Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 231134 AAA
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
634 AM CDT Sat Mar 23 2024

.KEY MESSAGES...

- Initial band of light snow moves east across the area today.
  Amounts from a dusting to as much as 2 inches. Onset of snow
  from the main storm system on Sunday, with as much as 4 to 6
  inches by the end of the day.

- Potential for icing has increased across portions of south
  central South Dakota, including the I-90 corridor, where a few
  hundredths to perhaps a tenth of an inch ice accumulation is
  possible.

- Snow will continue to be heavy at times Sunday night through Monday
  night, especially across portions of northeast South Dakota and
  west central Minnesota. Increasing northerly winds during that
  time will lead to deteriorating conditions such as blowing snow,
  low visibilities and potential blizzard conditions.

- Well below normal temperatures will persist next week in the
  wake of the winter storm. The potential exist for another storm
  system to move through the Plains toward the end of the work
  week. This will be our next chance for rain and or snow.

&&

.UPDATE...
Issued at 624 AM CDT Sat Mar 23 2024

Aviation discussion updated below for the 12Z TAFs.

&&

.SHORT TERM /TODAY THROUGH SUNDAY AFTERNOON/...
Issued at 419 AM CDT Sat Mar 23 2024

Initial band of light snow still on track to move east across the
area today. In fact, radar this morning shows this already
materializing across northwest SD. Hi-res simulated reflectivity
output shows this band moving eastward across north central SD
through the morning, and potentially making its way into the eastern
CWA by this afternoon, but more uncertainty with the eastward extent
is showing up in solutions. Still appears to be a light accumulation
event, with a dusting to as much as two inches. These amounts are
confirmed by HREF family of solutions, as well as various
probabilistic thresholds.

Focus then shifts to the onset of snowfall/mixed precip tonight and
through the day Sunday. Did push back the start time of the watch to
06Z tonight based on latest guidance showing most precip moving into
the region between 06Z and 12Z tonight, and closer to 12Z Sunday
across the east. One change that appears to have cropped up with
this forecast cycle is an increase in icing potential across the
southwest CWA, mainly along the I-90 corridor. Models are now trying
to bring a warm nose aloft up into the southwest CWA, thus nudging
the rain/snow line a bit further north towards I-90. HREF mean
freezing rain accumulation is a few hundredths along I-90, but as
high as 0.10-0.15 across the Lyman bootheel. HREF 25th-75th spreads
for icing do give some uncertainty to this though, with 25th
percentiles generally showing zero freezing rain. Something to look
at in more detail today and may want to consider possibility for an
advisory if icing potential continues to show up with higher
confidence.

Turning to the snow potential on Sunday, still appears snow will
fill in rather quickly across the CWA through the morning. High
confidence in this based on model reflectivity output through the
morning. HREF 24-hr mean snowfall (assuming 10:1 ratio) is generally
3 to 5 inches across the area, especially from north central SD
eastward into the Glacial Lakes region. Ratios may run close to 12:1
across the north, so perhaps a solid 4 to 6 inches just through 00Z
Monday, setting the stage for additional accumulations Sunday night
through Monday as we transition to the extended period.

As for headline decisions, with the onset of freezing rain potential
now over the southwest counties, and some uncertainty on amounts
across north central SD (possibly lower than forecast?), will
maintain the watch and day shift can upgrade. Still uncertain how we
will handle winter storm vs blizzard wording, as true blizzard
conditions (if things pan out as models suggest) may not get going
until Sunday night after several inches have already fallen Sunday
during the day. Felt one more model/forecast cycle should give
higher confidence on how to construct hazards. This seemed to be the
consensus for most other Northern Plains offices as well. All that
said, still potential for over a foot for much of the eastern CWA,
maybe even a foot and a half. Amounts across central SD may be
closer to 10 inches or less.

