Area Forecast Discussion
Issued by NWS Sioux Falls, SD
FXUS63 KFSD 201712
Area Forecast Discussion
National Weather Service Sioux Falls SD
1212 PM CDT Thu Oct 20 2016
.SHORT TERM...(Today and Tonight)
Issued at 429 AM CDT Thu Oct 20 2016
Main concern today is potential for stratus. High pressure will
drift across the forecast area, giving west to northwest winds both
at the surface and above. At this time, stratus is very extensive
throughout much of ND, as well as central SD. Although being
obscured by high clouds, surface observations are showing stratus
right now in our western zones at Chamberlain, Mitchell and Huron.
Currently there is a very weak mid level short wave moving down the
northwest flow aloft providing the mid level clouds, and some light
precipitation just west of our forecast area. Therefore kept
scattered to isolated pops going for light rain in our far west
through the early morning hours. After that, the HRRR, RAP and the
NAM are all strongly suggesting that the aforementioned stratus will
move into the western half of our forecast through the morning, then
into southwest MN toward midday and this afternoon. Aside from the
early morning rain chances due to the short wave in our far west
early today, the stratus should be dry, but it will impact highs.
Therefore went with the cooler RAP and HRRR readings.
Tonight will also be dry, but there is question in exactly where the
stratus will be by the end of today and early this evening. The NAM,
HRRR and RAP are all pointing at an erosion of the lower clouds west
of I 29, with a bulk of the stratus predicted to be around Brookings
and in southwest MN. Not sure if enough mixing will take hold to
evaporate the clouds, but with a dry westerly flow of air becoming
southerly this evening, that could be enough to usher the low clouds
to the north and northeast. Therefore have a decrease in clouds in
southwest MN through the evening hours. With a light southerly wind
tonight, lows should not plummet too far, with 30s common. This
despite being only in the upper 40s to mid 50s today.
.LONG TERM...(Friday through Wednesday)
Issued at 429 AM CDT Thu Oct 20 2016
A period of very pleasant mid-autumn weather will commence on
Friday, as long as one does not mind a periodically stronger wind.
Sharp upper ridge axis will move quickly across the area on Friday,
followed by a quick moving trough along the Canadian border. While
heights will generally continue to rise as the broader ridging takes
hold, there will be just a bit of concentration to mid-level warm
air advection along with this troughing feature which will act to
increase cloudiness. Pressure falling in the western plains will
result in a stronger southerly wind gradient as well, ahead of weak
surface trough pushing toward central SD late. Far southwest CWA
will get close to creeping into the much more dry and still windier
westerly flow, but should keep the fire danger below very high at
this point. If not for some thicker clouds during the afternoon
near/east of I-29, would probably make a run at 60s everywhere, but
will likely remain in upper 50s in parts of southwest Minnesota and
northwest Iowa. Weak surface ridge builds in behind weak front on
Friday night, diminishing winds and with only a little bit of higher
cloudiness, temps will fall back toward the upper 30s to around 40
even in this very mild airmass.
Saturday will be the warmest day under peak strength of upper ridge.
Southerly flow will again become established quite early as the next
upper wave pushing into the ridge starts to organize low pressure
across the western Dakotas by late day. Remains a fairly dry
airmass and with indications of a fairly stout southerly gradient,
will get some fire danger into the high category toward lower Brule
locations, and would not take much of a wind increase to reach very
Stronger upper wave will squash down the ridge somewhat as it digs
across far northern areas Saturday night and toward the Great Lakes
on Sunday. Another fairly strong but dry cold front will drop across
the area Saturday night, with a stronger ridge of high pressure
settling into the northern plains. Northwest to northerly winds
will be a bit brisk on Sunday by early to mid morning, and will have
to watch the hints that a stratocumulus field could again plague at
least the northeast half of the CWA through part of Sunday along
with temps just a little cooler than forecast, but likely still
at or just above normal.
Ridge rebuilds Sunday night and Monday, and like clockwork the
eastward shifting surface ridge will give way to a southerly
gradient which will again increase on Monday. Temps remain mild,
mainly in the 60s, but will get a fairly good increase in higher
mid- to high-level clouds late in the day.
The precipitation threat in the forecast will commence in some areas
by later Monday night, and appears to peak Tuesday night or early
Wednesday as an even stronger trough breaks down the upper ridge.
Southerly flow may bring enough moisture to produce a resurgence of
stratus Monday night into Tuesday. Warm advection could generate a
few showers Monday night over southwest Minnesota and northwest
Iowa. Consensus for best precip chances to hold off until Tuesday
night, lingering into early Wednesday. Enough elevated instability
to warrant a thunder mention from Monday night through early
Wednesday. Will have to watch to see if the storms developing ahead
of wave in warm advection wing on late Tuesday into Tuesday night
will bring any more significant weather threat. Temps remaining
mild for late October with highs upper 50s and 60s, and lows upper
40s and lower 50s Tuesday and Wednesday.
.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1157 AM CDT Thu Oct 20 2016
Stratus will only gradually dissipate this afternoon, and may even
linger into the evening. While ceilings have gradually risen to
VFR levels, pockets of lower ceilings continue along and west of
the James River.
With stratus gradually diminishing this evening (likely lingering
well into the evening east of I-29), winds will turn light and
variable, with an increase in mid- high level cloud cover by
daybreak Friday. There could be a bit of fog later tonight in
areas that clear, but impact to terminals should be minimal.