Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 141545
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
945 AM MDT Mon Aug 14 2017

.UPDATE...
Issued at 945 AM MDT Mon Aug 14 2017

Current surface analysis shows an area of low pressure over
eastern MT, with a warm front extending southeast into the Black
Hills area and a cold front extending southwest into southwestern
WY. Upper level analysis shows weak ridging centered over the
Northern Plains, with low pressure over southwest Canada. Trough
extends southward across the Pacific northwest and northern
Rockies, with embedded shortwave energy moving across MT and WY
into the western Dakotas. Clouds have increased across northeast
WY and much of western SD with isolated light showers mainly from
northeast WY into northwest SD. Temps are mostly in the 60s with
light and variable winds.

Have updated forecast mainly to make some adjustments to pcpn
chances through the afternoon. Models generally show the main wave
that will impact the area later today and tonight will be a bit
slower, so have lowered pops and pushed them further west for the
early afternoon. Still looks like northeast WY into parts of far
western SD will see the best chances late afternoon, spreading
east across the rest of the area this evening. A few strong to
severe storms with hail and strong wind gusts still look possible
later this afternoon into this evening from northeast WY to the
Black Hills area and southwest SD as MUCAPE values of 1000 to
around 1500 j/kg are progged over these areas near and ahead of
southeastward moving low pressure and frontal boundary. Will
leave high temps as is for now, but may need to make some
adjustments downward if more significant cloud cover hangs around
into the afternoon.

&&

.DISCUSSION...(Today Through Sunday)
Issued at 344 AM MDT Mon Aug 14 2017

Upper level analysis has low amplitude pattern across the CONUS
with a trof across the east and west, and weak ridge crossing the
northern Plains. Skies are partly to mostly cloudy across the area
with some very weak echoes showing up on radar across
northeastern WY. At the surface, weak low pressure resides across
eastern MT with frontal boundary to the west and south of the
area. Surface high pressure is located across ND with light east
to southeast winds across western SD. Temperatures at 3am are in
the 50s.

First of several upper shortwaves associated with the western
trof will move through central MT early this morning. The second
shortwave will track a little further south and east, and approach
the area late this afternoon. This will result in partly to
mostly cloudy skies with some weak showers/storms early in the
day, then a much better chance of stronger storms this afternoon
and evening. Southeasterly winds will become breezy across the
western SD plains ahead of the Montana surface low, and MLCAPE
will increase to 1000-2000 J/kg during the afternoon. Zero-6 km
shear of 40+ kts will support the development of strong to severe
thunderstorms. Showers/storms will continue tonight and through
Tuesday as a third wave slowly slides through the northern plains.
A low level jet will develop tonight, continuing to bring
moisture into the area, along with supporting continued favorable
shear. Heavy rain will be possible with any storm that develops,
especially late tonight into Tuesday, when PWATs are expected to
be 150% of normal.

Showers and storms will finally shift east of the area Tuesday
night, with Wednesday expected to be mainly dry. Another
shortwave trof will cross the area on Thursday, producing
additional chances for showers and storms.

Models are still showing a ridge towards the end of the week,
bringing drier weather.

Today and Tuesday will continue with below average temperatures,
but a slow warming trend will begin on Wednesday with near to
above average temperatures expected late this coming weekend.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Tuesday Morning)
Issued At 456 AM MDT Mon Aug 14 2017

An upper level trough will bring increasing chances for TSRA,
especially late this afternoon and tonight. Local MVFR conditions
will occur with the storms as well as hail/gusty winds. Outside
of storms, VFR conditions expected through this evening, but MVFR
CIGS may develop late tonight across the SD plains.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

Update...26
DISCUSSION...15
AVIATION...Helgeson


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