Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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397
FXUS63 KDTX 231451
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1051 AM EDT Mon Oct 23 2017

.UPDATE...

Morning update already out the door to provide additional temporal
detail on precipitation trends given latest near term observations
and hi res guidance. 1007 mb surface low now centered over west-
central Indiana remains projected to deepen roughly 15 mb over the
next 12 hours as it translates northeast to near Saginaw Bay by this
evening.  Well defined stripe of deep layer mid level forcing
marking the edge of the northern stream height falls and upper jet
sustaining a solid corridor of moderate rainfall from southern Lake
Michigan into central/northern lower MI.  The Tri-Cities will reside
within the eastern expanse of this forcing, supporting periodic
rainfall throughout the day. Elsewhere, meaningful ascent will
require the arrival of the inbound southern pv anomaly later this
afternoon. Recent model guidance centers the highest probability of
precipitation for most locations between 20z and 02z. Emerging
convection over the Ohio Valley does cast some doubt as to the
efficiency of the northward moisture flux with eastern extent, which
could temper qpf amounts to some degree. Roughly half inch of rain
in Midland County thus far, with 2+ inches still within reach given
close proximity to strengthening deformation forcing going forward.

&&

.PREV DISCUSSION...
Issued at 657 AM EDT Mon Oct 23 2017

AVIATION...

Moisture feeding into Lower Mi today will result in a region of rain
lifting across the area this afternoon and evening. MBS, which is a
little closer to a sfc frontal boundary, will see more persistent
rain through the day. The influx of moisture and rainfall will
result in steadily lowering ceiling heights through the afternoon
and evening, with low end MVFR and IFR conditions expected to be
prevalent by evening. The closer proximity to the frontal boundary
will result in more persistent low clouds at MBS in comparison to
locations farther south.

The forecast rapid deepening of the surface low as it lifts across
Lower Mi tonight will lead to increasing winds. The surface low will
track primarily west of the terminals this evening. The expectation
is that there will be enough of a shallow stable layer to keep
southeast wind gusts in the 20-25 knot range through the evening. As
the low lifts north (03 to 06Z time frame), winds will veer toward
the south-southwest. Deepening mixing depths may result in a burst of
stronger winds during this time frame.

For DTW...The influx of deep moisture will rotate into metro between
17Z and 19Z. While instability will likely not be high enough to
support thunder, there will be brief heavier bursts of rain this
afternoon through roughly 00-01Z. As winds increase this evening,
they should back toward the SSE, suggesting a low chance for
crosswind thresholds to be exceeded.

//DTW THRESHOLD PROBABILITIES...

* Low in ceilings at or below 5000 ft this morning. High this
  afternoon through tonight.

* Low for thunderstorms this afternoon and evening.

* Low in southeast winds exceeding crosswind thresholds this
  evening.

PREV DISCUSSION...
Issued at 316 AM EDT Mon Oct 23 2017

DISCUSSION...

Very moist airmass for late October standards has entered lower
Michigan, as the 00z APX sounding indicated an 850 mb dew pt of 12
C, with 4 C at 700 MB, both record values for this date, per SPC
sounding climatology page. The issue is frontal boundary/moisture
axis has pretty much exhausted its eastward progress, as upper level
energy/trough tracking through the Northern Plains is backing the
flow over the Western Great Lakes this morning.

