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KEY MESSAGES

- Warming trend continues, peaking Tuesday.

- Isolated thunderstorms possible through Tuesday.

- Dry and windy conditions Tuesday. *Fire Weather Watch Issued*

DISCUSSION

Current radar and infrared satellite imagery shows light returns extending across Deschutes, Grant, Crook, and Wheeler counties under partly cloudy skies. This is in response to an upper level shortwave trough moving into south-central Oregon and continuing into eastern Oregon through the day, as a large upper level trough strengthens along the British Columbia/Washington/Oregon coasts to enhance southwest flow aloft. The warming trend that was briefly paused on Sunday will commence through Tuesday as high temperatures reach into the low to mid-90s today and upper 90s to low 100s on Tuesday for areas of the Lower Columbia Basin, Blue Mountain foothills, Central Oregon, and the Yakima Valley. The NBM advertises a 30-50% chance for Walla Walla, Milton-Freewater, Tri-Cities, and Yakima reaching high temperatures of 100 degrees or greater on Tuesday. Little overnight relief is also expected with morning lows of around 60 degrees. These high and low temperatures allow for heat concerns to arise, as Moderate (2 our of 4) HeatRisk is present across The Dalles, Yakima, and Ellensburg on Monday, extending across the Lower Columbia Basin, Blue Mountain foothills, John Day Basin, and the Yakima/Kittitas Valleys on Tuesday. The moderate HeatRisk category primarily relates to concern for members of heat-sensitive groups, but it also extends to the general public who are exposed to the sun for longer periods of time. Make sure to stay hydrated and reduce time or move events during peak afternoon hours.

The earlier mentioned passing shortwave across central and eastern Oregon today, will be closely followed by a developing transient ridge and enhanced by an approaching upper level shortwave from the northwest. These synoptic features will promote instability across the region, especially through central and eastern Oregon during the afternoon and early evening. HREF members suggest CAPE values of 500-600 J/kg with lapse rates of 7-7.2C/km, which does suggest thunderstorm potential across Deschutes, Crook, Grant, and Wheeler counties - with highest potential centered over Deschutes county. The primary limiting factor related to these storms staying sub- severe is the lack of low level wind shear, which is only 10-18 kts. As a result, storm motion should be rather slow and updrafts should be limited in development. This does still relate to storm threats that include breezy outflow winds, frequent lightning, and small hail. The passing upper level trough to our north on Tuesday will again allow for isolated thunderstorm chances to extend across the Blue Mountains and Central/Eastern Oregon through the afternoon and early evening. The HREF suggests slightly better convective parameters on Tuesday, with CAPE values of 600-800 J/kg, lapse rates of 7.1-7.4C/km, and low level shear of 20-30kts. There is some uncertainty in how strong the transient ridge is able to build between today's shortwave and Tuesday's trough passage, and how efficient the trough will be in extending south from its parent low located in Northern/Central British Columbia. These inconsistencies are prevalent with observing the differences in trough/low strength between the ECMWF AI ensembles and the GFS AI ensembles. This will need to be further analyzed as the event nears, but at this time severe storms are not anticipated (70% confidence).

The passing trough on Tuesday, coupled with the transient ridge, will allow a pressure gradient to develop across the Lower Columbia Basin, Kittitas Valley, and the Warm Springs area promoting fire weather concerns related to strong winds and low relative humidities. Thus, a Fire Weather Watch has been issued between 11 AM and 10 PM on Tuesday across these areas.

Zonal, west flow aloft expected Wednesday and Thursday, returns high temperatures to near normal values. Another upper level trough will deepen offshore late in the week to shift west flow to more from the southwest. This will relate to a slight increase in high temperatures, but more importantly will bring breezy winds across the Lower Columbia Basin. Wind highlights are not currently expected, but fire weather concerns may arise Friday onward as dry conditions since Tuesday will keep afternoon humidities below 25%. 75

AVIATION /18Z TAFS THROUGH 18Z TUESDAY/

VFR conditions currently prevail through the entire forecasting period. There is small chances (5-15%) that temporary sub-VFR conditions with erratic wind gusts develop with thunderstorms in BDN and RDM at 20-24Z. Otherwise, light winds throughout with high ceilings expected for all TAF sites.

FIRE WEATHER

Primary fire weather concerns on Tuesday across the Lower Columbia Basin, Kittitas Valley, and the Warm Springs area as a passing upper level shortwave will develop a pressure gradient across the region. High temperatures will also coincide with this pressure gradient as afternoon values will approach triple digits and humidities in the mid-teens. Tuesday morning overnight humidity recoveries will also be moderate (30-55%) as west winds of up to 20 mph and gusts up to 30 mph are expected. The NBM highlights a 50-75% of Red Flag conditions being met across the aforementioned areas Tuesday afternoon and evening. Thus, a FIRE WEATHER WATCH has been issued for fire weather zones: WA690, WA691, OR691, and OR703 for strong winds and relative humidities as low as 13%.

In addition to the fire weather concern Tuesday, there is also concern later today related to dry thunderstorm potential across OR 705 and southern OR 700. The earlier mentioned shortwave passing through central and eastern Oregon, coupled with poor to moderate (25-40%) overnight humidity recoveries this morning, may provide concern for new fire starts due to the dry nature of any developing thunderstorms. The HRRR suggests this afternoon's dewpoint depression and vertical temperature changes at 850 mb are above critical dry thunderstorm criteria, however the NAM suggests more muted concerns - especially related to dewpoint depressions in the 850mb level. This parameter highlights how dry lower levels of the atmosphere are, with a normal threshold of 11C or above. A large spread of between 7C and 21C is suggested by the NAM and HRRR respectively, with the ECWMF and GFS advertising 11C and 6C. Guidance does suggest precipitable water amounts of 0.80-0.90" or 140-160% above normal. These conditions, along with the slow moving nature of any developing storm cells due to wind shear of 10-18 kts, may allows these areas to accumulate a wetting rain due to their longer residence time. This will be further analyzed as guidance is updated later this morning, as a Red Flag Warning may be necessary over OR705 this afternoon/evening. 75

Preliminary Point Temps/PoPs

PDT 89 57 95 58 / 0 0 0 10 ALW 91 62 97 64 / 0 0 0 10 PSC 93 58 99 61 / 0 0 0 10 YKM 95 61 98 59 / 0 0 0 0 HRI 92 59 98 62 / 0 0 0 20 ELN 92 59 92 56 / 0 0 0 0 RDM 89 53 91 50 / 10 10 10 10 LGD 87 56 94 58 / 10 0 20 10 GCD 87 53 94 54 / 60 10 10 10 DLS 96 64 93 61 / 0 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

WA...Fire Weather Watch from Tuesday morning through Tuesday evening for WAZ690-691. OR...Fire Weather Watch from Tuesday morning through Tuesday evening for ORZ691-703.