Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 152007
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
307 PM EST Mon Jan 15 2018

.DISCUSSION...

A lead mid level short impulse associated with an upper low now over
the Upper Mississippi Valley has been pivoting from south to north
across the forecast area this afternoon. Decent mid level height
falls and strongly difluent upper level flow have sustained light
snow throughout the day. While strong moist isentropic ascent has
shifted across Ohio within the warm conveyor, a narrow ribbon of
better ascent developed along the lead edge of the mid level PV max,
resulting in a brief period of higher intensity snowfall rates now
lifting northward across Se Mi as of 19Z. The water vapor loop shows
a region of mid level dry air now approaching the MI/IN/OH border,
set to lift across Se Mi during the evening behind the mid level
short wave feature. The result will be a gradual decrease in the
areal coverage of the light snow. By this time, total accumulations
since this morning of around 2 inches still seems reasonable.

The upper low is forecast to elongate as it rotates into Lower Mi by
Tues morning, while the associated surface low extends across
roughly the I-94 corridor as it stretches into srn Ontario overnight.
Although the mid level moisture will strip away this evening, ample
low level moisture will reside across Se Mi through the night. The
low level airmass will be cold enough for ice nucleation within
these lower clouds. So while the more widespread light snow
diminishes this evening with waning synoptic scale ascent,
increasing forcing in the boundary layer will result in nmrs to sct
snow showers. The greater focus will be across metro Detroit/Ann
Arbor and points south with the approach of the sfc low and
associated sfc trough axis. The region of ascent in these showers
will be focused more within the thermal regime for good dendrites,
so snowfall rates are likely to be greater than what occurred earlier
today. This suggests at least some additional minor accums tonight.

The elongated upper low will gradually push off to the east late
Tues/Tues night. The associated surface low will in turn exit to the
east of the region. There will however be a couple lingering surface
troughs holding back across Se Mi, providing a focus for sct to nmrs
snow showers to persist through Tues afternoon. Steepening low level
lapse rates due to diurnal contributions may support some brief
intense snowfall, warranting at least minor (less than one inch)
accums. Lake enhancement under northerly flow may however support
locally high totals across NE Huron County. Overall expect little
variation in temps this evening through Tuesday with persistent cloud
cover and limited thermal advections.

Weak ridging ahead of a quick-moving shortwave trough will bring dry
conditions midweek. Recent model guidance then indicates the
shortwave will quickly move across Lower Michigan Thursday night.
With limited moisture available, impacts will be limited to an
increase in cloud cover with a few light showers at most before dry
conditions return to end the workweek. High pressure centered over
the Southeast US will allow return southwesterly flow to begin
advecting milder air into the region during the late week period with
highs reaching the upper 30s by Friday.

An upper level trough digging into the Southwestern US on
Saturday will induce lee cyclogenesis over the Southern Plains. This
low pressure system will further strengthen southwesterly flow over
the region advecting warmer, moister air northward through the
weekend. This will allow temperatures to warm further into the 40s
both Saturday and Sunday. However, the increased moisture will lead
to cloudy, damp conditions with increasing shower chances late
Saturday into Sunday.

&&

.MARINE...

Southeast winds will gradually decrease during the night as low
pressure tracks into far southern Lower Michigan. The low will track
into southern Ontario late tonight into Tuesday morning. This will
result in a veering of the winds to the north-northeast. While
colder air will spill across the lake in the wake of this low, a
decreasing gradient will keep peak wind gusts under 25 knots. A
strengthening southwesterly gradient will develop mid week as strong
high pressure slides across the southern US and a slow moving cold
front pushes across northern Ontario. Warmer air advancing across
the lakes will inhibit mixing depths, thus the chances for gale
force wind gusts Wed and Thurs appear too low to carry a gale watch.

&&

.PREV DISCUSSION...
Issued at 1224 PM EST Mon Jan 15 2018

AVIATION...

IFR -sn will continue for the next several hours, winding down by
00z with improving vsbys as precipitation stops saturating the
boundary layer. Flow will remain on the light side, backing around
the compass from ESE at forecast issuance time to WSW Tuesday
morning - interrupted by several hours of VRB in the Detroit area as
the surface low passes overhead. Convergent flow lifting from the
south will bring potential for banded -shsn 03-12z for the Detroit
area while moist cyclonic flow supports 6sm -sn elsewhere. Second
trough will begin to pivot into the area from the north Tuesday,
reaching MBS/FNT by the end of the pd and likely producing another
transient window of banded showers. Cig forecast is uncertain, but
expect no better than MVFR at any time.

For DTW...If there is any potential for a modest increase in snow
rates it will likely fall 20-23z as the NW periphery of the warm
conveyor currently responsible for more organized echoes over NE
Indiana lifts NE and possibly clips the Detroit area. Included 3/4SM
tempo to acct for this potential. The banded showers that are
expected to pivot into the area from the south are already noted on
radar in the Chicago area.

//DTW THRESHOLD PROBABILITIES...

* High for cig aob 5kft


&&

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

$$

DISCUSSION...SC/JD
MARINE.......SC
AVIATION.....JVC


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