Pendleton forecast discussion

Discussion

Water vapor imagery shows a large closed low over central California, and a weak mid-level circulation wrapping north- northwest around the parent low is evident over southeast Oregon. Meanwhile, a shortwave trough is approaching the Pacific Northwest from the northeast Pacific Ocean, and is forecast to move onshore late this afternoon.

Widespread showers have developed across central to eastern Oregon and south-central to southeast Washington, and isolated embedded lightning has been observed over the Blue Mountains and their adjacent foothills. Modest instability is present (100 J/kg or less MUCAPE per SPC's RAP-based mesoanalysis), and 12Z HREF mean forecast values remain below 500 J/kg through the day (highest for the northeast Blue Mountains). Aside from the isolated lightning within the ongoing band of showers, the best chance for notable convection will be this afternoon and evening across the Blue Mountains region of Grant, Union, and Wallowa counties; convection developing across southeast Oregon and southwest Idaho will track north to northeast and the portion in Oregon may (50% confidence) clip the aforementioned counties. Main hazards associated with this activity will be lightning and gusty outflow winds.

Looking ahead to late this afternoon into the evening hours, breezy to windy westerly to northwesterly winds are forecast to ramp through the Cascade gaps and across wind-prone portions of north-central Oregon as pressure gradients increase in strength (8-11 mb difference between PDX and GEG in latest NWP guidance) and a modest low-level jet (25-40 kts) develops. Confidence is medium-high (60-90%) in reaching advisory-level wind gusts (45 mph or greater) through the Kittitas Valley, eastern Columbia River Gorge, and wind-prone portions of north-central Oregon; Wind Advisories are in effect from 7 PM this evening until 11 AM Friday morning.

Precipitation is forecast to taper off through Friday morning from west to east as the shortwave trough moves overhead, and a very dry air mass (PWAT of 0.15-0.40") replaces the moist air mass currently in place. This will set the stage for near- to sub-freezing temperatures Friday night into Saturday morning. While most public forecast zones have observed a season-ending freeze, a widespread freeze has not yet occurred in the lowest elevations of our forecast area. Given the anticipated dry air mass, predominantly light winds, and mostly clear skies, confidence is medium-high (50-95%) that a season-ending freeze will be observed in the lower Columbia Basin, eastern Columbia River Gorge, Yakima Valley, and the foothills of the northern Blue Mountains, so have issued Freeze Watches.

Uncertainty in the 500-hPa height field burgeons Sunday through early next week due to continued ensemble spread in solutions regarding the development and evolution of a closed low in the Gulf of Alaska region. Roughly 50-60% of ensemble members depict a compact closed low moving inland Monday through mid- week, while another 20-30% of members show a weaker low, and the remaining members advertise an upper-level ridge over the Pacific Northwest. The scenario that shows a stronger closed low would result in shower chances and cooler temperatures, while the weaker low would warmer and drier, and the upper-level ridge would be warm and completely dry.

Aviation /00Z Tafs Through 00Z Saturday/

VFR conditions expected through the period. A system passing to the south will bring rain shower impacts to site PSC through 02Z, and sites PDT/ALW through 08-09Z tonight. BKN-OVC CIGs are expected to lower after sunset at sites PDT/ALW this evening, but should remain AOA 4kft AGL. A cold front passage will bring breezy conditions to sites DLS/PDT/RDM/BDN/YKM. At site DLS, winds will increase to 15-25 kts with gusts up to 40kts in the next several hours and overnight. Sites RDM/BDN will see winds increase to 12-15 kts with gusts to around 20 kts between through 13Z. Site YKM winds will increase to 12-15 kts with gusts to around 23 kts between 03Z-15Z. At site PDT, winds will increase to 12-15kts with gusts to around 26kts between 08Z-22Z. Short-term model guidance is indicating low level wind shear impacts up to 2 kft AGL between 03Z-15Z tonight at site YKM, but confidence is low-moderate (30-40%) in persistent wind shear impacts over the site. Lawhorn/82

Preliminary Point Temps/Pops

PDT  36  56  27  58 /  70   0   0   0 
ALW  40  57  32  59 /  80   0   0   0 
PSC  38  64  29  59 /  50   0   0   0 
YKM  39  64  31  58 /   0   0   0   0 
HRI  39  61  30  58 /  50   0   0   0 
ELN  33  57  26  55 /   0   0   0   0 
RDM  25  51  19  60 /  10   0   0   0 
LGD  37  53  25  59 / 100  20   0   0 
GCD  36  53  25  62 /  90  20   0   0 
DLS  43  62  34  63 /   0   0   0   0 

Pdt Watches/Warnings/Advisories

WA...Wind Advisory from 7 PM this evening to 11 AM PDT Friday for WAZ024-026. Freeze Watch from Friday evening through Saturday morning for WAZ024-027>029. OR...Wind Advisory from 7 PM this evening to 11 AM PDT Friday for ORZ041-510. Freeze Watch from Friday evening through Saturday morning for ORZ041-044-507.

DISCUSSION...86 AVIATION...82