Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 182349

Area Forecast Discussion
National Weather Service Sioux Falls SD
649 PM CDT Mon Sep 18 2017

.SHORT TERM...(This evening through Tuesday night)
Issued at 347 PM CDT Mon Sep 18 2017

Stratus and fog across the eastern portions of the forecast area are
expected to become more widespread tonight as moisture becomes
trapped below the elevated inversion.  With increasing southeast
flow at the surface, expect the fog to expand back to the west. Flow
for the most part remains light enough through the night that fog
has the potential to become dense.  Confidence is not high enough at
this point to issue any headlines, but will need to keep a close eye
on it.

Pressure gradient really increases across the area on Tuesday
morning aiding in visibility improvement across the area.  Expect
the stratus to erode from west to east throughout the morning. Moist
boundary layer along with steep environmental lapse rates from 800-600
mb will lead to strong instability.  Elevated inversion between 800-
850 mb will keep a lid on convection throughout much of the day,
ahead of an approaching shortwave/cold front.  Expect convection to
develop across Central South Dakota, working east along the front
through the evening. Strong speed shear throughout the lower half of
the troposphere along with strong directional shear in the near
surface layer makes all modes of severe weather possible.  0-6 km
Bulk shear ranges from 40-50 knots, and with strong downdraft CAPE
near 2000 J/kg with deep dry layer in the mid levels of the
troposphere.  Cap strength is more of a question south of I-90,
possibly limiting storm coverage.  However, anything that is able to
develop has a strong potential of being severe late Tuesday night
into early Wednesday morning.

.LONG TERM...(Wednesday through Monday)
Issued at 347 PM CDT Mon Sep 18 2017

Warm and breezy Thursday and Friday, followed by a wet weekend!

Wednesday currently looks to be cooler and drier than Tuesday, as a
surface high pressure quickly moves northeast through the high
Plains in the wake of Tuesday evening`s shortwave. This change will
be short-lived, with a large, deepening upper trough over the
western United States and a building ridge over the eastern U.S. At
the surface, this will set up strong southerly flow over the central
U.S., advecting in an unseasonably warm and humid airmass into the
Northern Plains Thursday and Friday. Winds in the 15 to 25 mph range
with gusts in the upper 20s and 30s are likely Thursday and Friday

This weekend`s large scale pattern also favors a stalled frontal
boundary between the aforementioned trough and ridge. Current model
runs show this boundary setting up over the tri-state area Friday
evening through early next week, and with this would come the
potential for multiple days of moderate rain. Multiple shortwave
troughs will likely move through the large scale flow over the
weekend, and it`s possible that these could further enhance lift and
bring periods of heavy rain and perhaps strong thunderstorms as
well. Models do not resolve shortwave trough timing details well 5
to 6 days out, so it`s hard to pin down exact timing. Nevertheless,
plan for a wet and active weather weekend. Those with outdoor event
plans this weekend will want to pay close attention to how the
forecast evolves over the next several days.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 641 PM CDT Mon Sep 18 2017

Low stratus is expected to linger across portions of southwest MN,
northwest IA and far southeast SD this evening. Expect this layer
to expand westward overnight to cover much of the forecast area
by late tonight. MVFR and IFR ceilings will be possible at all 3
TAF sites, with enough low level moisture return to create MVFR
and IFR fog as well. Expect these low conditions to last through
much of the morning, but should improve towards the end of the TAF




LONG TERM...VandenBoogart
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