971 
FXUS65 KPIH 202011
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
211 PM MDT Sat Apr 20 2019

.SHORT TERM...Tonight through Monday. Upper trough shifting into
coastal states per satellite imagery. Low fairly wrapped with
center over northern California this afternoon. Weak shortwave
shifting northeast through Nevada ahead of main trough helping to
produce convection over East Idaho. Cloud cover left over from
weak convection overnight helped to limit instability most areas
but some convection able to fire early this afternoon over
southern highlands. This region was pinpointed to be favorable by
several of the convective allowing high res model runs, so have
given greater weight to these solutions for tonight into Sunday.
Initial convection looks to shift east through early evening then
dissipate in favor of broader area of weak convection taking shape
over western portions of the forecast area. In both cases,
soundings favor potential for gusty outflow winds in the 40-45mph
range and very small hail. Base of trough shifts into western Utah
overnight, with main low closing off over central Nevada. GFS and
NAM have come into better agreement regarding this feature, which
would maintain potential for convection continuing overnight
through more of the forecast area. Still believe best precip
potential will remain over southern highlands, given better
proximity to main dynamic feature. As upper low continues to shift
south through the day Sunday, expect deepest moisture and best
dynamics to wrap closer to the low, though extended trough axis
over the region will allow showers to continue through the day
Sunday. Best chance for thunderstorms appears to be along the WY
border, and especially toward the Bear Lake region. Cooler air
spilling into the central mountains and most of the Snake Plain
behind the trough axis will limit the instability potential for
those areas. Surface pressure gradient is still forecast to
tighten behind the trough with both NAM and GFS guidance hitting
15-20kts sustained for most of the Snake Plain. Precipitation
gradually diminishes Sunday night into Monday as effects from the
trough shift further south and upper ridge tries to nose into the
state from the west. DMH

.LONG TERM...Monday night through Saturday. Upper ridge tilts over
through East Idaho through Tuesday night. Fast moving but weak 
shortwave slides through PacNW across Idaho panhandle into Montana
mainly Tuesday. There are some differences in how the GFS and
ECMWF want to handle this feature, but in general, both models
portray weak orographic showers mainly across the Divide region.
GFS is a little more aggressive and pushes precip a little further
south toward the Utah border through Wednesday. Another ridge
tries to develop through the Great Basin, but the northern extent
of the ridge remains flat, and the GFS maintains a little bit more
instability and moisture than the ECMWF. Thus the blends trended
slightly wetter across the region. By Friday, next trough pushes
inland. GFS is able to capitalize on its already wetter solution,
and drastically increases the precip chances for Friday into
Saturday. DMH


&&

.AVIATION...Increased potential for thunder this afternoon as we 
approach peak heating. VCTS should continue through the evening 
hours as an upper low deepens over the West. Eventually the low will 
deepen further and drop to our south, however slight nuances between 
GFS/NAM. NAM is slightly faster to cut off the low, and the 
resulting precip wraps around the low about 3 to 6 hours earlier 
while the GFS pushes precip further eastward. Both models carry a 
significant bullseye of precip, though placement will be 
challenging. Will likely see ceilings drop to MVFR or even IFR late 
tonight through tomorrow with some heavier precip possible over KBYI 
and KSUN, particularly if the NAM solution holds true. Hinsberger


&&

.HYDROLOGY...Have maintained most of the hydrologic products today
as East Idaho rivers remain steady or with slow increases
indicative of the snowmelt and anticipated rainfall from this
weekend spring storm system. Portneuf River has already risen
above flood stage, and Big Wood River producing nuisance ponding
and seepage in residential areas along the banks. Snake River
remains elevated as well, and many others remain or approach
bankfull. Spring storm system beginning to make effects known 
with rainfall becoming more widespread across the hydrological 
area into tonight and Sunday. Cooler temperatures on Sunday will 
help to mitigate some of the mid and upper elevation snow melt, 
but the addition of the rain will continue to help things along. 
Expect a turn toward drier conditions for early in the week along 
with another warming trend, but East Idaho could be looking at 
additional precipitation later in the week. Regardless of the 
details, the message is this: 
spring snowmelt and river impacts are here and will continue for 
the upcoming weeks. DMH


&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$