Marine Weather and Tide Forecast for Lewiston, ID

DISCUSSION
...A MESSY WINTRY MIX OF WEATHER IS EXPECTED TODAY OVER PORTIONS OF THE INLAND NORTHWEST TODAY...

Today through Sunday night...This will be a very tricky forecast as a moisture laden warm front develops some precipitation over the Inland NW today and will be followed by a strong cold front and upper level trough late this afternoon and tonight. This front has a nice tap of sub-tropical moisture with precipitable water values approaching double the normal values for this time of year.
Precipitation isn’t the question, since most if not all places will receive some, the much tougher question to answer is what will the precipitation fall as? As of 3am this morning, all valley locations were solidly below freezing. Meanwhile a formidable inversion remains in place with 850 mb temperatures near the OR/WA border and southern portions of the ID Panhandle between 6-8C and closer to 0C near the Canadian Border, which represents an awfully strong thermal gradient and a will provide the impetus for significant precipitation event due to moderate S-SW winds aloft, abundant moisture and the resultant warm air advection. Given this thermal profile the resultant precipitation types are going to be quite messy, with a mix of rain, freezing rain, sleet and snow expected. The SREF chances for freezing rain continue to point to the best chances of icy conditions along an arc extending from the lee of the Cascades and western Columbia Basin then extending north and then east along the Canadian Border. How long this freezing rain will last is tough to answer as the NBM is quite insistent that most of the valley temperatures will rise to 32 or warmer by this afternoon. However given the lack of significant mixing with a hint of offshore surface gradients this could prove too rapid, especially for the northern valleys and in the lee of the Cascades where the mixing potential will be the most limited. Generating specific ice accumulations is a low confidence endeavor as the totals could not only be influenced by temperatures precariously close to freezing, but the cold air near the ground could also be deep enough to bring some sleet, which has much lesser impacts than accumulating ice. The NBM is forecasting most totals of 0.10” or less during the entire event which seems reasonable, however it could prove on the low side due to the aforementioned surface temperature warming. It’s worth noting the HRRR ensembles show some pockets around 0.20” west of the Methow Valley and the mountains between Colville and Sandpoint. If these heavier totals were in the valleys, we’d be dealing with a potential ice storm warning, however since these are generally sparsely populated areas, we will let the winter weather advisories ride for now. If the ice totals surge to this level or higher, we could see some minor tree damage, but the odds aren’t good. The HRRR chances of seeing 0.25” of ice or more are generally around 20-40% and again are limited to mountain locations, rather than the valleys. The threat of freezing rain will end rapidly with the passing cold front which is scheduled to move through the Cascades by mid- afternoon and into north ID by the early evening.

For locations like Spokane/CdA/Palouse/LC Valley/Camas Prairie, the mix out of cold air near the ground should easily occur with either brief or no freezing rain expected. In fact the latest HRRR runs are now showing no freezing rain with temperatures climbing above freezing before the onset of the precipitation. Again, this might be a little too quick and suspect brief freezing rain is still possible near Spokane and thus we will keep the winter weather advisory, but the impacts could be minor.

The weather behind the passing cold front for tonight into Sunday night will be vastly different than the weather we’ve endured over the past week or so as the strong ridge is replaced by a strong trough. This will generally result in an unstable and showery conditions with increased winds. The showers will generally be confined to the mountains, and appreciable snow is expected. Heavy accumulations are expected near the Cascade Crest with snow totals of 10-18 inches expected near Stevens Pass and 6 to 11 inches for Lookout Pass. Due to the latter we have expanded our winter weather advisories to encompass this portion of I90 over the Panhandle. The threat of showers will slowly dwindle Sunday night as ridging begins to build back over the region.
The chances for showers elsewhere will be significantly smaller due to strong cold air advection and rain shadowing in the lee of the Cascades. As is usually the case during this time of year, unless a cold front is unusually strong we could be left with an expansive blanket of low clouds and possible fog across most of eastern WA. fx

Monday and Tuesday: Models are in excellent agreement for an upper level ridge to move into the Inland Northwest on Monday. Northerly flow into the Idaho Panhandle early Monday as the upper level trough exits the region will allow for snow showers to continue through the afternoon before tapering off. As the ridge shifts into the region, a return to widespread stratus suggests temperatures on Tuesday may be cooler compared to what models are defecting. Like we've seen the last several days, persistent cloud cover has resulted in little variation between the high and low temperatures for many lowland locations.

