Area Forecast Discussion
Issued by NWS Grand Forks, ND

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FXUS63 KFGF 241108
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
608 AM CDT Sun Mar 24 2024

.KEY MESSAGES...

- A major winter storm will impact the Northern Plains beginning
  today and lasting as late as Wednesday.

- Heavy snowfall accumulations (exceeding 8 inches) are likely,
  particularly from the tri-state border up through north-
  central Minnesota. Snowfall totals decrease the further north
  and west you get.

- Blowing and drifting snow, potentially significant in
  southeast North Dakota and west-central Minnesota, will cause
  major impacts to infrastructure and response time.

- High uncertainty in the northern extent of heavy snow remains,
  although certainty is increasing. The range of the 25th to
  75th percentiles have shrunk on the northern end of heavy snow
  to 3-8 inches.

&&

.UPDATE...
Issued at 607 AM CDT Sun Mar 24 2024

Frontogenesis snow banding continues across the central portions
of the CWA. Reports have come in of between half an inch to an
inch of accumulating snow. Visibility remains a very low impact
as observations continue to hover at the lowest 2 miles. This
will continue through the morning, with an additional half inch
to inch possible. More widespread snow will arrive through the
day today.

&&

.DISCUSSION...
Issued at 307 AM CDT Sun Mar 24 2024

Very weak frontogenesis has allowed for transient snow bands to
develop. With some light slantwise instability associated with this
frontogenesis, there have been some areas of heavier snowfall rates.
Having said that, the worst impacts that have been apparent thus far
are light snowfall accumulations and periodic visibility falling as
low as 2 miles. Still, with the okay-ish forcing and weak slantwise
instability, there may be areas of rates getting below a mile, but
these should be fairly short lived should they arise. Attention
then turns to the major winter storm that will impact the area
through Wednesday.

Snow will start to overspread the area through the day today.
Forcing looks to primarily be driven by large scale isentropic
ascent. For this reason, snowfall rates should be fairly light
between 0.25-0.50"/hour. There exists some mid-level instability
with lapse rates exceeding 7 K/km that arises across the southern
part of the CWA late this afternoon. This is being picked up on with
HREF probabilities for greater than an inch per hour for snowfall
rates between 10-20%. There isn`t more organized mesoscale support
associated with this, so more likely than not we will see some
embedded cellular heavier snowfall rate areas within the main swath
of lighter snow. Snowfall rates should not exceed much higher than
an inch per hour and without organized mesoscale ascent, residence
time also shouldn`t be fairly long.

As we progress into the evening and overnight, winds should start to
increase as the pressure gradient magnitude increases. The bulk of
the momentum looks to come from the surface pressure gradient,
however weak low-level mixing may allow for higher wind gusts to get
to the surface. The wind direction for this timeframe isn`t entirely
favorable for valley enhancement or other larger scale terrain
impacts, so sustained winds right now in the forecast are between 25
and 30 knots, with gusts as high as 40 knots. Gusts don`t look to
get much higher than this barring rapid deepening of the low which
doesn`t appear too likely at this time.

The current timeframe for the worst impacts will likely be Monday.
This is when gusty winds will align with enhanced snowfall rates
pushing towards 0.50"/hour for the longest duration. It is because
of this that this is the most likely timeframe for blizzard
conditions to arise in the south. Having said that, confidence is
very low that snowfall rates will be high enough for a long enough
period that blizzard conditions will be achieved. There is the
potential for inch/hour snowfall rates to arise on Monday afternoon
as frontogenesis aligns with upright instability. The motion of this
band would be perpendicular to the long-axis of this, so residence
time in one location would be fairly short (on the order of 1-3
hours). Having said that, HREF probabilities for greater than
an inch per hour within this band are between 20-30%, so there
is the potential to push into blizzard conditions Monday
afternoon. The blowing snow model indicates that 0.75"/hour
would be enough to get whiteout. The main question is if it
lasts long enough to be a blizzard.

