Marine Weather and Tide Forecast for Heron, MT

SYNOPSIS
Snow increases around the mountains this evening into early Monday. Snow and rain chances develop over the lowlands this evening and overnight too, with a limited accumulation potential outside the northeast mountain valleys. A warmer, periodically breezy pattern settles in for much of the work week.
Precipitation, moderate to heavy at times will be possible along the Northern Cascades with increasing concerns for rises on rivers. Lighter precipitation with snow melt will lead to additional rises in the Idaho Panhandle.

DISCUSSION
Tonight and Monday: the first in a series of frontal waves that will usher in milder weather comes in. This evening a warm front lifts in, followed by a weak cold front overnight into Monday morning. Tapping into the leading edge of a plume of moisture, with PWAT rising to 200-300% of normal (or 0.75 to 1"). All this moisture converges best at the Cascade crest and secondarily over NE WA and the ID Panhandle, with the frontal wave. These areas will have the best chance of precipitation this period. The lowlands will see a definite risk for precipitation (generally between 30-60%, except 10 to 20% in the deeper basin); but precipitation amounts will be lower.

Precipitation-type: largely the temperatures profile support snow to begin with over the mountain and northern two-third of WA and ID, with rain or a rain/snow mix over the southern third of WA and the lower ID Panhandle. However overnight into Monday morning snow levels rise and push above most pass levels toward midday to afternoon. Before then some accumulation is possible.
The Cascade valleys, such as the Methow Valley, are expected to hold onto the cooler air a bit longer than areas further east and the NBM forecast was adjusted to keep them lower tonight/early Monday.

Snow amounts: some moderate snow amounts are forecast for the Cascade crest, with near snow rates potentially approaching 1"/hour this evening at Stevens Pass, before decreasing overnight into early Monday as the milder air starts in. Other light to moderate snow are possible late this evening into early Monday AM near the Central ID Panhandle, including Lookout Pass. Winter weather advisories were issued for the Cascade Crest this evening through early overnight; one was issued for the Central Panhandle Mountains above 3000 feet for later this evening to late Monday AM. There are some models showing a slight chance of freezing rain mixed in near the Cascade valleys and maybe some of the sheltered central Panhandle valleys, as the milder air surges in aloft before surface temperatures have a chance to catch up. Confidence is low, but it is a non-zero risk. Elsewhere some light snow accumulation is possible over the northeast WA and north ID valleys too, around 0.5 to 1.5".
Other areas could see trace to 0.5", but confidence leans closer to a trace to 0.1" if that.

Lows tonight are forecast to be in the mid-20s to mid-30s, with some upper 30s over the deeper basin and L-C Valley. Highs Monday are forecast to be in the 40s to lower 50s, with some mid 50s to mid-60s over the deeper basin and L-C Valley, with the warmer of those readings in the deeper basin.

Tuesday to Saturday: The regions becomes dominated by a strong zonal flow and progressive pattern, steeped in the atmospheric river with PWATs around 175 to 250% of normal. The jet stream remains largely centered north of the Canadian border until Saturday when it dips south into our region. Through the week impulses riding by the area are rather weak, but precipitation chances will continue. The best chances will be in the Cascades and Idaho Panhandle Mountain in the orographically-favored lift in this flow, with PoPs around 60-90%. Away from these areas the central and eastern third of WA and lower ID will see chances around 10-50%, with the lowest of that risk in the deeper Basin and L-C Valley. Regardless of chances, the higher precipitation amounts are focued in the Cascades and secondarily the ID Panhandle Mountains.

Here are the latest probabilities for storm total QPF amounts from Sunday night to Saturday (some of this will be snow Sunday night and Monday).

Total QPF (Mon-Fri) >1 inch >2 inches >3 inches >5 inches

Cascade Crest 100% 90% 80% 40% Idaho Panhandle 50% 10% 0% 0%

Additionally, recent heavy snow may be able to absorb the rain, but it may begin to ripen and start to runoff, especially with the mild temperatures and increasing wind which can enhance snowmelt. We will continue to monitor this. This could cause some hydrological issues.

Those winds mentioned will be breezy each day, with gusts in the 15-30 mph range. However if any of those shortwaves are a bit stronger, some locally stronger gusts are possible.

Temperatures are still forecast to trend warmer. Highs in the 50s are forecast for a large portion of the area Tuesday, with 60s in the lee zones, deeper basin and L-C Valley and maybe some areas near 70 in that deeper basin. Temperature edge warm Wednesday and by Thursday highs are forecast to be in mid-50s to mid-60s, with some lower to mid-70s in the deeper basin and L-C Valley. Temperatures then cool again toward next Saturday with highs in the upper 40s to 50s, with localized low 60s in the typically warmer spots. Compared to yesterday the potential for the Spokane area reaching 70 later this week had declined to only 10-30%. /Solveig



AVIATION
00Z TAFS: VFR conditions will continue at TAF sites through the remainder of this afternoon, but conditions start to change tonight into Monday morning with a system moving in. Clouds will start to thicken and lower this evening, with a chance of snow or rain/snow mix coming to TAF sites. The main chances near EAT and MWH will be before midnight and again later in the period near EAT. The GEG/SFF/COE/PUW/LWS sites will see the opportunity for precip between 00Z to 18Z, with better chances after 03Z.
As the evening progresses into Monday morning the chance for MVFR conditions will increase. Precipitation amounts look light, but some minor snow accumulations are possible near GEG/SFF/COE/PUW. By Monday afternoon, largely after 19-21Z primarily VFR conditions are projected with the potential for some light rain.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Moderate to high confidence in VFR conditions through 06Z, with moderate confidence in conditions dropping to MVFR during any precipitation. There may be some patchy freezing rain mixed in near the Cascade valleys overnight/early Monday AM, but the risk is low at any terminals.

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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 32 47 41 56 47 60 / 60 50 20 20 50 20 Coeur d'Alene 31 45 41 53 44 57 / 80 80 40 30 70 30 Pullman 35 51 43 58 45 60 / 60 70 20 30 40 20 Lewiston 40 56 44 65 47 68 / 60 60 10 10 30 10 Colville 28 47 35 58 43 60 / 80 60 20 20 40 30 Sandpoint 30 42 40 51 42 53 / 80 90 50 50 90 50 Kellogg 30 45 43 51 46 54 / 90 100 60 60 80 60 Moses Lake 35 60 44 68 48 67 / 20 10 0 10 10 0 Wenatchee 35 57 43 65 47 62 / 50 30 10 20 30 20 Omak 34 52 39 62 44 63 / 40 20 0 10 20 10

OTX WATCHES/WARNINGS/ADVISORIES
WA...Winter Weather Advisory until 2 AM PDT Monday for Western Chelan County.
ID...Winter Weather Advisory from 8 PM this evening to 11 AM PDT Monday for Central Panhandle Mountains.