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fxus63 kdtx 191952 
afddtx

Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
352 PM EDT Wed Sep 19 2018

Discussion...

Temperatures ranging from the upper 60s to low 70s over the area
this aftn. Cloud fraction varies greatly by location as the marine
layer has largely mixed out to a partial cu deck beneath thinner
high cloud over the southern half of the area and held firm across
the north with greater proximity to the lake and more insulation
from insolation by increasingly opaque cirrus. Recent satellite
trends don't show much reason for optimism regarding potential for
clearing of stratus roughly north of m43, particularly as onshore
flow becomes more organized overnight. Potential for temps to fall
well into the 50s in The Thumb may therefore be mitigated. Opted to
hold lows in the upper 50s/low 60s across the board.

Lee cyclogenesis will culminate in a cyclone ejecting into the
eastern Great Plains by early Thursday. Nocturnal forcing and
increased isentropic ascent within a strengthening right entrance
region will contribute to showers and thunderstorms along and north
of the warm front well upstream. Locally, a plume of weak elevated
conditional instability within which convection is ongoing invof the
Quad Cities will stream into the area after midnight. Meager forcing
suggests minimal pops are in order, though isolated shower coverage
is not out of the question, particularly north of I-69, by daybreak.
Wing of isentropic ascent supporting disorganized showers and
possibly some remnant convective activity will be ushered north and
east of the area during the morning, perhaps hanging on in The Thumb
into early afternoon. Attention then turns to lower-end potential
for convective initiation along the building instability gradient as
the warm sector overtakes Southeast Michigan coincident with
deepening surface low pressure translating toward southern
Minnesota. Strong signal exists among the convection-allowing
guidance that the gradient will feature some degree of activity, but
waning shear with time, a poor instability profile to begin with,
and a lack of organized mesoscale forcing will tend to limit
potential to nothing more than a transient modest to strong
thunderstorm threat. Any marginal severe threat will likely remain
tied to the corridor of ehi Max that will trail the warm front as it
lifts North.

County warning area remains in the warm sector until cold fropa Friday afternoon.
Poor moisture quality and a relatively shallow conditionally
unstable layer will again limit convective potential. Nonetheless, a
cross-section from approx Duluth to Detroit reveals a a healthy,
deep tropospheric jet-front system whose defining characteristic is
a bulging 850-700mb nose that will contribute to a rapid transition
to steep boundary layer lapse rates and convective instability
within the frontal zone itself. Ambient wind field and Post-frontal
conditions are not particularly impressive, but near-surface
dynamics and strong cold advection driving 292-300k isentropic
descent may be sufficient for a quick pop up to around 35 kts. Early
day highs in the upper 70s or low 80s will slowly fall through the
afternoon as 850mb temps tumble from the upper teens 12z Friday to
the single digits by 00z.

Final noteworthy modification to the week's forecast was to
drastically increase cloud cover for Saturday. As has been the case
with the last 2 cold fronts (including today), is clearly indicated
by nam12 forecast soundings, and is essentially autumn climatology
anyway, the clouds (or lack thereof) depicted by the blended grids
make little sense. The abundance sunshine has therefore been removed
for the first half of the weekend. Overcast stratus can be expected
for Saturday with the only question being if/when it Burns off and
subsequent impact on temps.

An expansive high pressure system will hold across the Great Lakes
region Saturday evening into Sunday, providing pleasant and dry
weather along with slightly below normal temperatures as daytime
highs hold in the 60s. A deepening trough located across The Rockies
will break the zonal flow pattern across the Great Lakes and will
allow a warm front to push into Michigan, noted by the increase in
h850 temperatures from 7c to 12c from Sunday morning into Monday
morning. The increase in warmer air will help push daytime high
temperatures back into the 70s for a high and will also provide
enough forcing for rain and thunderstorm chances as showalter
numbers turn neutral to slightly negative. Additionally, will have
the potential to see gusty conditions late Monday into Tuesday as
surface pressure gradient tightens in response to a strong high
pressure centered east of Maine and a low pushing across northern
Ontario. The European model (ecmwf) run also shows strong winds in the low-levels
with h850 SW flow ranging between 30 - 40 knots. However, GFS and
Gem runs display a weaker wind field aloft and a relaxed pressure
gradient relative to the ECMWF, so holding off on highlighting
wind potential.

