Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
348 am EDT Wednesday may 27 2015
Short term...today and tonight
Middle level trough will be swinging through the Great Lakes today
while a surface low over Ontario pulls a cold front across lower Michigan this
afternoon. This will finally provide some surface based forcing to a
warm and humid airmass thats been in place for several days now.
With the cold front off to the west at the start of the day...deep
layer swerly flow will once again rule over the region keeping
moisture levels elevated with dewpoints in the 60s. Overall the
moisture profile took a hit Tuesday when the wave lifted through the
state taking the >1.5 inch precipitable waters along with it...but still leaving
values over an inch to work with. Ll lapse rates will be decent once
again but we still struggle to get any notable in the middle levels.
The airmass looks to provide cape values of 800-1000 j/kg early in
the day but shear values are only 20-30 knots with veered low level
winds which will not help storm organization. The NAM/hrrr/gemreg
all suggest a prefrontal trough sliding through a few hours ahead of
the cold front. This looks to ignite a line of showers/thunderstorms
around 20-21z across far Southeast Michigan before the cold front slides through
around 22-00z producing another line. Not extremely confident in the
chances for severe weather as shear and midlevel lapse rates are
both weak and the prefrontal trough could work the atmosphere before
instability can maximize and it may not be able to recover for the
cold front. Regardless...there is enough potential to note a slight
chance of strong to severe thunderstorms this afternoon. Primary
threat will be damaging winds but slightly cooler middle levels will
allow for a hail threat as well. Primary area of concern will be
southeast of a line from Port Huron to Adrian due to the orientation
of the front...which lines up well with Storm Prediction Center and their new day 1
Outside of thunderstorm activity...winds will be gusty today as we
mix up to 850mb with gradient flow around 30 knots just off the
deck. This could result in gusts at or above 30 miles per hour at the surface
if the possible lines of convection do not disturb the flow too
much. Cooler and drier air will move in overnight as the cold front
exits eastward after 00z. Overnight lows will return to the
middle/upper 50s vs the 60s we have been dealing with of late. Low
amplitude ridge beginning to work into the region overnight will
result in more of a zonal western flow vs a cool nwerly Post frontal
flow. This will limit the amount of cooling.
Rising geopotential heights will push across the Great Lakes region
during the day on Thursday. Modeled absolute vorticity fields in
addition to bufr sounding data supports a signature for relatively
stable conditions due to subsidence. All of the County Warning Area begins the
Thursday period with no weather mention. The lower tropospheric
anticyclone center is prognosticated to slide eastward along/near Lake Erie
which will eventually southwesterly flow to spread across northern
lower Michigan Thursday afternoon and down into the Saginaw Bay/no
thumb vicinities towards the evening hours. Is is the higher Theta-E
content washing into the area from the west that allows the NAM to
generate some quantitative precipitation forecast for portions of NE lower Michigan and the tip of
The Thumb. Seems to be a classic example of the NAM convective
scheme kicking in...leading to a false representation of the
thermodynamic environment. Will include a chance pop for the areas
outlined...for the potential of some high based showers along lake
convergence boundaries. Warm advection...insolation...drier airmass
should allow temperatures to reach the lower 80s in many areas on
The high amplitude shortwave zipping quickly eastward across
northern Ontario on Friday will lead to a dominant 1000-500mb height
rise feature over much of the central United States. It is this far
reaching corridor of height falls that will strengthen...then drive
a strong surface cold front through southeastern Michigan during the
first half of the weekend. Still uneasy on when to begin
precipitation chances on Friday. Models are very bullish with
early quantitative precipitation forecast onset...but overall structure of the 300 mb heights and
some distance yet to the upper level jet axis suggests a lack of
forcing could yield quiet conditions. Soundings show no
capping...however...so confidence is low on timing. With that
stated...there is a strong confidence that overall frontal
dynamics will be high which brings a very good potential for
widespread meaningful precipitation amounts...primarily Friday
night and Saturday. The current forecast is including likely probability of precipitation.
With deep southwesterly flow...incrementally bumped Friday highs
upward into the middle 80s for much if not all of the County Warning Area.
There is high confidence in cooler temperatures for the weekend with
Saturday highs in the 70s...Sunday highs in the 60s.
South to southwest flow will continue over the Great Lakes region
today...ranging from 10 to 15 knots with gusts to 20 knots. With a
persistent southwesterly wind forecasted for today...will issue a
Small Craft Advisory for Saginaw Bay and the nearshore waters of
Lake Huron from Port Austin to Harbor Beach to account for the
funneling of winds. There is another chance for showers and
thunderstorms this afternoon as a cold front moves through the
area. Dry conditions are the expected Thursday and much of Friday.
Aviation...issued 1205 am EDT Wednesday may 27 2015
A stray shower will be possible during the early morning but
coverage will be too sparse to include at any one terminal location.
Intensity will also be considerably lighter compared to earlier in
the night. That leaves a mix of MVFR ceiling and fog as the main
aviation concern through the morning. After a brief clearing
trend...the latest data points to the best chance for ceiling to
occur at mbs down to fnt as southerly flow of humid air persists
during the night. There could be pockets of clouds at ptk through
dtw and any fog that develops will be limited to MVFR by wind
staying above 5 knots through middle morning. Convective potential
during the afternoon into Wednesday evening will support enough
shower activity for a probability group during peak heating but will hold
off on thunder at this point in the forecast.
For dtw... outside of a stray shower...weather concerns will be
limited to MVFR fog and patchy MVFR ceiling during the morning.
Confidence is high enough in shower coverage for a probability group during
the afternoon but not yet for thunderstorms.
//Dtw threshold probabilities...
* medium probability for ceiling below 5000 feet afternoon into
* Low potential for thunderstorms affecting the terminal afternoon
Lake St Clair...none.
Michigan waters of Lake Erie...none.
You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).