Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1207 am EDT Wednesday Jul 1 2015
The most recent surface analysis and satellite imagery indicates a
broad area of IFR/LIFR ceiling and visibility over northwest lower
Michigan. These conditions are set to slide across the rest of the
area in northwest low level flow trailing the low pressure system over
southern Ontario. The process will be enhanced by the clearing of
middle level clouds over wet ground within the terminal corridor.
Downslope flow into mbs and lingering gradient surface wind will be
factors complicating coverage and potential component of fog versus
stratus. The forecast sides with mainly stratus for now. Daytime
heating will then help lift ceiling into VFR during the afternoon
into Wednesday evening.
For dtw... confidence has increased on timing and coverage MVFR/IFR
transition during the night then lasting through the morning. Plan
to monitor ceiling trends that could bring the forecast down into
LIFR by sunrise.
//Dtw threshold probabilities...
* high for ceiling 5000 feet or less.
* Low for ceiling less than 200 feet.
Previous discussion...issued 326 PM EDT Tuesday Jun 30 2015
Short term...this evening and tonight
Forecast reasoning has changed relatively little for this afternoon
with the lone notable exception that a mesoscale low propagating
south over Lake Michigan has modulated the forcing field over
central and Southern Lower Michigan so far today. As a result, the
genesis region for showers and thunderstorms has been along and
south of this feature's pseudo-warm front. Probability of precipitation have correspondingly
been shifted south in accordance with the latest observational
evidence which suggests afternoon convective activity funneling into
the I-69 to I-94 corridor. 4km Storm Prediction Center WRF run appears usable through
late evening. Shear remains quite low while MLCAPE still looks to
maximum out around 600 j/kg near the Ohio border. No severe weather
anticipated, but a localized flooding threat could evolve in a
region of training of slow moving cells.
Large scale forcing associated with the middle-level circulation over
northern lower Michigan will shift east through the late evening
bringing a gradual end to showers after sunset. Marine modified air,
as noted streaming off of Lake Superior all day, will filter into
lower Michigan within westerly flow tonight supporting low clouds
through sunrise and containing lows to the middle and upper 50s.
Large upper level ridge centered over the central rockies this
afternoon folding over...with shortwave over extreme southern
Saskatchewan diving southeast into the Midwest tomorrow morning.
Southeast Michigan looks to be under general subsidence and drying
behind this evenings trough axis passage...with heights and surface
pressure on the rise. However...a northern stream shortwave trough
extending from the large upper level low over Hudson Bay is
forecasted to sweep through the northern Great Lakes late
Wednesday/Wednesday night...with second pv extension rotating
through Southern Lower Michigan during Thursday. Good 500 mb packing
of temperatures...but GFS indicating 925 mb computed lifted indices and
showalter index remaining solidly positive Wednesday evening...while
NAM lowers the lifted indices to around zero as weak frontal passage occurs.
With possible slight moisture contribution from Saginaw Bay...would
not discount the possibility of isolated cells going up....but even
NAM soundings show difficult time over coming the dry middle level air.
During Thursday...shortwave and surface low expected to track across
the Ohio Valley. Although middle level lapse rates will be rather steep
(6.5+ c from 700-500 mb noted) over lower Michigan...low level
anticylonic flow will be building in...and will continue with the
dry forecast. Light northeast flow in the low levels coupled with
partly cloudy skies (south) expected to hold maxes under 80 degrees
with 850 mb temperatures around 10 c.
Will maintain a dry forecast Friday into the Holiday weekend with
weak upper flow/lack of forcing...lack of moisture...and weak
surface high pressure shown by 12z model suite. Temperatures also
expected to warm back to near normal as the negative upper height
anomaly is replaced by slightly rising heights. Models disagree
substantially early next week with 12z Euro tracking a strong upper
low through Ontario while GFS/gefs mean cut this energy off over
northwestern Canada. Will keep probability of precipitation near climatology in the lower chance
range until better agreement can be reached.
Very light winds will persist through tonight with a few showers
around into the evening hours. A weak cold front looks to be
dropping south through the central Great Lakes late
tomorrow/tomorrow night. Little shower/thunderstorm activity is
expected with the front...with northwest winds increasing to around
15 knots over Lake Huron. High pressure building into the region on
Thursday with low pressure tracking through the Ohio Valley will
lead to northeast flow....but still holding around 15 knots.
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).