NWS Forecast Discussion

000
FXUS63 KDTX 201943
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
343 PM EDT Thu Sep 20 2018

.DISCUSSION...

A warm front will continue to lift across the CWA from south to
north this afternoon as deepening low pressure over Nebraska moves
into southern Minnesota this evening. Theta-e advection along this
frontal surface brought showers and a few storms across the area
this morning, leaving a narrow swath of locations from Livingston
County southeast through Wayne County that received over an inch of
rain with isolated reports of over two inches. Currently, a few
isolated showers remain over central Lower Michigan. Today`s early
activity, along with the edge of cloud cover currently along a line
from Downriver to Howell in satellite imagery, has left an
instability gradient across the southern CWA. Potential exists for
additional convection to develop off this boundary this afternoon
with ample surface based instability - SBCAPE of 2000+ J/kg -
inching in with the warm front. Warmer temps in the mid levels will
limit lapse rates aloft however. The best chance for a few isolated
updrafts to develop is over the next three or four hours, and there
are a few new storms noted across southwest MI at press time. Deep
layer shear of 45-50 kt will allow for organization of any updrafts
while 0-3km SRH values will reach or exceed 200 m2/s2, sufficient
for some rotation to develop. Any storms this afternoon will be
monitored for threat of damaging wind gusts and an isolated tornado.

As the warm front clears the area later this evening, SE Michigan
will be fully entrenched in the warm sector of the cyclone over the
northwestern Great Lakes. Any evening convective activity will
diminish with the loss of daytime heating, leaving a period of dry
conditions overnight for most of the area. The exception will be the
northern Thumb which has a slight chance for additional showers
overnight as a weak shortwave moves over northern Michigan. Lows
will fall to the low 70s with muggy dew points in the upper 60s.
Increased gradient flow will preclude any mention of fog tonight.

Main story for Friday will be the strengthening wind field as
deepening low pressure continues from Lake Superior into southern
Ontario and eventually Quebec by Friday night, leading to an
enhanced pressure gradient over the Great Lakes. Southwest winds
will increase to 20-25 mph by the afternoon with gusts approaching
30-35 mph. As the cold front pushes through the region during the
early afternoon, the potential exists for a relatively quick pop of
40+ mph over a 60-90 minute time period. Convection is not looking
likely along this front with limited moisture and better forcing
displaced behind the front, but any showers that do develop will be
able to tap into 50+ kt winds aloft and will have the potential to
produce damaging gusts at the surface. Will hold off on any long-
fused wind advisories and rely on short-fused statements tomorrow to
cover wind threat. After the front passes through, winds will veer
to the northwest and eventually north overnight as cold air
advection takes over. The pressure gradient will begin to let up as
well, leading to weakening winds by the evening. Highs tomorrow will
be influenced by the timing of the frontal passage, with more time
for eastern areas to reach the low to mid 80s. Western areas will
reach the mid 70s before the cooler air really begins to settle in.
850mb temps will fall from around 20C to around 7C, resulting in
surface temps dropping quickly during the evening; Friday night lows
will settle into the mid 40s to lower 50s.

High pressure slides in behind the front on Friday evening and
through Saturday, ushering in a quieter period for SE Michigan.
Highs Saturday remain cool in the 60s and a decent amount of clouds
are expected to stick around as cold air continues to filter in.
Lows Saturday night will again fall to the mid 40s/lower 50s.

Se Mi will remain under the influence of high pressure over Quebec
from the latter half of the weekend into Monday. The circulation
around the high, with east-southeast low level winds into Se Mi,
will support relatively cool and dry conditions. Long wave trough
amplification across the nrn plains and upper Midwest will lead to
southwesterly moisture transport into the area by Tuesday. Latest
model solutions indicate this trough, with the main pv center
lifting into Hudson Bay, will remain relatively progressive, driving
a cold front into Lower Mi Tues or Tues night. This will be the next
good chance for showers and thunderstorms. Post frontal cool air
will then dominate through at least mid week.

&&

.MARINE...

