Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 220414
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1214 AM EDT Sun Oct 22 2017


.AVIATION...

Low levels will remain dry for one more day (ceiling free below 12
kft), but increasing southerly flow (around 15 knots) ahead of a
cold front will allow for quickly lowering ceilings near and just
after sunset, with MVFR showers likely making it into MBS and FNT
toward midnight.


//DTW THRESHOLD PROBABILITIES...

* Medium confidence in cigs aob 5000 ft by 5z Monday.

&&

.PREV DISCUSSION...
Issued at 326 PM EDT Sat Oct 21 2017

DISCUSSION...

NEAR TERM / THROUGH 6 AM SUNDAY MORNING

As of 325 PM EDT...Another warm and precip-free day continues across
southeast Michigan, with temperatures warming well into the 70s
under partly cloudy skies. The region sits between a departing area
of surface high pressure/weak upper-level ridging centered across the
Mid-Atlantic region and a deepening longwave trough over the west-
central CONUS. This synoptic pattern has continued the
south/southwest low-level flow, which has helped keep temperatures
well above average the past few days. Atmospheric column continues to
be quite dry, with a PWAT of only 0.43 inch noted on the observed
KDTX 12z sounding and barren of moisture up through 500 hPa. Thus,
the partly cloudy skies are in the form of mid and high level clouds,
still allowing for relatively bountiful sunshine.

For tonight, dry weather will continue as a cold front slowly
approaches the region from the west. Broad diffluence aloft between
the departing high and approaching trough will allow for mid/high
clouds to continue streaming across the region, with low-level dry
air remaining entrenched. A persistent southerly breeze of 5-10 mph
will help keep low temperatures elevated and generally in the low
and mid 50s, although a few rural areas may briefly dip into the
upper 40s if low-level winds can slacken for any period of time.

SHORT TERM / 6 AM SUNDAY MORNING THROUGH TUESDAY NIGHT

Strong low pressure system to possibly bring heavy rain and gusty
winds to region Monday into Tuesday...

The short term period will mark a transition as the northern and
southern streams become very active. The forecast scenario continues
to be a complex one, but a solution that is now seeing increasing
confidence based on the latest suite of model guidance and trends
over the past 24 hours.

Sunday will mark the last day of the current unseasonably warm and
dry weather pattern that has been in place. High temperatures again
on Sunday will easily reach into the 70s, with several locations
making a run at 80 degrees (record highs possibly within reach) with
brisk southwest flow. The aforementioned cold front will continue to
slowly approach the region from the west as longwave troughing
continues to deepen as far south as the northern Gulf of Mexico, in
response to potent northern stream PV energy ejecting out of the
Rockies.

Meanwhile, the southern stream will also become active as the
subtropical jet intensifies in response to the expanding baroclinic
zone between the still warm Gulf waters and colder, northern stream
air diving south. Model guidance is now coming into agreement that a
split flow regime will set up for the latter half of Sunday into
early Monday, as initial northern stream energy moves north of Lake
Superior, while the southern stream briefly develops into a cutoff
low.

Heading into Sunday night, the cold front will continue to slowly
approach the region, and now looks to stall somewhere in the
vicinity of central Michigan. The front will gradually be weakening
with time as any lingering frontogenesis and dynamics are gradually
sheared away in the split flow. Still, enough moisture pooling out
ahead of the front riding a fairly impressive low-level moisture
transport surge should generate at least scattered showers,
especially after midnight. Rainfall amounts with the front itself
will generally be on the lighter side, a quarter of an inch or less.

Then attention shifts to the Monday-Tuesday timeframe, which will
literally feature dynamic weather. Despite a brief southern stream
cutoff low, the midlevel height fields will remain positively-
tilted, allowing for the low to become an open wave and the
combination of the lingering PV lobe and subtropical jet rounding
the base of the trough axis spawning cyclogenesis in the Tennessee
River Valley Sunday night. Further upstream, another shot of
northern stream PV will eject into the split flow, racing
southeastward across the upper Plains Sunday night and into Monday.

The resulting injection of potent dynamics looks increasingly likely
to evolve into a scenario more reminiscent of a classic Miller Type
A low pressure system, but instead of becoming a powerful
Nor`easter, is displaced roughly 600 miles west. As cyclogenesis
rapidly ensues during the day Monday, the intensifying low-level
southerly flow will open up the Gulf of Mexico, with increasing
tropical moisture rapidly moving north towards the lower Great
Lakes. In the upper-levels, a classic dual-jet structure will
develop, with southeast Michigan being placed within the favorable
northern stream right-entrance region and approaching left-exit
region of the southern stream jet. The end result of these features
will be a rapidly developing surface low as phasing occurs between
the northern and southern streams.

The 12z GFS, UKMET, and Canadian operational runs (along with a
large percentage of their ensemble members) are now in agreement
with the 12z ECMWF with the development of strong low pressure that
tracks somewhere between central MI and eastern OH. There still
remains some time for the model guidance to hone in on a more narrow
track, but the guidance is rather remarkably in good clustering with
this scenario despite the predecessor split flow regime. Guidance is
also in good agreement with the strength of the system, deepening
the rapidly intensifying low to sub 990 hPa in the vicinity of Lake
Huron as phasing occurs sometime late Monday into Tuesday.