&&

.LONG TERM /SUNDAY EVENING THROUGH FRIDAY/...
Issued at 419 AM CDT Sat Mar 23 2024

The main focus in this portion of the forecast will be the ongoing
winter storm that will be underway by the time this period begins
Sunday night. Not much of a change in the overall pattern among the
various guidance from the deterministic/ensemble solutions. If
there`s anything to pull away from the 00Z data, is that the GFS/CMC
guidance has ever so slightly shifted the track of the sfc low a bit
farther east and south from previous runs. It`s perhaps more of a
subtle shift and overall impact on expected weather conditions maybe
not noticeable, but it is something to take note of.

This storm, a classic Colorado sfc low is progged to shift eastward
into the Central Plains on Sunday night and then hook northeast
toward Omaha by Monday morning before making it`s way across Iowa
during the day and into southeast Minnesota and western Wisconsin
late Monday into Tuesday. Snowfall will be ongoing across this
forecast area Sunday night. Intensities of the snowfall will likely
vary depending on location, but the focus for heavier accumulating
snow will be shifting away from our western zones and into our
eastern zones overnight going into Monday morning. This could
possibly have an impact on anticipated snowfall for our western
zones(i.e. lower amounts, but it`s still too early to hang your hat
on that and confidence isn`t the greatest. NBM snowfall
probabilities for selected amounts remain impressive for this
period, particularly in our eastern zones. For example, chances of
seeing at least 12 inches or more between 7PM Sunday night and 7PM
Monday night are at least 50% or higher from parts of the James
Valley east into the northeast corner of South Dakota and west
central Minnesota. Parts of the higher elevation of the Prairie
Coteau sit at a 70-80% chance. So, confidence remains higher for
seeing higher snowfall values come out of our eastern zones,
especially in that I-29 corridor and points east.

Strong winds will be a concern still by the time Sunday night and
Monday rolls around. With the sfc low positioned south and southeast
of our region at that time, wind direction will have switched around
to the north and northeast. A tightening gradient across the region
will lead to increasing winds overnight. There remains at least a 70
to 80 percent chance of seeing sustained winds reach 25 mph or
higher Sunday night through Monday night across most of our forecast
area. These high chances translate eastward by Monday night into
Tuesday night where forecast zones along and east of the I-29
corridor have at least a 70 to 80 percent chance of seeing 25+ mph
sustained winds. The larger impact time frame will be from Sunday
night through Monday night where we will still have falling snow
forecast. Visibilities will be reduced significantly due to the
intensity of the snowfall but also because of the strong winds as
the snow is falling. The Winter Storm Watch will remain posted and
continue to include the mention of possible Blizzard conditions.

Conditions will begin to quiet down toward the middle of next week
as a ridge of high pressure at the sfc and aloft take over our local
weather pattern Temperatures will remain well below normal thanks in
part to the new snowpack on the ground and a cold air mass being
reinforced from the north. By late in the period, another system
will possibly be tracking into the Plains. This may bring our area
the next round of precipitation, some of which could be in the
wintry form. Plenty of time still exists between now and then to
keep an eye on what may become of that disturbance.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 624 AM CDT Sat Mar 23 2024

Terminals KABR,KATY,KPIR,KMBG

An area of -SN moving into central SD this morning may bring a
period of MVFR/IFR VSBY to KPIR/KMBG, along with CIGs approaching
or into MVFR. Elsewhere across KABR/KATY, VFR conditions to start
off the TAF period, but CIGs are forecast to decrease into MVFR as
the day/evening progresses. Onset of -SN/SN with the approaching
storm system for Sunday may arrive as soon as the tail end of the
TAF period.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from late tonight through Tuesday morning for
     SDZ007-008-011-019>023.

     Winter Storm Watch from late tonight through late Monday night
     for SDZ003>006-009-010-015>018-033>037-045-048-051.

MN...Winter Storm Watch from late tonight through Tuesday morning for
     MNZ039-046.

&&

$$

UPDATE...TMT
SHORT TERM...TMT
LONG TERM...Vipond
AVIATION...TMT


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