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Area Forecast Discussion (AFD)

Issued by Boise, ID (BOI)

FXUS65 KBOI 251951

Area Forecast Discussion
National Weather Service Boise ID
151 PM MDT Fri May 25 2018

.SHORT TERM...Tonight through Sunday...An upper level low will
move slowly from nrn CA into NV today into tomorrow, before slowly
drifting toward the NV/UT line Sunday. Convection initiating on
the northern side of this system is likely to grow into a 
significant convective event across much of the area this 
afternoon and evening. Thunderstorms are forming in southeast 
Oregon at this time, and they are expected to move/develop 
northward and expand into southwest Idaho generally west of 
Mountain Home. High resolution models indicate that a large and 
strong outflow boundary will form ahead of these storms as the 
afternoon wears on. Wind gusts could go over 60 mph along and 
behind this boundary. Also, multiple boundaries are possible. The 
main boundary is forecast to move through northern Harney and 
Malheur counties and into Baker County late this afternoon and 
early this evening. It will also likely extend east into southwest
Idaho, entering around 6 pm and moving through the Boise and 
Mountain Home areas by 8 pm. It is unclear at this point how 
strong the boundary might be once it gets east of Mountain Home.

In addition to the wind threat, flash flooding is possible in
eastern Oregon as multiple rounds of storms occur. Recent burn
scars are most prone to problems, but flash flooding may occur
elsewhere as well. Rainfall totals may reach 2 inches in a short
amount of time this afternoon/evening. A flash flood watch has
been posted for our eastern Oregon zones.

After this event ends late tonight, additional showers and storms
are forecast Saturday and Sunday, but they are not expected to be
nearly as strong. The location with the best chance of
precipitation will shift east Saturday and especially Sunday, and
by then there is very little chance of precip in the Oregon zones.
Temps will cool down 10-15 degrees tomorrow compared to today,
then warm up slightly Sunday.

.LONG TERM...Sunday night through Wednesday...Lingering showers in 
the central Idaho mountains to start the period as our upper level 
low still sits just east of us. By Monday night a short wave ridge 
builds over the Pacific Northwest which helps increase temperatures 
to above normal again. This is a short lived break as yet another 
long wave trough begins to drop down along the coast bringing moist 
southwesterly flow aloft to our area. Showers begin to hit SE Oregon 
again by early Wednesday increasing in coverage by late afternoon.

Wednesday night through Friday...Models show the trough along the 
coast moving inland during this period, but the latest runs differ 
on timing and amplitude. They do agree on at least a slight chance 
of showers Wednesday night through Friday, and there will be enough 
instability for isolated afternoon and evening thunderstorms. 
Temperatures will be close to normal.


.AVIATION...VFR. Scattered showers and thunderstorms this afternoon 
with much of the coverage staying in SE Oregon. KBNO and KBKE will 
see heavy rain and thunderstorms through late afternoon. 
Thunderstorms have the potential of producing gusty winds up to 
50kts. Outflow winds are of concern this afternoon after 22z and 
could travel up into the Treasure Valley. Surface winds outside of 
thunderstorms, variable 15-20kts. Winds aloft to 10k feet MSL, 
southerly 25-35kts.

Weekend Outlook...Shower and thunderstorms continue through the 
weekend with coverage migrating more to SW Idaho for Saturday and 
Sunday with the Oregon side starting to dry out. Gusty winds with 
thunderstorms of 25-35kts.


OR...Flash Flood Watch through late tonight ORZ061>063.