Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
659 PM EDT Sat may 30 2015
Precipitation trends will shift toward concentrated areas of heavy
downpours rather than thunderstorms and mainly from ptk southward
during the night. Conditions will otherwise vary widely from
borderline MVFR/VFR at mbs to LIFR at times at dtw. The other
concern will be increasing northeast wind Post cold front that will
gust to 25 knots through Sunday.
For dtw... IFR restriction in persistent heavy showers over IFR
ceiling and increasing NE wind tonight. IFR will alternate between
low visibility in showers and low ceiling as waves of rain sweep
through the terminal area tonight through Sunday. Excessive rainfall
amounts and localized flooding remain possible.
//Dtw threshold probabilities...
* high for ceiling 5000 feet or less.
* Low potential for thunderstorms... but high for heavy rainfall.
Previous discussion...issued 349 PM EDT Sat may 30 2015
Flood Watch continues.
High moisture content in place over Southeast Michigan today...as
evidence by 13 c dew pt at 850 mb (per 12z DTX sounding)...surface
dew points in middle/upper 60s...and precipitable water values above 1.6 inches...per Storm Prediction Center
mesoscale analysis. Surface cold front has pushed toward the Ohio
border this afternoon...but has been losing it southward momentum.
There was ripple/thunderstorm complex which rode the front early
this afternoon along the M-59 corridor. Dte outage map suggests a
few power outages due to wind damage. Rainfall totals from this
complex ranged from half an inch to one and half inches in a very
short period. High wet bulb zero height conducive to torrential
rainfall...with still enough instability around across the south
half of the County Warning Area to support torrential producing thunderstorms into
early evening hours. Attention tonight turns to the well defined
circulation over extreme southeast Missouri lifting northeast into
the central Ohio Valley tonight. This feature is expected to support
blossoming rain shield tonight...despite the loss of daytime
heating...as low/middle level fgen ramps up. Increasing pressure
gradient as high pressure over south central Canada begins to build
in while surface low tracks into Ohio Valley...supporting gusty
northeast winds of 25 to 30 miles per hour...likely higher downwind of Saginaw
Bay. Enough low level drying is expected north of I-69 to support
showers tapering off.
It was decided to issue a Flood Watch earlier today to cover
localized flashy type rainfall late this afternoon...but also to
capture the long duration rainfall tonight into Sunday...as storm
totals should reside in the 1 to 3 range across a wide area....with
potential for localized higher amounts. Fortunately...rainfall has
been lagging over the past week or two leading into this event...and
6 hour flash flood guidance is running above 2 inches. Thus...this
looks to be more of urban type flooding if there are any issues.
Guidance temperatures suggests temperatures falling into the middle 40s to
lower 50s south as the stiff northeast winds off Lake Huron and low
level cold advection takes hold.
Long term...Sunday through Friday
Main height falls associated with a dynamic wave lifting through
southern Ontario will largely hold to the north...leaving the
dampening middle level trough extension to work into Southeast Michigan over
the next 24 hours. This evolution will hold the trailing middle level
frontal slope firmly across a good portion of Southeast Michigan on
Sunday. Strong middle level dynamic response already underway by early
Sunday...ellicited by interaction between this trough and a
stronger inbound southern stream wave now lifting out of Missouri.
Prolonged strong middle level frontal forcing within favorable right
entrance region upper jet support and increasing deformation
continues to lend support toward seeing an extended period of
widespread rainfall impacting a sizable portion of Southeast
Michigan /Ohio border to I-69 corridor/ right through the afternoon.
The existing deep layer moisture profile /pw at 1... 850 mb
dewpoints/ and overall magnitude and longevity of this forcing field
will lead to some higher end rainfall amounts on the order of an
inch through the daylight hours. Column carries a higher degree of
stability relative to today/tonight...suggesting a limited
convective element to this precipitation. Flood Watch now in place
through the afternoon on Sunday along and south of I-69 to highlight
the possible additive impact of this rainfall on top of the multiple
rounds expected through tonight.
The extensive moisture and a strengthening northeast low level
gradient will keep any diurnal temperature recovery to a extreme
minimum on Sunday. 925 mb temperature remain projected to hold at
the 2 to 4c level through the day. This suggests temperature
readings will struggle to climb much above the lower 50s...with some
locales over The Thumb and Tri-Cities immediately downstream of the
colder lake waters holding in the 40s. This will be a good 20 to 25
degrees below normal for the last day of may. The increasing wind
field will contribute a gust component to this raw looking
day...gusts into the 20 to 25 miles per hour range common. Some stronger gusts
possible into the Tri-Cities given the direct flow off Saginaw Bay.
Slow exit of the deformation into Sunday evening with an associated
reduction in shower coverage across southeast sections through this
time. Clearing of low level moisture likely an even slower
process...expansion of drier air from the north taking a good
portion of the night to make better inroads. With that...enough
moisture tucked into the inversion to suggest some persistent
stratus/stratocu may linger into Monday /particularly south/ with
ensuing daytime mixing on Monday then working to scour out this
moisture. Regardless of the cloud coverage...Southeast Michigan will
remain under the influence of cool northeast flow south of high
pressure building across Ontario on Monday. Temperatures will
continue to run well below normal...readings largely middle 50s to
Warming trend is on tap Tuesday through Friday as upper level
heights rise in the wake of the exiting upper trough. High pressure
looks to maintain dry conditions through at least Thursday. Low
chance probability of precipitation Friday/Friday night to account for Euro depiction of
well-defined shortwave moving through the westerlies.
Thunderstorms will remain possible through tonight across Lake St
Clair and the Michigan water of Lake Erie as a cold front lifts
across the region. A Post-frontal northwest wind this evening will
transition to northeasterly and strengthen tonight as low pressure
tracks through the Ohio Valley. The combination of gusty
winds...onshore flow and corresponding building wave heights will
result in small craft conditions at all nearshore locations. The
potential does exist for a period of gales to emerge late tonight
into Sunday across Saginaw Bay given the favorably long fetch on
northeast flow. This corridor will be upgraded to a warning.
Winds and waves will diminish Sun night into Monday as the low
departs to the northeast and high pressure builds into the region.
Michigan...Flood Watch through Sunday afternoon for miz055-060>063-068>070-075-
Lake Huron...Small Craft Advisory until midnight EDT Sunday night for lhz441>443.
Gale Warning from 2 am to 8 PM EDT Sunday for lhz421-422.
Lake St Clair...Small Craft Advisory from 10 PM this evening to 8 PM EDT Sunday for
Michigan waters of Lake Erie...Small Craft Advisory from midnight tonight to 8 PM EDT Sunday for
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