Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1034 am EDT Tuesday may 5 2015
The detailed forecast reasoning provided in the early morning
discussion was well done and remains valid.
A mature mesoscale convective system and complex of shortwave energy supported by right
entrance region dynamics aloft is currently lifting into
southeastern Michigan this morning. Surface precipitation reports
support light to moderate rain occurring within the
forward...warm air advection quadrants of the mesoscale convective system. With the rain
moving in...southeastern Michigan can expect categorical/periods
of rain throughout the afternoon hours. Forecast uncertainty for
the short term period centers on when exactly synoptic
support/rain will end over the area. Latest NAM and rap are really
painting different pictures. NAM suggests subsidence in the wake
of this mesoscale convective system...may bring a quiet period early this evening...while
the rap drags out the duration of this categorical rain into the
evening. There remains a window of opportunity for some thunder
chances late this afternoon...Post 20z. Those chances appear to
hinge on the likelihood of the NAM solution verifying. Negligible
convective instability yields no support for severe thunderstorm
Aviation...issued 707 am EDT Tuesday may 5 2015
A notable decrease in lightning within ongoing activity moving in
from the west for the morning. A few stray lightning strikes will be
possible but more likely an MVFR restriction due to a heavier
downpour of rain. Expect MVFR restriction due to low visibility in
showers initially...especially ptk to fnt where the most persistent
rain will set up...and then MVFR due to low ceiling later today and
this evening. IFR ceiling is then likely overnight into Wednesday
morning as the surface front drifts northward from Indiana and Ohio.
For dtw... showers will move into the terminal area shortly after
issuance time and then persist through the morning. The heaviest and
most frequent activity will then shift north during the afternoon.
There is a late afternoon or early evening window for thunderstorms
but this will depend on the extent of development along the surface
front during peak daytime heating. Otherwise...ceiling will lower
steadily into MVFR through the evening and then IFR overnight
through Wednesday morning as the surface front drifts closer from
Indiana and Ohio.
//Dtw threshold probabilities...
* high for ceiling below 5000 feet today and tonight.
* Low for thunderstorms today and tonight.
Previous discussion...issued 355 am EDT Tuesday may 5 2015
Short term... today and tonight
A wet 24 hour period is setting up over Southern Lower Michigan
today through tonight. The Tri Cities and northern thumb may escape
the bulk of the rainfall today but the rest of Southeast Michigan stands to
pick up a prolonged period of showers and thunderstorms. The surface
front will remain south of the Ohio border through the period so
thunderstorms will be the elevated variety and mainly contribute to
locally heavy downpours. Rainfall totals locally around 1 inch will
be possible by sunrise Wednesday.
Widespread showers and thunderstorms are already underway over the
Midwest and will spread to the north and east during the rest of the
morning. This area of convection originated along the front over the
Central Plains during peak heating yesterday and has since grown
large enough and persistent enough to support mesoscale convective vortex development aloft
and an area of surface pressure falls over Iowa and Missouri. The
wave is just strong enough to support a weak low level jet fueling
activity over northern Illinois which will continue to develop into
Southern Lower Michigan as the low to middle levels of the front
strengthen with time. Convective debris in satellite imagery shows
the entrance region of a 120 knot upper jet analyzed in model data
at 250 mb. This feature is positioned favorably over the Great Lakes
to generate strong frontogenesis through today while the large scale
frontal position remains stationary and then drifts slowly northward
tonight. Cross sections of model data from the position of the
surface front extending northward to the upper jet axis show the
steepest portion of the ageostrophic circulation setting up through
central sections of our area by afternoon through this evening. Deep
convective instability above the frontal surface combined with
precipitable water rising to about 1.25 inches will ensure
widespread coverage of showers with embedded thunderstorms. Locally
heavy rainfall will also be possible due to the persistence of this
textbook upper jet enhanced frontal circulation and the convective
nature of the precipitation pattern. So then what could go wrong in
this forecast? The potential for surface based convection over
Indiana and Ohio...combined with the tendency for elevated
instability to become less effective during the day...could lead to
a weakening trend in the pattern over Southeast Michigan during the
afternoon into this evening. Should this occur...it would do more to
reduce the potential for heavy rainfall in our area and possibly
alter the location of the most persistent showers and overall quantitative precipitation forecast...
but most areas will see a lengthy period of rain as the morning ramp
up would carry over into the afternoon just based on inertia.
Progression of the upper level ridge and the entrance region of the
upper jet will nudge the middle level frontal zone to the north and
east overnight. This will bring the Tri Cities and northern thumb
more into the pattern of remaining showers and will be the favored
location for any nocturnal redevelopment. The axis of likely probability of precipitation
over Southeast Michigan is placed along the 850 mb Theta-E gradient which
is well agreed upon by the 00z model cycle and is the most favored
location for renewed development of nocturnal elevated instability
and support for showers through sunrise Wednesday.
Long term...Wednesday through Monday
Southeast Michigan will reside along the immediate downstream side of an
elongating upper ridge on Wednesday. The resulting weakly sheared
west-northwest flow will sustain a perturbed 850-925 mb frontal zone
across the region. Models trending slightly stronger with the
ascent/convergence through this layer...suggesting the existing
frontal slope will potentially be more active. This will support
the inclusion of a lower end pop. Greater potential for shower
production during the morning period...before both the moisture
quality and available forcing diminishes as the inbound upper ridge
makes better inroads. Substantial cloud cover within persistent low
level easterly flow will cap the diurnal temperature response to
some degree...regardless of shower coverage. Absent of a larger and
persistent coverage of showers to disrupt the heating cycle...
existing thermal profile will support highs generally within the middle
and upper 60s range.
Upper ridge steadily builds into lower Michigan Wednesday night...
580 dm at 500 mb marking the ridge axis centering overhead by early
Thursday morning. This progression will send Southeast Michigan into the
warm sector by Thursday...solid depth to the emerging southwest flow
continuing to point toward highs making a run into the lower 80s.
Dry/stable profile still influenced by exiting upper ridging will
result in little more than a Standard diurnal cumulus field.
Closed low currently parked over the Desert Southwest will lift
northeast through the latter half of the week...eventually shearing
over the top of the upper ridge stagnating along the East Coast.
This lowering height field will begin to ease an attendant frontal
boundary eastward within the Friday and Saturday window. General
timing across the model spectrum suggests Friday will remain
charactered by the pre-frontal warm sector...with any possible late
day/evening convective chances arriving too late to disrupt another
day with highs in the 80s. Potential is there for an active period
heading into the upcoming weekend...frontal boundary eventually
aligning over or near the region for an extended time as the axis
becomes parallel to the mean flow. This pattern would shave a few
degree off prospective highs this weekend relative to the
Thursday/Friday warmth...still near to above normal through this time.
Showery conditions with the chance of thunderstorms will exist under
persist east to northeast flow over the Southern Lake Huron basin
and points southward through tonight. Favorable marine conditions
across Northern Lake Huron during this time as high pressure builds
into the region. Modest wave and wind conditions through the
midweek period as high pressure sustains a weak gradient.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
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