You don't appear to have any favorites yet, or your cookies may be disabled.
Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
653 am EST Friday Mar 7 2014
VFR conditions will persist through this evening as deep layer of
dry air remains over the area as high pressure moves off to the
east. A weak cold front will approach from the northwest overnight which
will gradually moisten the column. Clouds will thicken and lower
through the night as the front drops south generally between 06-12z.
Winds will increase toward 10 knots out of the SW ahead of the front
before veering to the northwest behind the front. There is a low chance of
some light snow showers with the frontal passage but may be to
sparse to include at the tafs at the moment. Will drop ceilings down to
MVFR with the frontal passage overnight.
//Dtw threshold threats...
Previous discussion...issued 343 am EST Friday Mar 7 2014
Short term...today and tonight
Pleasant day on tap as we become located in between the exiting surface
ridge and a weak cold front entering the western Great Lakes. This
puts US into a region of deep layer SW flow...surface to 800 mb...for
much of the day. Thermal profiles in model soundings advertise a
layer from about 950mb-700mb above zero maxing out around 3c.
Combined with the substantially dry airmass from the exiting ridge
and we will have a good amount of heating today. High clouds...at or below
15kft...will stream into the later in the day but only after several
hours of advective and radiative heating has worked over the area.
Only minor tweaks to todays highs...still in the upper 30s to low
40s which is slightly above most guidance.
Changes coming during the overnight hours as a weakening frontal
boundary drops through lower Michigan. The best frontal forcing lifts off
to the northwest as the front drops through after 06z. The fgen band that
does pass through looks very fractured and disorganized. Looking at
a cross section view there is a layer of Omega aloft that tries to
work its way to the low levels while the front passes but even that
weakens during the trip. Much of this is tied to the fact that the
jet energy is transitioning. The original jet maximum aiding in the
precipitation upstream currently lifts off to the NE with the low but
another maximum develops right overhead starting close to 12z
Saturday. The depth of dry air with a layer of dewpoint depressions
around 15-20c from the surface through 700mb...much of the moisture
early on will go to saturating the column from the top down during
the evening hours. The deep warm layer brought up the question of
ptype. By the time saturation of the low levels occurs...the column
has already begun to cool off leaving only a shallow layer above 0c
aloft. With the saturation stemming from aloft all the precipitation is
starting off as snow with the front. The wet bulb potential should
force the last of the warm air to cool off allow the flakes to fall
to the surface. There is a chance that the lead edge of the band could
produce a quick burst of snow pellets before flakes take over but
that should be the extent of the influence of the warm layer. So
will keep the ptype as snow through the front. All this said...only
expecting a dusting up to a tenth or two before sunrise as the dry
air should use up most of the moisture. A chance pop will continue
for 06-12z tonight.
The cold front will finish sliding through Southeast Michigan during
Saturday morning. Moderate surface temperature contrast will be
maintained as the boundary exits the region but the dynamics of the
frontal circulation will continue to suffer from being left behind
by the wave over eastern Canada. Model indications of moisture
quality are good judging by specific humidity around 3 g/kg in the
850-700 mb layer but this will be offset by the dry near surface
layer and the shallow nature of the frontal structure with height.
Model cross sections indicate a very shallow frontal slope in
Theta-E from the surface to 700 mb by 12z Saturday with plan view
depictions of 2d fgen weakening with time in response to the
weakening upper jet and eastward departure of short wave dynamics.
In this environment, the NAM indicates a surprisingly upright
ageostrophic circulation near the Ohio border, likely too much so
considering the broad and weak upper jet left over the area in
locally zonal flow aloft. Prefer the GFS depiction in this case
which is considerably more shallow leaving the vertical motion
field mostly within the higher static stability region below 500
mb. On the other hand, and typical of the GFS, its 6 hour quantitative precipitation forecast is too
generous with nearly 0.10 inch liquid equivalent during Saturday
morning. Instead, plan to maintain a chance pop along and south of
Interstate 69 lingering into early afternoon near the Ohio border.
The front will drop maximum temperatures back down below normal values, which
are ranging from the upper 30s at mbs to lower 40s at dtw heading
into the second week of March, but anything above freezing is
welcome compared to recent days even if coupled with min temperatures
near single digits by Sunday morning. A rebound is then expected
Sunday into Monday with good model agreement on the arrival of the
next northern stream low pressure system. This will generate some
southwest return flow in the low levels before the wave washes out
over western Quebec in favor of yet another system from central
Canada by Monday. This system will maintain a similar surface wind
pattern over lower Michigan adding some confidence to the
temperature rebound during the first half of next week. The key
here is that, while predictability is low on the individual short
waves, the track of each system is solidly over Canada with
limited potential for substantial cold advection until middle week.
That is when the large scale upper air pattern is shown to amplify
enough for a meaningful surge of cold air by Wednesday, just as
the next southern stream wave moves into lower Mississippi Valley.
Forecast confidence is much greater on the resulting below normal
temperature trend in the Great Lakes compared to precipitation
chances. The GFS and European model (ecmwf) have a lot of spread in the upper air
forecast in terms of phasing of the westerlies. The European model (ecmwf) is
stronger in that regard with its solution brushing our area with
snow on the north flank of the system Wednesday. The GFS is less
amplified with some support from the Canadian and both are far
enough south for snow to miss our area. Recent history leans the
forecast toward the GFS given the tendency for the long wave ridge
out west to easily/quickly take on Omega block character shortly
after a new round of amplification.
Modest southerly flow and mild temperatures by recent standards
will develop over the region today and tonight. This will be
followed by a cold front of average strength passing through the
northern lakes tonight and southern lakes Saturday morning. Moderate
northerly flow over the mostly ice covered Lake Huron will be
generally less than 25 knots. Southwest winds will increase again
Sunday and Monday as two waves of low pressure tracks through
southern Canada. This will keep temperatures on the mild side until
a stronger front moves through the region by Wednesday.
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).
|National Weather Service Glossary of Abbreviations
||Back to forecast page
Non-Expanded Version (with abbreviations)
Copyright © 2014 Weather Underground, Inc.
Copyright © 2014 Weather Underground, Inc.