Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 252341
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
641 PM EST Fri Nov 25 2016


.AVIATION...

Shortwave trough now digging southeastward across portions of NE IL
and the southern basin of Lake Michigan is driving an area of
midlevel saturation 800-700mb across much of southeastern Michigan.
Very subtle increase to cyclonic flow immediately in advance of
the vorticity maximum is providing slight theta e advection which
is resulting in an organized area of neutral stability in the 5.0
to 10.0 kft agl layer. Breaking it down forcing at the whole is
very weak to almost non-existent, relying on more favorable
thermodynamics. The lack of dynamics is supported by the
overwhelming surface observations with -ra but remaining at 7 to
10SM for vsby. With the back edge lining up to recent HRRR output
feeling is looking at shorter duration to rain activity this
evening, should end by 04-05Z. Surface dewpoints remain to warm
and precipitation rates remain too weak to support a changeover to
snow. Will be monitoring that this evening. No near surface dry
air advection will keep MVFR stratus to possible VFR stratus Sat
afternoon locked in place through the end of the period.

For DTW...Better subtle theta e advection thus far has remained
north of the Detroit metro area. Progs continue to support increased
near surface theta e and saturation pushing southward into DTW yet
this evening. delayed MVFR cigs and -RA until 01Z and should exit
the region by 03-04Z. Still could be looking at some potential
overnight for -DZ given degree of saturation in the lower
troposphere.

//DTW Threshold Probabilities...

* High for cigs aob 5kft.
* very low for ceilings aob 200 ft 00-04z.

&&

.PREV DISCUSSION...
Issued at 347 PM EST Fri Nov 25 2016

DISCUSSION...

The short term forecast is focused on the small but intense short
wave moving toward the south end of Lake Michigan late today and
over southern Lower Michigan during the evening. The system has a
much different look compared to yesterday`s projections in terms of
mesoscale structures, mainly in the strength of the circulation and
associated dynamic forcing. This will allow it to make the most of
the limited Pacific moisture supply and possibly even overachieve
with a boost of low level moisture and low level instability from
Lake Michigan. There are already signs the lake effect forcing will
couple with the mid levels of the circulation that is already
favorable for enhanced vertical motion. Mid afternoon satellite
observations indicate good texture in visible imagery and colder IR
enhancement over the north flank of the system which is
representative of enhanced vertical motion due to steeper mid level
lapse rates and deeper elevated instability. Synoptic and mesoscale
model solutions capture this in terms of convective instability in
theta-e cross sections centered around 700 mb with 700-500 mb lapse
rates in the 7.5 to 8 C/km range from 00Z-03Z this evening. This
will allow a good burst of vertical motion to develop as the larger
scale trough takes on more of a neutral tilt while the circulation
moves over SE Michigan. The resulting increased precipitation rate
will then open the door for at least a mix of rain and snow post
sunset as the freezing level drops below 1000 ft on the downside of
the diurnal cycle but with no more than a slushy coating on unpaved
surfaces. Likely POPs during the late afternoon through evening is
expected to represent the pattern well as it will be more of a
cellular convective pattern or possibly fragmented bands before
exiting west to east overnight.

High pressure is on track to build into the Great Lakes during
Saturday and low clouds continue to look stubborn prior to the
surface ridge passage. Satellite imagery today adds some confidence
to a more pessimistic cloud forecast through the day. This goes in
hand with colder afternoon temps just in time for the clearing trend
to begin during Saturday evening. Good radiational cooling will
likely take temps down into the 20s by Sunday morning, at the low
end or below guidance, as the sharp upper level ridge to our west
should guard against too much high cloud coverage. The rest of
Sunday will be a transition day as high pressure is driven
eastward by the large and strong low moving in from the west
coast. Southerly flow developing during the day will help boost
temps slightly above normal with the warming trend continuing
through Sunday night as clouds thicken over the region.

As low pressure over the Central Plains slowly approaches the Upper
Mississippi Valley region, deep southwesterly flow will set up over
Michigan bringing milder, wet conditions through midweek. With
shortwave energy ejecting towards the Great Lakes region Monday
night and favorable jet dynamics, large scale ascent will support
a broad area of rain Monday night. Drier conditions are expected
late Tuesday before another shortwave ejects northeastward towards
the Eastern Great Lakes bringing a chance of precipitation to
Southeast Michigan on Wednesday. As the low slowly moves eastward
across the Great Lakes region, westerly flow will setup keeping
unsettled conditions and seasonal temperatures in place through
Friday.

MARINE...

Moderate west to northwest will persist through the weekend bringing
a period of quieter marine conditions. Southeast wind will ramp up
early next week in advance of a large low pressure system over the
Upper Mississippi River Valley. Persistent southeast flow will
likely warrant small craft advisories for waves for the nearshore
zones during this time. There will also be a slight probability for
wind gusts to marginal gales by Tuesday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....CB
DISCUSSION...BT/JD
MARINE.......DRC


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at www.weather.gov/detroit.



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