Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191133
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
633 AM CDT Fri May 19 2017

.SHORT TERM...(Today and Tonight)
Issued at 352 AM CDT Fri May 19 2017

Currently, a large upper low is centered over Colorado, with surface
low pressure situated over the southern plains. To the north, an
unseasonably large and cool high pressure system extends throughout
the entire northern plains and Great Lakes region, and into southern
Canada. Another short wave is clearly lifting northward this
morning right across the forecast area, evident per the mid and
upper QG forcing, PV 1.5 pressure surface, and thetae ridging in
the 305-320K layer. The problem is, the aforementioned high is
draining a lot of dry air into the low levels across the area,
especially north of I 90. The various short range models and CAMs
have picked up on this, and thus we have reduced pops in our north
to either slight chance or low end chance. POPs along and south of I
90 of high end likely to categorical are still warranted, and indeed
the rainfall is finally beginning to fill in now across our far
southern zones. As the wave moves northward this afternoon, chances
for rainfall will become more skittish. Because of this and the dry
air in the low levels, decreased QPF amounts today to generally less
than a half inch south of I 90, with very minimal amounts north of I
90.

The upper wave moves eastward tonight into the western high plains,
with surface low pressure strengthening and moving toward the Kansas
City area by late tonight. Deeper moisture begins to once again form
ahead of this next wave along with strong trowaling for this time of
year taking shape on the backside of the surface low in the 305-320K
layer. After a fairly quiet evening, high pops will drift northward
once again into the forecast area, with the highest pops warranted
along the trowal, along an axis from Windom MN to Yankton SD.

The other big story today and tonight is obviously the cold
temperatures. Moderated highs slightly today by not using any
bias corrected data, which seemed very cold given the current
temperatures. Therefore have highs today very close to our current
readings right now, and also noting that if we go rain free this
afternoon, that should allow for slightly more mild temperatures
then what the bias corrected values are advertising. Therefore have
a lot of mid 40s to lower 50s forecast, and many of these readings
will be in the top 5 coldest maximum temps for May 19. Tonight due
to the cloud cover, lows will not drop off a lot with a lot of lower
40s forecast.

.LONG TERM...(Saturday through Thursday)
Issued at 352 AM CDT Fri May 19 2017

Cool pattern expected to continue into the weekend.  TROWAL looks to
impact the region on Saturday keeping temperatures in the 40s. Blend
came in cooler for Saturday, which looks reasonable with wet and
cloudy day across the area.

Temperatures look to moderate some on Sunday into Monday, however,
another cold front is expected to move in late in the day knocking
temperatures back.  For now, have struck with the blend on forecast
highs, but if the front moves in quicker as GFS suggests, forecast
highs will need to be dropped north of I-90.  Could see some showers
and thunderstorms with the frontal passage, but instability is
somewhat limited.

Cooler conditions expected on Tuesday before temperatures moderate
in the middle and late portions of next week. Temperatures remain
below normal through the period.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 629 AM CDT Fri May 19 2017

VFR will likely prevail at KHON until very late in the TAF period,
when MVFR rainy conditions may move into east central SD.
Otherwise heading southward, along and south of I 90, conditions
will become increasingly MVFR to possibly upper end IFR at times
with rainfall this morning, and again tonight.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...MJ
LONG TERM...BT
AVIATION...MJ



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