Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 232316
AFDDTX

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


.AVIATION...

VFR conditions will be replace by MVFR and then IFR cigs and vsbys
late this evening into the overnight as warm front encroaches from
the south in response to approaching low pressure. This will bring
an expanding area of showers/fog into area in the 04z-06z. These
conditions will continue KFNT/KMBS into Friday, but expect warm
front to clear the I-94 terminals and reach about KPTK by mid/late
afternoon. This will bring a break in showers/lower cigs before
additional showers (and thunder after 00z) expand back through the
area as the low pressure tracks through southern lower Michigan.
Winds will veer from northeast to south in the warm sector by early
afternoon, but remain northeast further north where fog/rain will
persist with LIFR conditions perhaps becoming VLIFR @ KMBS.

For DTW...MVFR cigs should spread back into terminal from the SW by
around 03z with IFR cigs/vsbys in showers/fog after 06z as moist air
overruns approaching warm front. This front will lift north of the
terminal by 18z-20z with a period of low VFR cigs perhaps scattering
out briefly in the late afternoon. Convection will then expand into
area by early evening as low level forcing increases substantial due
to a strong low. This strong forcing, acting on minimal/modestly
unstable airmass will likely result in scattered thunderstorms by
mid evening.

//DTW THRESHOLD PROBABILITIES...

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

* Low in thunderstorms tonight with increasing confidence by 00z
  Friday evening.

&&

.PREV DISCUSSION...
Issued at 335 PM EST Thu Feb 23 2017

DISCUSSION...

Key larger scale features expected to drive a period of active
weather locally to finish the week coming into greater focus now as
further sampling solidifies confidence in the general forecast detail.
Weak frontal boundary to settle just to the south by this evening.
Backed northeast to easterly flow locally to yield some renewed low
level cooling across the lowest 3000-4000 ft given streamlines
flowing off the cold lake waters.  This will bring overnight
temperatures down into the 30s across many locales north of the
ridgeline.  Meanwhile...strong height falls and associated 990 mb
surface low on track to enter the southern plains by this evening.
Emerging corridor of isentropic ascent within the ensuing increase
in low-mid level southwest flow to steadily translate northeast
through the night.  Surging 850-700 mb theta-e gradient immersed
within this ascent and lodged on the nose of 50 kt 850 mb inflow
suggests a solid precipitation response from south to north between
06z and 15z Friday morning.  Steepening mid level lapse rates under
improving theta-e quality will provide an elevated thunder risk.

Retention of cool pre-warm frontal easterly flow to define the early
portion of Friday - particularly with northward extent.  Surface
warm front will then make a more concerted northward penetration
into southern lower Michigan through the afternoon period.
Associated low level destabilization thereafter to leave at least a
weakly unstable profile prior to 00z south of the M-59 or I-69
corridor - differences in northward placement of the boundary still
carries some uncertainty.  Potential for MLCAPE to climb over 1000
J/KG across the warm sector.  However, well defined elevated mixed
layer to accompany this inbound warm sector.  Potential for some
degree of capping to dictate convective prospects will therefore
establish a very conditional risk for convective development through
the early evening hours, contingent on achieving an adequately
buoyant parcel to work through this cap.  This risk does give pause
however, as well established deep layer flow and the likelihood of
localized enhanced vorticity at the warm frontal interface provides
a background environment conducive to discrete rotating updrafts
should the thermodynamics prove adequate to support low topped
development.  Greatest potential for development/expansion early may
focus near the triple point as the low lifts over southern lake
Michigan, where any downstream propagation may anchor on the
slightly elevated warm frontal zone draped across the tri-cities.
Freezing levels remain low enough to present a hail threat for such
activity.

Greatest potential for organized convection still centered within
the 00z-05z window early Friday night, owing to a sizable increase
in mass/frontal convergence into an existing modestly unstable
profile.  Any nocturnal downturn in available instability at the
surface may become essentially negligible, given the high degree of
instability /for late February standards/ that will still exist just
off the surface.  General model signal suggests a lead pre-frontal
trough anchoring weak mid level cold air advection/steeper lapse
rates could become an initial focus, either just upstream or
directly overhead.  Development is expected either on this feature,
the trailing cold front or both.  In any case, magnitude of the wind
field across both the lowest 6 km and 1 km could support a mixed
mode, tending to be discreet/supercellular early before congealing
into linear segments. Greatest risk with southward extent, with
areas south of M-59 appropriately highlighted within an enhanced
risk.

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

Long duration of cold air advection lasting well into Saturday,
beneath a high amplitude upper trough passage.  Wrap around moisture
appears lean enough to maintain simply lower precipitation chances
on Saturday, but a few light rain/snow showers or flurries will be
possible given the cyclonic flow.  While temperatures will be
noticeably cooler relative to this recent stretch of record warmth,
readings will remain in the vicinity of normal for late February.

EXTENDED PERIOD SUNDAY THROUGH FRIDAY: Broad area of high pressure
centered over the Tennessee Valley, will begin ridging northward on
Sunday. This will allow for a brief dry period over southern
Michigan Sunday. However, several weak waves of low pressure will
migrate through the ridge Monday and Tuesday, resulting in at least
slight chances of precipitation. Meanwhile, developing low pressure
over the central plains will push into the Great Lakes Region on
Wednesday; bumping up the chances for precipitation and pulling in
gusty southwest winds. Gusty winds swinging northwest for Thursday
on the backside of the front. High Temperatures start out
predominantly in the low 40s Sunday and Monday; rising near 50 for
Tuesday and Wednesday.

MARINE...

Northeast winds will increase late tonight into Friday night as low
pressure lifts from the Central Plains to the Great Lakes. Winds
will become stronger over Lake Huron, particularly across the
northern half of the lake where gale force wind gusts are expected
to develop Friday into Friday evening. The persistent strong
northeast winds will also allow large waves to grow over Saginaw Bay
and off the tip of the northern Thumb into late Friday night.
Thunderstorms will also be possible tonight, and especially late
Friday through the first half of Friday night as a warm and unstable
airmass is replaced by much cooler air. Thunderstorms may become
strong to severe late Friday and Friday night, with the highest
probability of storms over Lake St Clair and Western Lake Erie.

The low will lift from Michigan to Quebec on Saturday, ushering in
cooler air and gusty westerly winds behind a strong cold front. Wind
gusts are expected to reach 30 knots across all marine areas on
Saturday, and could potentially reach gales. The best potential for
a period of gale force winds will again be over Lake Huron Saturday
and Saturday night.

HYDROLOGY...

Rounds of showers and thunderstorms are expected tonight through
Friday night as low pressure tracks across the area. First, a
steadier round of rain will spread into the area late tonight and
early Friday. There then looks to be a break in activity from at
least Flint southward during the late morning and early afternoon
before additional showers and thunderstorms redevelop during the
late afternoon and first half of the night. Precipitation will
meanwhile continue to affect Central Michigan during the day,
keeping a higher coverage and longer duration of showers over the
Saginaw Valley and northern Thumb. Rainfall totals will vary widely
as more intense showers and thunderstorms bring locally heavy
downpours from late tonight through Friday night.

Total average rainfall is expected to range from one-half to three-
quarters of an inch between the Ohio border and M-59 by Saturday
morning, with totals closer to three-quarter of an inch to one inch
north of M-59.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Saturday morning through late Saturday night for
     LHZ361>363-441>443-462>464.

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

     Gale Warning from 10 AM to 10 PM EST Friday for LHZ361-362.

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

$$

AVIATION.....DG
DISCUSSION...MR/DE
MARINE.......HLO
HYDROLOGY....HLO


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