textproduct: Pendleton

DISCUSSION

Key Messages:

1. Quiet weather through Thursday, with air stagnation and the potential for fog being the only concerns.

2. Precipitation returns from Thursday through the weekend.

3. Low (10-30%) freezing rain or sleet Thursday in central and north-central Oregon Thursday morning and Thursday night into Friday morning for portions of the Columbia Basin, Yakima Valley and colder lower elevation locations.

A large upper level ridge was located over the western CONUS. This ridge will remain over the region today before moving eastward tonight into Wednesday. Due to the ridge overhead, and light winds, air stagnation remains a concern and advisories have been issued. Otherwise, dry weather with temperatures about 5 degrees below normal will remain in place through Thursday.

By Thursday, a more south to southwest flow will be in place and moisture will begin to move northward from California during the day on Thursday, and continue into Thursday night before what looks to be a lull on Friday. The highest QPF amounts will be over the mountains, where 0.25 to 0.50 inches could fall while generally less than 09.10 inches will occur over the lower elevations.

Depending on how much precipitation moves in and how quickly there is a low (10-30 percent) potential for freezing rain or sleet in in central and north-central Oregon Thursday morning and then across portions of the Columbia Basin, Yakima and Kittitas Valleys and other lower elevation cold spots Thursday night into Friday morning due to the cold air trapped at the surface. Models differ on how much QPF falls during these times and we will see how the situation evolves over the next few days. Overall QPF is expected to be light and temperatures are expected to get above freezing during the day.

On Friday, low pressure is forecast to move north off the Oregon and Washington coasts and will bring trough across the region by Saturday, with some additional precipitation, mainly over the mountains, with mostly dry weather for the lower elevations Sunday and Monday.

Snow levels will initially start off in the 2500-3000 foot range in the Washington Cascades on Thursday, 3500-4500 foot range in the Blues and Wallowas and the 5500-6500 foot range in the Oregon Cascades. Snow levels will rise to 4500-5000 feet in the Washington Cascades and over 6500 feet elsewhere on Friday. Any precipitation that falls over the mountains would not be snow during this time, except at the higher elevations. Snow levels will fall again on Saturday into Sunday and will be between 2500-3000 feet by Sunday morning in the Washington Cascades and 3500-4000 feet elsewhere. Some snow accumulation is possible during this period, and it looks like generally 4 inches or less in the Cascades, at pass level, and perhaps 6 or more along the crests with an inch or two in the Blues.

AVIATION

12Z TAFS...VFR conditions are expected through the day today, with the possible exception of some lower condition (IFR/LIFR) in the BDN/RDM area. Tonight into Wednesday morning, there is a chance (30-40 percent) at PDT, YKM and ALW. Winds will be light through the period.

PRELIMINARY POINT TEMPS/POPS

PDT 37 21 35 23 / 0 0 0 0 ALW 34 25 34 25 / 0 0 0 0 PSC 39 22 34 23 / 0 0 0 0 YKM 39 22 36 22 / 0 0 0 0 HRI 37 23 33 23 / 0 0 0 0 ELN 33 20 34 22 / 0 0 0 0 RDM 41 22 47 26 / 0 0 0 10 LGD 41 30 44 31 / 0 0 0 0 GCD 45 28 48 30 / 0 0 0 10 DLS 39 27 37 28 / 0 0 0 10

PDT WATCHES/WARNINGS/ADVISORIES

OR...Air Stagnation Advisory until noon PST Thursday for ORZ044-507- 508-510-511.

WA...Air Stagnation Advisory until noon PST Thursday for WAZ026>029.