Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
923 PM EDT Monday Sep 15 2014
Forecast on target...and most recent update was to take care of
some wording and minor timing issues. Strong wave on water vapor
continues to exit lower Michigan this evening. This will end the
light rain around midnight for the far eastern areas. Surface
front at 01z was from phn to adg. This will leave the area in the
frontal zone for much of the early night with mostly cloudy skies
expected. However a steady northwest flow will continue to advect the 40s
dew point air into lower Michigan and give way to at least partial
clearing by daybreak Tuesday morning. This drier air should also
proclude much fog development overnight...in addition to the lack
of boundary layer moisture as indicated by the current observation and the
00z upstream soundings.
Aviation...issued 656 PM EDT Monday Sep 15 2014
Strong shortwave moving across lower Michigan this evening helping to
expand the rain across the region. Clouds with this feature will
lower through the evening hours becoming MVFR by 03z-05z for all taf
sites. This should be short lived as drier air advects into the
region during the overnight hours...eventual scattering out the
clouds by daybreak for Southeast Michigan. Expect a good diurnal cumulus day on
Tuesday with ceilings between 3k and 5k feet.
//Dtw threshold threats...
* high confidence in ceiling below 5kft after 02z.
* Low confidence in ceiling below 5kft after 09z.
* Moderate confidence in ceiling below 5kft after 15z Tuesday.
Previous discussion...issued 348 PM EDT Monday Sep 15 2014
Short term... tonight
The main moisture/850-700 mb Theta-E axis ahead of the cold front is
south of the Michigan border...which has drawn concern about the
coverage of showers along the front as it tracks through Southeast
Michigan early this evening. However...the upper wave over Wisconsin
remains formidable...and with the current upstream radar coverage
and expected quick ramp of 700-500 mb fgen/middle level cooling over
south half of the County Warning Area...planning on maintaining the current
inherited higher probability of precipitation...which is also supported by 17z hrrr.
The maximum quantitative precipitation forecast location of the 12z GFS looks pretty good...but is
likely way overdone in magnitude. Showers look to be tapering off
from northeast to southeast shortly after 00z...with pronounced
surface ridge arriving shortly after midnight. Enough moisture
around in the low levels to introduce uncertainty with the exact
amount of clearing. Banking on just enough drier air feeding in and
light winds to keep fog mention out of the forecast as temperatures
expected to fall into the 40s...per blended guidance. NAM surface
dew points in the lower 50s at 9z appear to be too high.
Long term... Tuesday through next Monday
Surface high pressure will be firmly centered over Iowa/Missouri
Tuesday morning with a ridge extending into lower Michigan. Dry
conditions will be ensured by this high pressure feature and
lingering middle level subsidence trailing today's system...but the
cloud forecast will be less straightforward or at least less
optimistic than what might be expected with high pressure taking
control. As mentioned in the previous discussion...this past weekend
demonstrated how much influence the warm Great Lakes water can have
on the cloud pattern in these early season cool air masses that are
not devoid of boundary layer moisture below the subsidence
inversion. This is expected to be in play again Tuesday with a
couple of subtle differences mainly tied to model timing on the
southward placement of the low level thermal trough and broad
anticyclonic low level flow. These elements add some uncertainty to
the prospects for nocturnal stratocu migrating into our area from
northern lower after sunrise. However...there is more confidence in
low level cold advection combining with daytime surface heating to
fill in remaining open sky during the late morning through the
afternoon. Forecast soundings across all model runs indicate steep
lapse rates below 800 mb with enough moisture for generous coverage
of shallow cumulus. The resulting partly sunny sky and low level
cold advection will limit maximum temperatures to the lower and middle 60s vs the
lower half of the 70s that is normal for middle September.
A significant diurnal component in the Tuesday afternoon cloud
pattern will give way to a clearing trend with sunset Tuesday
evening. This will open the door to a cold night as surface high
pressure becomes centered over Indiana and Ohio and Fosters strong
radiational cooling with light wind under mostly clear sky. Late
developing westerly flow below 850 mb will be on the trailing flank
of the low level thermal trough and thus represent warm advection...
too weak and late to disrupt radiational cooling but enough to
lessen concern for stratocu from Lake Michigan. About the only other
cloud potential is associated with the next short wave embedded in
the broad long wave trough over eastern Canada. Model consensus
indicates this will be too late to provide more than thin cirrus
toward sunrise Wednesday.
The cold front trailing the northern Great Lakes short wave will
bring a glancing chance of light rain to the Saginaw Valley
Wednesday afternoon. The front will be losing moisture integrity as
it moves south but will have a respectable temperature contrast
judging from model low level thickness and thermal fields. After a
run toward 70f Wednesday afternoon this front will kick the legs out
of the warming trend by Thursday with highs back down toward the
lower end of the 60s...and possibly confined to the upper 50s in The
Thumb given low level flow from the northeast.
As quickly as the surface high is reinforced through lower Michigan
Thursday...it is displaced eastward equally fast Friday in global
model solutions heading into the weekend. Substantial upper level
height falls over the Gulf of Alaska are shown to initiate
progression in the long wave trough over eastern Canada. The short
wave trough helping this along is trending stronger in the latest
solutions as it moves through the plains...just in time to pick up
tropical remnants of "odile" from The Four Corners area. This will
set the stage for a warmer but wetter weekend as the frontal system
moves through the Great Lakes.
Marine conditions will be favorable in terms of wind and waves
despite the passage of two cold fronts during the week. The first
will be tonight which will have just a weak wind shift passing
through along with the rain pattern. The second will bring a chance
of light rain Wednesday and renewed surge of cooler air by Thursday
with modest northerly flow possibly ramping up waves a bit more over
the relatively warm water of Lake Huron. After that...the next
stronger low pressure system will bring warmer air Friday and
Saturday but possibly stormy conditions later Saturday and Sunday as
the associated cold front moves through the region.
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).