Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 240528 AAC

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1228 AM CDT Wed May 24 2017

Issued at 1225 AM CDT Wed May 24 2017

See updated aviation discussion below.

UPDATE Issued at 932 PM CDT Tue May 23 2017

Based on the reduction in cloud cover per sat pix and expected
light winds overnight, have decided to expand the frost advisory
to the James Valley, where it typically gets cold under these
circumstances. No other major changes at this time.


.SHORT TERM...(This Evening through Wednesday Afternoon)
Issued at 231 PM CDT Tue May 23 2017

Forecast challenge will be areal coverage of killing frost or
killing freeze tonight.

Currently, under a cloudy sky with scattered showers dotting the
radar, temperatures are generally in the 50s, with some low 60s
noted across portions of north central South Dakota. Winds are out
of the north at 15 to 25 mph with some higher gusts.

Tonight: looking at somewhere between 6 and 9 hours of an ideal
radiative cooling set up, where temperatures will spend a long time
(more than 3 hours) down in the low to mid 30s.  This will likely
place a good portion of central and north central South Dakota in a
potential killing frost (or if temperatures drop below freezing, a
killing freeze) situation. Setting the bar for now, at frost
advisory. Much depends on how quickly the cloud cover goes away this
evening, and whether or not the clouds clear off completely or stick
around overnight. Started off with the coldest guidance and then
knocked a few degrees off it, especially over central/north central
South Dakota where guidance has the clouds completely dissipating by
mid-evening timeframe. Some of the model output/guidance suggests
that even portions of northeast South Dakota go clear by midnight or
1 AM CDT tonight, leaving the door open for 3 to 5 hours of
radiative cooling potential and perhaps a shot at some 20 to 50
percent coverage of frost in some areas there as well. For now,
setting the bar at "areas to widespread" areal coverage of frost and
a frost advisory for the counties in this CWA west of the James
River valley (west of Brown/Spink counties). With a noticeable
warm bias in the numerical guidance the past several mornings when
temperatures dropped to between 25 and 35 degrees, taking note of
tonight`s coldest guidance advertising readings down to between
35 and 38 over a large area, and going ahead and knocking those
temperatures down some. Lows tonight are currently forecast to
range from 32 to 36 degrees over central/north central South
Dakota, with warmer readings further east where cloud cover is
supposed to hang on longer into the night. Won`t be surprised if
it turns out that some locations out over central/north central
South Dakota overnight drop below the freezing mark, too.

Dry weather conditions on Wednesday and much more sunshine should
have most locations rebounding nicely into the 60s for high

.LONG TERM...(Wednesday Evening through Tuesday)
Issued at 231 PM CDT Tue May 23 2017

Light rain showers return mainly west river Wednesday night, as the
500mb ridge shifts east across MN and the next low swings into
southern Saskatchewan. At the sfc, the main low will be across
Saskatchewan, with a cold front draped southward near the ND/SD
border around 00Z Thursday. An additional low will form along this
boundary over western SD Wednesday night. The main concern may be
just above the sfc, as a 35-45kt south to southeast LLJ sets up
(lightest over our eastern counties overnight).

The sfc low to our west will weaken as we move through the day
Thursday, with the cold front swinging west to east over the
forecast area. Quick mixing into the strongest winds will result in
some gusty morning winds ahead of the cold front, over and east of
the James River Valley. Will keep Thursday mostly dry, with slight
chance pops at most, as an area of clouds accompanies the cold
front. Look for this boundary to only slowly exit into MN during the
day Friday as the low moves across Manitoba. The result will be
mainly dry weather Thursday night through Friday night.

The 500mb low will settle in across south central Canada through
Saturday, before possibly dipping across the international border
(per the 12Z GFS) and exiting eastward. The Canadian and ECMWF are
not as south with this feature, but they all agree that the
circulation will continue over the Great Lakes and Ontario from
Sunday afternoon through at least early next week. For our area this
will mean west to northwest flow from 06Z Sunday on, and no return
of well above normal temperatures.

Saturday afternoon could bring a round of mainly afternoon showers
as the 500mb trough slides overhead, and is assisted by daytime
heating/instability as a secondary through swings around the main
low. Additional waves will rotate in Sunday and Monday. Can`t rule
out a rumble of thunder later Saturday afternoon, Sunday afternoon,
or Monday afternoon with isolated/slight chance showers.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
Issued at 1225 AM CDT Wed May 24 2017

VFR conditions will prevail across the area overnight and through
the day Wednesday.


SD...Frost Advisory until 9 AM CDT /8 AM MDT/ this morning for



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