Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
1149 PM EDT Sat may 18 2013
Aviation...
//discussion...
A region of very dry low level air circulating around the high
pressure system to the east of the region has held surface dewpoints
in the 40s across much of the area this evening. The low level flow
will turn more toward the southeast during the course of the morning
which will advect the middle to upper 50s dewpoints now over north
central Ohio into Southeast Michigan. It now appears that the bulk of the surface
moisture advection will occur after daybreak. This supports the
removal of any mention of haze in the tafs. Otherwise...an upper
level ridge will maintain benign aviation weather conditions through
the afternoon.
//Dtw threshold threats...
* none
&&
Previous discussion...issued 338 PM EDT Sat may 18 2013
Short term...this evening and tonight
Downstream amplification of the eastern North America ridge will
occur tonight as strong jet energy digs through The Four Corners.
The main consequence of this during the short term is the apex of
the ridge will sharpen and lift into James Bay guiding the
consolidated jet core along with it. The net result is for the pesky
altostratus that has been harbored within the shearing deformation
axis over northern lower Michigan...Saginaw Bay...thumb to exit with
time. Meanwhile...water vapor imagery shows smaller wavelength
anticyclonic motion over portions of northeastern Illinois and Lake
Michigan folding over. This highlights the increasing ridging aloft
through tonight.
Some modification to the air mass in combination with a trend very
late for some increasingly southerly flow suggests a good 2 to 3
degrees warmer tonight. Lows should dip into the lower 50s
north...middle 50s south.
Long term...Sunday through next Saturday
Upper ridge will hold over Michigan for one more day tomorrow.
Subsidence under the ridge should prevent showers/thunderstorms from
developing...although we will build up some instability during the
day as the ground heats and lapse rates steepen. Southeast winds
will reduce potential for low-level convergence near the lakeshores
(no convective trigger)...and will also hold temperatures closer to
the shorelines into the 70s. Low 80s are expected well inland as
good insolation combines with rising temperatures at 925-850mb. Frontal
boundary by this point will also be poised north of Lake
Huron...having little impact on weather for Sunday but will become a
factor in our weather through most of next week.
Upper wave off the Pacific northwest coast will become absorbed in
the broad upper trough now along and west of The Rockies. Models
show this system closing off over the Central Plains on
Monday...then very slowly working east into the Great Lakes/Ohio
Valley by Thursday. Arrival of this system has been hard to
predict as it will either cut-off from main flow or remain under
very light steering flow. Models continue to display some timing
differences with its entrance and exit into the area...in part due
to timing/placement issues with another upper low dropping through
eastern Canada. Overall confidence in the forecast Monday afternoon
through Thursday is not high...other than to say rounds of showers
and thunderstorms will likely roll through and affect everything
from cloud cover to temperatures.
Chances for precipitation will start to increase late Sunday night
and Monday morning as a weak low-level jet noses towards Lake
Michigan. Weak push of Theta-E advection coinciding with this could
trigger a few showers/tstorms...and have maintained low probability of precipitation through
the first part of the day on Monday. Models then in great agreement
with clipping shortwave energy up through lower Michigan as it rides
southwest flow preceding the large low pressure system. Model timing
of this wave varies from late morning through early evening.
Consensus does bring it through during the late afternoon...right at
peak heating...and will continue to anticipate a fairly good
coverage of showers/thunderstorms developing. Chances may be better
further north over central Michigan...as the frontal boundary north
of Lake Huron starts to drop southward. The front will then become
quasi-stationary over lower Michigan (best support for either
central or northern lower) Tuesday through early Wednesday...while
additional upper shortwaves ejecting out of the low and mcvs from
upstream convection ride over the front or within the warm sector
and trigger additional rounds of convection. The front should lift
back north late on Wednesday as the low approaches and crosses the
region...before dropping back south as a strong cold front on
Thursday.
Temperatures will be very uncertain with rounds of showers/thunderstorms
and cloud remnants from upstream convection crossing overhead.
Assuming a pretty dry start to the day...temperatures on Monday
should warm into the 80s. Remained a little conservative with
forecast highs the low/middle 80s. If convection waits until very late
afternoon or evening to fire...some locations could see upper 80s.
Severe weather may become a concern Monday through Wednesday. Model
sb cape values of 2500-3000 j/kg may be realized on Monday in a
"fresh atmosphere" that has not been worked over...assuming no
upstream remnants arrive early in the day. Stronger storms may also
fire overnight and especially Tuesday as the front looks to sink
slightly southward into the area...and upper jet support becomes
more favorable. Model sb cape values continue to look conducive for
strong/severe storms Wednesday as the low approaches and the front
lifts back north. Shear values do not look overly impressive any of
the days however. Locally heavy rainfall will also be a threat with
precipitable water values running between 1.25-1.50+ inches during this entire
timeframe.
Quiet weather is then expected towards the end of the week as high
pressure builds over the area...but temperatures will run much
cooler with maximum temperatures only in the 60s.
Marine...
Easterly flow will continue through tonight as we remain under the
influence of high pressure over Canada. The favorable fetch will
allow waves to be slightly high over Western Lake Erie...but a
decreasing trend will keep them below Small Craft Advisory criteria.
Winds will veer to southerly by Sunday night before increasing
next week as low pressure lifts out of the plains. This low will
bring a chance of thunderstorms Monday through at least Wednesday
and will be accompanied by a modest increase in southerly winds by
Monday afternoon. A middle-week wind shift to northerly can then be
expected as a cold front pushes across area waters.
&&
DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&
$$
Aviation.....SC
short term...cumulonimbus
long term....hlo
marine.......cb
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