Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 161728

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1128 AM MDT Wed May 16 2018

.DISCUSSION...(Today Through Tuesday)
Issued at 310 AM MDT Wed May 16 2018

Current surface analysis shows weak high pressure to the
east-southeast of the region, with southerly low level flow
developing over our area behind the high. Upper level analysis
shows high pressure and ridging in place over the northern High
Plains, with mostly clear skies in place. A couple of light
showers/sprinkles have developed overnight over far south central
SD in closer vicinity of low level jet across western and central
NE. Temperatures range from the 40s over parts of northeast WY and
the BLack Hills to the 60s over south central SD.

The next couple of days will be quite warm as ridge moves over and
then east of the area. Highs will mostly be in the 80s across the
Plains, with 70s over the Black Hills. Southerly winds will persist,
bringing increasing low level moisture and instability to the area.
A surface trough will move into western parts of northeast WY during
the afternoon, bringing some drier air into Campbell County. Remnants
of the upper low that has sat to the west of the region for almost a
week will be pushed northeast into the northern High Plains today by
another low moving onto the CA coast. With the ridge over the area,
a fairly strong cap will be in place and could be tough to break.
Any activity over our area later today should be fairly isolated,
with the northern Black Hills area and parts of northeast WY near
the trough seeing the best potential for a storm this afternoon into
the evening. A couple of showers/storms could also develop over
south central SD this afternoon in vicinity of weaker energy
expected to cross the Central Plains. Surface based CAPE of 500 to
1000 j/kg would be sufficient for a potential strong storm or two,
but shear will remain relatively weak across the area for more
robust activity. Low level southerly jet will re-intensify tonight
from western NE into central SD, so could see isolated
showers/storms overnight over mainly south central SD.

Thursday will be the prime day for severe storm potential this week
as a cold front to the north of the region starts to sag south into
the forecast area later in the afternoon and evening. Low pressure
along the front over eastern MT will bring a surface trough/dryline
across northeast WY during the afternoon. Aloft, the upper low off
the west coast will move inland, bringing a fairly strong shortwave
northeast across CO/WY into NE and the Dakotas Thursday afternoon
and night. CAPE values of 1500 to over 2500 j/kg are progged for
later in the afternoon, with shear at least adequate for supercell
development. Cap will weaken through the afternoon from northwest to
southeast as the front approaches. The best potential for severe
storms will be late afternoon into the evening from the Black Hills
area into west central and southwest SD, but stronger storms are
possible across much, if not all of the forecast area, as the
shortwave moves across the region. Large hail and strong wind gusts
will be the main threats from severe storms, with an isolated
tornado not out of the question. Some areas of heavier rainfall are
also likely, especially from the Black Hills east-northeast to
central SD.

As the front slides south across the area Thursday night and Friday,
cooler air will filter into the area. Upper low to the west will
bring additional shortwave energy across the area, with chances for
showers and thunderstorms Friday and some lingering showers on
Saturday. A few strong to severe storms are possible south and east
of the Black Hills on Friday. Temperatures by Saturday will only be
in the 50s and lower 60s.

What is left of the upper level system is forecast to move east of
the region by Sunday, with weak ridging bringing drier and warmer
weather Sunday into Monday. Yet another low is progged to move into
the western US early next week, with southwesterly flow likely
bringing some unsettled weather into the region later Monday through
midweek. Temperatures look to be near seasonal averages, mostly
upper 60s and 70s for the first half of next week.


.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1127 AM MDT Wed May 16 2018

Scattered showers and thunderstorms are possible after 20Z this
afternoon. Otherwise, VFR conditions are expected.




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