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FXUS66 KPDT 040546 AFDPDT

Area Forecast Discussion National Weather Service Pendleton OR 946 PM PST Sun Dec 3 2023

EVENING UPDATE
The development of areas of dense fog in the Yakima Valley has prompted the issuance of a dense fog advisory until 7AM PST Monday morning. Confidence in dense fog persisting beyond the onset of precipitation in the morning is low, hence the end time. Dense fog has also developed in the lower Columbia Basin, but remains patchy so have held off on an advisory and instead issued a special weather statement to cover the hazard in the near term. Minor updates to precipitation timing and other forecast variables were made based on latest 00Z guidance and observations, but the overall message remains consistent with the previous shift (see previous discussion for more details).
Plunkett/86


AVIATION
06Z TAFs...Mostly VFR conditions have been observed this evening with the exception of VLIFR at YKM due to +FG and periodic MVFR at PDT due to -RA and BR. The next round of precipitation (-RA) overnight through Monday morning is forecast to bring a mix of conditions, mostly MVFR and VFR, but IFR CIGs cannot be ruled out. Confidence is low in prevailing IFR at any site except for YKM this evening and overnight due to FG. S-SW winds of 10-15 kts with higher gusts are forecast at KPDT/KRDM/KBDN/KALW with mostly light, variable winds at KDLS/KYKM/KPSC. Plunkett/86

PREV DISCUSSION
/issued 238 PM PST Sun Dec 3 2023/

SHORT TERM
Tonight through Tuesday
With winter weather now almost in the rear view as our primary hazard, rainfall and hydrologic concerns become our focus over the next few days, alongside a final risk of freezing rain in the higher elevations thanks to warm air overrunning cold air, and breezy winds due to tightening pressure gradients in the region.

Overnight into tomorrow a warm air mass will lift across the region as a transient ridge sets up. As moisture continues to lift across and bring mostly rainfall to the region with snow levels expected to lift above 5-6k feet, cold air could continue to become trapped across the valleys of the Washington Cascades.
Model soundings point out the continued risk of freezing rain in these locations, but overall totals are expected to remain on the lower side, generally under a tenth of an inch.

As the ridge then shifts eastwards, moisture continues to stream in under a southwesterly flow, leading to weak to moderate atmospheric river conditions. Integrated Water Vapor Transport models, or IVT, show a weak to moderate inland atmospheric river event. Over the next 72 hours, this is translating to around a quarter to three-quarters of an inch of rainfall in the lower elevations, heaviest in the upper Columbia Basin, with the mountains and higher terrain expected to see anywhere from 1-3 inches, possibly higher in the peaks of the Washington Cascades.
The NBM shows a 20-40% chance of greater than or equal to 2 inches of new rainfall in the Blue Mountains through Wednesday morning, while the most of the Cascades have an 80-100% chance. The heaviest rainfall should thankfully be shadowed by the Cascades in this instance, but with rain falling onto the snow we just received, some concerns for local runoff and downstream rivers is present. Most area river and stream forecasts just show a very noteworthy rise to levels leading to increased flow and cold waters, but a few such as the Naches and possibly the Umatilla are expected to reach bankfull stage and will need to be monitored.
Currently, we continue to issue a hydrologic statement on these rivers and their expected rises, but with the heaviest rainfall expected to remain on the west side of the Cascades or further to our northeast, flood products such as watches are not currently being issued. This could easily change in the next day or two as we continue to monitor the incoming systems and as river forecasts change.

This strengthened pressure gradient should also lead to some breezy winds for the region, strongest in the Grande Ronde Valley and Ladd Pyles Canyon where pressure gradients between Baker City and Meacham are expected to near 8-9 mb of difference, a good indicator at stronger winds nearing advisory levels. Winds should start increasing for this area starting Monday evening and continuing into Tuesday, strongest Tuesday morning into the afternoon. Southwesterly flow also keeps us on the warm side, with widespread highs on Tuesday for our population centers in the upper 40's to upper 50's, with Central Oregon possibly reaching 60. Overnight lows will also be seasonably above normal, ranging from the mid 30's to upper 40's. Some of these overnight lows could set records for their warmth.

LONG TERM
Wednesday through Sunday
Wednesday will see the last of the atmospheric river as more rain is expected during the day and into the evening before tapering off at night. There will be some lingering precipitation on Thursday, mainly over the mountains. A drying trend settles in for Friday and Friday night over most locations before the next systems moves over the ridge for Saturday into Sunday.

The forecast QPF associated with the Tuesday/Wednesday rain has decreased again and is now generally less than 1.5 inches over the mountains and less than 0.50 inches in the lower elevations.
However, we continue to expect responses on the rivers and streams in the form of in-banks rises due to the rain on snow and very warm temperatures.

Snow levels Wednesday morning will be around 5000 feet in the Washington Cascades and 6000-7000 feet elsewhere. By Thursday morning, snow levels will have fallen to below 3000 feet in the Washington Cascades and to 3000-4000 feet elsewhere. By Friday morning, all locations will be below 2500 feet, with the Washington Cascades below 2000 feet.

Therefore, as we approach the end of the week, any precipitation that falls will become more of a mix in the mountains and then just plain snow at and above pass level. QPF is much less with this next system, but ultimately how everything plays out with moisture, snow levels etc will determine snow amounts. Lower elevation QPF is no more than a few hundredths.

There is considerable uncertainty as we approach the weekend. The deterministic ECMWF has a stronger ridge over the Pacific Northwest in response to a deeper low downstream over the southern Great Basin/southern plains. While both the GFS and ECMWF bring a system over the ridge Saturday into Sunday, the the strength of the ridge and its exact position could end up being a factor in where the precipitation shield sets up. Given the natural variability in a forecast at this time scale and the guidance having difficulty with the downstream features over the last few days, confidence is average at best. However, the ensemble clusters are more supportive (about 67%) for the stronger ECWMF ridge as we get toward the end of the extended period.

Temperatures will be above normal at the start of the period and return to near normal by the end of the period. Highs on Wednesday will mainly be in the low 50s, with some 40s in portions of Washington. On Thursday, highs will mainly be in the 40s with the Columbia Basin possibly reaching 50 degrees. High temperatures on Friday will generally be in the low to mid 40s. Highs on Saturday and SUnday will be in the upper 30s to lower 40s. Overnight lows will mainly be in the 30s Thursday night and Friday night, then 20s and 30s the remainder of the period. Once again the ECMWF EFI is keying in on both high and low temperatures on Wednesday (though more on the lows). High temperatures are 0.6 to 0.7, mainly across the Columbia Basin, whereas there is a widespread area of 0.7 to 0.8 across much of eastern Washington and a large portion of eastern and central Oregon, and there is an area of 0.8 to 0.9 centered on the Columbia Basin and surrounding areas.

PRELIMINARY POINT TEMPS/POPS
PDT 44 55 47 57 / 40 70 50 70 ALW 46 55 46 56 / 40 70 60 80 PSC 37 46 40 55 / 10 70 60 80 YKM 31 44 35 51 / 10 90 90 90 HRI 44 54 46 58 / 20 70 50 70 ELN 31 43 35 48 / 10 80 90 90 RDM 44 57 46 58 / 70 60 30 50 LGD 36 47 40 48 / 60 70 40 60 GCD 38 51 39 53 / 60 70 20 50 DLS 45 54 49 57 / 80 90 90 90

PDT WATCHES/WARNINGS/ADVISORIES
OR...None.
WA...Dense Fog Advisory until 7 AM PST Monday for WAZ027.

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