This forecast scenario will result in a period of potential high
impact weather beginning during the day Monday and continuing into
Tuesday, ultimately dependent on the exact track of the surface low.
As tropical moisture is drawn into this system, PWAT values will
rebound to around 1.5 inches and along with plenty of jet dynamics,
frontogenetical, deformation, and isentropic forcing, there is the
threat for heavy rainfall Monday into Tuesday. It is still too early
to pinpoint exact ranges of possible amounts, but several inches of
rain (2+) will be possible in a roughly 12-18 hr time window
sometime late Monday into early Tuesday morning. The Weather
Prediction Center has now highlighted all of southeast Michigan in a
Marginal Risk in their Day 3 Excessive Rainfall Forecast.

Depending on the eventual track of the surface low, high winds may
also come into play for at least a portion of the area. This
scenario will highly be dependent on the surface low track. For now,
agree with the thinking of Weather Prediction Center and will side
with a track favoring a blend of the 12z GFS/ECMWF, which takes the
low just east of the area.

Forecast becomes more uncertain heading into Tuesday as the likely
sub 990 hPa low pressure lifts north into Ontario. GFS continues
with a more progressive solution, while the ECMWF stalls the low
solidly in the mid 980s hPa in the vicinity of the Straits, which
would lead to a prolonged period of deformation rain showers and
gusty winds continuing into Wednesday. Temperatures will trend back
towards normal after Sunday, with highs Monday in the 60s and in the
50s Tuesday, with lows generally in the 40s.

While model guidance is now in much better agreement with the
overall synoptic pattern accompanying this low pressure system,
there are mesoscale factors that have yet to be resolved and will
play a significant factor in the intensity of impacts we may face.
Still, interests in southeast Michigan should continue to pay close
attention to the forecast over the coming days, as a more clearer
picture evolves.

With the trough axis over the region on Wednesday, cool and
unsettled conditions will continue as a northern stream wave
dropping through the region keeps a chance for showers in the
forecast. Ridging then noses into the region on Thursday as the
trough shifts east yielding drier conditions. Southerly flow will
increase during the late week timeframe ahead of a strengthening low
pressure system over the Northern Plains bringing a warming trend to
the region with highs approaching 60 by Friday. This low is then
progged to slowly track over the Northern Great Lakes next weekend.
Model solutions diverge late next week on the the timing of a cold
frontal passage and associated shower chances across SE Michigan.
The GFS favors a quicker frontal passage on Friday while the ECMWF
keeps the front just to the west of SE Michigan through Saturday.
Regardless, upper level troughing looks to return next weekend
bringing a return to cool, unsettled conditions across the region.

LONG TERM / WEDNESDAY THROUGH FRIDAY

With the trough axis over the region on Wednesday, cool and
unsettled conditions will continue as a northern stream wave
dropping through the region keeps a chance for showers in the
forecast. Ridging then noses into the region on Thursday as the
trough shifts east yielding drier conditions. Southerly flow will
increase during the late week timeframe ahead of a strengthening low
pressure system over the Northern Plains bringing a warming trend to
the region with highs approaching 60 by Friday. This low is then
progged to slowly track over the Northern Great Lakes next weekend.
Model solutions diverge late next week on the the timing of a cold
frontal passage and associated shower chances across SE Michigan.
The GFS favors a quicker frontal passage on Friday while the ECMWF
keeps the front just to the west of SE Michigan through Saturday.
Regardless, upper level troughing looks to return next weekend
bringing a return to cool, unsettled conditions across the region.

MARINE...

Moderate to fresh southerly flow will persist until a cold front
traverses the waters Monday into Monday night. Winds will quickly
become strong and westerly in the wake of the front gusting to at
least near-gales as low pressure lifts from the Ohio Valley into
Ontario. The exact track and strength of the low are uncertain at
this time and a wide range of outcomes remains possible because the
low will track directly over or very near the lakes...the former
resulting in a more subdued wind scenario. Despite uncertainty...the
potential for significant impacts Monday evening through Tuesday
evening warrants the issuance of a Gale Watch. Note that the wind
and wave forecast is likely to undergo noteworthy changes over the
next few forecast cycles.

HYDROLOGY...
A cold front carrying a band of showers will work into the area late
Sunday evening into early Sunday morning before a rapidly
strengthening low develops along it causing it to become nearly
stationary over Lower Michigan. Although the strength and track of
the low remain uncertain at this time, there is potential for more
than 2 inches of rain over a portion of the area Monday into Monday
night as strong deformation forcing lifts through the peninsula.
Urban and small stream flooding will be a possibility during this
time.

CLIMATE...

Record high temperatures will be possible across portions of
southeast Michigan Sunday. Here is a look at record high
temperatures for Sunday, October 22nd:

Record temperatures for October 21...

Detroit                80 (set in 1979)
Saginaw/Tri-Cities     82 (set in 1979)
Flint                  81 (set in 1920)

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Monday afternoon through late Tuesday night for
     LHZ361>363-421-422-441>443-462>464.

Lake St Clair...Gale Watch from Monday afternoon through late Tuesday night for
     LCZ460.

Michigan waters of Lake Erie...Gale Watch from Monday afternoon through late Tuesday night for
     LEZ444.

&&

$$

AVIATION.....SF
DISCUSSION...IRL/JD
MARINE.......JVC
HYDROLOGY...JVC
CLIMATE...IRL/JD


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.


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