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FXUS66 KPDT 260544 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 1044 PM PDT Thu Apr 25 2024

Updated Aviation Discussion

UPDATE

Key Messages:

1. Thunderstorm potential Friday afternoon.

2. Breezy afternoon winds over the weekend.

The upper level trough that is currently pushing onshore will quickly drop through California as upper level ridging occurs to our east. This will lead to the possibility of thunderstorm development along the Blue Mountains and east during the afternoon and early evening, as advertised by being included in the General Thunderstorm (0 of 5) category via the Climate Prediction Center's Convective Outlook. The best chances will be over Grant, southern Union, and Wallowa counties. The HRRR highlights surface CAPE values between 150-200 J/kg and 0-6km shear of 35-45kts, which is enough forcing to expect some discrete cells to form.
However, lingering cloud cover and delayed mid-level moisture arrival will lead to storm development peaking between 3 PM and 8 PM, with high confidence (80%) in sub-warning storm cells.

A building upper level ridge offshore will flatten as a shortwave travels onshore on Saturday, tightening a pressure gradient along the Cascades to allow for breezy winds to occur across the Simcoe Highlands and Lower Columbia Basin as gusts of 25 to 35 mph out of the west will be possible. Confidence in these values is high (80%), as the NBM suggests a 70-90% chance of wind gusts of 30 mph or greater over the aforementioned areas. An approaching upper level closed low off the British Columbia coast on Sunday will further tighten isobars to provide even windier and more widespread conditions across the Simcoe Highlands, east slopes of the Cascades, Eastern Gorge, Lower Columbia Basin, northern Blue Mountain foothills, and the Yakima/Kittitas Valleys. Highest gusts are anticipated through the Simcoe Highlands, with gusts up to 40 mph. Elsewhere, gusts of 25-35 mph are expected out of the west.
The NBM showcases a 75-95% chance of 35 mph gusts or greater over the Simcoe Highlands, and a 50-75% chance across the Lower Columbia Basin and northern Blue Mountain foothills. 75



AVIATION
06Z TAFS
VFR conditions currently at all sites, which will stay the course through the period. Ceilings will drop to between OVC035-060, with KDLS/KBDN/KRDM/KALW near the lower bounds into the Friday morning hours before improving through the afternoon and evening. KPSC/KPDT should stay between OVC050-060 overnight before improving Friday afternoon. Winds will stay light and below 10 kts for all sites except KDLS, which will increase to between 10 - 15kts in the afternoon and extend through the remainder of the period. 75

PREV DISCUSSION... /issued 243 PM PDT Thu Apr 25 2024/

SHORT TERM
Today through Sunday
Current radar is showing the leading edge of the upper level trough has begun to push along the Cascades and spill over to the east slopes. Current observations show 0.01 inches along the OR east slopes and 0.01-0.03 inches along the WA east slopes. Current satellite shows mid and high level clouds over the majority of the CWA This will assist with keeping temperatures more on the seasonal side. However, if there is a break in the clouds this afternoon, daytime heating coupled with some orographic lift and mid level moisture, the eastern mountains could hear a rumble or two of thunder this afternoon.

Today through Friday night models are in relatively firm agreement with the upper level trough moving slowly across the region. With it there are elevated probabilities of some high mountains snow and low elevation rain. Looking at the raw ensembles 6 hour potential for 0.05 inches of rain, there is and 80-95% chance along the Cascades crests and along the east slopes of the Cascades through tonight while only a 5-15% chance along the eastern mountains, while the lower elevations will see a 30-40% chance of 0.01-0.06 inches of rain. Snow levels will decrease from over 5000 ft to near 4500 ft allowing the crests of the Cascades and portions of the Wallows to see an 80-90% probability of 0.5 inches along the Cascades and higher at the crests while the Blues and Wallowas have 60-80% probabilities for 0.5 inches as well. Along the OR Cascades through Santiam Pass there is a 30% probability of up to 3 inches. Not enough to warrant an advisory but still enough to say be cautious while driving across. As for the temperatures, due to the increased cloud cover, temperatures will remain near seasonable with the EFI showing seasonable temperatures and the ensembles showing low to mid 60s along the lower elevations and high 50s elsewhere. With the majority of the cloud cover currently over the eastern portion of the CWA, there is a slight chance with low confidence (5-15%), that the daytime heating coupled with a lifted index of above 6.5°C/km and MUCAPEs of 150J/kg, the eastern mountains could here a rumble or two this afternoon. CAMs models show these ingredients to uptick Friday afternoon to MUCAPEs of over 200 J/kg, lifted index cresting 7.4°C/km, mid level moisture and southwest flow, isolated thunderstorms could pop up over the eastern mountains Friday afternoon with confidence slightly higher (15-25%).

