Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1142 PM EDT sun Aug 30 2015
Area observations have begun to fall to 1/4 mile visibility under
good radiating conditions and elevated surface dewpoints. Increasing
cirrus debris from the northwest casts some uncertainty on the
scenario, but expectation is for other locations to follow. Have
trended visibilities downward at the airfields accordingly, with rapid
improvement expected by 14-15z. High based cumulus potential during peak
heating Monday, but coverage should be limited given drier airmass
and some inhibition to heating by high clouds.
//Dtw threshold probabilities...
* medium for visibility at or below 1/2 mile 09-12z.
Previous discussion...issued 1101 PM EDT sun Aug 30 2015
Clear skies, calm winds, and dewpoints solidly in the 60s are giving
way to increasingly widespread dense fog at 03z, mainly north of
M59. In the immediate Detroit area, lingering clouds and slow
cooling from warmer daytime temperatures are delaying onset. When all is
said and done, however, expect that all counties will see a
noteworthy component of dense fog worthy of an advisory. A dense fog
advisory is therefore forthcoming for the I-69 counties north valid
immediately. Seeing as though sites such as adg, dtw, and arb have
yet to dip below 1sm (6sm in the case of dtw) will start the
remainder of counties at 06z with the understanding that there may
yet be some further delay in urban corridors.
Previous discussion...issued 353 PM EDT sun Aug 30 2015
Short term... afternoon and tonight
Middle afternoon mesoanalysis indicates lowest 100 mb MLCAPE in the
1000-1500 j/kg range from the Ohio border to about the I-96/696
corridor. The radar trend has been relatively soft given the
instability suggesting either daytime heating needs a little more
time or that middle level subsidence is sufficient to cap deep
convection. Most recent rap model soundings support the latter with
the depiction of surface based parcels just barely clearing the
deepening 700-500 mb warm/stable layer. At the same time...the
newest mesoscale models allowing explicit convection produce
scattered coverage of showers with an isolated thunderstorm during
peak heating from late afternoon through early evening. The most
prudent approach is to allow for an isolated thunderstorm
considering a wind hazard is in play conditional on storm
development. WV satellite imagery continues to support model
soundings that show very dry air centered around 500 mb. The dry air
is symptomatic of the larger scale subsidence occurring through the
middle levels that is inhibiting convection in the first place...but
which also could provide strong negative buoyancy potential should
instability become adequate for deep convection.
Points north of the I-69 corridor will be dry but cloudy during the
late afternoon and evening...and any showers/storms that are able to
form south of that line will quickly dissipate with the loss of
surface heating by middle evening. That will leave a clearing trend to
occur through the overnight as surface high pressure becomes re-
established over lower Michigan. The northern fringes of the surface
high remains set to establish areas of fog in Southeast Michigan toward
sunrise as min temperatures settle in the lower to middle 60s. Projected min
temperatures are about a category below surface dewpoint observed during
the afternoon today with just a few degrees of dry advection
expected tonight. This adds some confidence to fog coverage in our
area combined with the widespread observational evidence over
Wisconsin/Iowa/Illinois from this morning.
Long term...Monday through Saturday
The anticipated wavelength broadening of upper level ridge is
forecasted to accelerate Monday with a centroid of the upper level
geopotential height maximum breaking off of the ridge peak and rolling
eastward into much of eastern Canada. Models forecast a weakness
developing within the ridge for the beginning of the week
period....but all indications are the slightly cooler midlevel
temperatures will remain confined to the Ohio River valley. A very
weak baroclinic zone...with no jet support aloft...will materialize
to south of the Ohio/Michigan border. It is along this feature and
to the south where a better chance exists for a pop-up shower or
thunderstorm. For southeastern Michigan the forecast will remain
dry. With the ridge expanding east...the heat and uncomfortable
conditions will get started with temperatures and dewpoints
combining to push heat indices into the middle to upper 80s for Monday.
The weak temperature discontinuity or frontal zone is then
forecasted to lift slightly northward Tuesday. This will occur has
loosely defined and weak shortwave energy consolidates into some
organization over greater portions of Ohio and Indiana. A plan
view of midlevel or 700 mb temperatures shows this with some sub
7c 700 mb temperatures lifting across the Stateline into far
southeastern Michigan. It is a steep lapse rate pocket from
800-700mb that may then allow for some convective shower/tstorm
development Tuesday afternoon as the boundary lifts northward.
Will refrain from adding a precipitation mention this forecast
issuance as potential still appears to be less than 20 percent.
Working off warmer mins...temperatures are expected to moderate a
few more degrees with highs easily reaching the middle to upper
80s Tuesday afternoon. Heat indices are forecasted to eclipse the
90 degree mark.
Extended...by the middle of next week...Southeast Michigan will be
firmly under the upper level ridging and southerly flow. This will
bring a continuation of above normal temperatures along with dry
conditions which will persist through the remainder of the week and
into the weekend.
Very quiet marine weather will persist on the Great Lakes through
much of the upcoming week. A strong ridge of upper level high
pressure will maintain light flow and a general southwest wind
Michigan...dense fog advisory until 9 am EDT Monday for miz047>049-053>055-
Dense fog advisory from 3 am to 9 am EDT Monday for miz068>070-075-
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).