Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
349 am EDT Sat Oct 25 2014
Short term... today and tonight
With the mostly clear skies which developed by midnight...the good
radiators across Southeast Michigan have fallen to around 40
degrees...with mins in the low to middle 40s across most of the
County Warning Area. There is some fog around...locally dense in spots...but
conditations should actually improve slightly toward sunrise as
clouds spread in from the west with the approaching cold front.
Based on the 295k isentropic ascent and upstream light radar
returns...could see a few sprinkles this morning as the front
crosses the area. However...the pronounced dry slot punching in this
afternoon will quickly scour out any remnant low level
moisture...with breezy westerly winds developing as the boundary
layer depth easily exceeds 925 mb. Based on forecasted 925 mb temperatures
in the lower teens this afternoon...looking as maxes in the
60s...with the Standard northwest to southeast gradient under the
west-northwest flow. Speaking of gradients...there looks to be sharp
850 mb relative humidity gradient with the cold northwest flow tonight...but
outside of the northern thumb region...there looks to be little
support for clouds...per 12z NAM. However..with westerly winds
persisting through the night...close to 10 knots...should see bit
more uniform mins...with continued cold advection through the night
allowing mins to dip to around 40 degrees.
Long term...Sunday through Friday
Ridging will bring dry weather and abundant sunshine to Southeast
Michigan on Sunday. However cold air will be in place behind the
departing cold front as 850 mb temperatures fall to 3 to 5 celsius.
Despite sunny skies the cooler airmass will mean highs will
generally be limited to the middle to upper 50s Sunday afternoon.
The ridge will track east Sunday night...allowing for warm air
advection to commence in earnest across the central Great Lakes on
Monday as 850 mb temperatures rise to 12 to 14 celsius. A warm front
will lift north through the southern Great Lakes on Monday...acting
as a trigger for showers and a few thunderstorms...mainly across the
north where the front will settle during the afternoon hours. A
shortwave tracking through the southern Great Lakes will assist in
precipitation development on Monday...with warm moist air and
increased instability bringing the chance for thunderstorms embedded
within the shower activity. Despite the passage of the warm
front...highs will vary greatly on Monday...with highs around 60
across The Thumb given a southeasterly surface wind off Lake
Huron...with highs around 70 near the Ohio border away from the main
precipitation and deeper into the warm sector.
The parent low associated with the aforementioned warm front will
develop over the western Great Lakes Monday night...then track east
through the northern Great Lakes Tuesday...dragging a cold front
through the central Great Lakes on Tuesday. Models have come in
better agreement this run...mainly the European model (ecmwf) which has come in
faster this run...more in line with the GFS in clearing the front
through by Tuesday. Although it is not lock step with the faster GFS
solution...it is in the same Ball Park to allow for likely probability of precipitation on
Tuesday given the higher degree of confidence in frontal passage sometime on
Tuesday rather than a possible frontal passage solution that is a whole 12
hours apart per the previous runs. Continued instability will keep
thunderstorms in the forecast during this time period until the cold
front clears the area.
With the better model agreement...it appears rain chances will be
off to the west by Tuesday night given the subsidence behind the
departing front. Therefore will dry out the forecast on Tuesday
night...with ridging expected to keep the weather dry into Wednesday
as a shot of cool dry air sends afternoon highs back down into the
lower to middle 50s on Wednesday.
A weak low pressure system is then on track to affect the central
Great Lakes during the later part of the work week. Given the weak
nature of this low pressure system it is not surprising that the
models are having a hard time resolving the timing and impact of
this low pressure system. There are hints that this system could
bring the first taste of winter to Southeast Michigan late in the
week...with some snow flakes possible across the area. However the
aforementioned issues with the exact timing and strength of this
system does not lend itself to a great degree of confidence this far
out...but it does bear watching with the upcoming Halloween Holiday
at the very end of the extended forecast.
Strong upper level wave of low pressure will dive into the Great
Lakes region today. The dynamic system will lead to a tight pressure
gradient over the northern Great Lakes with cold air possibly
building into the northern 1/2rd of Lake Huron. It is behind this
developing frontal boundary that conditions will become quite
unstable. These unstable conditions will lead to gale force
northwesterly winds across the northern 1/2 of Lake Huron tonight.
Further south...although winds are not expected to reach gale
force...waves are expected to build up over Southern Lake Huron
tonight...then slowly subside on Sunday as high pressure builds in
from the west and the pressure gradient slowly loosens. Elevated
wave heights will necessitate the issuance of a Small Craft Advisory
for outer Saginaw Bay as well as the Lake Huron nearshore waters
from Port Austin to Port Sanilac. The extreme southern zone from
Port Sanilac to Port Huron will need to be monitored closely as it
is forecast to reach Small Craft Advisory criteria for a brief time
on Sunday. However given the brevity...uncertainty...and the fact
that conditions will not be met until Sunday will hold off on
headlines with this issuance.
Aviation...issued 1208 am EDT Sat Oct 25 2014
Skies have cleared out across the taf sites...which will allow for
some fog to develop overnight...as southwest winds are very light at
the moment. Before the fog is able to get dense or dense for
any extended period...looking at a return of MVFR/VFR clouds and an
increase of southwest winds ahead of a cold front between 9-12z. The
lower clouds will then likely prevail through the morning hours.
However...a good deal of drying will occur during the afternoon
hours...as westerly winds also ramp up...with gusts likely reaching
and exceeding 20 knots. Once skies clear out by late in the
day...should see mostly clear skies persist tonight. However...cold
northwest flow will attempt to send some lake induced clouds toward
the area...mainly up toward mbs.
//Dtw threshold threats...
* high confidence in ceilings below 5000 feet 10-18z.
* Low confidence wind gusts will exceed crosswind thresholds
Small Craft Advisory...outer Saginaw Bay and the nearshore waters
from Port Austin to Harbor Beach...from 8 PM Saturday to 4 PM
Small Craft Advisory...nearshore waters from Harbor Beach to Port
Sanilac...from midnight Sunday to 4 PM Sunday.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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