Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
714 PM EDT sun may 19 2013
Aviation...
//discussion...
Ridging at all levels will maintain mainly VFR conditions, clear
skies, and light southeast winds through Monday morning. However, increasing
low-level moisture will pose a risk for fog development after about
07z tonight...particularly for the Detroit area including
kdtw/kyip/kdet where there will be a contribution of cool & moist
low-level air from Lake Erie. Should fog form, visibilities in the MVFR
range can be expected along with the lake contribution delaying
burn-off until at least 12z Monday morning.
By late morning...diurnal mixing will allow southeast winds to gust up to
20 miles per hour as a scattered cumulus field develops around 4kft...becoming
broken around 4-5kft by late afternoon as showers and thunderstorms
push into the area. Confidence in showers and thunderstorms
affecting the area after 21z remains rather high, but confidence
remains low with regard to the potential impact for any one
location. Will therefore only introduce showers to the forecast for
Monday afternoon, except for the addition of thunder to kmbs where
the probability is highest.
//Dtw threshold threats...
* medium confidence in ceilings below 5kft after 21z Monday
* low confidence that a thunderstorm will impact kdtw airspace
&&
Previous discussion...issued 407 PM EDT sun may 19 2013
Short term...tonight
Upper level ridge axis will slide over lower Michigan this
evening/tonight providing an expected dry night. There is a warm
front which lifts north...but much better defined to our west.
A weakening surge of 925 mb Theta-E arrives toward 12z Monday...with
computed lifted indices of -7 c...but that layer looks to remain unsaturated.
If activity does materialize...it would more than likely be after
12z anyway.
With the increase in low level moisture...already reflected in this
afternoon dew points pushing into the lower 60s (south)...comes the
potential for fog...especially with cooling influence of Lake Erie.
On the flip side...Southeast Michigan has had an extended period of
dry weather...and not fully convinced we will see much more than the
Standard 3-5 sm fog/haze...as mins expected to only drop back into
the upper 50s to lower 60s.
Long term...Monday through Saturday
A broad upper level trough that initially sets up across the
northern plains Monday morning will persist across the northern
plains/upper Midwest through midday Thursday. The models have come
into an agreement that despite 500mb heights closing off within the
center of the trough...behavior of the system will not act like a
true cutoff low. Instead...various shortwaves will rotate around the
diffuse low allowing for varying bouts of baroclinic like activity.
Southeastern Michigan will remain on the periphery of the
low...tucked within a burgeoning warm sector. With episodic moisture
transport expectations are for an active work week period with a low
predictability regarding specifics.
Monday...longwave ridge axis is expected to hold across the area
throughout much of the day which supports another day of
predominately quiet weather. First thing was to incrementally
increase high temperatures. A strong Ava signal remains west of
the County Warning Area through 18z...before a shearing deformation axis slams
into the area between 18-21z. Magnitude of upward vertical velocity forcing will not be
that high from the shearing deformation itself...however...the
feature should serve as a focus mechanism for a secondary Theta-E
surge or active warm front. Difficult this far out to pin down
timing and trajectory of inbound shortwave energy (which will
emerge out of Oklahoma convection tonight). There is enough model
guidance suggesting the shortwave energy may initiate showers and
thunderstorms across western lower Michigan before setting a
course for the Saginaw Valley between 21-00z. The Theta-E surge
behind this warm front should be significant...surging above 1.30
inches by Monday evening. Typical surface moisture biases exist in
the model soundings but steepness of 700-500mb lapse rates
suggests for potential of MLCAPES of 2000 j/kg. 0-6km bulk shear
of 30-40 knots will support mesocyclone and supercellular
behavior...good for potential of damaging wind gusts and large
hail. Latest swody2 expanded the slight risk into much of
southeastern Michigan.
Exit region dynamics...responsible for the secondary warm front and
moisture surge late Monday...will lift northward causing a wave
break and energy to spill over the ridge axis on Tuesday. This
dampening is important as it should allow precipitation activity
to sink into the ridge a little more and cause activity to focus
over the area. Tried to offer some timing resolution...but the
whole Monday night through 00z Wednesday timeframe will require a
high likely pop. Confidence is high that semich will reside in the
warm sector which requires a forecast of low to middle 80s.
Understand some hesitation there...with overall clouds limiting
insolation. However...given late may any breaks will cause
temperatures to soar. The potential will continue for strong to
severe storms on Tuesday...but guidance is too muddied at this time to
offer much insight.
Wednesday...last of the upper level jet energy should lift out of
the trough and cause remnant midlevel low circulation to become
absorbed in westerlies to the north. The surface reflection is
expected to meander over the state in time for the afternoon. We
will need to monitor this outcome as it could provide some shear
focus to augment the environment for strong to severe
thunderstorms. However...at this time thermal forecasts suggest that
instability will less than both Monday and Tuesday.
There is a strong signal the upcoming Holiday weekend will be one
dominated by a stable Canadian anticyclone. What has been typical
for this Spring...quiet but very cool conditions for Memorial Day
weekend.
Marine...
Weak southeasterly flow over the lakes will provide favorable marine
conditions through tonight. A low pressure system over the Central
Plains will drift toward the Great Lakes laying out a warm front
across central Michigan. This front will stall over the area through
midweek as the low tracks along it. Winds south of the front will
remain southerly while winds to the north will back to the east and
increase slightly. This will lead to higher waves over Northern Lake
Huron while the southern Basin...Lake St Clair and Lake Erie keep
waves generally under 2 feet. This low will bring scattered
thunderstorms Monday through Thursday. Winds will shift to the north
across all waters by Thursday morning...as the low and trailing cold
front push off to the east.
&&
DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&
$$
Aviation.....Jvc
short term...sf
long term....cb
marine.......sf
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