Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 190449

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1049 PM MDT Tue Apr 18 2017

Issued at 742 PM MDT Tue Apr 18 2017

The 00z KUNR sounding showed a drier boundary layer than the
RAP/NAM/GFS short-term forecasts indicated. This is consistent
with a slower development of rain. Based on these observations and
trends, have trimmed back PoPs this evening. Otherwise, water
vapor images showed substantial cloud development over WY, and
blended TPW and RAOBs indicated 150-200% of normal PW upstream of
the CWA. Thus, the widespread rain area still appears on track
for later tonight. Most models also indicate a strong coupling of
Q-vector convergence and frontogenesis over western SD by 12z, so
would expect a narrower band of relatively heavy precipitation to


.SHORT TERM...(This Evening Through Wednesday Night)
Issued at 222 PM MDT Tue Apr 18 2017

Southwest flow has developed aloft across the western half of the
CONUS including over the northern plains ahead of a longwave trough
slowly moving onshore the Pac NW coast. A strong shortwave is
crossing the Great Basin at this time with weaker waves downstream
into WY and the CWA. Clouds have increased as the waves have moved
into the region. KUDX radar shows reflectivity aloft moving in from
northeast WY into the Black Hills area. This precip is not reaching
ground...yet...except across parts of the higher Black Hills. Precip
coverage will increase through the night across the region as the
main shortwave moves into the area late tonight. Rain will begin
across southern portions of the CWA...then slide northward Wednesday
morning into the the northeast WY plains...the Black Hills area and
SD plains east of the Hills. This band of rain will sit across the
central CWA through much of the day Wednesday before gradually
decreasing through the afternoon and evening hours. Temps will be
cold enough late tonight for rain to mix with or change to snow
across the higher Black Hills. The ECMWF/NAM/WPC/SREF are all going
with 1 to 4 inches of accumulation while the GFS is the outlier
showing about a foot of snow. Will throw out the GFS and stick with
1-4" with no need for headlines. Total QPF is expected to range from
0.75" to 1.25" across most of the CWA...with some areas possibly
seeing QPF amounts approaching 2". Temps tomorrow will be limited
with the precip and clouds cover...reaching into the 40s to low 50s.

.LONG TERM...(Thursday Through Tuesday)
Issued at 222 PM MDT Tue Apr 18 2017

A brief break from the unsettled weather can be expected on Thursday
before the next system digs southeast from the northern Rockies to
the Central Plains Thursday night and Friday. Latest model runs have
trended back farther south with the track of the system with precip
barely clipping the southwestern portions of the CWA. This solution
also decreases or eliminates snow accumulation that would have
occurred over the Black Hills. This system will exit the region
Friday night, but additional energy in active flow could bring
isolated showers across the area through Saturday. Dry and warm
weather is still anticipated for Sunday before another series of
disturbances brings chances for showers Sunday night into early next


.AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night)
Issued At 1044 PM MDT Tue Apr 18 2017

Widespread MVFR/IFR CIGS and VSBYS will develop by Wednesday
morning as showers spread across the area. Some LIFR CIGS are
expected around the BLKHLS and adjacent areas where the rain is
heaviest. Some snow also will fall over the highest BLKHLS.
Conditions will gradually improve Wednesday afternoon and evening
from west to east.




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