Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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937
FXUS63 KDTX 191534
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1134 AM EDT Wed Jul 19 2017

.UPDATE...

The main governing factors in the weather today will be an
innocuous col that will emerge today on the southeastern periphery
of high pressure over the western Great Lakes as well as the ribbon
of midlevel moisture that will reside within it. Surface observations
are a little baggy, but latest obs have it residing near the I 69
corridor. NWP solutions suggest the col may be good enough to begin
generating agitated cumulus early this afternoon before a theta e
discontinuity (relatively more favorable for convection) will
develop on the backside of the midlevel moisture due to a more
concerted midlevel dry air push. Otherwise, dynamics are severely
lacking with no active potential vorticity advection, convergence,
frontogenesis, or favorable upper level jet dynamics. Merely, looking
at a thermodynamic type event with marginal instability (upper limit
at 1000 J/KG SBCAPE) developing with no subsidence/capping
inversion. The strongest of convective cells will be capable of a
wind gusts to 40 mph and brief heavy downpours.

Narrative to remain isolated to scattered showers and thunderstorms
developing between the I 69 and I 96 corridors by 1 PM EDT...with
convection then contributing to additional hit or miss development
southward through the Detroit Metro area 18-00Z. Slow moving storms
(northwest to southeast at 30 mph), likely not clearing southeastern
Michigan or dieing off until dissipation with loss of daytime
heating. No severe weather anticipated.

&&

.PREV DISCUSSION...
Issued at 703 AM EDT Wed Jul 19 2017

AVIATION...

A weak front will move through the MBS area during the morning and
then move slowly southward into the DTW corridor during mid
afternoon. Low clouds trailing the front over northern Michigan could
hold together long enough for some MVFR ceiling at MBS but will
develop into VFR coverage as daytime heating works on the weak
frontal zone. VFR in a mix of mid and high clouds ahead of the front
will move eastward and be replaced by scattered to broken cu through
the afternoon. The front will reach PTK to DTW around peak heating
and a tempo group for just showers is added to this forecast package.
Thunderstorm potential will be monitored while currently expected to
be isolated and less likely to affect any of the terminals.

The front will stall south of the Ohio border during the evening
with the question then being if it will move back north during the
night and the extent of any associated ceiling or visibility
restriction. At this point, the most likely scenario has the front
lingering to the south during the night while upstream thunderstorm
complexes approach from the Midwest. Thickening high clouds will
precede any storms which will hold off until after sunrise Thursday.
MVFR fog around sunrise is the most likely restriction as the
surface layer will remain warm and humid on either side of the
boundary.

For DTW... The front moving through lower Michigan today will be weak
but still able to produce scattered showers and isolated thunderstorms
during mid to late afternoon. The 18-22Z timing is the best window
for rain to affect the terminal. A thunderstorm is possible within
D21 but likely not at DTW. These trends will be monitored for later
updates.

//DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling at or below 5000 ft during afternoon

* Low for thunderstorms during afternoon

PREV DISCUSSION...
Issued at 344 AM EDT Wed Jul 19 2017

DISCUSSION...

Forecast remains potentially active through Sunday as a pair of
fronts and series of waves target the Great Lakes region. Will
continue to try to refine timing of convection as it crosses the
area. Otherwise, warm and humid airmass will linger through this
active stretch with highs in the 80s (low, mid, or high 80s decided
by convection, convective debris, and position of wavering fronts).

Today will be another warm and muggy one as temps and dewpoints
elevate ahead of a weakening cold front. The front will drop through
SE MI this afternoon supporting mainly isolated showers as
instability appears low and very elevated. A narrow ribbon of higher
theta e along the front will drop through SE MI supporting these
slight chance pops. Best chance comes later in the afternoon as the
front approaches the Detroit area and points south. Instability will
have longer to build being later in the afternoon so could see a
better coverage of showers and thunderstorms down that way.

Attention then turns toward Thursday morning and a possible MCS
tracking SE from WI, through southern Lake MI, and possibly clipping
a portion of SE MI. Forecast will be based more on what`s more
likely to happen verses what models say will happen based on
confusion from convective feedback. Large scale pattern shows a
trough rotating around the large parent low over Canada while a mid
level shortwave, currently over the Rockies, shoots through the
strong zonal westerly through northern MI. Models suggest a strong
dynamic response upstream with 850mb jet upwards of 50 knots
Wednesday night through thursday ahead of this wave. Potential MCS
then develops and wants to dive southeast along the instability
gradient which is more than reasonable, but convective feedback seems
to spin into a stronger surface low which tracks due east across
northern MI. This with the upper level support and strong low level
jet turns into a large area of precip in the models. Doesn`t seem
reasonable for this to play out if the convectively induced low dives
south with the MCS. So will tweak the grids down to more of a
showery scenario, assuming the MCS plays out and forcing is more
large scale with potential outflow from the MCS ejecting north as
well. Model continuity says MCS since its been advertised for days
now.

Unstable air will attempt to be pulled north into lower MI later
Thursday but it the MCS plays out, the main axis of instability
could be too far south to make it here. Will assume the
outflow/boundary laid out from the MCS will spark stronger late day
convection to our south, with some activity across southern MI. With
the boundary now well to our south, appears Thursday night into
Friday will see a lull in activity. This will be short lived as the
next wave to round the ridge, now flattened even more from the train
of waves, will target southern MI, pulling the front back north as a
warm front during the afternoon. Several shortwaves embedded in the
flow combined with the surface convergence, isentropic ascent,
height falls and potential low level jet will all lead to a higher
confidence forecast in thunderstorm occurrence. Location will likely
need amendments are features come into view. This will be the
highest pops of the forecast late Friday through Saturday. Biggest
threat appears to be heavy rainfall as pwats soar to 2+ inches with
the strong return flow into the front. Something to monitor in the
coming days.

Forecast quiets down for the start of the new work as pattern
amplifies bringing an upper level trough overhead with surface ridge
building. This should lead to a cooler and drier stretch.

MARINE...

A weak front moving slowly through the area will be the focus of
marine weather today through the second half of the week. The main
impact from this system will be thunderstorms and locally higher
wind and waves. The wind with the front itself will be weak and will
have little impact on waves as it moves from central Lake Huron
during the morning to western Lake Erie by evening. The front will
stall just south of Lake Erie tonight and then waver between Lake
Erie and the south end of Lake Huron Thursday through Friday while
weak high pressure develops over the northern Great Lakes. The front
will remain the focus of thunderstorms through Friday and possibly
even into Saturday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

UPDATE.......CB
AVIATION.....BT
DISCUSSION...DRK
MARINE.......BT


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