Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 231811
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
111 PM EST Thu Feb 23 2017


.AVIATION...

Winds will gradually weaken, turning more to the northwest and then
northeast by this evening as low pressure tracks away from the area
and through Quebec. Daytime heating and lingering moisture will
provide some clouds between 2500-3500 feet this afternoon, mainly
towards MBS and FNT, behind a weak cold front that is washing out as
it drops slowly southeast through Michigan.

The next low pressure system will approach the area tonight,
allowing MVFR ceilings to develop during the evening. Showers and
IFR conditions will spread into the area between 02-07Z as elevated
portions of a warm front begin to lift into the area and strong
moisture advection commences with the low-level jet. A few
thunderstorms may also occur late tonight from FNT south. Ceilings
could lower to LIFR as early as 09Z, but confidence is greater after
12Z when showers will also transition to drizzle south of FNT. Low
ceilings and rain/drizzle will be slow to exit MBS and FNT tomorrow
on the north side of the surface warm front. Locations further south
should see a break in activity and MVFR to even VFR conditions
during the very late morning and early afternoon, before another
round of showers and thunderstorms (possibly severe) move in during
the very late afternoon and evening.


For DTW...VFR conditions are expected through the afternoon and
early evening, while winds weaken and gradually shift to the
northwest and then northeast. Showers look to lift into the Detroit-
area after 02Z, with best potential for rain tonight between 06-10Z,
before activity transitions to drizzle. A few thunderstorms are also
possible overnight. There could also be a window of LIFR ceilings in
the 09Z-16Z timeframe, but confidence is not high enough to include
in the TAF at this time.

//DTW THRESHOLD PROBABILITIES...

* High in ceilings below 5000 feet late this evening through mid-
  morning Friday.

* Low in thunderstorms tonight, and again 21-00Z Friday.

&&

.PREV DISCUSSION...
Issued at 357 AM EST Thu Feb 23 2017

DISCUSSION...

Orphaned cold front currently stretching from Northern Lower through
Chicago to just south of Kansas City will slow and veer out today as
forcing diminishes and it becomes increasingly aligned with the
flow. Low clouds are established beneath the frontal zone as
evidence by low cloud observations stretching the Upper Midwest into
APX`s CWA. These clouds will make inroads into the CWA early this
morning, likely gradually eroding through the late morning as low-
level convergence weakens and unfettered insolation takes its toll.
As a result, suspect a mostly sunny character to the day will be in
store south of I-69 with high clouds moving in late. High
temperatures for the day have probably already been established
across the north with current readings at 08z in the upper 50s with
dewpoints near 60. Sun and decreased clouds will allow southern
areas to increase back into the low 60s. Cool northeast flow will
develop tonight as the front settles south, spelling overnight lows
in the upper 30s, which is still near average high temperature
values for late February.

Attention then turns toward the energy digging into the 4-Corners
region early this morning. Pressure falls have already commenced
along the front range with a lee cyclogenesis episode anticipated
this morning. Subsequent longwave feedback will establish a coupled
jet and attendant broad low-level isentropic response late this
afternoon through tonight as the surface low lifts toward the Quad-
Cities, Iowa. Antecedent dry air will delay precip onset locally,
even while stronger forcing allows showers to blossom to the west.
Eventual saturation will bring showers to all of Southeast Michigan,
mainly after 03z tonight. Surging 40-kt LLJ and steepening lapse
rates aloft will likely bring elevated thunder and some locally
heavy showers as the front lifts north.

Warm frontal showers lift north during Friday morning, exposing a
yet uncertain portion of the CWA to the system warm sector as the
low lifts toward southern Lake Michigan by 00z Friday. Non-GFS
guidance suggests the the southeastern half of the CWA making it
into the warm sector. Lack of significant convection does increase
confidence that the front won`t be shunted further south as it prone
to happen in the warm season.

