Area Forecast Discussion

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FXUS66 KPQR 280518 AAA AFDPQR Area Forecast Discussion...Updated National Weather Service Portland OR 917 PM PST Wed Feb 27 2019 Updated Aviation/Marine .SYNOPSIS...An unseasonably cold weather pattern will continue for much of the next week with snow possibly mixing down to the lowest elevations Thursday and Friday mornings in spots. && .SHORT TERM...830 PM UPDATE: Today through Saturday...Water vapor satellite imagery this evening showed a 528 dm upper low spinning off the Washington coast. It also appeared another short-wave feature was rounding the base of the low headed toward the Oregon coast. Infra-red imagery showed an area of enhanced shower activity 100 to 200 nm off the coast. KLGX radar showed a cluster of showers along the South Washington and far North Oregon coast as of 04Z. Some of the 12Z higher resolution models suggest this short-wave and associated shower activity will reach the coast late tonight and rotate through the area Thu morning. The main focus for showers looks to be north of a Newport to Salem line. It will be a hit and miss type situation. Not all areas will experience showers, but those that do could pick up a quick inch or so of snowfall. Another issue will be potentially icy conditions on wet secondary or untreated roads. Temperatures will fall below freezing overnight, for those areas that are not already 32 deg or lower. One big change from 24 hours ago is the much weaker offshore gradient. Last night the KOTH-KGEG large-scale gradient was near -20 mb, which was why many areas away from the Gorge experience windy conditions, i.e. more of a downslope event. At 04Z tonight, the KOTH-KGEG gradient was 2.1, or light onshore. The KTTD-KDLS gradient was -3.0, but this is forecast to hover around -2 to -3 mb through Thu morning. Otherwise, models are in decent agreement the aforementioned upper level low pressure currently found across the northeast Pacific, will shift southeastward across southern Oregon late Thursday into Friday. This could bring another round of light precipitation Thursday night into Friday morning, particularly to the southern half of the CWA. Given the time of day (ie overnight) precipitation will be arriving and pressure gradients will remain weakly offshore, will need to monitor this for additional low elevation snow potential, but at this point, it appears this should not be a big deal. Models are in good agreement that the area will dry out, but remain unseasonably cool Friday night into Saturday. Weishaar/Neuman .LONG TERM...Saturday night through Wednesday...The 12Z GFS and ECMWF are in pretty good agreement with the general trend through the extended forecast. A weak high pressure moves in Saturday night off the coast and will reduce precipitation. A low that propagates south along the west coast of B.C. Sunday will stay to the north due to a ridge growing progressively stronger over the Great Basin through Monday. This will alleviate the chance of precipitation through Tuesday. However, precipitation returns Tuesday night as a low comes up from the south. The pressure gradient across the Cascades brings easterly surface winds along the gorge and down the mountains turning into a northerly flow within the valley. This pattern of strong surface winds looks to be pretty constant Saturday night through Wednesday. Snow is expected in the Cascades and northern Coast range when precipitation returns Wednesday but was not a feature for any time before. -BPhillips && .AVIATION...Band of showers moving ashore on the north Oregon and south Washington coast is less organized than the band this afternoon. It is likely producing MVFR conditions with some snow mixed in. Some models show this area of precipitation making it inland north of KSLE. Other models do not. The models that do not are handling the current situation better so will follow them more closely. Satellite shows mid and high level clouds departing to the east, revealing expanding stratus deck over the interior. This is expected to continue into Thu for MVFR to IFR conditions. NAM model sounding indicate the areas south of KSLE will have LIFR fog/stratus into at least midday Thu. Next system approaches the coast later Thu with precipitation in the evening, possibly as low elevation snow. KPDX AND APPROACHES...Turning the forecast around to have MVFR conditions through the night and likely through at least midday Thu, the VFR developing after that. /mh && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory for winds from 6 AM Thursday to 6 AM PST Friday for Coastal Waters from Cape Shoalwater WA to Florence OR out 60 NM. Small Craft Advisory for hazardous seas from 9 AM Thursday to 11 AM PST Friday for Coastal Waters from Cape Shoalwater WA to Florence OR out 60 NM. Small Craft Advisory for Rough Columbia River Bar from 10 AM to 2 PM PST Thursday. && Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. This area is commonly referred to as the forecast area.