Rain and thunderstorm chances will persist into Wednesday as a cold
front pushes across Michigan, however, confidence is low at this
time regarding timing and strength of front. Additional convergence
will be needed across model runs to pinpoint potential weather
threats with the passage of the front.

&&

Marine...

Light easterly marine wind continues tonight from high pressure over
northern Ontario and while a front remains stalled over the Ohio
Valley. This front begins a northward move tonight as low pressure
organizes over the Midwest and both features will support an
increase in showers and thunderstorms overnight through Thursday.
Worsening marine conditions also include increasing wind and waves
as the low strengthens over the northern Great Lakes. The system
will pull much warmer air across lower Michigan and Lake Huron that
increases stability and limit wind gusts to some extent. The
exception could be greater gustiness through Saginaw Bay if boundary
layer mixing becomes established over land areas during Thursday
afternoon. Gusts to 30 knots will even be possible over the cooler
open waters of Lake Huron through Thursday night.

Once the low crosses the northern Great Lakes and continues into
Ontario early Friday morning, an associated cold front will push
southeast through the central Great Lakes. The front will bring
veering winds to the northwest with considerably colder air
reversing the thermal profile to an unstable configuration. The
strength of the low likely supports a wind field capable of gusts
exceeding gale force over the open waters of Lake Huron and a gale
watch is now in effect Friday through Friday night. High pressure
then builds in quickly behind this system by Saturday and brings
improved marine conditions during the weekend.

&&

Hydrology...

Showers and thunderstorms will increase coverage and intensity,
mainly after sunrise Thursday, and continue through mid afternoon as
a warm front lifts northward through the region in response to low
pressure developing over the Midwest. The warmer and more humid air
moving into the area with this front will supply enough moisture for
rainfall totals averaging around a half inch for the Thursday
period. Locally higher totals are more likely across the Tri Cities
and northern thumb where isolated rainfall amounts around 1 inch are
possible. While widely scattered showers or thunderstorms may occur
into Thursday afternoon and evening, the most significant activity
should shift north of the region with the warm front. The potential
for flooding is minimal with this activity, limited to ponding of
water on roads and minor flooding of poor drainage areas.

&&

Previous discussion...
issued at 105 PM EDT Wed Sep 19 2018

Aviation...

Widespread morning MVFR stratus eroded in spots and then partially
filled back in during late morning. These clouds will continue to
dissipate through early afternoon and leave VFR under a mix of mid
and high clouds for the rest of the day and much of tonight.
Thunderstorms dissipating over the Midwest during the afternoon
supply the mid and high cloud debris before a new round of storms
develops later tonight. This activity is associated with the front
stalled south of the Ohio border that begins to move northward as a
warm front tonight. The elevated nature of new convection is
expected to direct activity north of the Southeast Michigan terminals with
the exception of mbs and possibly as far south as fnt, and then with
timing mainly after sunrise Thursday morning.

Until then, and for points farther south, the northward pace of the
warm front dictates aviation conditions late tonight through
Thursday morning. Surface dewpoint projections suggest the surface
front takes on a northwest to southeast configuration across lower Michigan and
spends much of the morning moving south to north through the region.
Given the slow pace, the most likely scenario is some borderline
IFR/MVFR fog around sunrise lifting into a ceiling during Thursday
morning until the front moves north of each terminal location.

For dtw... MVFR ceiling continues to erode during early afternoon
giving way to VFR under thickening mid and high clouds for mid
afternoon through tonight. Surface wind maintains a light easterly
component until the front stalled to the south moves back north of
dtw Thursday morning.

Dtw threshold probabilities...

* moderate for ceiling 5000 ft or less redeveloping Thursday
morning.



&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...gale watch from Friday afternoon through late Friday night for
lhz441-462-463.

Gale watch from Friday morning through late Friday night for
lhz361>363.

Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&

$$

Discussion...jvc
marine.......bt
hydrology....bt
aviation.....Bt

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