Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191908

Area Forecast Discussion
National Weather Service Sioux Falls SD
208 PM CDT Sat May 19 2018

.SHORT TERM...(Today and Tonight)
Issued at 312 AM CDT Sat May 19 2018

There have been some significant changes in the overall forecast
over the past 24 hours. Thanks to the impacts of convection Friday
morning, we`ve seen a fairly significant shift in the synoptic
features that were expected to impact the area this weekend.

Early this morning, the surface frontal boundary has pushed well
south of the CWA and into eastern Nebraska and west central Iowa.
Cool and dry air has continuously filtered southwards overnight,
with lows in the 40s in most locations.  Aloft, a fairly stout upper
wave is pivoting through the Central Plains this morning, inducing
showers and thunderstorms across much of the state of Nebraska.

A branch of mid-level vorticity extending eastward out of the more
defined upper circulation in eastern Colorado will lift northward
through the day, and attempt to pull a narrowing band of showers
with it. The problem however is that there is a continuous flow of
drier low-mid level air advecting in from the northeast today,
and this dry airmass may significantly reduce the northward extent
of precipitation up to the I-90 corridor by this afternoon.

Speaking of this afternoon, a fairly widespread area of convection
will form over the lower and mid Missouri river valley early in the
afternoon with a convective system quickly moving east into Iowa
this evening. This convection may further shunt the moisture
transport back northward. All that said, have confined PoPs to the
southern half of the CWA through the evening hours.  Models do
indicate the potential for upwards of 700 J/KG MUCAPE approaching
Sioux City this afternoon, but shear overall is fairly weak and
severe weather risks should remain on the lower end. Clouds and
light rain will make the temperature forecast today tricky.
Certainly potential that if stratus doesn`t mix out, we could see
highs stuck in the 50s in many areas. Some areas north of Highway
14 could break into the 60s if stratus mixes out.

The upper wave slowly pivots eastward tonight and into Sunday
morning. Again, the highest risk for light rain or perhaps
drizzle will stay across NW Iowa and adjacent areas of MN/SD/NE
into daybreak. Have trimmed PoPs southward.

.LONG TERM...(Sunday through Friday)
Issued at 312 AM CDT Sat May 19 2018

At this point, Sunday doesn`t look like a washout by any means.
There should be fair amount of stratus that builds back into the
area. However, the upper system should now be moving east of the
area by mid-day and there is potential that we could jump back into
the 60s.

A fairly messy pattern develops across the CONUS for most of next
week. Subtle embedded shortwaves under the broad upper ridge will
produce a nearly daily chance for convection. Temperatures will
climb back above the seasonal normals into the upper 70s and 80s for
most of next week.

Nocturnal convection certainly looks possible Monday night and again
Tuesday night with a slightly stronger LLJ developing those nights.
Severe weather chances may be present each day from Monday night
through Thursday, albeit not a great setup for widespread severe
weather. Shear remains marginal for most of the week, but with dew
points rising back to the lower 60s, daily MLCAPE values approach
1000-1500 J/KG each afternoon and evening.  As mentioned earlier,
the LLJ could induce additional convection each overnight period.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1259 PM CDT Sat May 19 2018

The main concern will be low clouds and rain spreading across the
Missouri Valley into areas along and south of I-90. MVFR and
visibility and ceilings will be possible with this activity. At
this time, TSRA potential is very low. MVFR conditions will
persist tonight south of I-90, with improvements in southeast SD
Sunday morning into midday Sunday. Northwest Iowa will likely
remain within MVFR conditions through the bulk of the TAF period.




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