Scientific Forecaster Discussion
fxus63 kdtx 221039
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
639 am EDT sun Sep 22 2019
Southwest winds sustained around 15 knots with gusts to 25 knots are
expected this afternoon as mixing depths increase.
Mbs will experience the longest duration of showers and MVFR/IFR
conditions today and tonight, as cold front will be slow to track
through Southeast Michigan late tonight. The southern taf sites look
to be predominately VFR through the day, but also dropping into
MVFR/IFR tonight as widespread showers and embedded thunderstorms
tracks through Southeast Michigan. Low instability is expected to
lead to a low coverage of thunderstorms, and not enough confidence
to include in the tafs at this time.
For dtw... VFR ceiling above 5000 ft expected to hold through the
day. Lower cigs and showers track through tonight, with westerly
wind shift occuring Monday morning.
Dtw threshold probabilities...
* low for ceiling at or below 5000 ft this evening, high tonight
into Monday morning.
* Low for thunderstorms tonight.
issued at 329 am EDT sun Sep 22 2019
A plume of tropical moisture (characterized by precip water values
ranging from 1.8 to 2 inches) extends from Missouri into lower Michigan.
There continues to be good moisture transport into lower mi
(particularly central lower mi) as the core of highest moisture is
still back over nrn Illinois. The persistence of weak moist
isentropic ascent and several short wave impulses embedded within
the mid level southwest flow will support showers across portions of
the area this morning. The greatest coverage of which will be across
the Saginaw Valley, with showers tapering to scattered or isolated
across Metro Detroit and points south where forcing will be much
Amplification of a longer wavelength trough approaching Minnesota
this afternoon will back the low-mid level flow a little more toward
the south across se mi. This will nudge the deep layer moist axis a
little farther north and bring a temporary slug of drier (and more
stable) mid level air into Metro Detroit/Ann Arbor and possibly into
the Port Huron areas this afternoon/evening. This will diminish
precip chances if not end them entirely in this region. Diurnal
mixing, especially across the south, combined with a decent south-
southwest gradient (925mb winds 25-30 knots) will support warm and
windy conditions this afternoon/evening over the forecast area.
Model soundings suggest temps will again rise into the 80s (with the
exception of the Tri Cities where clouds and showers are expected to
be more persistent), while wind gusts top 30 miles per hour.
The longer wavelength trough is forecast to evolve into a closed mid
level circulation tonight as it drifts across Wisconsin into Upper
Michigan. This will result in not only decent mid level height falls
overspreading lower mi, but will also support favorable upper jet
dynamics and decent upper level divergent flow. The result will be a
tightening of the low to mid tropospheric front stretching from SW
lower mi into NE lower mi. A 50 to 60 knot low level jet forecast to
nose into this front will support an axis of showers and some
thunderstorms. With the tropical moisture still over a good chunk of
lower mi, high rainfall rates will result. The way the forcing is
expected to set up would suggest the Tri Cities and points north and
west would stand the better chances for prolonged heavy rain.
Several cams solutions do however suggest convection actually
building farther south into the better elevated instability pool,
enough to still suggest portions of the I-69 corridor or thumb
region may see some locally high rainfall totals. A surface wave
forecast to develop along the associated sfc front and lift into
Lake Huron overnight will drive a cold front across the southern
portions of the area. This front combined with the mid level height
falls and weak elevated instability will support a good chance of
showers/thunderstorms (likely more brief in duration) across the
southern portions of the forecast area.
Cooler and drier air will advect into se mi late tonight into
Monday, supporting Monday high temps a little closer to seasonal
norms (around 70). The upper low will traverse the northern Great
Lakes on Monday. Low level cyclonic flow and diurnal processes will
support a chance of showers Monday. The chances will be greatest
across the northern half of the forecast area where remnant moisture
will be better and convective cloud depths will be a little greater.
Mid level ridging and sfc high pressure will support dry conditions
Tuesday. A broad long wave trough forecast to advance across the
upper Midwest and Great Lakes region Wed/Wed night will drive
another cold front across the area. Moisture content will be much
less but enough to support a chance of showers. After a brief cool
down in the wake of this front, medium range guidance is suggesting a
warming trend toward the end of the forecast period.
Southwest winds today ahead of a cold front will be mostly in the 15
to 25 knots range, as warm and humid airmass will lead to stable
surface profiles, limiting mixing. None-the-less, gusts to 25 knots
can be expected across most of the nearshore waters, and small craft
advisories are in effect through this evening. Showers and
thunderstorms will be around through tonight. Moderate west to
northwest winds on Monday behind the cold front, with winds becoming
light on Tuesday as a ridge axis arrives over the central Great
Lakes. Southwest winds of 15 to 25 knots then look to return on
Wednesday, ahead of another cold front.
Showers and possible thunderstorms during the day will be mostly
confined to north of I-69. However, widespread showers and embedded
thunderstorms then expected this evening and tonight as a cold front
tracks through Southeast Michigan. Tropical moisture in place will
present a heavy rain threat with any thunderstorms. Regardless,
total rainfall amounts of 1-2 inches are expected north of a line
from Lansing to Sandusky, with less than 1 inch south. Localized
flooding is possible in the Tri-Cities region, where the heaviest
rain is expected.
Lake Huron...Small Craft Advisory until 8 PM EDT this evening for lhz421-422-441.
Lake St Clair...Small Craft Advisory until midnight EDT tonight for lcz460.
Michigan waters of Lake Erie...Small Craft Advisory until midnight EDT tonight for lez444.