Meanwhile, strong southern upper wave over the central Mississippi
River Valley now tracking off to the northeast, as the upper level
PV linkage is underway. Merging of 500 mb height falls occuring over
the Central Great Lakes late today into Tonight, with greatest max 6
HR fall center looking to track through Lake Huron late tonight.
Cyclogensis/rapid deepening of the surface low will occur as it
arches northwest toward the Straights-Beaver Island late tonight. 12z
Euro ensembles were still indicating close to a 8 MB spread at 12z
Tuesday in that region. However, the 00z NAM/regional GEM have come
in pretty good agreement with the operational 00z Euro, deepening the
low toward 980 MB. With the 850 MB low/circulation tracking west of
southeast Michigan, heavy rain axis will be situated over western
Lower Michigan (supported by latest HRRR and RAP) as the impressive
negative tilted trough/trowel lifts north within the high
end/moisture rich environment. The progressive nature of the system
lifting north should help keep rainfall amounts in check on this side
of the state as well, as main window for heavy rain will be late
today through this evening. With just about everybody in southeast
Michigan expected to see total rainfall amounts under 2 inches, and
probably even at or below 1 inch, just minor flooding/standing water
concerns. The exception looks to be northern Bay/Midland counties,
where 2-3 inches still looks possible. However, with 6 hr flood
guidance in the Tri-Cities region near 3 inches, probably only
looking at low end flooding if anything. None-the-less, will issue
flood watch in case amounts reach and exceed 3 inches.

Active lake enhanced showers Tuesday-Wednesday across the
region as the large scale upper level trough/circulation remains in
place, with good low level cyclonic flow and 850 MB temps dropping
below zero. Lake Michigan water temperatures still mostly at or above
60 degrees. Steep low level lapse rates and marginally unstable
environment during Tuesday, as showalter index drops to around zero.
Mid levels are dry, but lapse rates are very steep, 7.5 C/KM from
700-500 MB. Graupel is possible with low freezing levels, with an
isolated thunderstorm not totally out of the question as well. One
more shot of cold air to arrive for Wednesday as jet streak/upper
wave slides southeast through the Central Great Lakes, with 850 mb
temps falling to -3 C. Westerly flow off Lake Michigan should be
sufficient for continued shower activity, with graupel likely.

Short lived upper level ridge to build in for Thursday, as another
large upper level trough/low encompasses the Great Lakes Region over
the Weekend, and not out of the question to see first few snowflakes
in the air if the -6 C at 850 MB (per EURO and GFS) materializes.

MARINE...

Model trends have come into better agreement on the track and
strengthening of low pressure forecast to lift into Lower Michigan
later today. The surface low is now expected to rapidly deepening as
it lifts across Lower Mi tonight and into the straits region by
early Tuesday morning. The result will be an increase in the winds
on Lake Huron to gale force tonight. The surface low is forecast to
weaken over the region on Tuesday. However, an influx of cold air
wrapping around the low will increase the instability over the lakes
and support continued gale force winds through the day Tuesday.
Probabilities of wind gusts to 35 knot gales on Lake Huron are now
high, while probabilities of high end gales to 45 knots over the open
waters of Lake Huron have increased to roughly 30 percent. Winds and
waves will undergo a gradual weakening Tuesday night into Wednesday
with further weakening of the low pressure system.

HYDROLOGY...

Model solutions have had a noted trend toward farther north and west
with the axis of heaviest rainfall today and tonight. In fact, the
highest rainfall totals are now expected to fall northwest of the
Saginaw Valley, with most of Southeast Michigan likely seeing total
QPF of an inch or less. Rainfall amounts in excess of an inch are
still possible in Midland and Bay Counties. There will be some
concern for river levels on the Tittabawassee River in Midland
County as some of the highest rainfall amounts are now forecast
across its headwaters in central Michigan.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 4 PM EDT this afternoon through Tuesday morning
     for MIZ047-048.

Lake Huron...Gale Warning from 8 PM this evening to 8 AM EDT Wednesday for
     LHZ362-363-462>464.

     Gale Warning from 8 PM this evening to 8 PM EDT Tuesday for
     LHZ441>443.

     Gale Warning from 1 AM to 4 PM EDT Tuesday for LHZ361.

     Gale Warning from 2 AM to 10 PM EDT Tuesday for LHZ421-422.

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

     Small Craft Advisory from 6 PM this evening to noon EDT Tuesday for
     LCZ460.

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

     Small Craft Advisory from 4 PM this afternoon to noon EDT Tuesday
     for LEZ444.

&&

$$

UPDATE.......MR
AVIATION.....SC
DISCUSSION...SF
MARINE.......SC
HYDROLOGY....SC


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