A seasonably strong low pressure system will track across the
northern Great Lakes tonight and into Friday, initially lifting a
warm front through lower Michigan today into tonight before dragging
a strong cold front eastward across the region Friday. Scattered
showers and a few thunderstorms will accompany the low pressure
system, along with rapidly worsening marine conditions. A Gale
Warning is now in effect for all of Lake Huron midday Friday through
Friday night, with Small Craft Advisories in effect for Lake St.
Clair and western Lake Erie. A rush of warm air behind the warm
front will initially hamper peak wind gusts out of the southwest
tonight into Friday morning at or below 30 knots, but as colder air
rushes in behind the cold front passage under northwest flow, gusts
will reach gale force across all of the Lake Huron waters, peaking
around 40 knots by Friday afternoon. The strong winds will lead to
rapidly increasing waves over the open Lake Huron waters as well,
with peak wave heights exceeding 10 feet over northern and central
Lake Huron. High pressure will then build in quickly behind this
system by Saturday, bringing improved marine conditions during the
weekend.

&&

.HYDROLOGY...

Morning and early afternoon showers and thunderstorms across
portions of southeast Michigan, especially around the Detroit metro
upwards to the Flint region, resulted in a swath of 1-3 inches of
rainfall. This rainfall resulted in localized areas of poor drainage
flooding in low lying and urban areas, along with significant within
bank rises on area fast-responding creeks and rivers. An additional
threat for widely scattered thunderstorms late this afternoon and
this evening will be possible, especially south of the M-59
corridor. Rainfall amounts with this activity may exceed a quarter
of an inch and lead to renewed poor drainage flooding concerns if
the storms move over the same areas that saw earlier rainfall.
Elsewhere, water levels will continue to rapidly recede the rest of
today with a brief period of drier conditions.

A strong cold front will sweep east across the region Friday, with a
line of showers and thunderstorms possible midday Friday through
Friday afternoon. Rainfall amounts will generally be under a quarter
of an inch with this activity, and the progressive nature should
limit any flooding concerns. Dry high pressure will then build into
the region for the weekend.

&&

.PREV DISCUSSION...
Issued at 134 PM EDT Thu Sep 20 2018

AVIATION...

Lead surge of shortwave energy along northward-moving warm front
this morning has resulted in chaotic aviation conditions at times so
far today, with periodic MVFR/IFR/LIFR flying conditions associated
with rounds of rain/thunderstorms. Remnants of this activity
continues to push across areas mainly north of I-96 to start the TAF
period, and will affect primarily KPTK and KFNT. Second piece of
shortwave energy noted in water vapor imagery over southern Lake
Michigan will traverse the region 20z-00z as the warm front attempts
to push north across the Ohio border. Where breaks in near
persistent cloud cover can occur towards the Ohio border, rapid
destabilization will be possible in a favorable low-level
shear/helicity environment. Confidence in coverage is not high
enough at this time for a TEMPO group, but did go with VCTS for the
southern TAF sites 20z-23z as several hi-res guidance members have
honed in on this period for potential redevelopment of widely
scattered thunderstorms between M-59 and the Ohio border. After this
activity exits around 00z, concern will then shift to winds
overnight into Friday as strong low pressure moves across northern
Great Lakes and swings a cold front through towards late in the
period. First concern will be LLWS potential 04z-14z, with strong
SSW LLJ around 45 knots near 2 kft. Daytime mixing ahead of front
Friday will push SW wind gusts to near 30 knots by the end of the
period, with higher gusts possible after 18z.

For DTW...Ongoing convection north of terminal expected to remain to
the north through 20z, with 20z-23z the timeframe for renewed
potential thunderstorm development. LLWS concerns much of tonight
with SSW LLJ around 45 knots near 2 kft. Mixing Friday leads to SW
wind gusts peaking at 30-35 knots by the afternoon hours lending to
potential crosswind concerns. Broken line of convection during the
early afternoon may briefly enhance these winds as well.

DTW THRESHOLD PROBABILITIES...

* High for ceilings 5000 ft or less through this evening. Low
  tonight and moderate after 12z Friday.

* Low for thunderstorms impacting terminal in the 20z-23z time frame.

* Moderate for crosswind threshold exceedance 240-260 degrees 30-35
  knots after 16z Friday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement from Friday morning through Friday evening
     for MIZ049-055.

Lake Huron...Gale Warning from 8 AM Friday to midnight EDT Friday night for
     LHZ361>363-421-422-441>443-462>464.

Lake St Clair...Small Craft Advisory from 8 AM Friday to midnight EDT Friday night
     for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 8 AM Friday to midnight EDT Friday night
     for LEZ444.

&&

$$

DISCUSSION...TF/SC
MARINE.......IRL
HYDROLOGY....IRL
AVIATION.....IRL


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NWS DTX Office Area Forecast Discussion