Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 202015

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
315 PM EST Mon Feb 20 2017


Record setting upper level ridge (574 DAM at 500 MB per 12z DTX
sounding) in place over Southeast Michigan Today, but backed surface
flow out of the southeast holding temps in the 50s this afternoon.
Upper level ridge axis will continue to slide East, reaching the
East Coast Tomorrow. Rather narrow upper level trough over Central
North America, to break off into two pieces, as 500 mb cutoff low
develops over southern Mississippi River Valley, with northern
stream energy lifting through western Great Lakes Tonight into
Ontario Tomorrow. Narrowing moisture Axis and weakening nature of
the front draws into question the rain chances over southeast
Michigan, as bulk of upper level forcing and low level jet lifts
north of the CWA. Still, it appears there will be enough low level
convergence for some light showers (likely) to hold together on
Tuesday as the front moves across the area, as Local SREF weighted
probabilistic guidance suggest pops around 70 percent are justified
across most areas. However, 12Z HRRR gives one pause, as front comes
through mainly dry with lack of instability. Regardless, does look
like bulk of shower activity arrives after 12z, and will be trimming
pops for Tonight, and a bit for tomorrow. Clouds and showers working
through during peak heating will limit maxes, but still expecting
temps around 60 degrees.

Another surge of warm air on Wednesday with increasing southwest low
level flow as low tracks through Lake Superior. 850 mb temps progged
to rise back to around 10 C over southeast Michigan, with lower
teens at 925 mb level, supporting highs well into the 60s to
potentially 70 degrees at the high end. However, there is some
concern in the development of low clouds Tuesday night into
Wednesday morning behind tomorrow`s frontal passage, but suspect
models over doing the low level moisture just a bit, especially if
rain is very light tomorrow or does not materialize, and planning on
skewing the forecast on higher end of guidance, mid to upper 60s.
With the higher chance of low clouds in the south, along with the
surface flow potentially more southerly vs southwest, would subsequently
favor Flint Vicinity to be the warmest location during the day.

Well above normal temperatures will continue through the end of the
week before cooler air works in bringing temps back down closer to
seasonal for this time of year.  The main issue in the extended is
the strong storm system that models are showing coming up from the
plains for Friday and into the beginning of the weekend.  Chances
for precipitation are expected to increase by later in the day on
Thursday as the warm front approaches.  Currently, the low is
advertised to move northeast over the central Great Lakes area
keeping Southeast Michigan in the warm sector.  A chance for
thunderstorms will be possible as well as gusty winds as the cold
front slides across the area this weekend.   Conditions then look to
dry out for a brief time to round out the weekend as high pressure
settles in to the south.



East-southeast winds will continue to increase this afternoon and
persist into Tuesday morning as strong high pressure retreats to the
eastern Great Lakes. A frontal boundary will approach the western
Great Lakes. Warm air above the cold lake surface will keep fairly
stable conditions. Nonetheless, the gradient will increase with a
long trajectory over the lakes. This suggest winds will become
rather gusty tonight. Gusts in the 25 to 30 knot range knots are
forecasted across much of Lake Huron tonight. A cold front will
slowly move across Lower Mi and Lake Huron on Tuesday. There will be
substantial weakening in the winds as the broad frontal boundary
moves into the region. Further weakening of the post frontal
gradient will support light and variable winds Tues night. Modest
southwest winds are then forecast to develop on Wednesday.


Issued at 105 PM EST Mon Feb 20 2017


High amplitude upper level ridging will remain in place over the
central Great Lakes through the next 12 hours, but the surface
ridge axis aligned under greater geopotential height rises will
retreat out of the state more quickly. Models show a definitive
cold, stable lake aggregate response late this afternoon and early
evening, bringing with it a greater easterly flow this afternoon.
Have already been seeing this signal in surface observations. Dry
air will remain entrenched in the lowest 20 kft agl, bringing no
cloud considerations this afternoon and tonight. LLWS remains a
concern for tonight as the lead edge of low level jet axis pushes
towards southeastern Michigan. Followed the lead of the inherited
tafs which errs on the side of greater gradient flow at the surface.
Otherwise, the main change in this taf issuance was to push back
timing of prevailing showers. Narrowing ribbon of moisture in the
700- 600mb layer will not arrive over the Saginaw Valley until after
12Z, not over the Detroit terminals until as late as 17Z. Given
weakening dynamics and narrow deep moisture, the Tuesday event is
appearing to be a short duration one, with perhaps a 3 hour window
for rain shower chances.

For DTW...Daytime heating and an increasing, stable lake aggregate
response will lock wind directions in the 110 to 120 deg fetch with
high confidence flow of 10 knots. This will increase probabilities
for northeast flow operations.


* None


Lake Huron...Small Craft Advisory from 10 PM this evening to noon EST Tuesday
     for LHZ441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.



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