep the
low gets, with the ECMWF weaker than the GFS (mid 990s vs. sub 990
hPa), which will have implications on the strength of the frontal
features.

Either way, this system will be quite dynamic as it ejects out of
the central Plains and into the upper Midwest. Significant
atmospheric mass adjustment will occur over the weekend as coupled
upper-level jet with strengthening subtropical branch brings a surge
of warmer and more moist air that will be infused with moisture of
both Pacific and Gulf origins (PWATs increasing to near 1 inch).
Impressive midlevel height falls will occur as a result as the
system moves into the upper Midwest Monday, with a significant
response in the low-levels noted by a 60-70 knot LLJ that looks to
move over lower Michigan Monday.

Diving into the forecast details first starting with Sunday,
increasing low-level warm air advection and IVT ahead of the
system`s developing warm front will overspread the region leading to
a period of likely light overrunning precip Sunday morning. Forecast
soundings reveal a pronounced warm nose warmer than 3 C and surface
temps near freezing Sunday morning. Freezing drizzle appears
possible given the synoptic setup and have added mention of this to
the grids. Increasing dewpoints and boundary layer saturation will
also lead to areas of possible fog Sunday into Sunday night aided by
melting snow. Continued advection of milder air should end any mixed
precip threat by the afternoon with a transition to rain showers as
temperatures rise into the 40s as the warm front attempts to lift
northward through lower Michigan, but the northward extent may be
limited by the influence of cold lake temps/ice.

Precip chances continue to increase Sunday night and into Monday as
increasing isentropic ascent in the warm conveyer belt arrives along
with sharp diffluence aloft in the left-exit region of the upper-
level jet. Diurnal effects may allow for some wet snowflakes to mix
in Sunday night especially across northern lower Michigan before a
transition back to all rain Monday. Monday looks to be a damp day
with periods of rain. There are signals in the model guidance,
coupled with the strength of the LLJ, for a convective line of gusty
showers accompanying the cold frontal passage late Monday with some
elevated instability present but overall lack of insolation
precludes any mention of thunder at this time. Will need to watch
this potential closely over the coming days.

As the low pressure system lifts north into Canada Monday night and
into Tuesday, colder air will rush back across the region as 850 hPa
temps plummet from a peak of 5-7 C on Monday to near -15 C Tuesday
behind the strong cold frontal passage. Lake effect snow machine
will get ramped back up to round out the long term period with
chances for snow showers. Despite the plummeting low-level temps,
increasing sun angle will help offset this as high temperatures will
be near to slightly below normal for late January.

MARINE...

Persistent southwest flow will continue into the weekend. While
winds will be a bit gusty at times, the overall gust potential will
diminish due to increasingly warm, stable fetch. Winds will back to
the southeast by Sunday as low pressure begins to organize over the
central plains. As this low deepens and moves into the western Great
Lakes, wind gusts will increase. That said, stable conditions should
cap gusts at 30 knots or less over Lake Huron.

&&

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

$$

AVIATION.....SF
DISCUSSION...IRL
MARINE.......DG


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