Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 270943
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
343 AM MDT Sun May 27 2018

.DISCUSSION...(Today Through Saturday)
Issued at 340 AM MDT Sun May 27 2018

Water vapor shows broad upper low over the Great Basin, with upper
ridge extending from the Central Plains northward through the
eastern Dakotas. At the surface, low center is over northwest SD
with frontal boundary extending southwest through the Black Hills
into southeast WY. Early morning convection is ongoing from
northeast WY/southeast MT through western ND (where an MCS is
tracking eastward).

Majority of elevated morning convection should be confined to
northeast WY, with the rest of the forecast area remaining mostly
cloudy. Rather staunch capping inversion should remain in place
much of the day, with most daytime convection limited to the Black
Hills. Convection will become more widespread and lift northeast
across the forecast area this evening as strong upper wave ejects
northeast from the Great Basin low. CAPE values up to 3000J/KG
along with increasing deep layer shear will support rotating
updrafts, with the only downside to a widespread severe wx
outbreak being the late arrival of the short wave. Either way,
even elevated storms will have enough energy to be severe. Also,
due to the slow moving nature of any supercells there is a flash
flood threat from northeast WY into far western SD. Hi-res cams
showing good potential for widespread rain totals near an inch
with locally much higher amounts within stronger slow moving
supercells in this area. Thus, will hoist a Flash Flood Watch for
the aformentioned area. Precipitation will linger well into the
night as 700mb low closes off somewhere near the northeast
WY/western SD state line.

Upper low shifts very slowly eastward on Monday, with another late
day upper wave ejecting into the forecast area. Plenty of
CAPE/shear available for more severe storms, but the late arrival
of the wave could dampen the overall severe threat somewhat.
Another flooding threat will exist, especially given the
widespread soaking rains expected tonight across the area, and we
may need to consider another flash flood watch. Stay tuned.

Upper low/trof crosses the Northern Plains Tuesday with continued
unsettled weather, but the severe/heavy rain threat should be
east/south of our area.

Expect a brief dry/warm period mid week, but southwest flow aloft
returns late in the week for more unsettled weather.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Sunday Night)
Issued At 1116 PM MDT Sat May 26 2018

Mainly VFR conditions will prevail through Sunday afternoon.
Isolated thunderstorms are possible tonight, especially over
northeast WY into far northwest SD. Low level wind shear is
expected on the SD plains due to a low level jet. Higher coverage
TSRA will develop late Sunday afternoon and persist into Sunday
night - in/near storms expect IFR conditions.

&&

.HYDROLOGY...
Issued at 340 AM MDT Sun May 27 2018

Widespread thunderstorms are expected to cross the area this
evening and overnight. Some stronger to severe storms will be slow
moving and will have the potential to produce very heavy rain
rates and possible flash flooding across northeast WY and far
western SD this evening.

Another round of widespread thunderstorms with the potential for
heavy rains will cross the area Monday night.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Flash Flood Watch from 3 PM MDT this afternoon through this
     evening for SDZ024-028-029.

     Flash Flood Watch this evening for SDZ012-025>027-041-072-074.

WY...Flash Flood Watch from 3 PM MDT this afternoon through this
     evening for WYZ057.

     Flash Flood Watch this evening for WYZ054>056-058-071.

&&

$$

DISCUSSION...Johnson
AVIATION...Helgeson
HYDROLOGY...Johnson


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