Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 211951
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
351 PM EDT Thu Mar 21 2024

.KEY MESSAGES...

- A Winter Weather Advisory is in effect tomorrow for a greater
  portion of SE MI. Snowfall accumulation ranging between 2-6 inches
  will be possible for counties within the advisory, with the higher
  end totals favored along or north of I-69.

- Dry weather prevails this weekend

- Warmer temperatures and rain showers return for early next week.

&&

.DISCUSSION...

An upper-level shortwave now tracking across Montana will weaken
across the northern Plains before funneling into a sheared wave that
will drop in from the western Canadian Provinces. This wave will
push into the northern Plains before extending into the northern
Great Lakes tonight. Weak trough amplification will take place
across the Plains as a result, which will strengthen an already
pronounced temperature gradient downstream across southern Michigan.
This will elicit a strong frontogenetic response across southern
Michigan starting tomorrow morning, with a secondary frontogenetic
response sweeping across SE MI later in the day and into the night
in response to the increased proximity to the upper-level trough and
cold front. This will bring two possible windows for snow
accumulation across the Metro region, with locations along or north
of I-69 having higher probability for more of a sustained snow
through the day.

Given the bulk of the forcing will be driven by mesoscale processes
which can lead to snow banding, and noting proclivity for NWP to not
handle mesoscale banding/saturation location well, high uncertainty
for localized higher snow amounts will exist leading into the event
until upstream observations can be leveraged. There will however be
two main windows for higher impact accumulating snow:

The first window for snow will arrive during the rush hour commute
as elevated warm air advection/isentropic ascent commences across
all of SE MI, with frontogenetic forcing maximized somewhere across
the I-94 to I-69 corridor. Generally lighter snow around an inch is
expected through the morning, but a localized maxima of 2-3 inches
cannot be ruled out given the strength of the fgen with near neutral
values of epv above the strongest forcing. The HRRR/ARW exhibit a
southern solution for possible snow maxima across the I-94 corridor,
while the NSSL holds the bulk of snow accumulation north of I-94.
The NAM solutions are the outliers that hold all snow accumulations
along or north of M59 through the morning and afternoon hour as low
level moisture in the model solutions remain dry. There are some
small uncertainties regarding onset of snow given the very dry
conditions, noted by the single digit dew points at present time.
Plenty of virga will likely initially be observed on radar as top-
down saturation commences, with snow reaching the ground between
the 6-9 AM window.

The corridor of best forcing will pivot north into the afternoon
hours which will likely produce a lull in precipitation across the
Metro region, possibly as far north to the M59 corridor, as a dry
slot works in and temperatures increase in the mid to upper 30s.
Locations north will likely see concurrent snowfall through the
afternoon hours with possible rain mixing in across the I-69
corridor. Low-level frontogenetic forcing will become deeper in
response to the frontal boundary pivoting to a cold front, with
added support from the approaching shortwave and upper-level right
entrance dynamics from a jet streak over Lake Huron. The deeper
ageostrophic response will increase snowfall rates through the
afternoon and evening hours along or north of the I-69 corridor. Mid-
level lapse rates aoa 6 C/km and potential for a convective response
with mid-level instability will bring the possibility to see a 1-3
hour window of 1-1.5 in/hr snow rates, favored along or north of I-
69. The HREF highlights the Tri-Cities with the highest probability
to see this (80% chance for >.25" qpf in a 6 hour window). This is
where the higher confidence window exists for snowfall totals
reaching 3-6", with localized higher amounts possible if sustained
banding is maintained.

The cold front will return snow chances back into the Metro region
later in the evening, but the increased translation speed of the
front and warmer temperatures should aid in capping snowfall totals
relative to the northern counterparts. Still an additional quick
inch of snow in a 1-2 hour period cannot be ruled out late tomorrow
night, respecting the forcing of the front. There is also a low
(<20%) chance for sleet mixing in around or south of I-94 as the
front nears tomorrow evening, pending how strong the warm nose aloft
is. This would cut down snow totals to end the event.

Overall, taking into consideration the late March sun angle eating
away at some accumulation potential, with uncertainties regarding
location of mesoscale banding potential, will opt with a high-end
winter weather advisory for M59 north, with totals generally ranging
between 3-6 inches. Localized totals in excess of 6 inches will be
possible pending location of snow banding. Snow rates of 1+ inches
will be possible with banding and can rapidly coat roadways. Snow is
expected to come to an abrupt end after the passage of the front as
deep dry layer subsidence builds in tomorrow night.

Dry air holds over the Great Lakes Saturday with a high pressure
system building over the northern Great Lakes, with dry conditions
prevailing through Sunday.

&&

.MARINE...

High pressure passes over the area this afternoon and evening with
continued light wind and waves. Winds then organize out of the east
and southeast overnight through the day Friday as low pressure
tracks from the Midwest into the Ohio Valley/southern Great Lakes.
This system will bring widespread snow and rain across the region on
Friday with wind speeds around 15 to 20 knots. Waves will build
along the shoreline of the Thumb and a new round of Small Craft
Advisories has been issued there. Stronger north to northwest wind
develops on Saturday as the system passes east and ushers in an
arctic air mass. Potential for gales remains low, but winds of 20 to
25 knots will be possible. A ridge of high pressure then builds in
Saturday night into Sunday with weaker winds expected.

&&

.PREV DISCUSSION...
Issued at 1238 PM EDT Thu Mar 21 2024

AVIATION...

Elevated frontal forcing in advance of approaching wave of low
pressure will bring an expanding lower VFR cloud deck by early
Friday morning with eventual snow developing as the low levels
saturate through Saturday morning. The best forcing/lift looks to
focus north of I-94, but questions remain as to where the heaviest
and most persistent snow banding sets up between KPTK and KMBS.
Highest confidence remains in the KMBS/KFNT corridor with lesser
(but not zero) chances of decent snow into KPTK Thursday morning.
IFR or lower vsbys will occur in the heaviest snow with MVFR to
lower VFR cigs/vsbys south of main banding. Even a chance of dry
conditions along I-94 throughout the event.

For DTW...Other than FEW-SCT cigs around 4kft this afternoon, only
expect increasing/thickening mid clouds into this evening and the
first part of tonight. Cigs will drop below 5kft in the 11z-14z and
will tentatively add p6sm -sn during this period. From late morning
into Friday afternoon, most snow should pivot north of terminal and
will shape the forecast that way for this issuance.

DTW THRESHOLD PROBABILITIES...

* Low for ceilings aob 5kft early Friday morning.

* High for precipitation being all snow Friday morning, but low on
  the degree of snow actually impacting the terminal


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 6 AM Friday to midnight EDT Friday
     night for MIZ047>049-053>055-060>063-068>070.

Lake Huron...Small Craft Advisory from 10 AM to 7 PM EDT Friday for LHZ441-442.

Lake St Clair...None.
Michigan waters of Lake Erie...None.
&&

$$

DISCUSSION...AM
MARINE.......TF
AVIATION.....DG


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