Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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041
FXUS63 KDTX 230239
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
939 PM EST Thu Feb 22 2018

.UPDATE...

Not planning to make any drastic changes to the forecast for
tonight. Offered thoughts in aviation discussion earlier with more
emphasis on potential freezing drizzle the latter half of the night
versus freezing rain. Even then, low confidence in timing and
coverage of drizzle exists with a lack of activity immediately
upstream. Surface temperatures are expected to be very marginal with
current surface temperatures this evening still running some 2 to 4
degrees above the freezing mark. Dewpoints are in the 20s across the
Detroit Metro area, but wet bulb cooling potential expected to be
minimal as near surface theta e content increases. From this vantage
point, not expecting any significant impacts.

&&

.PREV DISCUSSION...
Issued at 658 PM EST Thu Feb 22 2018

AVIATION...

The last few peeks of satellite imagery suggests that MVFR cloud
that initially lifted into southeastern Michigan has been thinning
and scattering out due to an increase in easterly dry air advection.
Rolled back timing on MVFR/IFR by a couple of hours, although
suspect that lowering of cig heights will be very rapid once low
level moisture advection kicks in. The big question for tonight is
both timing and overall potential for freezing rain. Looking through
a collection of forecast soundings suggests some potential for
drizzle development as early as 05-06Z tonight. Upstream,
observations over central Illinois are reporting drizzle. Given
amount of near surface saturation with IFR/LIFR conditions in central
Ohio, decided to introduce TEMPO FZDZ at all sites after 06Z. Warm
air advection is forecasted to be very strong around daybreak and the
idea is to carry a warm rain forecast once depth of moisture
increases. Confidence in areal coverage of freezing drizzle tonight
is low. IFR/LIFR cigs to remain in place early Friday before lifting
during the late afternoon.

For DTW...Pushed back timing of MVFR/IFR a couple of hours, but
suspect rapid onset of conditions late this evening. Introduced
earlier timing with TEMPO for -FZDZ between 06-10Z with warm rain
after 10Z. Low confidence in coverage of -FZDZ.

/DTW THRESHOLD PROBABILITIES...

* High for cigs aob 5kft

* Moderate for -FZDZ 06-10Z. High for rain after 10z.

PREV DISCUSSION...
Issued at 315 PM EST Thu Feb 22 2018

DISCUSSION...

Mid clouds stripped away this afternoon allowing temperatures to
reach into the upper 30s to around 40 degrees across most areas. Dew
pts are predominately in the mid 20s. Mid clouds should be returning
this evening, limiting the diurnal fall expected tonight (upper 20s
to lower 30s), as low level warm advection ramps up as well.

Strong upper wave coming out of the Four Corners region, tracking
through southern Minnesota late tonight, with max 6 hr height fall
center tracking through Lake Superior during Friday. Thus, max
forcing/better isentropic ascent displaced to our northwest, but
there looks to be adequate moisture advection/isentropic ascent
arriving close to 12z Friday to support developing precipitation. It
does not appear the surface high over Eastern Canada will put up
much of a fight, and models indicating a real good surge of warm air
in the 925-850 MB layer, rising into the upper single numbers to
near 10 C. Thus, even if surface temperatures are marginally below
freezing, it is not a given the warm drops will freeze on contact.
Either way, freezing threat looks limited/brief early Friday
morning, and certainly not enough confidence to issue an advisory.
If anything, could see a little freezing drizzle with shallow
moisture before 11z, but otherwise latest HRRR keeps it mainly dry
through 11z, and then expect dew pts to rise above freezing, with
temperatures climbing well into the 40s during the day.

Mid level dry slot arriving Friday afternoon will shut off the rain,
with low level cold air then bleeding in Friday night with northerly
drainage flow. Depth of cold air and southern placement in question
on Saturday as baroclinic zone aligns in west-east fashion, with
high pressure (1026 MB) sliding through the northern Great Lakes.
This high will tend to help maintain the low level cold feed during
the day, as precipitation potentially overspreads southern lower
Michigan (see 12z Canadian/Euro). A light wintry mix is not out of
the question, but surface temps likely well above freezing and with
the daytime timing, probably looking at mainly just a cold rain.

Strong low pressure progged to be centered over the UP of Michigan
Sunday morning will pull an occluded frontal boundary across
Southeast Michigan early Sunday. Behind this front, a much drier air
mass will quickly end precipitation as better mixing allows gusty
winds to overspread the region throughout the day on Sunday. High
pressure will then build into the region early next week as upper
level ridging slowly rebuilds over the Eastern US. This will bring a
period of dry, calm conditions to Southeast Michigan through Tuesday
as temperatures remain above average with highs in the mid 40s to
lower 50s while lows fall into the upper 20s to lower 30s. As the
high moves to the Eastern Seaboard by midweek, return southwesterly
flow will advect moisture across the region ahead of the next wave
moving into the central US. Shower chances look to increase starting
Wednesday as temperatures remain above average with highs in the mid
40s to near 50.

MARINE...

Light to moderate flow will dominate through Saturday. Strong east
to southwest wind will develop on Saturday night as a deepening low
lifts toward the Straits. Stability may be sufficient to probhit
gusts to gales on Saturday night, but the gradient will ramp up as
wind turns southwesterly on Sunday. A gale watch may eventually be
needed for the waters of Lake Huron...particularly the central axis
of the Lake including part or all of Saginaw Bay.

HYDROLOGY...

Flood warnings remain in effect for most of the primary river basins
over southeast Michigan. Additional rain on Friday is not expected
to contribute to additional flooding or prolong existing flooding as
amounts will remain generally one quarter of an inch or less.
Another round of rain will then impact the area late Saturday into
Saturday night with perhaps one half of an inch of rainfall falling
in some areas. This may impact river and stream levels to some
degree, but should not exacerbate flooding to any great degree.

&&

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

$$

UPDATE.......CB
AVIATION.....CB
DISCUSSION...SF/JD
MARINE.......JVC
HYDROLOGY....JVC


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