Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 260904

Area Forecast Discussion
National Weather Service Boise ID
304 AM MDT Wed Jul 26 2017

.SHORT TERM...Today through Thursday...PW values remain above
average as a warm moist atmosphere sits over the area in advance
of a slow and weakening upper level low situated over northern CA
this morning. The low is forecast by all models to move slowly
northeast today and tonight, passing through the area Thursday.
The EC is a fast outlier, pushing the low through by 00Z Fri,
while the GFS and NAM have it over the area at that time. With the
recent trend toward slower movement, have sided with the slower
runs. This increases the convective threat, mainly on the Idaho
side, for Thu. Therefore have raised pops for that period. In the
meantime, have also slightly raised pops for today and tonight
based on activity from yesterday and the continued presence of
unstable air and hot temperatures. Slow storm motions will result
in the potential for heavy rain amounts. Also, SPC has placed
extreme southern Harney and Malheur counties in a marginal risk
for severe downburst winds today, and most of southwest Idaho in
the same category for tomorrow, again for winds.

With PW percentiles forecast to be less than 90th percentile,
will not go with another flash flood watch at this time. However,
if any storms undergo training this afternoon/evening, especially
in mountainous terrain, isolated flooding is possible. The
temperature forecast continues to be a couple degrees above
normal, and changes were minimal this morning.

.LONG TERM...Thursday night through Tuesday...The weakening upper
low will be moving to the northeast out of our area late
Thursday, which will bring an end to showers and thunderstorms for
the overnight hours. For Friday and beyond, a monsoon pattern is
likely to set up, however, some differences exist in the latest
GFS and ECMWF model runs with regards to how much moisture will be
present for showers and thunderstorms. This is especially true
for Sunday through next week. Also of note, two tropical systems
are depicted to be off the coast of Western Mexico during this
time, and while both models show a northward trajectory for these
features, by next week the placement of them varies greatly. These
two tropical storms could be a large contributor to moisture
moving up, or not moving up the coast around the Four Corners
High. The bottom line is that there is much uncertainty in the
precipitation forecast, but there is high confidence in
temperatures remaining above normal through the period.


.AVIATION...VFR with showers and thunderstorms through Thursday
afternoon. Gusty outflow winds near thunderstorms may exceed 40
kts. Otherwise, surface winds are expected to be variable at 10
kts or less. Winds aloft near 10 KFT MSL: South 5-10 kts, becoming
west 10- 15 kts after 26/21Z, then becoming south once again at
10-15 kts after 27/12Z.





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