Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1157 PM EST Wednesday Nov 25 2015
South-southwest flow will bring increasing moisture into area into Thursday with
rain showers increasing in coverage...particularly kptk north. Ceilings will
also lower to MVFR. I-94 terminals will be on the edge of this surge
of higher Theta-E area as surface ridge gradually builds into far
Southeast Michigan...and this activity shifts north. Will maintain
borderline MVFR/VFR visibilities/ceilings at this time...with progressively worse
conditions north into kmbs.
For dtw...ceilings will drop to 5kft or below by 10z-12z or so with
rain showers/virga developing. Most light rain should lift north of
terminal by 14z with ceilings edging back above 5kft into the afternoon.
//Dtw threshold probabilities...
* medium in ceilings at or below 5kft Thursday morning...lower
confidence by afternoon.
* High that any precipitation will fall as light rain.
Previous discussion...issued 326 PM EST Wednesday Nov 25 2015
The short term period will be dominated by increasing warm advection
as deep layer southwesterly flow increases downstream of the high amplitude
longwave trough over the western United States. A deep tropospheric
cold front is in the process of taking shape well upstream over the
Great Plains and upper MS valley as jet dynamics strengthen.
Preceding low-level jet will undergo only modest nocturnal
strengthening tonight, drawing north into lower Michigan in response
to a cyclone tracking north of Lake Superior. Despite a lack of
organized large scale forcing, significant Theta-E advection within
deep layer isentropic ascent will drive broad weak ascent and
steepen 850-700mb lapse rates. Lead edge of the Theta-E gradient
will fold across Southeast Michigan 6-12z with 700mb dewpoints
modeled to rise by nearly 30 degrees between 00z this evening and
09z Thursday morning. This will be sufficient to at least generate a
partial coverage of shower activity upon the arrival of steeper middle-
level lapse rates 6-12z. Lack of upstream showers at this time precludes
raising probability of precipitation into the likely category for tonight...though an upward
adjustment is not out of the question depending on coverage during
the time frame leading up to 12z. After warm temperatures in the low
to middle 50s today, strong SW gradient and increasing clouds will
limit nocturnal temperature falls to the middle 40s.
Southeast Michigan firmly entrenched within a moistening/moderating pre-
frontal southwest flow on Thanksgiving. Lead surge of higher Theta-E
lodged within an initial period of greater isentropic ascent arrives
late tonight...maintenance of this forcing likely still in place
through the latter half of the morning. Greatest potential for
shower development south of a Howell to Port Huron line centered on
the morning period...before settling into a seemingly benign 'warm
sector' void of tangible ascent. Points toward the north will
retain the potential for additional pockets of light showers to lift
through during the afternoon/evening...given closer proximity to the
main axis of deeper and persistent moisture advection. Some
refinement of probability of precipitation for the Holiday made accordingly. Perpetuation
of weak low level warm air advection within a 10-15 knots southwest
wind will counter the extensive cloud canopy and lack of insolation
potential. This will translate into afternoon readings a good 10+
degrees above normal for the day...highs arriving in the middle and
Frontal zone parked across the upper MS valley over the next 36
hours will make a more progressive move eastward Thursday night and
Friday as stronger height falls push into the upper Great Lakes.
Frontal timing locally still centered on the Friday period. Strong
frontal forcing and accompanying favorable upper jet support working
into the deep resident moisture axis providing high confidence for
widespread rainfall to shift through during the daylight period.
Degree of forcing and overall residence time will support quantitative precipitation forecast
amounts within the .25-.50" inch range. Window for a possible brief
mix/change over to snow appears very limited early Friday night...
progressive downstream movement of the moisture gradient outpacing
the greater cold air advection and a corresponding adequate
reduction in freezing levels by several hours.
Lower tropospheric frontal zone expected to continue sinking south
of the region Saturday and weaken Sunday Onward as faster upper flow
is reoriented over the northeastern US. The result is expected to be
dry weather through at least the weekend. Some model solutions are
slower with the front departing Saturday so can/T totally rule out
some showers toward the Ohio border. 12z nwp suite shows the deep
upper low over the intermountain west finally moving out over the
plains early next week. Associated cyclogenesis forecast to occur
with a potential track through the Great Lakes providing increased
chance for wet weather Monday night Onward. Temperatures are
expected to gradually warm from near normal Saturday to a bit above
normal by Tuesday.
Sustained southerly winds of 20 to 25 knots in place across Lake
Huron through tonight. Gust potential contained by increasing
Overlake stability...with gusts holding at 30 knots or less. This
will result in a period of marginal small craft conditions for
elevated waves along the nearshore waters of the northern thumb
region. A moderate pre-frontal southwest flow will remain in place
on Thursday. Expect a long duration rain event late Thursday into
Friday as the front slowly tracks east. Cold frontal passage on
Friday brings a wind shift to north...with gusts between 20 and 25
knots to perhaps as high as 30 knots over Southern Lake Huron with
longer fetch into Friday night.
Lake Huron...Small Craft Advisory until 7 am EST Thursday for lhz421-441-442.
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).