Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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122
FXUS63 KDTX 221943 CCA
AFDDTX

Area Forecast Discussion...CORRECTED
National Weather Service Detroit/Pontiac MI
343 PM EDT Sun Oct 22 2017

.DISCUSSION...

After an extended stretch of seasonably warm and dry conditions to
finish the weekend, the pattern will turn increasingly complex
heading into the upcoming work week as a dynamic mid autumn system
enters the region. Early stages of this process initiated by the
inbound elongated corridor of height falls now stretching from
western Ontario to the lower MS valley. Northern portion of the
effective pv advection will break off and shear northeast, a
response to the acceleration of the primary subtropical jet core
into eastern Canada. This will leave an orphaned strong piece of
energy to briefly drift across MO/AR over the next 12-18 hours.

In the meantime, the near term focus centers on the attendant
frontal boundary forecast to ease southeast into the lower
peninsula. This boundary will eventually stall in the vicinity of
central lower MI on Monday, any additional forward progress now
hindered as frontal boundary positioning becomes increasingly
parallel to the mean thickness field. Established northward moisture
transport vectors within the existing deep layer pre-frontal
southwest flow will anchor an axis of high moisture content - pw
near 1.5 inches - along the inbound frontal slope. The combination
of modest mid level frontal convergence and a period of favorable
entrance region jet dynamics will subsequently support an increase
in rain chances from northwest to southeast tonight through Monday
morning. Proximity to boundary places the highest rain potential
during this time window northwest of a Flint to Bad Axe line. One
final mild night under this pattern, with temperatures still
hovering in the mid-upper 50s by daybreak Monday.

Rapid northeast acceleration of the aforementioned southern negative
height anomaly will commence Monday and Monday night, both the speed
and trajectory strongly governed by the pace and amplification of
trailing energy anchored to an inbound stout pacific jet. Model
sensitivity analysis suggests this remains a source of uncertainty,
lending to some model differences yet in terms of both magnitude and
location of the eventual dynamic response as potential vorticity
interacts/phases between these waves.  With that said, there is
growing consensus for an associated surface low track across or near
southeast Michigan late Monday, before more pronounced deepening
commences over northern Lake Huron Monday night. Strengthening mid
level dynamics - pv advection, fgen, coupled upper jet, mass
convergence - into a moist deep layer profile will subsequently
support an increase in rainfall coverage on Monday. Low element of
elevated instability noted in the soundings will contribute to
moderate to occasionally rainfall, but limited enough at this stage
to preclude a defined mention of thunder. A generally damp but mild
day, the lack of greater insolation potential with extensive
clouds/rain capping highs but with readings still above normal /60s/.

Widespread rainfall expected throughout Monday night as lower
Michigan becomes the pivot point for strengthening mid level
deformation in the presence of persistent fgen and large scale
divergence.  Rainfall amounts carry the greatest uncertainty in this
event, with a strong dependence on both placement and longevity of
the greatest axis of ascent. High confidence exists in a widespread
swath of 1 to 2 inches with this event, but with diminishing
confidence in witnessing a higher end stripe of 2+ inches. Certainly
a plausible scenario exists to see higher amounts given the overall
setup, a solution space supported for several cycles now by the
ECMWF/Canadian. 12z NAM is trending in this direction, depicting a
much greater overall response now. Outside of the GFS, model
consensus points to the higher rainfall amounts across the Tri-
cities, or roughly anchored along and enhanced by the expected
frontal positioning. 6 hour flash flood guidance on average hovers
near 2.5 inches. With this being a long duration event and still
more than 24-36 hours out from any potentially impactful amounts,
will hold off on issuance of a flood watch.

Lingering deformation rainfall may persist for a time early Tuesday
while upper jet placement remains favorable, particularly with
northward extent. An uptick in moisture advection tied to the
passage of the trailing closed mid level circulation will then
maintain unsettled conditions late Tuesday and Tuesday night. Colder
air rotates back into the region during this time, with 850 mb
temperatures in the lower single digits through the day. High in the
50s.

Cyclonic flow will remain over the region on Wednesday as low
pressure departs to the northeast while a weakening shortwave drops
through the region. These features will keep a chance for showers in
the forecast throughout the day as highs struggle to reach 50 with
the thermal trough axis overhead. Ridging will then nose into the
region on Thursday bringing dry conditions as the trough pivots
eastward. A slight warming trend will commence Thursday into Friday
with highs approaching 60 ahead of the next low pressure system over
the Northern Plains. 12Z model solutions are coming into better
agreement that low pressure will track across the northern Great
Lakes during the Friday-Saturday timeframe pulling a cold front
through SE Michigan late Friday. A period of showers will accompany
the frontal passage late Friday before troughing digs into the
region for the weekend bringing a return to unsettled, cooler
conditions with showers and highs in the lower 50s and lows in the
upper 30s.

&&

.MARINE...

Southerly winds in the 15 to 20 knot range today ahead of a cold
front, which will slowly work into southeast Michigan tonight. Wind
gusts up around 25 knots over southern Lake Huron this evening will
diminish as we head into Monday as the frontal boundary stalls over
the area. Low pressure coming up from the south on Monday will be
deepening, but it appears the track will be too close to the Central
Great Lakes to generate Gale Force winds, and the Gale Watch was
dropped. Although, there still is a less than 30 percent chance the
low could deepen greater than advertised to produce gales, but would
not be until late Tuesday as colder air wraps into the system.

&&

.HYDROLOGY...

A cold front carrying a band of showers will work into the area
early Monday morning before a rapidly strengthening low develops
along it causing it to become nearly stationary over Lower Michigan.
Confidence has increased in the potential for more than 2 inches of
rain over the Saginaw Valley 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.

&&

.PREV DISCUSSION...
Issued at 1254 PM EDT Sun Oct 22 2017

AVIATION...

Southerly flow in advance of a slowly approaching front has begun to
gust to around 20 kts with diurnal heating. Expanding high cloud
canopy and loss of heating should end gusts no later than 22z.
Conditions then deteriorate to MVFR with light rain at MBS for
tonight as the front stalls while low pressure develops well to the
south. Showers will then overspread the remainder of the area from
south to north between predawn and mid-morning Monday. A band of
heavy rain accompanied by IFR or LIFR will develop by the end of the
forecast period. ATTM, this band is expected to setup between KMBS
and KPTK. Wind will veer to northerly west of the front tonight into
early Mon, mainly affecting KMBS/FNT.

//DTW THRESHOLD PROBABILITIES...

* Moderate for cig aob 5kft after 06z. High after 12z.


&&

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

$$

DISCUSSION...MR/JD
MARINE.......JVC
AVIATION.....JVC
HYDROLOGY....JVC

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



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