Area Forecast Discussion
Issued by NWS Pendleton, OR
Issued by NWS Pendleton, OR
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289 FXUS66 KPDT 171727 AFDPDT Area Forecast Discussion National Weather Service Pendleton OR 1027 AM PDT Mon Jun 17 2024 Updated aviation discussion .MORNING UPDATE...Current Satellite and radar imagery show isolated showers with low stratus across portions of the Cascades and eastern mountains, while mid to upper level cloud cover is pushing in from the north across the lower elevations. This activity and the well below normal temperatures this morning is associated with an upper trough that is currently sitting over eastern WA/OR. This trough will continue to bring isolated to scattered showers across the mountains today, where higher areas along the Wallowa and Strawberry mountains will see snow showers through the afternoon. There is also increasing chances (15-30%) of isolated thunderstorms across the eastern mountains this afternoon, thanks to modest mid-level lapse rates between 7-8 C/km and increasing surface CAPE values of 100-300 J/kg. Otherwise, an increasing cross Cascade pressure gradient will result in breezy to locally windy conditions through the Cascade gaps and the Lower Columbia Basin, with the strongest winds along the Simcoe Highlands, the eastern Columbia Gorge, and the Kittitas valley. Lawhorn/82 && .PREV DISCUSSION... /issued 405 AM PDT Mon Jun 17 2024/ SHORT TERM...Today through Wednesday...Bottom Line Up Front: 1. Mountain snow/rain showers over higher terrain with slight chance thunderstorms along the eastern mountains. 2. Breezy conditions Monday through the Cascade Gaps. 3. Below normal temperatures through today then a slight warm up. Short term models are good agreement with the upper level low moving over the area bringing with it precipitation. Along with the precipitation, the upper level low is bringing in a rush of cold air which will bring the snow levels down to near 4500-5500 feet. Therefore some of the precipitation falling, especially above 4500 feet, will be snow with 80-100% probabilities of more than a trace along the higher peaks of the WA & OR Cascades, Blues, and along the Ochoco-John Day Highlands. With the influx of instability associated with the system, model derived soundings are showing an increase in MUCAPEs of 150-300 J/kg and lapse rates of 8.4 C/km and higher. However, the remaining ingredients needed for better lift and mid level moisture is lacking. PWATs are low at only 0.4-0.5 inches, bulk shear is below 15 kts, and LIs are only -1. However, cannot rule out a few lighting strikes. With the incoming cold air and the amount of cloud cover that is over the area, confidence in thunderstorms is low (less than 25%) along the aforementioned areas. Another round of breezy conditions will swing through as the upper level low continues to push across the region. Looking at surface model data, there is a clear tightening of the isobars as the low pushes towards and over the Cascades. Looking at the pressure gradient tool using the NAM40, GEFS, SREF and RAP40, it clearly shows a strong gradient between PDX and GEG. The NAM is the most bullish with an 11 to 12 mb change while the others are 9 to 10 mb. This leads to the conclusion that we will see yet another round of breezy conditions along the Gorge, Kittitas and Yakima Valleys as well as along the Southern Blues. 40-60% of the raw ensembles show probabilities of these areas seeing 25 mph or greater sustained winds with gusts to 50-70% probabilities of seeing gusts to 40 mph. After today, winds will settle and return to normal. Lastly, EFI show temperatures to take a bit of a dive with this upper level low. Snow levels drop, as do the surface temperatures. Equal amounts of the raw ensembles have temperatures in the upper 50s to mid 60s for today across the PDT area, 56% show the Basin in the mid 60s to low 70s, 67% show the Gorge and surrounding areas in the low to mid 60s and lastly the higher terrains in the low 60s. Today will be the coolest day of the forecast period through the weekend and next Monday. Models show the upper level low to push the moisture and cooler air off to the east by Monday night. Models show a broad trough move over with northwest to westerly flow. This will warm the region up a bit bringing temperatures back