Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
658 am EDT sun Apr 26 2015
The upper level low pressure system over portions of eastern North
America will expand in size today and tonight. This low will again
influence the weather across southeastern Michigan. Two waves of
midlevel energy and moisture will rotate back into southeastern
Michigan from the northeast. This evolution will yield VFR ceilings this
afternoon eventually transitioning to MVFR overnight. Will leave
precipitation out of the taf forecast at this time as a considerable
amount of uncertainty still exists.
//Dtw threshold probabilities...
* high confidence in ceilings below 5000 feet late tonight. Low confidence
in exact timing.
Previous discussion...issued 358 am EDT sun Apr 26 2015
Short term...today and tonight
Suppression of geopotential heights over the southeastern United
States will allow the broadening closed low anomaly over portions of
northeastern North America to reach back and regain an influence
over the Great Lakes region today and tonight. The strengthening
deep column cyclonic flow will not only bring another day of below
normal temperatures...it will backdoor a couple of spokes of weak
shortwave energy and ribbons of deeper moisture from northeast to
southwest across Lake Huron and down into interior sections of the
For the short term forecast...the item of uncertainty is when clouds
will push across the lake and spread across the County Warning Area. The model
consensus is very bullish...suggesting clouds will be here already
by middle to late morning. The problem is current...infrared satellite
imagery shows areas void of cloud clear back across Lake Huron to
Ontario. The Pocket of cloud that had advected down into the Saginaw
Bay area earlier has since eroded. For sky cover...went with a blend
of guidance and gut feeling. Overall...favor more sun during the
morning before clouds increase during the afternoon either for
increase in diurnal cumulus/stratocumulus or advection of midlevel
cloud. Adding to this point...first spoke of cva is expected to
collapse down upon the area during the late afternoon. The second
spoke of midlevel energy will arrive late tonight...likely after 9z.
Model soundings support a fairly respectable depth of deeper
moisture with this second wave. Kept the slight to low chance of a
rain shower for the eastern Thumb/Lake Huron shoreline. The
potential may hinge on more of a drizzle/weak stratiform rain
potential with very weak lapse rate structures forecasted.
Long term...Monday through Saturday
Deep layer north-NE flow will persist across Southeast Michigan on Monday as the
forecast area will remain under the influence of an upper low and
elongated trough axis extending from northern New England across the
Great Lakes. A region of middle level subsidence will however
overspread the forecast area Monday afternoon in the wake of a middle
level short wave impulse dropping south across eastern lower Michigan. While
some degree of low-middle level moisture will continue to stream into
Southeast Michigan from the northeast...middle level warming will limit convective
cloud depths. This will support a dry afternoon forecast...although
there is likely to be a healthy cumulus/strato cumulus field. The continued
northerly flow and daytime cloud cover will keep Monday highs in the
50s. A high amplitude middle level ridge will then build into lower Michigan
on Tuesday. The loss of the northerly wind component as surface high nudges
into the area from the west and better diurnal heating potential
will support Tuesday highs into the low 60s after relatively chilly
early morning temperatures in the 30s.
The 00z GFS has trended stronger with middle level short wave energy
dropping into the Great Lakes from central Canada in the Wednesday-Thursday
time frame and actually develops a cut off circulation over lower Michigan
Wednesday night. The 00z European model (ecmwf) /whose solution for the Wednesday-Thursday time
frame has been by far more consistent/ is along a similar line
although not nearly as deep as the GFS. A dry forecast will be
maintained in the Wednesday-Thursday time frame with continued model
discrepancies and in consideration of the overall dry airmass
residing over the Great Lakes. In light of the trends...high temperatures
for Thursday have been nudged down a few degrees /upper 50s to low
60s/. The departure of this system late in the week will then offer
an opportunity for a substantial warming trend heading into next
weekend as middle level heights rebound across the region.
Persistent low pressure along the East Coast and strengthening high
pressure building across the western Great Lakes today into tonight
will actually strengthen the north-northwest gradient across lakes Huron and
Erie today into tonight. The net result will be a strengthening of
the north-northwest winds this afternoon through tonight. Winds and waves will
then slowly decrease Monday into Tuesday morning as high pressure
builds into the region from the west.
Lake Huron...Small Craft Advisory from 8 PM this evening to 6 am EDT Monday for
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).