Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
702 PM EDT Monday Apr 20 2015
There has been some slight decrease in wind speeds over the last
couple of hours. Ongoing low level cold air advection should keep
some degree of gustiness to the winds during the first hour or two
of the taf forecast. There should then be some degree of decoupling
in the boundary layer during the evening which will decrease wind
speeds further and help suppress some of the gustiness. An axis of
low level moisture will track across Southeast Michigan this evening...sustaining
some residual light showers and MVFR based ceilings. Ceilings will
lift during the overnight as the depth of cold air dramatically
increases across southern Michigan. Rapid deepening of the daytime mixed layer
will lead to strong SW winds late Tuesday morning through Tuesday
afternoon. The current set of of tafs accounts for some potential
for strong convective wind gusts out of afternoon showers on Tuesday.
This still looks reasonable.
For dtw...in light of current observations...there is a chance that
ceilings will drop below 2k feet briefly this evening. The better low
level moisture will lift north of metropolitan overnight...possibly opening
the door for some brief clearing toward daybreak. Confidence of this
is not high enough to add a period of clear skies at this time.
//Dtw threshold probabilities...
* high in ceilings below 5000 feet this evening. Low overnight and
* Medium in wind gusts exceeding cross wind thresholds Tuesday
Previous discussion...issued 346 PM EDT Monday Apr 20 2015
MUCAPES did reach 500 to 800 j/kg earlier this afternoon (per laps
data)...which was able to support a couple thunderstorms over
Southeast Michigan. Bulk of convective activity has pushed
east...and continued drying behind the cold front will occur...as
surface dew points crash into the 40s. With that colder/dry air push
is some gusty winds...as some peak winds of 35 to 45 miles per hour can be
expected for another couple of hours as we exit peak heating.
Noted peak wind at dtw of 42 knots at 1855z. NAM/GFS model
soundings/low level relative humidity fields suggest we are going to partially
clear out tonight...but upstream observations extending back west
over the western Great Lakes/upper Mississippi River valley to not
look very encouraging...and preference was to increase the cloud
cover forecast for tonight with the cold cyclonic flow. Also...as
850 mb temperatures fall into the middle negative single numbers over Lake
Michigan...some Lake Michigan moisture/instability contribution is
not out of the question. Have carried isolated-scattered light
rain shower mention to account for lingering activity late this
afternoon/early this evening and then the possible activity toward
morning. Despite clouds...cold advection expected to send mins
into the upper 30s to lower 40s.
A broad upper level trough with a closed low in close proximity to
the Great Lakes region will keep southeastern Michigan under the
influence of cold cyclonic flow throughout the midweek period.
Nature of individual shortwaves rotating around this upper level
feature keeps the specific timing confidence inherently low.
Magnitude of cold and dry air advection from tonight will
precondition the atmosphere and allow for the growth of a deep
diurnal near surface mixed layer on Tuesday. Forecast soundings
suggest mixing heights reaching as high as 6 to 7 kft above ground level by the
afternoon hours. Geostrophic flow in the region will remain elevated
with the low height anomaly over central Ontario which supports
widespread westerly wind gusts late Tuesday morning/Tuesday
afternoon in excess of 30 knots. Combination of shortwave energy
working through the trough axis and spokes of enhanced midlevel
convergence/saturation will bring the potential for rain showers by
Tuesday morning. Potential medium impact weather scenario may
unfold...contingent on overall efficiency of individual convective
circulations of the showers to enhance downward momentum transport.
Aforementioned forecast discussion made reference to a classic
inverted v structure...which suggests some possibility that
windgusts could reach 50 miles per hour. Future shifts can assess the
feasibility of a possible Wind Advisory for Wednesday afternoon.
Persistent cyclonic flow around upper level closed low will bring
another day of cold and showery to southeastern Michigan on
Wednesday. A secondary surge of cold air advection is forecasted
between 15-18z. The cold air advection should work against diurnal heating...with
highs some 7 to 10 degrees cooler than the day before. Highs on
Wednesday are only expected to reach the middle to upper 40s. Better
message to communicate is that wind chills for many areas will not
climb out of the 30s.
The region will still be under the influence of longwave troughing
on Thursday before weak upper level ridging builds in for the start
of the weekend. This trough will continue to lead to cooler temperatures
Thursday before temperatures moderate a tiny bit back into the 50s
by the weekend. At this point the only potential for possible
precipitation in the extended would be on Saturday...although most models
are keeping the bulk of the precipitation south of the County Warning Area with the
exception of the 12z European model (ecmwf).
Strong southwesterly wind field is now in place with windspeeds area
wide ranging between 25 and 35 knots. Small craft advisories remain
in effect for all nearshore zones. Breezy westerly winds will
persist through midweek as conditions remain relatively static with
low pressure parked over the Great Lakes. Additional small craft
advisories for wind will likely be needed on Tuesday and possibly
once more on Wednesday.
Lake Huron...Small Craft Advisory until 10 PM EDT this evening for lhz421-422-
Lake St Clair...Small Craft Advisory until 10 PM EDT this evening for lcz460.
Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for lez444.
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at www.Weather.Gov/Detroit (all lower case).