Large scale forcing will increase dramatically over the western
Great Lakes Friday afternoon into Friday evening as very strong PV
advection allows the mid-level circulation to begin acquiring a
negative tilt. Current forecast assumes a warm frontal location
somewhere invof the M59 corridor during Friday afternoon, north of
which the temperature will be suppressed in the 40s and 50s and
south of which low 70s may be approached conditional on clearing.

Pristine EML progged by the 00z ECMWF to begin spreading overhead as
early as Friday morning, effectively capping the free warm sector as
instability builds to 500 to 1000 j/kg in response to 7.5 C/KM or
greater lapse rates aloft. As the LLJ translates east during peak
heating, deep layer forcing for ascent will become maximized
immediately in advance of the descending dry conveyor and within the
pre-frontal trough. Deep layer bulk shear vectors oriented about 45
degrees to the initiating boundary within the strongly sheared
environment may support an initial supercell mode over western Lower
Michigan. 0-1km shear in excess of 30 kts and 0-6km in excess of
50kts will be adequate for maintaining any surface-based
mesocyclones for a period of time. A tornado threat will present
itself west of US 23 within the 0-1km EHI maximum
(Livingston/Washtenaw/Lenawee) as either this activity encroaches on
the area or with any preceding low-topped warm sector convection
that pops up, the latter of which may be too shallow to produce
thunder given 800mb cap strength. Tornadic showers certainly not
unheard of in these parts. Tornado threat forecast to remain on the
lower end of the spectrum given uncertainty in convective mode and
overall weak instability.

Strongly unidirectional wind profile and an abundance of mid-level
dry air suggest a high likelihood of an eventual transition toward
downdraft modulated convective modes carrying primarily a straight-
line wind threat as storms speed from SW to NE at 50+mph. Some
uncertainty in the magnitude of the wind threat as convection
marches east owing to developing nocturnal inversion as well as a
more pronounced lake shadow east of a Flint-Pontiac-Warren line.
Suspect legitimate downdrafts would have no problem punching through
the young nocturnal inversion further west, which includes the
majority of the Detroit Metro area. All said, a marginal tornado and
wind threat Friday afternoon is expected to transition to a straight-
line wind threat with time after 00z. Convection exits east by 06z.

Gusty WSW winds in the wake of the cold front drop high temperatures
to seasonably mild readings in the low to mid 40s. Strong cold
advection through a deep layer will bring the potential for a very
gusty day. Attm, however, high quality moisture within the column is
progged to limit mixing depths and cap gusts below 30 kts.

MARINE...

Low pressure will lift northeast of Lake Huron early this morning
and will drive a cold front across the lake later this morning. The
front will settle into Lake Erie this afternoon. The strength of the
prefrontal southerly winds early this morning are being limited by
strong over lake stability. Neutral to slightly unstable conditions
will develop across Lake Huron during the day within post frontal
cold air advection under north-northwest winds. These winds should
primarily remain below 20 knots as the gradient relaxes.

The cold front will push south of Lake Erie during the evening
before lifting back north as a warm front late tonight into Friday.
This will occur as low pressure lifts into the midwest. The easterly
gradient will tighten to the north of the warm front, resulting in
gusty winds across Lake Huron on Friday. There is high probability
that easterly gusts across nrn and cntl Lake Huron will reach 30
knots Friday, with lower probabilities for a brief period of gusts
to gale force. The low is forecast to weaken a little as it lifts
across Lower Mi Friday night. This system will provide a good chance
for thunderstorms, especially across Lakes St Clair and Erie within
the warm sector. The departure of the low to the northeast on
Saturday will bring colder air into the region within gusty west-
southwest winds. The strongest winds are expected Sat night, where
gusts to gale force are possible.

CLIMATE...

Record high low temperatures for February 23
DTW...43 (1930)
FNT...47 (2000)
MBS...42 (1930)

Record high temperatures for February 24
DTW...59 (1976)
FNT...57 (2002)
MBS...60 (1930)

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Friday morning through Friday evening for LHZ361-
     362.

     Small Craft Advisory from 4 AM to 10 PM EST Friday for LHZ421-441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....HLO
DISCUSSION...JVC
MARINE.......SC
CLIMATE......JVC


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