Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 182039

Area Forecast Discussion
National Weather Service Sioux Falls SD
339 PM CDT Thu May 18 2017

.SHORT TERM...(This evening through Friday)
Issued at 324 PM CDT Thu May 18 2017

Cool and wet will be the dominant theme over the next 24 hours.
Most confident in precipitation chances late tonight into Friday
morning for areas along and south of I-90, with greater question
across the northern portions of the area due to uncertain influence
of dry east-northeasterly flow.

For tonight, lingering stratus will gradually be overtopped by an
increase in mid-high level clouds as a shortwave rotating around a
deep trough over the Rockies swings north-northeast into the region.
Deepest saturation in response to strong isentropic lift expected to
reach the Missouri River Valley around 06Z. This then progresses to
the north late tonight, and rain should reach the I-90 corridor by
12Z Friday. Isentropic flow north of I-90, especially into southwest
Minnesota, initially appears to come from a drier source region to
our east, so will be much slower to increase pops into the northeast
portion of the CWA. Still some model discrepancy regarding just how
strongly the dry air holds vs. the ability of stronger lift to fully
saturate the column, though greater consensus shows light showers
nearing the Highway 14 corridor at some point Friday morning, so
will stick with at least likely pops to Highway 14, limiting the
chance range to our extreme northeast around Ivanhoe/Marshall.
Rainfall amounts averaging a half inch to an inch still appear
possible through Friday for areas along and south of I-90, with
amounts quickly tapering to less than 0.20 inch in our far north.

The upper wave lifts north of the area through Friday afternoon,
taking the leading band of mid-level moisture with it. While broad
scale isentropic lift appears to persist, the loss of deep moisture
should result in decreasing coverage of rain through the afternoon
hours. Low level moisture remains locked in place, however, along
with a very cool air mass and brisk east-northeast winds. Despite
the diminishing precipitation trends, the widespread clouds will
keep temperatures well below seasonal normals. Currently projecting
highs in the mid-upper 40s, with a few locations perhaps breaking
records for coldest high temperatures for May 19.

.LONG TERM...(Friday night through Thursday)
Issued at 324 PM CDT Thu May 18 2017

Cold and wet conditions continue into Saturday as the cool upper
level low continues to slowly lift north northeast across the
Plains. With seasonally cold mid and low level temperatures, cloudy
conditions and rainfall, highs on Saturday will be similar to Friday
in the mid 40s to lower 50s. Superblend came in cooler and more in
line with thermal profiles. There is some potential that the dry
slot will lift north across portions of Iowa, lowering rain chances
there on Saturday, however most model solutions paint a high chance
through the morning hours. Went ahead and boosted pops into
categorical Friday night and Saturday.

Upper low lifts into northern Minnesota Saturday night, which will
allow rain chances to gradually taper off. With the low potentially
stalling over the upper Midwest into Sunday, a few showers may
linger across southwest Minnesota. Clouds will linger through the
day Sunday with stratocumulus clouds lingering and breezy northwest
flow. Mixy conditions will improve temperatures, however did lower
highs a few degrees across southwest Minnesota and parts of
northwest Iowa where greater cloud cover and colder temperatures
aloft linger.

A secondary trough drops across the northern Plains on Monday,
bringing yet another round of scattered showers, mainly in the
afternoon and evening. Instability is fairly weak, but forecast
soundings suggest that isolated thunder mention is reasonable.
Models a bit less certain with the timing of this system and how
quickly the trough swings out of the region. Not enough confidence
to alter pops at this point.

Models further diverge Wednesday and beyond, however it does look
like overall that conditions remain dry. The GFS does bring a quick
wave through Minnesota Thursday afternoon or night, however will
leave the forecast dry for now. The good news is that temperatures
will slowly climb back into the 70s.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1210 PM CDT Thu May 18 2017

Stratus will continue to dominate the forecast for much of the
region. HON will be on the edge of the cloud shield with VFR
conditions prevailing by the mid afternoon. FSD and SUX will
continue with MVFR/low VFR conditions through the period.
Widespread rain will approach from the south and eventually impact
all TAF sites overnight Thursday into Friday morning.




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