Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
FXUS63 KDTX 180754
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
354 AM EDT Sat Mar 18 2017
Amplifying upper level trough has dug southeastward into the central
Great Lakes this morning. Overall, the upper level trough is pseudo
barotropic with just some minor baroclinicity. A narrow, surface
warm sector ribbon is attached to the emerging vorticity center and
has led to widespread fog this morning across the region. The
vorticity center, now over far southwestern lower Michigan, has been
forcing some very weak midlevel instability showers due to direct
differential cyclonic vorticity advection. The showers are set to
push into and plague the forecast area (particularly the southern
cwa) throughout much of the morning.
As for the fog, surface observations have been solidly at less than
1 mile for much if not all of the cwa. Been trying to get a handle
on what sort of disruption the shower activity may have on it. With
that said, there has also been some transient behavior of the fog
with just enough vsbys bouncing around to lend some uncertainty.
Despite the stagnant conditions brought on by a sluggish inverted
trough, the vorticity center now pushing through suggests a time
tendency to both uvv forcing and some differential moist advection
(some observing sites in northwestern Indiana have seen a marked
improvement in visibilities this morning). With this in mind and a
lack of any M1/4SM observations will continue to monitor trends and
delay a decision on a dense fog advisory.
Today...The vorticity center is progged to track directly over the
cwa by 12Z, before spinning out and drifting southeastward through
the early afternoon. Bulk of the hi-res guidance highlights this
direct cva in developing additional showers mainly south of the I 96
corridor prior to 18Z. In-house probability metrics continue to
hammer a very high likelihood (at/above 90 percent) to measure 0.01
before noon. Made the decision to go with a higher pop/lower qpf
forecast or narrative today.
Lower level wind directions at 925mb and 850mb will veer uniformly
to the northwest by 18Z allowing for cold air advection. The cooling
temperatures will then set the stages for steepening lapse rates in
the lowest 6 ft agl this afternoon. Add a diurnal component and much
of the hi-res guidance is suggesting a blossoming of shower activity
over all of the cwa 20-04Z. Will honor the probability metrics and
prob trends and go with a chance PoP for this timeframe, but will
most likely verify as a numerous coverage. Now for precipitation
type. Stout midlevel dry air will be down to 6000 ft agl with gives
virtually no shot at ice nucleation. However, freezing levels down
to a very low 1000 J/kg and qualitatively a good looking little CAPE
sounding in the boundary layer points somewhat emphatically towards
snow pellets or graupel showers this afternoon.
Sunday...Incredible amount of 1000-500mb geopotential height rises
will be ongoing late tonight and early Sunday with a high amplitude
mid to upper level ridge pushing across the state. Surface ridge
axis will grip the state as well with a very strong signal for
stable lake aggregate processes. Little to now wind Sunday. Dry.
Sunday night and Monday...Models are advertising a progressive
northern stream pattern with a strong shortwave racing across
portions of northern Canada. Models suggest a cold front, but more
accurately described as a col region, to descend into the central
Great Lakes the latter half of Sunday night. There will be little to
connection to the main potential vorticity reservoir bottled up
north along with unfavorable right exit region dynamics off of a
straight jet. This really shows itself in the forecast model
soundings with a relatively stable frontal zone between 6 and 15 kft
agl in addition to substantial questions in moisture quality. Very
mild midlevel air points to rain as a ptype. Chance to low end
likely PoPs appear reasonable for this setup.
Low pressure dropping through the area will bring an increase to
east winds for several hours early today over northern Lake Huron,
with gusts reaching about 20 knots. Otherwise, winds will be fairly
light today and tonight. A ridge of high pressure will bring even
quieter conditions on Sunday. Modest south winds will then develop
Sunday night into Monday as low pressure tracks into Hudson Bay.
This system will pull a cold front through the Central Great Lakes
Monday afternoon, flipping winds to the northwest.
Issued at 1150 PM EDT Fri Mar 17 2017
Ample low level moisture streaming into an occluded front over SW
Lower Mi has triggered widespread LIFR and VLIFR conditions in fog
and stratus across srn Mi. These conditions will prevail through 10
or 11Z. An upper low will move overhead by daybreak. The resultant
cooling aloft will lift cloud bases during the morning. Periodic
drizzle during the overnight will then give way to increasing
chances for rain showers toward daybreak as convective cloud depths
For DTW...Upstream observations are suggesting a period of 200 ft
ceilings and sub 1 mile visibility is possible during the early
morning hours. The approach of the upper low will slowly bring
deeper moisture and better chance for rain showers into metro around
or shortly after daybreak.
//DTW THRESHOLD PROBABILITIES...
* High in ceilings below 5000 ft tonight and Saturday.
* Moderate in precip type being all liquid the remainder of tonight
through the day Saturday.
* Low in ceilings and/or visibilities dropping below 200 ft/ one
quarter mile overnight through early Sat morning.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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