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Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
717 PM EDT Wednesday Aug 20 2014
Early evening observations indicate showers and thunderstorms
diminishing rapidly with the loss of daytime heating and a
substantial loss of cloud cover as well. This will leave VFR
conditions in place at all terminals through midnight before open
sky with light and variable wind promotes another round of fog.
Expect IFR conditions to result in similar restriction to last
night but with less stratus and more of a fog component. A typical
trend of morning improvement in visibility is expected along with
another round of showers and storms mainly during late Thursday
afternoon into the evening.
//Dtw threshold threats...
* medium confidence in ceiling below 5000 feet as fog lifts Thursday
* Low confidence in 200 feet ceiling and 1/2 sm visibility.
Previous discussion...issued 323 PM EDT Wednesday Aug 20 2014
Stacked low pressure continues to churn over northern Michigan this
evening. The upper low will pivot some this evening to create a
negative tilt within the broad 500mb trough pattern tonight. As the
feature tilts...an area of surface convergence will help spark
continued showers with isolated thunderstorms. While there is a
forecasted maximum of 25kt 850 mb winds over southern Michigan this evening...the
overall flow is very weak. This will lead to slow moving single
cells. Both moisture content and instability are modest today /1.5
inch precipitable water and 1500 j/kg cape/ but are ample to produce some
convection with little potential for damaging winds or hail. 0-6 km
bulk shear of 25 knots is unidirectional...leading to little risk for
The diurnally driven convection will come to an end after 01z this
evening. Just a few isolated showers will be possible thereafter
from the Saginaw Valley to eastern thumb region...closer to the
center of the surface low. Mins tonight range from the upper 50 to
low 60s. There is potential for am fog again tomorrow...which may be
mitigated by cirrus blowoff from upstream warm frontal system. Areas
north of I-69 hold the better chance for fog development with light
winds and increased moisture content.
Longwave feedback associated with strong jet energy digging into the
Pacific northwest and intermountain west will force the subtropical ridge
anchored over the southern United States to undergo noteworthy
amplification heading into the weekend. During this time, Southeast
Michigan will reside on the edge of the instability gradient with no
appreciable upper forcing to speak of. As a result, the forecast
features several periods consisting mainly of chance probability of precipitation to account
for the rather nebulous period of convective potential.
During the next 48 hours, the highest probability scenario will be
for Southeast Michigan to feast upon the leftovers of upstream
remnants. High res solutions suggest that this will be the case on
Thursday as High Plains remnants propagate into the Ohio Valley,
experiencing a diurnal flare-up during the day which is most likely
to occur to our south...or at least south of the metropolitan area...given
the prognosticated location of higher instability. However...diurnal
heating in vicinity of northward advancing warm front will present the
opportunity for isolated to perhaps scattered thunderstorm develop
during the day. Adequate shear around 35 knots will be sufficient to
support organized updrafts depending on cloud cover/instability, so
a marginal risk for severe weather does exist.
A similar situation will be present Thursday night into Friday
morning, except with the favorable thermodynamic environment
creeping closer. The genesis region for any Thursday night activity
will likely be in vicinity of the apex of the ridge/edge of the cap somewhere
in the upper MS River Valley per pattern recognition/high-res model
signals. Mesoscale details that will not become clear until Thursday
or Thursday night enter the near term will be critical. The
potential for cold pool development and maintenance will need to be
monitored as it will determine the potential for upscale growth as
this activity dives into the lower peninsula, where the
thermodynamic environment will be favorable - but not exceptional -
late Thursday night.
High temperatures will creep upward through much of the long term period as
the aforementioned ridge amplifies and 850 mb temperatures increase
into the upper teens by Friday...and then further heading into the
weekend. No arguments with the guidance which suggests daytime highs
highs in the middle to upper 80s and lows in the upper 60s.
Rain chances will likely stick around through next week as a series
of shortwaves track over Southeast Michigan. Model agreement is
still quite low and have therefore only put in low chance probability of precipitation into
the forecast through Wednesday. Timing and coverage of the rain will
be refined as models get a better handle on these disturbances.
Temperatures will remain fairly consistent with highs in the low 80s
and low temperatures in the middle to upper 60s through Wednesday.
A weak pressure gradient will maintain light winds across the waters
through the first half of the weekend. Thunderstorm chances will
persist mainly for Central Lake Huron and points south through this
evening. There is then a chance for thunderstorms for all marine
areas late Thursday night into early Friday...and severe weather is
not out of the question during this time. Moderate southwesterly
flow will develop by Sunday but wind gusts will remain at a
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).
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