Wednesday through Saturday: Discrepancies begin to emerge in the model ensembles on the progression of the ridge and the overall pattern for the end of the week. About 66% of ensembles in the WPC Cluster Analysis show the next system moving into the PNW between Wednesday and Thursday with the other 34% holding onto the ridge. By Saturday, 82% of the ensembles suggest the PNW to be under the influence of an upper level trough with unsettled weather continuing into the weekend. Examining various members do reveal significant differences in the exact strength and position so the specific details remain uncertain. For Wednesday and Thursday, enough models show a trough moving through the region to give better confidence for rain or snow across eastern Washington and north Idaho. The track of this system suggests there to be a decent shadow in the lee of the Cascades, but cold air at the surface Wednesday night into Thursday may allow for some light lowland snow. We will continue to monitor this potential over the upcoming days. /vmt

AVIATION
12Z TAFS: Confidence is high that widespread LIFR/IFR stratus will persist through the morning at all TAF sites aside from PUW. The cigs and visibilities will slowly rise through the day as precipitation from an incoming warm front begins to perturb the stratus and fog however we suspect cigs won't rise above MVFR levels for all the sites. The precip will arrive at the TAF sites between 17z-20z for EAT/MWH/GEG/SFF/COE and between 22z-00z for LWS and PUW.
For EAT our confidence is high that it will begin as freezing rain with the same being true for most of the valleys north of I-90. For MWH we have moderate confidence that it will begin as freezing rain with a transition to rain expected around 20z. GEG will be tricky as the models have been trending toward no or only brief freezing rain with little threat at the remaining sites. If freezing rain does occur it would likely only be for an hour or two with accumulations less than 0.05 expected. A cold front will pass over the region late this afternoon and evening resulting in snow showers for the mountains, but little threat for the TAFs. What could be problematic is what to expect behind the front. We suspect we should see widespread IFR/MVFR cigs forming overnight (provided they ever break up)which is typically the case behind all but the strongest cold fronts.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is high of stratus remaining over most of the sites through the day. For tonight the confidence is moderate that we should see low clouds reforming provided they ever break up today. Meanwhile precipitation type confidence is listed above. fx

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Confidence descriptors: Low - Less than a 30 percent chance Moderate - 30 to 70 percent chance High - Greater than a 70 percent chance

For additional probabilistic information for NWS Spokane airports, please refer to the Aviation Dashboard on our webpage: https:/ www.weather.gov/otx/avndashboard

PRELIMINARY POINT TEMPS/POPS
Spokane 35 31 37 25 33 25 / 90 60 20 0 0 0 Coeur d'Alene 37 31 36 26 34 24 / 90 90 40 10 0 0 Pullman 40 32 37 28 36 23 / 70 90 60 30 0 0 Lewiston 48 36 46 32 42 27 / 40 70 40 20 0 0 Colville 32 27 35 18 32 18 / 90 30 20 0 0 0 Sandpoint 35 30 35 25 32 24 / 100 90 50 10 10 0 Kellogg 42 32 35 28 33 23 / 80 100 90 30 20 0 Moses Lake 33 30 37 24 35 23 / 80 10 10 0 0 0 Wenatchee 35 33 41 28 38 26 / 70 20 20 0 0 0 Omak 35 29 37 24 37 22 / 50 10 0 0 0 0

OTX WATCHES/WARNINGS/ADVISORIES
ID...Winter Weather Advisory until 10 PM PST this evening for Northern Panhandle.

Winter Weather Advisory from 2 PM this afternoon to 4 PM PST Sunday for Central Panhandle Mountains.

WA...Winter Weather Advisory until 10 PM PST this evening for Northeast Mountains.

Winter Weather Advisory until 7 PM PST this evening for Central Chelan County-Moses Lake Area-Okanogan Highlands-Okanogan Valley-Spokane Area-Upper Columbia Basin-Waterville Plateau- Wenatchee Area-Western Okanogan County.

Winter Storm Warning until 4 PM PST Sunday for Western Chelan County.