Saturday the models show the upper level trough to have its axis over the region with the backside of the trough over the Cascades and the leading side over the eastern mountains. Guidance shows the majority of the precipitation will linger along the Cascades as well as the northern Blue Mountains and the Wallowas. With temperatures above freezing overnight and snow levels above 5000ft, much of the precipitation that will fall will be rain at pass levels and rain snow mix along the crests with 20-40% chances of up to 0.10 inches of rain. EFI continues to show the temperatures at seasonable with temperatures increasing ever so slightly to the upper 60s to low 70s through the lower elevations and mid to upper 50s elsewhere.

Lastly, winds through the period will be west to southwest with a diurnally driven increase. Ensembles show high confidence (70-90%)
that sustained winds will be 10-20 mph through the Cascade Gaps and along the lower Columbia Basin daily through Saturday and decreasing overnight. Bennese/90

LONG TERM
Sunday through Thursday
The long term period is expected to see unsettled weather as a trough will be over the area through most of next week and a series of weak and moisture deprived fronts bring light amounts of rain mainly to the mountains.
Temperatures will be generally near to slightly below normal for late April. There are growing differences between model ensemble members over time so while the overall pattern is clear, the finer details are less certain. The Extreme Forecast Index indicates little in the way of unusual weather aside from elevated winds Sunday and Monday afternoons in the Columbia Basin (EFI = 0.75-0.80).

Models start out in good agreement on Sunday in having a strong upper low off the northern British Columbia coast and a shallow trough centered off the Pacific Northwest coast. A weak front and a generally west to southwest flow into the Cascades will bring a 60- 80 percent of rain along the Cascade crest with amounts of about a tenth of an inch or less. The northern portions of the eastern mountains will have a 20-35 percent chance of very light rain. Snow levels will be around 4500-5500 feet. The main concern will be winds with the frontal passage and 15-30 mph westerly winds are expected over most of the area in the afternoon. Temperatures will be slightly warmer than Saturday with highs in the upper 50s to mid 60s with 50s in the mountains.

For Monday, models are still in reasonable agreement though model ensemble clusters show some differences. 70 percent of model ensemble members have the upper low over Vancouver Island or southern British Columbia though they have significant differences about the strength of the closed low. The other 30 percent have the low offshore near central British Columbia. They also indicate some form of weak front crossing the area in the afternoon with a 40-60 percent chance of light rain in the Cascades and a 15-25 percent chance in the eastern mountains. Amounts will be mainly just a few hundredths of an inch. The front will again bring some breezy westerly winds of 15-25 mph in the afternoon. Temperatures will be 3- 5 degrees cooler in the afternoon.

At this point, model ensemble members diverge significantly. On Tuesday and Wednesday, 55 percent of the members have the upper low and trough centered off of Vancouver Island and the other 45 percent favor a shallower trough and low centered over Montana. The National Blend of Models favors the solution with the trough off the coast, though with pops of 20-40 percent in the Cascades and 15-25 percent in the eastern mountains with the rest of the area dry. Rain amounts will be minimal. The version with the trough offshore gives us a southwest flow which pumps up a ridge off to our east, which will warm temperatures a few degrees each day reaching the mid 60s to lower 70s with upper 50s to mid 60s in the mountains on Wednesday.
It should be noted that the 25th percentile and 75th percentile of temperatures have a broad range of 10-15 degrees, so forecasted temperatures have low confidence.

For Thursday, 62 percent of the models continue to keep a strong trough along the Pacific Northwest coast and a strong ridge over the Midwest with a southwest flow over our area while the others show a weak trough developing along the coast or just ashore and a weaker ridge over the midwest. Both solutions favor rain, so have a 40-60 percent chance of rain over the mountains and a 15-30 percent chance over the lower elevations. The mountains will get up to a tenth of an inch of rain while amounts will be very light in the lower elevations. The trough and greater precipitation and cloudiness will cool temperatures to the 60s with 50s in the mountains.
Temperature spread remains wide and confidence remains low. Perry/83

PRELIMINARY POINT TEMPS/POPS
PDT 42 61 42 62 / 80 20 0 0 ALW 47 65 45 65 / 80 20 10 0 PSC 50 68 46 69 / 50 10 0 0 YKM 42 64 40 66 / 30 20 0 0 HRI 45 67 45 67 / 60 10 0 0 ELN 40 62 40 62 / 50 30 10 0 RDM 38 54 34 58 / 20 20 0 0 LGD 41 57 39 58 / 90 40 10 0 GCD 40 56 37 59 / 90 50 20 0 DLS 47 62 46 61 / 60 30 0 0

PDT WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...None.

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