Scientific Forecaster Discussion
fxus63 kdtx 172324
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
724 PM EDT Sat Aug 17 2019
Attention over the first few hours of the forecast is on a weak
system current passing over Southern Lake Michigan. Much of the hires
resolution guidance has been over doing the developing convection
tied to this feature thus far with little to no development showing
up on radar at this point. Satellite shows some enhancement to the
cloud field so there is still potential for things to come together
by the time it reaches Southeast Michigan. Will lean more toward showers and
vicinity ts for the time being to give it some credit for the
potential going forward. Area should remain dry early Sunday morning
into the afternoon before a stronger system looks to produce
widespread thunderstorm activity across lower Michigan.
For dtw...confidence is decreasing for thunderstorm activity for the
first few hours of the forecast. Activity between dtw and Chicago is
much weaker than anticipated suggested by earlier guidance. Still a
shot at some precipitation between 00-04z but thunder chances seem
lower so will hold it out at this point to see if anything can get
going. Higher confidence in storms Sunday afternoon and evening with
the next system that sweeps through.
Dtw threshold probabilities...
* low for ceilings aob 5k ft late tonight into Sunday morning.
* Low for thunderstorms between 00z to 06z. Moderate Sunday after
issued at 352 PM EDT Sat Aug 17 2019
A subsidence bubble bringing capping along with some dry air has
effectively prevented much precipitation from developing across
southeast lower Michigan through the early afternoon. A few high
clouds with scattered cu is allowing for a good amount of sunshine,
which has resulted in temperatures increasing into the low to mid
80s. Main adjustment for today has been to increase pops and quantitative precipitation forecast
slightly and expand coverage northward into the M-59 corridor for
this evening. Scattered convection early this afternoon is
developing in response to a shortwaves moving along the frontal
boundary to the south. Later day convection will come from a mesoscale convective vortex
taking shape across Southern Lake Michigan and eventually moving
across lower Michigan. These will bring convection along and south of
the I-94 corridor over the next few hours with activity developing/moving
across the M-59 area closer to the 6-8 PM time frame. Greater severe
potential resides south of the Michigan border. However, MLCAPE
values of around 500-1000 j/kg with 0-6 bulk shear values of 20-30
kts and steep low level lapse rates will bring the possibility of
strong storms with gusty winds and brief heavy rainfall across
portions of the southern County Warning Area. Loss of daytime heating and weaker mid
level lapse will decrease thunderstorm potential into tonight as
activity moves out of the area by around 2 am.
Longer wave trough axis will move across the northern plains tonight
as a deep low pressure system rotates across central Canada. A
nocturnal mesoscale convective system is forecast to move across the upper Midwest into the
western Great Lakes region tonight supported by a strengthening low
level jet at the base of this trough. Remnants of this system are
expected to generate a mcv(s)/multiple shortwave impulses that are
forecast to move across lower Michigan tomorrow. Ahead of this
system will be increased southwesterly flow advecting greater
moisture into the area. A conditional severe risk will be present
for tomorrow as a moderately unstable airmass sets up with 0 to 6 km
shear of at least 30-40 knots supporting this risk. Still some
uncertainly with how early day convection and cloud cover will limit
instability later in the day. If early day convection dissipates
across Lake Michigan or western part of the state, this will lead to
a greater potential for severe weather across Southeast Michigan
during the late afternoon early evening time frame. Strong low level
flow with the negatively tilting trough axis will make damaging
winds the main threat with large hail and locally heavy rainfall
also possible. A well mixed boundary layer tapping into strong
southwest flow aloft will also bring some windy conditions with
gusts of 20-30 mph kicking in by early afternoon. Convection will
push east of Michigan by around midnight tomorrow night as the
trough axis pivots into the eastern Great Lakes.
High pressure slides into the region on Monday with a weakening
surface front helping push the instability axis across far lower
Michigan. This will limit precipitation potential to the I-94
corridor and south. The lack of cold air advection will keep high
temperatures for Monday in the 80s for much of the area. The
exception will be in The Thumb where temperatures along The Thumb
shoreline will be in the mid/upper 70s.
Pseudo zonal flow over the northern conus, with 588 dam heights over
Southern Lower Michigan on Tuesday, leading to another very warm
day (upper 80s). The upper level flow will then buckle, with a
seasonable strong upper level trough/cold pool taking residence over
the Great Lakes region by Thursday morning, sending 850 mb temps
into the single numbers.
Sprawling high pressure (1022+ mb) will build in for Thursday-
Friday, and likely last through much of the weekend, supporting dry
and pleasant temperatures for August, but a bit
cool/below normal on the mins (40s not out of the question for
normally colder locations Friday morning).
Toughest part of the extended forecast remains in the Tuesday
evening-Wednesday morning time frame with the frontal passage, as
12z models (euro/Canadian/icon) have all trended drier. Two reasons,
one there looks to be a thunderstorm complex developing over Iowa on
Tuesday, disrupting moisture transport into our area. Second, warm
mid levels ahead of the front (10 c at 700 mb) could be sufficient
to provide a cap, and a dry/limited convection frontal passage is on
The Table. Thus, no reason to deviate from the blended low chance
pops in the 18z Tuesday-18z Wednesday time frame at this time.
Weak southwest flow will continue tonight. There is a chance of
thunderstorms with a weak system for the southern lakes region.
South-southwest winds increase on Sunday afternoon as a low pressure
system lifts into the western and northern Great Lakes. This system
will bring a good chance for showers and thunderstorms Sunday
afternoon into Sunday evening, with locally higher wind and waves to
be expected in any thunderstorm activity. The cold front will push
through lower Michigan before weakening on top of the Michigan/OH/in
border region. High pressure fills back in and leading to lighter
wind and waves on Monday and Tuesday. However, that stalled frontal
boundary may lead to additional shower/thunderstorm activity over
Lake Erie and Lake St. Clair.
Scattered thunderstorms this evening and during the early overnight
hours will be developing on the next moisture surge. Precipitable
water in excess of 1.75 will reach into Metro Detroit by midnight.
While storm motion will be east-northeast around 30 mph, new
convection will develop on the upwind side of ongoing convection
which could lead to some short-fused training. Isolated locations
across Metro Detroit south to the Ohio border could get in excess of
an inch of rain with the thunderstorm activity tonight.
Scattered showers and thunderstorms will likely again on Sunday
afternoon and evening as a low pressure system tracks through the
Great Lakes. The air mass over the region will be characterized by
dew points near 70 degrees and precipitable water values of 1.50 to
1.75 inches. This will likely result in heavy downpours within any
convective activity. Activity is likely to have forward propagation
with good cold pools developing leading to storm movement at least
40 mph. Basin-average rainfall totals of 0.25 to 0.50 inches look to
be the most likely. Isolated higher totals will be limited to any
location that see repeated thunderstorms.
Lake St Clair...none.
Michigan waters of Lake Erie...none.