Area forecast discussion...updated 
National Weather Service Pendleton or 
1045 PM PDT Thursday may 21 2015 

Updated aviation discussion 

Update...scattered showers and thunderstorms have moved through 
the area dropping significant rainfall in short times. Many areas 
reported over 1 inch rain in less than an hour. Multiple flash 
flood warings were issued. The storms have decreased in intensity 
but a Flash Flood Warning remains in effect in the upper Yakima 
Valley. Expect scattered showers to continue overnight. Showers 
and thunderstorms will redevelop tomorrow afternoon. The heaviest 
showers are expected from the ochoco John Day Highlands to to 
Blue Mountains Friday afternoon. A Flash Flood Watch will continue 
for this area until Friday evening. Coonfield 


Aviation...06z tafs...generally VFR conditions will prevail over 
the next 24 hours except at krdm and kbdn. IFR ceilings and mvfrpossible 
until 15z due to low level moisture from heavy rain. Moist unstable 
airmass remains over the area with scattered showers and 
thunderstorms expected to redevelop after 20z with MVFR conditions 
and gusty winds possible near the thunderstorms. Confidence to put 
thunderstorms in specific locations is low. 76 


Previous discussion... /issued 220 PM PDT Thursday may 21 2015/ 

Short term...tonight through Sunday. Busy shift today as we focus 
on the flash flood potential over the next 24 hours as well as the 
Holiday weekend forecast. Scattered thunderstorms have developed 
over northeast Oregon and southeast Washington. Convection is a 
little farther north today compared to yesterday...and rainfall 
rates near 0.5 inch per hour have already been observed. The Flash 
Flood Watch was extended to include the northwest Blue Mountains of 
Washington. The main upper low off the Southern California coast 
will move inland tonight and will approach The Four Corners region 
late Friday. Most of the showers and thunderstorms on Friday will 
be over the eastern portion of the John Day-ochoco Highlands and 
from the Blue Mountains east to the Idaho border. The Flash Flood 
Watch for the east slopes of the or Cascades will end tonight as a 
westerly flow aloft stabilizes the air mass in this area on Friday. upper low off the Washington coast may allow showers 
and thunderstorms to develop along the east slopes of the Washington 
Cascades...and easterly winds aloft and a weak steering flow may 
result in slow moving thunderstorms north of Montana. Adams. This will 
need to be monitored by future shifts to determine if a watch is 
needed. The latest GFS seems too strong with this upstream low as 
it tracks across Washington/or on Saturday compared to the NAM and ecwmf. 
This has a strong influence on the forecast for the Holiday weekend. 
Will consider the GFS as an outlier with the stronger low and more 
bullish quantitative precipitation will keep probability of precipitation in the 20-40 percent range and light 
quantitative precipitation forecast amounts. On Sunday...the next upper low over Vancouver Island 
will bring a chance of showers to the northern zones but drier 
conditions to the south. Wister 

Long term...Sunday night through Thursday. Unsettled weather with 
a chance of showers for much of the area will continue through the 
extended period as another upper level low moves south from Canada 
over the area Sunday and Monday. The GFS model keeps the low over 
the area the entire period...while the European model (ecmwf) kicks the low east by 
late Tuesday and Wednesday. Thus will little confidence...I have 
kept some slight chance of shower and afternoon thunderstorms 
through Thursday...mainly over the mountains. Temperatures will be 
slight warmer than normal. 


Preliminary point temps/pops... 
PDT 57 75 53 72 / 50 50 50 60 
alw 56 78 56 74 / 50 50 50 50 
psc 60 83 59 80 / 40 50 50 30 
ykm 59 78 58 78 / 40 70 40 40 
hri 59 81 59 78 / 30 50 50 40 
eln 56 76 54 75 / 40 70 40 40 
rdm 51 68 42 66 / 70 50 20 30 
lgd 52 69 54 67 / 50 80 50 50 
gcd 48 69 51 66 / 70 80 60 40 
dls 59 76 57 72 / 50 40 20 30 


PDT watches/warnings/advisories... 
or...Flash Flood Watch through Friday evening for orz049-050-502-503- 

Flash Flood Watch until 9 PM PDT this evening for orz509. 

Flood Watch until 9 PM PDT this evening for orz510-511. 

Washington...Flash Flood Watch through Friday evening for waz030. 


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