Sunrise 5:32 AM   Sunset 8:07 PM Moonrise 3:59 AM   Moonset 4:02 PM

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FXUS66 KPDT 041549 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 849 AM PDT Sat May 4 2024

SHORT TERM UPDATE
A deep low is centered about 140 miles west of Coos Bay with an increasing moist southerly flow ahead of it. A weak deformation band has set up ahead of the low with stratiform light rain along the WA/OR border in the Lower Columbia Basin.
PoPs were increased in this area to reflect the more widespread rain. Previous models, including the high resolution CAMS, did not handle this deformation band well.

A wind advisory is in effect for the base of the Oregon Blue Mountains east-southeast of Pendleton and Milton-Freewater. There have been gusts 55-60 mph along Cabbage Hill and near Cayuse which is on the low end of warning criteria. Current MSLP difference between BKE and MEH is 2.7 mb and has decreased slightly since 12Z when there was a 3.3 mb difference. Will keep the advisory as is due to the gradual weakening of pressure gradients. Wister/85

PREV DISCUSSION
/issued 505 AM PDT Sat May 4 2024/

SHORT TERM
Today through Monday
The deep closed upper level low off the PacNW coast continues to move to the SE and will move over SW Oregon late today then into the Great Basin overnight into Sunday. Meanwhile, the cold front will be very slow to progress eastward as the low passes south of the forecast area. Will see increasing rain over the western portion of the forecast area this morning with precipitation slowly spreading east into the rest of the forecast in the afternoon and overnight. Models continue show a deformation band forming on the north side of the low where the cold front stalls across northern Oregon and southern Washington overnight into Sunday. This will lead to some significant rainfall amounts on the oder of .5 to 1.0 inches across the Columbia Basin and 1 to 2 inches over portions of the eastern mountains and along the east slopes of the southern Washington Cascades. Central Oregon should see the least amount of precipitation at .25 to .5 inches.
The low begins to move off into the Rockies late Sunday and Monday which will cut off the wrap around moisture and allow a northwest flow to develop. This will shift the focus of precipitation Monday mainly to the Cascade crest and over the eastern mountains in the form of showers including a slight chance (15%) of thunderstorms over the eastern mountains in the afternoon and evening. River forecast continue to show that this significant rain will cause rivers to rise but with no flooding expected as rivers remain below bankfull.

Snow levels start out this morning around 6000 to 7000 feet but begin to lower across central Oregon through the day and overnight as the low center migrates into SW Oregon and then into the Great Basin. Snow levels across central Oregon will lower to 2500 to 4000 feet overnight through Sunday morning before rising back to around 4000 feet Sunday afternoon. This will allow for some minor snow accumulations in the lower elevations but more significant accumulations above 4000 feet. Snow advisories are in effect for the East Slopes of the Oregon Cascades for 4 to 10 inches mainly along the crest and 3 to 6 inches in the John Day Highlands.

South to southeast winds have increased across the region overnight with winds of 10 to 25 mph and some gusts 30 to 40 mph. These winds will continue through the morning and then decrease as a transition to westerly winds starts to develop this afternoon and evening.
These westerly winds will increase overnight and continue through Sunday at speeds and gusts that warrant the issuing of a wind advisory for some zones across the Lower Columbia Basin mainly from the eastern Columbia River Gorge to Pendleton.

LONG TERM
Tuesday through Saturday
Deterministic models and ensembles are in good agreement particularly through Thursday then there are some differences in the evolution/amplitude of an upper ridge over the area for Friday and Saturday. On Tuesday a shortwave trough will move SEWD across the region in NW flow aloft. This will result in mountain showers and snow showers (50-75% chance) with snow levels 3500-4500 feet. QPF amounts are expected to be mostly less than .2 inches so significant impacts from snow are not anticipated. The NBM has low probabilities of TSTMS for Tuesday afternoon for the eastern Mountains (15-18%) but forecast soundings show little CAPE and insufficient depth for lightning so chose to keep TSTMS out of the forecast for Tuesday.

The winds will be a concern on Tuesday as well. The GFS is forecasting the PDX-GEG surface pressure gradient to increase to 11- 14 mb which is indicative of significant west winds across the lower elevations. NBM 24 hour max gust probabilities of 45+ mph across the lower elevations exceed 70% over a wide area Tuesday.

After Tuesday the weather will become quiet with the available guidance indicating a warming and drying trend. On Wednesday high temperatures will still be around 5 degrees below normal for this time of year but by Thursday high temperatures will be near normal.
For Friday and Saturday the majority of the ensemble clusters favor a building upper ridge though there is a lower probability scenario (15-30% chance) for weak troughing over the area on Friday and then flatter westerly flow on Saturday. This is similar to the operational GFS and the GEFS ensemble and results in slightly cooler temperatures than what the NBM and ECMWF ensemble are forecasting.
Given that the more likely scenario is for the upper ridge went with the warmer NBM temperatures for now. By Saturday the NBM probabilities of 85+ degree high temperatures in the Columbia Basin and Yakima Valley are 40-70%. 78

AVIATION
12Z TAFS
Increasing aviation concerns today that continue tonight between deteriorating flight categories, light gusts, and possible snow at central OR terminals tonight. Latest satellite shows an extensive cloud shield with clouds moving north across the area and obs denoting mainly high-end VFR ceilings (greater than 7 kft). This system will prompt precipitation to spread gradually north and east over the next 12-18 hrs as moisture gets ushered north and low levels and moisture in the column increases from increased forcing for ascent over time.

Chances start ramping up (to and >80%) through 18Z across the western-most terminals before ramping up (>80%) across the KPSC, KPDT, and KALW terminals around 0Z-6Z, the former having the longest wait. High confidence (>80%) in MVFR conditions developing with precip and worsening flight categories, initially over central OR and the KDLS terminal today then KYKM, KPSC, KPDT, and KALW tonight with a strong consensus seen in the forecast envelope composed of hi- res deterministic guidance runs and past runs. Of note, winds will be gusty but confidence is high (80%) in peak gusts staying around 20-25 kts, except at KPDT when gusts are expected to increase late tonight to 30 kts. Lastly, snow mixing in with rain will be a possibility at KBDN and KRDM, however, confidence in predominately snow as the p-type is currently low (less than 30%) through 12Z tonight.

PRELIMINARY POINT TEMPS/POPS
PDT 71 40 50 40 / 30 100 100 70 ALW 73 42 52 44 / 30 100 100 80 PSC 74 47 57 47 / 60 100 100 40 YKM 63 44 60 39 / 90 80 80 10 HRI 72 42 55 43 / 70 100 100 40 ELN 59 43 56 40 / 70 60 60 10 RDM 51 32 47 34 / 80 70 60 10 LGD 66 38 47 38 / 30 100 90 90 GCD 62 34 45 35 / 90 100 100 70 DLS 58 45 56 46 / 90 80 60 20

PDT WATCHES/WARNINGS/ADVISORIES
OR...Wind Advisory from 2 AM to 11 PM PDT Sunday for ORZ041-044-507- 508-510.

Winter Weather Advisory from 5 PM this afternoon to 11 PM PDT Sunday for ORZ506.

Wind Advisory until 2 PM PDT this afternoon for ORZ507.

Winter Weather Advisory until 11 AM PDT Sunday for ORZ509.

WA...Wind Advisory from 2 AM to 11 PM PDT Sunday for WAZ024-521.

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