Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 201650
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1250 PM EDT Thu Jul 20 2017

.AVIATION...

Weak instability developing this afternoon expected to limit
convection, and much of southeast Michigan will likely remain dry.

Surface dew pts have also fallen into the 65 to 70 degree range as
we mixed well late this morning. Still enough low level moisture to
support SCT diurnal cu, but not sure on BKN. Lack of rain and drier
air calls into question the fog development tonight, and will err
more on the optimistic side. Otherwise, surface high pressure moving
overhead should promote mostly clear skies and light and variable
winds well into Tomorrow.

For DTW...Any thunderstorm activity which develops is expected to be
isolated and likely missing terminal to the south. Still will hold
onto BKN skies near 5000 Feet this afternoon, but confidence is not
high.

//DTW THRESHOLD PROBABILITIES...

* Low for cigs aob 5000 ft this afternoon.

* Low for thunderstorms this afternoon

&&

.PREV DISCUSSION...
Issued at 1113 AM EDT Thu Jul 20 2017

UPDATE...

Weak synoptic scale boundary remains draped across the region,
through southeastern Michigan between the I 69 and M 59 corridors.
The front, is a very weak one with no discernible active frontal
dynamics locally for the past 24 hours plus. No active frontal
dynamics are anticipated later today either. Very difficult to
pinpoint any potential triggers or large scale forcing for ascent
this afternoon. Upstream MCS activity last night heavily favored the
sharp and deep CAPE gradient that was in place over the mid and upper
Misssippi River Valleys. With regards to convective potential this
afternoon, the questions then distill down to 1. whether or not the
CAPE gradient will overtake southeastern Michigan today, and/or 2.
can any pop-up type thunderstorm activity develop similar to what
happened yesterday.

Plan views show and forecast soundings suggest that low level
subsidence and some equaterward advection of dry is occuring in the
wake of a weak cyclonic circulation that is passing into Ontario. It
is reintroduction of anticyclonic flow trajectories that will push
the most rich of near surface theta e content southward into northern
Ohio and northern Indiana by this afternoon. Thus, the max gradient
of the deep CAPE is not expected to be over southeastern Michigan.
All signs then point to residual latent heat bubble aloft (likely
above 600 mb) pushing across or rolling over southeastern Michigan
during the afternoon. Indications are this subsidence will be deep
enough and strong enough to extend down into the middle to possibly
lower levels. The uncertain part is if the subsidence will extend
down into the 4-9 kft agl layer. If it does, chances for any
precipitation will be isolated at best. If the subidence is not as
deep, the atmosphere remains soupy enough down low to bring some
prospects for convective initation and shower/thunderstorm
development. Latest 3km NAM hints at this, while the vast majority
of the NWP suite keeps things dry. If pop-up activity were to develop
it appears that it would be along the front edge of 700 mb dry
advection. Most favorable timing would be 18-21Z. The potential for
severe weather appears low. Aforementioned subsidence will limit the
steep lapse rates, limiting CAPE to around 750 J/KG. Swody1 update
this morning lowered the outlook to marginal for a bulk of the cwa.

PREV DISCUSSION...
Issued at 336 AM EDT Thu Jul 20 2017

DISCUSSION...

Potentially active stretch of weather begins this morning, with
periods of showers and thunderstorms possible now through Sunday
night. On a large scale we have fast zonal/westerly flow lined up
along the northern tier of counties with large upper low over
northern Canada and impressive 597dam subtropical high sprawled
across the southern/central plains. On a slightly smaller scale, a
train of mid level waves will race through this westerly flow over
the next few days.  And on an even smaller scale we will have
convective systems complicating the matter, displacing instability
pools and laying out numerous boundaries. Low confidence forecast as
models struggle to initialize well due to the current MCS activity,
thus many models become instantly useless. Will have to rely on
pattern recognition with short fused updates to handle much of the
activity.

Early this morning through this evening: Plenty of activity ongoing
upstream as a train of convective systems have been flaring up
through the night. Most of the activity will stay to the west as it
tracks southeast out of the Midwest following the instability
gradient. Outside of some showers on the northeast flank clipping
our southern counties, this will largely not impact SE MI. What is
in play will be a shortwave tracking across the northern Great
Lakes, phasing with a trough rotating around the Canadian low. In
addition, the strong 850mb low level jet currently forcing much of
the convection upstream is supposed to push eastward through
northern MI today. A combination of the HRRR and 3km NAM (which seem
to have the best handle on the current pattern), would suggest
nothing more than scattered showers developing under the broad mid
level forcing as the lead gradient of the enhanced theta e plume
passes through. Better coverage of storms would lift north of the
area while the train of MCS`s robs the area of better moisture
return and instability.  With this line of thinking, reduced pops
to chance and slight chance thinking drier overall. So we shift
attention the the afternoon where we then have a front pulled south
through the area. less precip early on leads to more heating and
build up of instability thus better chance for storms to develop
later in the day. Best chance will be generally south of M59, maybe
as far north as I69, as the boundary runs into around 1000 J/kg of
surfaced based CAPE and 40 knots of bulk shear (per RAP).  Could see
some organized storms develop. Primary threat with any strong to
severe storms will be wind and hail. Will need to watch cloud debris
from upstream convection to make sure it doesn`t prevent better
destabilization though.

Thursday night and Friday: We`ll dry out for a spell as surface high
slides through the region under a shortwave ridge aloft.
Temperatures will hold in the mid 80s as the airmass changes very
little with the passing cold front.

Friday night through Saturday night: Another active period appears
to be in the works as the boundary draped across the Ohio Valley
gets pulled back north as a warm front ahead of a developing low
over the central Plains. this slow moving east to west oriented
front look to pose a threat for heavy rain and maybe flooding as a
very moist airmass is forced up the front. PWATs of 2+ inches
continue to show up in the models as a connection to the monsoonal
moisture over the SW conus is made. A series of shortwaves could
ride along the front leading to enhanced periods of showers and
storms throughout this stretch.

Sunday through early week: Sunday could feature a few more rounds of
showers as the deformation region of the low wraps around through
the area along with a trailing trough axis. SE MI will dry out
Monday and Tuesday as high pressure builds back into the area with
ridging upstream beginning to nose into the region as well.

MARINE...

A weak center of low pressure will slide through southern Michigan
today along an equally weak front that is stalled over the region.
These features will be able to produce thunderstorms which will be
the primary marine weather impact today. Outside of thunderstorms,
marine wind will be light and have little impact on waves. The low
pressure center will move east of the Great Lakes and the front will
move south of the Ohio border tonight. This will allow high pressure
to temporarily develop over the region. The next low pressure system
is then projected to approach the area Friday night and interact
with the front for another round of stormy conditions during much of
the weekend.

&&

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

$$

AVIATION.....SF
UPDATE.......CB
DISCUSSION...DRK
MARINE.......BT


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