Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
FXUS63 KDTX 181926
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
326 PM EDT THU AUG 18 2016
Poorly organized convective complex over Minnesota will take a turn
southeast this evening into tonight as it rides along the
instability gradient provided by the lead edge of an EML over the
upper MS valley. There is low potential for showers or an isolated
thunderstorm to graze the area late tonight or early Friday morning,
but am comfortable maintaining just a schc pop for now given timing
during the instability minimum. Better chance of convection will
arrive Friday evening into early Friday night as a better push of
higher mid-level theta-e content arrives coincident with the
instability gradient folding into the area. ARW/NMM/ECMWF have a
good handle on this potential, developing tstorms north of I-69 21-
03z. Large scale environment is certainly supportive, so nudged pops
up to higher chc category. Clouds will exacerbate muggy conditions
during the day by limiting mixing. Thus, despite highs only in the
mid 80s, heat indices will hover in the low 90s by afternoon.
Upstream trough will undergo significant amplification Friday
evening through Saturday as it is fed energy from an active jet in
the far NE Pacific. Surface low is progged to lift across the
climatologically favored straits area on Saturday evening. SE
Michigan will be poised for severe convection depending on limiting
effects of cloud cover on instability. As it stands, 12z EC has
trended a couple MB stronger by Saturday eve. This will be important
as the rate of deepening will determine the wind field response.
Ambient shear around 30 kts, as currently modeled, will be
sufficient for some organization convective clusters/multicells
conditional on instability. Deep tropospheric forcing will extend
from the surface front up to the lead edge of the PV anomaly aloft
yielding a high probability of line of showers/tstorms working
through the area Saturday night.
While the initial mid level impulse and deep moisture plume will be
east of the forecast area by the latter half of the weekend, the
long wave trough axis is forecast to pass overhead on Sunday.
Despite the advection of cooler and drier air, model solutions
suggest steep mid level lapse rates resulting from the mid level
cold pool will support weak instability within some residual mid
level moisture. This will warrant a low chance of daytime convection
even though afternoon temps will be relatively cool (in the 70s).
Mid level subsidence and sfc high pressure will expand across Lower
Mi in the wake of the mid level trough and will maintain cooler and
much drier conditions through the first half of next week. Medium
range models do suggest the large scale flow will remain
progressive, suggesting building mid level heights and a
modest warming trend during the middle to latter half of next week.
High pressure will maintain light wind conditions today with only a
modest increase in southerly flow expected late tonight into Friday
as this high settles southeast of the area. A weak frontal boundary
will sag south into Lake Huron late Friday and stall over the north
portion of the lake Friday night, bringing a chance of showers and
Low pressure will then track into the Great Lakes on Saturday,
bringing a better chance of showers and thunderstorms over the
weekend. South to southwest winds will likely increase to some
degree in advance of this system on Saturday. A strong cold front
will then sweep through the area in its wake, bringing gusty
northwest flow late Saturday night into Sunday.
While a small chance of showers and thunderstorms will exist over
parts of the area late this week, the next chance for widespread
convective activity will hold off until Saturday and Saturday night
as a low pressure system lifts through the central Great Lakes.
Even though uncertainty in the strength and track of this low
remains somewhat high, it appears that rainfall of one quarter to
one half of an inch will be common as this system crosses the area
with localized amounts of up to one inch possible.
Issued at 1259 PM EDT Thu AUG 18 2016
Scattered diurnal cumulus will fade with loss of daytime heating
this evening. With light winds and partly cloudy skies, low level
moisture should be sufficient for at least patchy fog (and perhaps
stratus). It should be noted that an area of dense fog and LIFR
stratus formed over Thumb region early this morning (but missed the
terminals). Slight SSW gradient flow on backside of high pressure
tonight should limit a repeat performance of that magnitude.
For DTW...Scattered cumulus will lift above 5kft between 18z-22z and
dissipate this evening. Patchy fog should develop late tonight, but
not very confident in degree of VSBY reductions or whether low CIGS
accompany for a time. Will maintain MVFR VSBYS for this forecast.
//DTW Threshold Probabilities...
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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