Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1238 PM EST Thursday Dec 25 2014
High pressure advancing from the Midwest through the Great Lakes
will bring improving sky conditions and diminishing wind tonight...
and ensure dry weather through Friday. A clearing trend tonight will
be preceded by borderline MVFR/VFR ceiling during the afternoon that
will tend to sink back below 2000 feet before the clearing line
arrives. The western edge of clouds moving out of Illinois at press
time is on pace to reach Southeast Michigan during the evening leaving just
patchy high clouds in place through Friday morning. Relatively mild
air for this time of year will be subject to some stratus
redevelopment after midnight. This is hinted in model data but needs
some observational support before being added to the forecast...
especially considering clear sky over the middle Mississippi Valley
during the afternoon.
For dtw... MVFR ceiling will be the prevailing condition through middle
evening. A downward trend below 2000 feet is more likely than any
meaningful time of VFR...suggested by recent observations...until
the clearing line exiting Illinois at press time reaches the
terminal. After clearing...conditions will be monitored over the
Ohio Valley for potential stratus redevelopment after midnight
through Friday morning.
//Dtw threshold probabilities...
* high for ceiling below 5000 feet through early evening.
Previous discussion...issued 1113 am EST Thursday Dec 25 2014
Late morning observational data contain somewhat mixed signals regarding
clouds trends for the rest of the day. There are some breaks in
the low clouds over the northern thumb and Southern Lake Huron
that suggest holding out for mixing to erode coverage. There is
also solid coverage over western lower to the western edge of
stratus over Illinois. This has the look of inversion locked
conditions pointed out by our 12z sounding which leaves the
clearing trend tied to the surface ridge axis and cloud edge
migrating slowly eastward into the evening. The minor morning
update adjustment will be toward a more pessimistic cloud
forecast...or delayed diminishing trend...for the Detroit area
through the afternoon.
Previous discussion...issued 250 am EST Thursday Dec 25 2014
Short term...today and tonight
Remaining light rain and snow showers tracking through area with
weak wave that is wrapping around main low pressure circulation to
the north will end early this evening...leaving mostly cloudy skies
into much of the day. Even with a decided lack of sunshine...cold
air advection in the wake of overnight storm system will end by 18z
with warm air advection already in place prior to 00z. This will
lead to relatively mild temperatures and will edge highs closer to
guidance readings by adding 1 or 2 degrees area-wide to the going
forecast. West to southwest flow will persist within decent pressure
gradient through today...aiding in keeping temperatures relatively
mild for late December.
Weak shortwave ridge will build into area already tonight as upper
trough evolves over the southern rockies and southern High Plains.
This feature...along with surface high pressure should lead to some
break in the clouds by evening. Winds will also slacken considerably
with the loss of boundary layer mixing...so temperatures should be
able to drop off fairly well tonight...even in the moderating air
mass. Will leave low temperatures...upper 20s to lower 30s...as is
for this forecast package.
Long term...Friday through next Wednesday
Friday will have building heights as a shortwave ridge builds over
the eastern U.S. Moisture profile suggest that Southeast Michigan should
at least start with partly sunny skies. There will be some hi
altocumulus and cirrus ahead of the shearing shortwave that will be
ejecting from the inter mountain system. By late Friday...a good
surge of 850-700 mb moisture will also ensure that mostly cloudy
skies return by the evening. With some sun...went closer to the
warmer mav/GFS guidance numbers.
As the moisture advection continues ahead of that sheared out
shortwave with deep and strong SW flow...the chance of showers
increase on Friday night especially for the northern part of the
forecast area. Given the moisture surge that eventually gets into
the boundary layer later Friday night...added patchy drizzle for
late Friday night and early Saturday. Expect a Friday evening low
before temperatures are steady or even slowly rise during the
overnight hours with the good SW flow. The entire forecast area
should not have any issues staying above freezing and thus keeping
the precipitation liquid.
Finally by Saturday...that sheared out shortwave moves through lower
Michigan accompanied by the a cold frontal passage late in the day.
There really is not much going for Saturday other than that weak
shortwave...cold front and some upper divergence as we get into the
right rear entrance region of a 140 knots jet by late in the day. For
the most part will keep the probability of precipitation in the chance category. Guidance
numbers for Saturday were close and do not see any reason to differ.
Saturday should have a familiar feel with plenty of clouds...a damp
feel but mild for December.
By Saturday evening the cold front passes through the region and
should bring in enough cold air that any residual showers mix with
and change to snow showers before ending. Again with the lack of
any forcing...probability of precipitation will continue to be in the chance category.
The remainder of the forecast remains very quite. A general
lowering of The Heights with more of a troughing northwest flow will
bring in colder temperatures for Sunday through Wednesday...nothing
extreme just closer to average for the end of December.
Low pressure just northeast of Lake Huron will continue to pull away
today. As it does...gales will remain for portions of Lake Huron
early in the day and then diminish. By late in the day today...a
ridge of high pressure will build over the central and eastern
lakes. After the passage of this weak ridge...moderate southwest
winds up to around 20 kts will develop for Friday and Friday night
ahead of a cold front. That cold front will pass through late
Saturday and Saturday night. Behind the cold front will be another
round of moderate winds from the northwest.
Lake Huron...Small Craft Advisory until 4 PM EST this afternoon for lhz421-441.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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at www.Weather.Gov/Detroit (all lower case).