Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1154 PM EDT Friday Jul 25 2014
Subsidence in wake of passing shortwave will maintain VFR conditions
overnight into Friday...delaying rain showers chances into midday/afternoon
Saturday. By that time...lift from next approaching shortwave trough
encroach on the area and coincide with better peak daytime heating.
Location of best chance of convection remains nebulous...but models
seem to focus nearer the I 94 terminals. Will include a prob30 for
this area by middle/late afternoon as a start...which hopefully can be
refined based on overnight/morning trends.
//Dtw threshold threats...
* low confidence in ceilings at or below 5000 feet after 00z Saturday night.
* Low confidence in scattered thunderstorm development near terminal
Previous discussion...issued 328 PM EDT Friday Jul 25 2014
Short term...through tonight
Main challenge in the short term is overall coverage and timing of
precipitation early Saturday morning. Clouds from last night/early
morning convection over the upper Mississippi Valley has been
streaming in over the area since this morning. Any precipitation
related to this has quickly diminished...especially given the amount
of drier air in the atmosphere and the fact that Southeast Michigan was still
under the influence of the high pressure off to the east. The drier
air was prevalent on the 12z DTX sounding and the latest
observations support this as showers continue to diminish over far
western Michigan. Only thing that may be seen is some virga as
clouds overspread the area. Clouds will continue to hang around and
once again increase late tonight ahead of the next system in
association with the warm front. The diurnal cumulus field that is
currently over the area will dissipate with the loss of heating this
evening with only high clouds sticking around. The main story will
be the increasing rain/thunderstorm chances with the warm
front. Entrance region of the upper level jet will be encroaching
on Southern Lower Michigan overnight which will assist in starting
to create better overall dynamics. This will carry into the weekend
as the chance for unsettled weather continues. Southerly winds will
bring in better moisture overnight as precipitable water values slowly increase to
around 1.5 inches by Saturday morning. The best isentropic ascent
still looks to be between 06-09z as the low level jet gets going.
Latest hires models do not have precipitation coming in until closer to 10z
with the better chances looking to be over the northern County Warning Area.
Overall...the only change to the grids was to account for the later
onset of precipitation. Given cloud cover...low temperatures will not drop too
much with lows in the lower 60s.
Plenty of 12z model (nam/gfs) initialization issues when looking at
upstream radiosonde observations over the central/northern plains extending northeast
in the western Great Lakes...as sharp temperature/moisture gradients exist
at 850/700 mb. In addition...based on water vapor imagery...the
upper level low over Alberta/Saskatchewan looks to be shearing apart
a bit more aggressively than the models suggest. We are left with
the strong jet core in place over the northern plains diving
southeast into the western Ohio Valley...which looks to be the most
likely location (max instability) of the next organized convection
during Saturday...as we continue to be partially under the influence
over the massive upper level low over eastern Canada...which
eventually directs the remnant middle level cold pool/circulation over
western Canada through lower Michigan later Sunday. Still...there is
room for subtle upper level disturbance to track through Michigan
during Saturday with surface trough in place to tap into likely
moderate instability...MLCAPES of 2000+ j/kg and severe threat
remains conditional on activity developing...with the more than
adequate wind fields (0-6 km bulk shear of 45 knots). 700 mb temperatures
warming toward 10 c may be just enough to cap activity
however...and/or we will be in the shadow of the previously
mentioned activity over the Ohio Valley...and have actually lowered
probability of precipitation slightly...as not fully convinced the 70 surface degree dew points
will make it into the County Warning Area with the disruption in moisture transport.
850 mb temperatures in the middle teens support middle 80s...but cloud concerns
and low level moisture increase worthy of shaving a couple degrees.
Saturday night into sunday's forecast hinges on the location of the
nearly west-east moisture axis/850-700 mb Theta-E ridge...as the 500
mb low dropping into the western Great Lakes will help draw some of
the moisture back north...but not sure how much past the Ohio border
the deeper moisture will get before the middle level dry slot punches
into lower Michigan during Sunday. That will not end the rain
chances all together...as deformation axis swings through Sunday
evening/night likely producing additional rainfall. At the very
least (in line with day 3 Storm Prediction Center outlook)...there does look to be a
severe threat toward the Ohio border as better moisture/instability
feeds up from the Northern Ohio valley...and it appears the triple
point will reside in that vicinity.
Amplified upper level eastern North American trough next week will
assure below normal temperatures...as -2.5 to -3 standardized 500 mb
height anomalies reside over the Middle Atlantic States. With the
trough axis near or overhead...scattered diurnal convective activity
seems likely in our neck of the Woods.
There is an increasing chance of showers and thunderstorms over the
weekend...with the potential for severe weather...especially over
Western Lake Erie. Outside of the thunderstorms...winds we be light.
Southerly winds Saturday...becoming variable on Sunday as low
pressure tracks through the area. Behind this low Sunday
night...northerly winds will ramp up...peaking around 30 knots over
Southern Lake Huron on Monday....and small craft advisories will
likely be needed as long fetch allows for good wave build up
impacting the nearshore Lake Huron marine zones.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).