As we progress into Monday night and Tuesday, snowfall rates should
diminish but accumulating snow will continue. With the elevated
winds continuing through the overnight hours, impacts will likely
shift more towards drifting snow. Significant drifting is likely in
southeast North Dakota and west-central Minnesota as winds will
remain elevated for a very prolonged period and accumulations will
exceed 8 inches. As we progress through Tuesday, winds will remain
elevated but with snowfall rates diminishing, so blizzard conditions
are unlikely at this time. Without falling snow, we would need to be
substantially higher on winds to achieve continued whiteout
conditions. Still, drifting will be a major concern through Tuesday.
The low begins to exit the area Wednesday and with it conditions
should improve.

Snow totals have been the main driving force of attention as we
have approached this event. As you may have already noticed,
ranges are very high between the 25th and 75th percentiles. This
has been because of ongoing uncertainty with the residence time
of snow and how tight the northern snowfall gradient is. The
message remains the same:

DO NOT LOOK AT ONLY THE HIGHER NUMBER FOR SNOW TOTALS.

This event will be lasting over multiple days. Not all of the
snow will fall all at once and in a 24 hour window. 24 hour
snowfall totals generally look to be at most 8 inches in even
the heaviest locations. Additionally, snowfall on the northern
end is very likely to fall closer to the 25th and 50th
percentiles as certainty increases in how tight the snowfall
gradient is. For this reason, we want to reiterate: Heavy
snowfall WILL cause significant drifting and blowing snow in
southeastern North Dakota and west-central Minnesota.
Accumulating snowfall WILL occur across the region. Not everyone
will receive a foot of snow. Not everyone will even receive six
inches of snow. There are areas within the Winter Storm Warning
that may not even achieve 4 inches of snow. This system is very
volatile and our snowfall totals are highly dependent on track,
duration, and strength of the low, which ensemble members
continue to struggle to gain a handle on. If you are in a Winter
Storm Warning, prepare now for warning impacts, but they are not
a guarantee the further north and west you get. Do not wait for
a Blizzard Warning to prepare, as this will likely end up as a
lower lead time event for blizzard conditions. Regardless of if
blizzard conditions arise, drifting snow and heavy snow will
cause major impacts, so if a Blizzard Warning is not issued that
does not mean major impacts are not occurring. We haven`t had a
lot of these systems this year, but now is a good time to
prepare.

Through the remainder of the week, impacts look to be fairly low
with the potential for additional rain/snow by next weekend, but EFI
signals aren`t raising any alarm bells at this time.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 607 AM CDT Sun Mar 24 2024

Ceilings will fall to MVFR for most TAF sites through the day
today, with VFR ceilings likely lingering at DVL. Snow will
begin to overspread much of the area, although how significant
visibility impacts will be is very much in question. There is
high confidence that IFR visibilities will at least make it to
Fargo, but may not progress much further north than that. Winds
will increase through the day today with gusts approaching 30
knots at times, especially further south.

&&

.FGF WATCHES/WARNINGS/ADVISORIES...
ND...Winter Storm Warning from 10 AM this morning to 10 AM CDT
     Tuesday for NDZ024-026>030.
     Winter Storm Warning until 7 AM CDT Tuesday for NDZ038-039-049-
     052-053.
     Winter Weather Advisory from 1 PM this afternoon to 10 AM CDT
     Tuesday for NDZ008-016-054.
MN...Winter Storm Warning from 10 AM this morning to 10 AM CDT
     Tuesday for MNZ001-002-013>016-022-023-027-028-030-031-
     040.
     Winter Storm Warning until 7 AM CDT Tuesday for MNZ003-029.
     Winter Weather Advisory from 1 PM this afternoon to 10 AM CDT
     Tuesday for MNZ004-005-007-008.
     Winter Storm Warning from 1 PM this afternoon to 1 PM CDT
     Tuesday for MNZ006-009-017-024-032.

&&

$$

UPDATE...Perroux
DISCUSSION...Perroux
AVIATION...Perroux


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