Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
352 PM EDT Thursday Oct 23 2014
Short term... afternoon and tonight
A pleasant fall late afternoon and evening is in store for Southeast
Michigan as high pressure drifts eastward across the region. The
surface high will give way to influence from the next wave of low
pressure sliding through the Midwest and northwest Ontario...but
this system will bring little more than an increase in clouds and
perhaps evidence of virga on radar late tonight. Temperatures will
follow the typical evening slide after sunset but then stabilize
overnight as clouds thicken for mins ranging from the middle 30s to
Afternoon satellite imagery and radar mosaics support model analysis
fields that indicate a well developed upper trough extending from
central Canada through the Midwest. Distinct circulations and
filaments of deformation within the trough are well resolved in the
500 mb vorticity field among the model solutions within an otherwise
pedestrian upper jet circulation downstream of the trough axis.
The primary forcing for the showers over the Midwest depicted in
radar mosaics is a combination of DCVA and middle level Theta-E
advection...the bulk of which remains well west of our area during
the night. Dry air in the low levels is expected to remain
substantial enough to erode any light rain with middle level origins
which adds confidence to the dry forecast but it will do little to
stop the substantial increase in high clouds that will precede the
primary middle level moisture axis. These clouds will be the main
influence on our weather during the night...which along with light
southwest wind...will help buoy min temperatures in the middle 30s to
around 40...which is about 10 degrees warmer compared to last
Shortwave trough currently over the upper MS River Valley/western
Great Lakes will continue to migrate within prevailing westerly flow
over the next 24-36 hours. As noted on WV imagery, the trough is
anchored on the northern end by a closed circulation over southern
Canada along with a second circulation over Wisconsin this evening.
Strongly diffluent flow in advance of a 150+kt jet maximum from the
Pacific northwest will cause the trough to fracture into two separate
waves with Southeast Michigan poised to be positioned safely between
them. This will result in nothing more than meager Theta-E
advection and spill- over of remnant cloud debris for Saturday.
Favored the 12z guidance which offers a slower intrusion of clouds
and a virga type scenario that will struggle to produce anything
that reaches the ground.
Middle-level subsidence will increase Friday night into Saturday as
this wave departs and a much stronger shear maximum on the northern
periphery of the aforementioned jet begins to close off upstream.
The resultant ramp up of 925-850mb Theta-E advection will help
reinforce the middle-level subsidence inversion. 12z guidance indicate
a healthy amount of moisture trapped beneath the 900mb level for the
balance of the daytime hours. This seems adequate to warrant a
considerably more pessimistic cloud cover forecast for Saturday. The
healthy southwest gradient should be sufficient, however, to bump
850 mb temperatures up to 10c and surface temperatures up to 60-65 even in a poorly
mixed environment. The strengthening wind field will support gusts
of 20 miles per hour. Showers in advance of the cold front are a poor bet at
the moment with the best forcing prognosticated to remain to our north and
east. Nonetheless, the relatively close proximity of rather potent
dynamics warrants monitoring, especially given that the bulk of the
associated remains offshore at this time. For now, the forecast remains dry
through the first half of the weekend. Cold and dry advection will
get underway on Saturday evening sending dewpoints down about 20
degrees from their daytime peak values and helping overnight lows
settle near 40 degrees...average for late October.
Expecting a pleasant fall day Sunday with diminishing
winds...though cooler compared to Saturday as the low level thermal
field is slow to recover from Saturday/S frontal passage. Warm
advection ensues in earnest Sunday night into Monday as upper ridge
builds in response to upstream near-full latitude trough over
intermountain west. Rather mild Monday and Tuesday with 850 mb temperatures
of 12-14c advertised. Global models and gefs members have come into
a little better agreement with this trough...showing it assuming a
negative tilt northwest of the local area and dragging a cold front
through Southeast Michigan. Still targeting Tuesday/Tuesday night for highest
likelihood of precipitation...with a cool-down for Wednesday Post front.
A tranquil wind regime will give way to developing moderate
southwest flow on Saturday in advance of an approaching system. The
associated cold front will traverse area waters Saturday afternoon
into Sunday evening. Winds will quickly turn northwest and become
fresh to strong...especially over the open waters of Central Lake
Huron...with gusts to near-gales likely over the northern half of
Lake Huron on Saturday night. A gusty moderate to fresh northwest
breeze will then persist across the waters through Sunday.
Aviation...issued 1240 PM EDT Thursday Oct 23 2014
Some lingering stratus in the mbs area will continue to lift and mix
out through the afternoon. This will leave VFR for the rest of today
through tonight in the form of clear sky followed by thickening high
clouds tonight...and then a slow progression toward MVFR between
sunrise and noon Friday. High pressure over Southeast Michigan today will
slide eastward tonight in favor of the next wave of low pressure
resulting in a light southwest wind along with the increase in
clouds. There could also be a sprinkle late tonight through Friday
morning...or at least some evidence of virga on radar...until
increased low level moisture arrives toward noon Friday.
//Dtw threshold threats...
* medium confidence on ceiling below 5000 feet Friday.
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).