to near normal with the EFI in agreement. NBM and 77% of the raw ensembles have the PDT area and central OR in the low to mid 70s, the Basin, adjacent valleys and the Gorge in the mid to upper 70s. Temperatures are expected to slowly increase through the remainder of the period. Bennese/90 LONG TERM...Thursday through Monday...The main sensible weather highlights include: a warming trend with departures peaking 10-15 degrees above average on Saturday, a cold front and windy conditions Sunday, low-end precip chances returning to the mountains late Sunday-Monday, and elevated fire weather potential Sunday revolving around winds and low RH. The large scale pattern early Thursday will be characterized by weak mid-level cyclonic flow and a weak upper-level trough over the west coast. Elsewhere, a modest upper-level ridge is forecast in the eastern North Pacific with the high centered north of Hawaii. Ensemble clustering and guidance are in good agreement through Saturday. A 500 mb shortwave ridge is anticipated to build and move east to be overheard 48-hrs later/by 12Z Saturday while a closed upper-low develops and moves east across the far northern east North Pacific/Gulf of Alaska. This will continue to support a warming trend Thursday through Saturday. The greatest departures from average are expected to be seen on Saturday at mainly 10-15 degrees area-wide. We are seeing widespread moderate HeatRisk (category 2) across much of the Lower Basin into the Gorge Friday and Saturday driven largely by daytime temperatures, though mild lows are forecast around 5-10 degrees above average. While highs are above seasonal averages, they are not that unusual and heat will affect mainly those sensitive to heat and those without cooling/hydration. High confidence (>80%) in 90 F or higher highs across the lower Basin with moderate-high confidence (50-80%) across the foothills Friday and Saturday. Low confidence (< 15%) in exceeding 100 F on the warmest day (Saturday). The spread in gudience increases by Sunday with the differences centered on the strength and timing of a shortwave trough and its attendant front associated with the evolving upper-level trough in the eastern North Pacific. While some differences exist with the timing of the front/marine push, fairly confident in Sunday being cooler (60-70%) with highs trending to drop 5-10 degrees from Saturday. Limited moisture is anticipated on top of dry conditions with a marked increase in winds expected Sunday. Chances for 24-hr daily highest wind gust exceeding 34 kts/39 mph exceeds 60-80% across much of the eastern Gorge spilling into the Lower OR Basin with Kittitas Valley and Simcoe Highlands exceeding 90%. Meantime, reasonable high-end gusts, the 75th percentile wind gust forecast (1 in 4 chance of higher gusts), are seen to exceed 40 mph at spots in the afternoon Sunday such as KPDT, KDLS, and KELN with the latter seeing gusts reach 50 mph. Even so, most likely gusts of around 30-45 mph will combine with low afternoon humidity across the Lower Basin and north central OR that will promote potential for elevated fire weather concerns. Cooler air will then continue to move into the region on Monday. Finally, there will be low-end chances for precip (~30% and less) across the upper slopes of the eastern WA Cascades Sunday- Monday. Of note, there could be conditional potential for showers/isolated thunderstorms across the far eastern mountains but confidence is very low (less than 15%). AVIATION...18z TAFs...Main aviation concerns with be the breezy to windy conditions today followed by some shower this afternoon through evening but overall expecting VFR conditions. Will see sustained winds 10-20kts with gusts 30-35kts this afternoon through evening before decreasing overnight becoming less than 10kts. Showers will produce some local CIGS 040-080 into this evening before showers dissipate and clouds decrease. && .PRELIMINARY POINT TEMPS/POPS... PDT 67 43 73 45 / 30 30 0 0 ALW 70 47 76 48 / 40 40 0 0 PSC 74 49 79 49 / 20 30 0 0 YKM 72 43 77 46 / 10 10 0 0 HRI 73 47 79 48 / 20 20 0 0 ELN 67 45 75 46 / 10 10 10 0 RDM 61 33 72 39 / 10 0 0 0 LGD 60 39 69 41 / 50 40 20 0 GCD 59 36 71 41 / 50 20 10 0 DLS 68 48 77 51 / 10 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ MORNING UPDATE...82 SHORT TERM...90 LONG TERM....80 AVIATION...80