Scientific Forecaster Discussion
Area forecast discussion...corrected typo
National Weather Service Detroit/Pontiac Michigan
1155 PM EDT Tuesday Sep 30 2014
IFR ceilings continue to spread southward through the County Warning Area within moist
cyclonic flow on west side of low pressure. Ceilings/visibilities are expected
to edge down through the night with IFR/LIFR common by dawn as low
level moisture remains trapped under lowering inversion. Conditions
will then gradually improve on Wednesday as the aforementioned low
shifts to the east/southeast.
For dtw...expect ceilings to lower below 1000 feet between 06z-08z with
areas of fog possible after 08z. This will linger through 14z or
so...before gradual improvement in ceilings/visibilities ensues for the rest of
the forecast period.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet through Wednesday
morning...with medium confidence for Wednesday afternoon/evening.
Previous discussion...issued 354 PM EDT Tuesday Sep 30 2014
Current model runs are still showing surface ridging to build across
the region tonight helping to put an end to any lingering drizzle
and rain showers by early evening. Original thinking was for clouds
to also diminish slightly...however...with abundant moisture around
from rain and drizzle throughout the day...confidence is high in cloud
cover remaining overcast tonight. This low level moisture combined
with some weak warm air advection will result in a higher
probability of low stratus and even some fog development especially
in low lying areas. Only major concern to watch out for during the
next forecast shift is the uncertainty about how dense the fog will be
overnight. Overcast skies overnight will result in temperatures to
only drop a few degrees from tuesdays high temperatures into the upper 40s
and low 50s across Southeast Michigan.
Long term...Wednesday through Monday
Longwave pattern to undergo some big changes through the rest of the
week leading to a very cool weekend as a deep trough overtakes the
region. The midweek time frame will be the transition period for the
pattern as the first strong wave comes ashore the northwest Continental U.S.. this
will carve out a trough over the mountain west which will absorb a
cutoff low as it moves further east. Narrow ridge axis in advance of
the trough will pass over the Great Lakes Wednesday. Meanwhile at
the surface...southern periphery of high pressure over Ontario/Quebec
remains in control at the surface but will drift east loosening its
grip. Cool easterly flow will be replaced with warmer and moister
south/southwest flow as the deepening trough reaches the plains
states. Pattern stalls over the upper Mississippi Valley as a strong
wave dives through the trough changing it from a positive to neutral
tilt. As the wave lifts NE out of the trough...cyclogenesis will
ensue along a cold front extending from Ontario down into Oklahoma.
This surface low will undergo rapid intensification...deepening from
1004mb 18z Thursday down to 988mb 12z Friday which is nearly 1
mb/hr. But this surface low will have little forward propagation due to
the upper level trough stalling.
Now for what this means for Southeast Michigan Wednesday and Thursday. Wednesday
will start off very cloudy as model consensus keeps a very moisture
laden bl under a strong but weakening inversion. Increasing heights
with ridge axis overhead will promote some warming along with the
more favorable wind direction. East winds will veer around south
late in the day becoming southwest overnight. Current thinking keeps
temperatures in the middle 60s. Will be some additional potential for temperatures to
jump a degree or two with warmer air aloft but model soundings show
mixing depths struggling to tap into that air. There will likely be
a sharp gradient in precipitation on Thursday as a prefrontal line
of showers approaches Southeast Michigan. The ridge will hold it to the west for
most of the day with only our western counties having a chance for
any showers. The strongest of the shortwaves thus far will surge
through the trough forcing the cold front through lower Michigan late
Thursday night into Friday. Good SW flow ahead of the front will
ramp up warm air advection with temperatures in the middle 70s which will drop off
significantly Post frontal as we experienced today from the most
recent cold front.
Deepening low pressure and associated cold front will cross through
the state on Friday. This feature will spread rain...brisk
winds...and a sharp temperature drop to the area. Expect low
stratus...drizzle...and much cooler temperatures for the weekend in
the Post frontal/upper trough pattern. Temperatures will be hovering around
10 degrees below normal for the weekend into early next week.
A tight northerly gradient remains over Central Lake Huron keeping
wind speeds up longer into the afternoon. These winds will continue
to build waves up to small craft criteria along the Lake Huron
shoreline of The Thumb. Small craft advisories will continue through
this evening to account for that. Winds will diminish tonight as
high pressure lessens the gradient over the lakes but the persistent
northeast flow will keep some elevated waves heights into the
overnight hours. Light winds and low waves will persist Wednesday
into Thursday. However...southerly winds developing ahead of a cold
front late Thursday could result in winds up to 30 knots over the
open waters of Lake Huron Thursday night.
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...until 4 am Wednesday.
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).