textproduct: Pendleton

DISCUSSION

Current radar and satellite imagery show light returns mainly over portions of Umatilla and Union Counties, which is due to the orographic lift from the post-frontal system. These returns is associated light snow along with mist and breezy winds, mainly along and south of Cabbage Hill earlier today. Surface observations also showed precip amounts of 0.10 inch or less fallen at Union County, Joseph State Airport (KJSY) and other mountain areas. Webcams are now showing improved VSBYs across the aforementioned area. For the rest of today, scattered to broken high clouds will be left behind as the upper trough exits further eastward with the upper ridge moving in. Light snow may linger for the crest of WA/OR Cascades tonight from the approaching light returns, but chances are low (<20%).

A high pressure system will then enter the PacNW Sunday through Wednesday. However, fog will be the main highlight for these days during morning and overnight hours. With near to below freezing nighttime temperatures and dewpoints, there is a good chance (>50%) that freezing fog could also develop. Visibilities may be affected, but with lower confidence (<20%) of 3 statue miles or lower based on the NBM. In addition to that, air stagnation could be a concern for Monday through Wednesday with the mixing heights below 1.5 kft during morning and afternoon hours and transport winds less than 10kts. For now, we will monitor for that and see if an advisory will be needed. Winds will be light across the forecast area with some breezes over the mountains.

Thursday onward is when precip will returns with an approaching trough. The Cascades will see showers before becoming more widespread with light rain for the low elevations with mountains rain/snow mix to all snow. QPF amounts may exceed to 0.10 inch or higher along the Cascades and other mountain areas but less for the low elevations. There seemed to be a timing disagreement with the frontal system coming Thursday. The Euro has been favoring an early onset of the system whereas Canadian has it come in later Thursday and Friday for GFS, making it challenging on when the system will actually arrive. Feaster/97

AVIATION...00Z TAFs.

VFR conditions currently area-wide with stratocu across central and northeast OR and clearing across south central WA. A disturbance embedded in the periphery of a building ridge will move in tonight prompting increasing sky cover. The main aviation concerns revolve around fog and low stratus overnight-early tomorrow as precip chances are confined to the crest of the Cascades and upper slopes.

Light winds and limited mixing expected tonight, however, much drier air than anticipated is in place with dew points in the teens in the Basin and foothills. While there is added moisture in the way from recent rainfall over the last 36 hours, this drier air mass in tandem with increasing clouds and warm ground based on soil temperatures is lowering the overall threat. As such, forecast favors more stratus than fog presently as this setup will more likely delay or limit any fog development. Currently have left visibility restrictions out beyond 6SM and emphasis on potential MVFR-IFR ceilings with stratus. Highest confidence in KPDT, KDLS, and KALW.

PRELIMINARY POINT TEMPS/POPS

PDT 26 38 24 35 / 10 10 0 0 ALW 26 36 26 35 / 10 10 0 0 PSC 24 36 23 35 / 10 10 0 0 YKM 21 36 22 36 / 10 0 0 0 HRI 25 38 23 35 / 10 10 0 0 ELN 19 31 21 32 / 10 0 0 0 RDM 20 40 21 43 / 10 0 0 0 LGD 24 36 23 38 / 10 10 0 0 GCD 23 38 24 43 / 10 0 0 0 DLS 31 40 29 38 / 20 0 0 0

PDT WATCHES/WARNINGS/ADVISORIES

OR...None. WA...None.