Scientific Forecaster Discussion
fxus63 kdtx 221915
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
315 PM EDT Mon may 22 2017
Not much time to dry out, as rain chances increase over next 48
hours. 70-75 knot jet core at 500 mb/110-115 knots at 300 mb exiting
northeast early this evening, with moisture advection and right
entrance forcing already bringing a scattered/chance of showers
(remote chance of a thunderstorm) as we become modestly unstable
with Li's nearing zero over Tri-Cities region. Narrow, but
potentially sufficient 850-700 mb Theta-east moisture axis, with precipitable water
values around 1 inch move over northern half of the County Warning Area toward 12z
Tuesday, stalling out over the rest of Southeast Michigan during
Tuesday. Modestly unstable daytime instability (mlcapes 250-500
j/kg), weak low level convergence, and leftover moisture axis could
be enough to trigger convection but upper level shortwave ridge axis
in place may be enough of a deterrent. Overall, will be favoring
mostly scattered coverage for both tonight and tomorrow, with pops
hovering around 50 percent north of M-59 and quickly tapering off
south. Mav/met guidance appears to be too aggressive tomorrow
compared to local probabilistic sref weighted guidance. Inherited
temps tonight/tomorrow looked reasonable with low level warm
advection pushing 925 mb temps at or above 15 c by tomorrow
afternoon, supportive of lower 70s.
Large upper level low centered near northern Minnesota will sink
south into the Midwest tomorrow and reaching the southern
Mississippi River valley late tomorrow. With placement of warm
conveyor belt arching back through the Northern Ohio valley. One
issue is the lead low/storm system riding along the Gulf Coast
today/tomorrow, reaching the mid Atlantic coast early Wednesday
morning. How much impact will this system have on the moisture
transport? It appears it will just have a temporary disruption/
impact, as the main low deepens over the Ohio Valley during
Wednesday. Good deformation showers expected to spread over
Southeast Michigan by Wednesday evening, with substantial rainfall
(greater than half an inch), as 850 mb dew pts of 10 c arrive, per
12z Canadian/Euro. Although, 12z Euro may be exhibiting its typical
bias of over-deepening the low by Thursday morning, as NAM is 8 mb
weaker, so not fully on-board with quantitative precipitation forecast in excess of 1 inch just yet
with what looks like modest instability.
An upper-level cut-off low situated over the Ohio Valley will bring
on and off rain chances throughout Thursday as temperatures peak in
the mid to upper-60s. The cut-off low is expected to push northeast
throughout Friday as an upper-level ridge pushes across the Great
Lakes. This will act to scour out some cloud cover and bring
relatively dry conditions through the day, although some embedded
short-waves could provide the low chance for a light pop-up shower.
For the extended weekend, a weak surface low is expected to move
east across in/Ohio Saturday into Sunday as an upper-level ridge
pushes across Michigan, which would hold prolonged rain chances just
south of the Michigan/Ohio border. Slightly better rain chances are
forecasted for Sunday as a trough swings through from the Central
Plains, pushing a weak warm front through southern Michigan.
Additional rain chances possible on Monday, although only the GFS
run is picking up on measurable rain chances as it develops and
pushes a low from Iowa into northern lower Michigan. Overall, the main
message for rain chances -- weak forcing will allows for spotty rain
chances during the extended period, with Friday into early Saturday
bring having the highest odds to stay relatively dry. Higher
confidence regarding forecasted highs -- mid to upper-60s for
Thursday, with a warming trend into the 70s starting Friday and
continuing through Memorial Day.
Diminishing wind during the late afternoon and evening will allow
ongoing small craft advisories to expire on time. The wind will
continue to ease after sunset and remain light through Tuesday. A
weak surface trough will separate northern and Southern Lake Huron
until dissipating Tuesday night in favor of the next low pressure
system. This low will produce an active rain pattern and bring up a
moderate northeast wind over marine areas Wednesday into Thursday.
The long fetch of onshore flow could result in waves exceeding Small
Craft Advisory threshold for Saginaw Bay and Southern Lake Huron.
Low pressure developing over the Ohio Valley Wednesday will stall
and intensify through Wednesday night. A band of rain is expected to
develop on the north side of the system and spread into lower
Michigan mainly during Wednesday night. Rainfall totals averaging
0.75 inch are likely with locally higher amounts in thunderstorms.
Ponding of water on area roads and in other prone areas is likely,
especially considering the widespread activity over the weekend that
left the ground saturated in most areas.
issued at 1233 PM EDT Mon may 22 2017
A pocket of dry air will bring clear sky to scattered cloud
conditions through the remainder of the afternoon. That will leave
the gusty southwest wind as the most significant aviation condition
until early evening. The gust component will diminish with the loss
of daytime heating while light gradient wind continues through the
night. A noticeable increase in mid clouds is expected during the
evening associated with low pressure moving through the border
region of Iowa/Wisconsin/Illinois. Some high based showers are
projected by most model solutions to move across Southeast Michigan mainly
affecting the fnt to mbs area with some VFR rainfall. Upstream
observations also indicate some lightning over Iowa during the late
morning in support of some of the models with greater rainfall
intensity over lower Michigan tonight. The forecast approach for now
will be to carry a tempo group for VFR light rain at mbs and monitor
trends for coverage and intensity in later updates.
//Dtw threshold probabilities...
Lake Huron...Small Craft Advisory until 8 PM EDT this evening for lhz421-422-
Lake St Clair...none.
Michigan waters of Lake Erie...none.