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FXUS66 KOTX 021747 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 1047 AM PDT Sun Jun 2 2024

SYNOPSIS
A vigorous storm will make its entrance today bringing widespread rain through Monday, and windy conditions for Monday afternoon.
There will also be a chance for thunderstorms late Monday morning into Monday afternoon. Next week will see a significant warming and drying trend. Confidence is increasing for temperatures warming into the 80s and 90s.

DISCUSSION

...DYNAMIC SPRING STORM TO BRING WIDESPREAD RAIN SUNDAY INTO MONDAY FOLLOWED BY A CHANCE FOR THUNDERSTORMS AND WINDY CONDITIONS...

Today through Monday night: Satellite imagery early this morning shows a vigorous storm system making its way toward the Northwest.
A strong jet streak of 160-170 mph will fuel a surface low that will track toward Vancouver Island today, across southern BC tonight, and then into southern Alberta on Monday. With the upper level jet will be a moderately strong Atmospheric River (AR 2 level event) that will focus moderate to heavy precipitation along the Cascade crest and over the Idaho Panhandle. Widespread stratiform rain will be upon the region by Sunday afternoon with heaviest amounts falling Sunday night into early Monday. The trailing cold front will move into the Cascades Monday morning and into the Idaho Panhandle by mid afternoon friday. The cold front will bring strong dynamical forcing that will be supported by strong dynamics aloft: bot with a strong vorticity maximum at the base of the upper level trough and within the left exit region of the upper level jet.

Lapse rates will steepen at lower to mid level of the atmosphere.
Rain will moisten the boundary layer with a -22 Celsius cold pool aloft with the upper level trough also advancing in over the region.
lower levels will destabilize right ahead of the surface front with favorable conditions for at least low topped convection to form.
Model guidance shows the potential for 200-500 J/kg of surface based CAPE, which will be enough for thunderstorms to form as strong dynamics will compensate for only modest amounts of instability. A tight pressure gradient will also result in windy conditions and these winds may be enhanced by any developing convection. Cold air advection will also result in lowering snow levels over the Cascades with slop over precipitation changing over to snow over the higher mountains above 4,500 to 5,000 feet. This will be low enough for snow to fall over Washington Pass in the north Cascades with 2 to 4 inches of accumulation expected.

* Rain Amounts: Not much has changed with the precipitation forecast. In general, the Cascade crest is looking at storm total rainfall of between 2.0-3.25 inches south of Lake Chelan and between 1.25-2.0 inches north of Lake Chelan (with a transition to snow above 4,500 feet). Places such as Omak, Wenatchee, and Moses Lake will see downsloping off the Cascades with much less in the way of total accumulations in the range of 0.05-0.20 inches.
Rainfall amounts will then precipitously increase over extreme eastern Washington and in the Idaho Panhandle with around 0.5-1.0 inches expected for Colville, Spokane, Pullman, and Lewiston. Then between 1.25-2.25 inches expected in the Idaho Panhandle with the westerly slopes of the higher terrain likely wringing out the most moisture. Probability of at least an inch in the Idaho Panhandle varies wildly as there will be a strong orographic component. The Silver Valley, Bonners Ferry, Lewiston-Clarkston Valley, and western portions of the Camas Prairie will see a fair amount of downsloping off of the higher terrain. A near certainty for at least an inch for the higher terrain especially westerly and southerly slopes, including the Cascade crest. Probabilities fall to 50-60% for getting at least 2 inches of precipitation; although the Cascade crest will see at least a 70% chance for 2 inches of precipitation.

* Winds: Trended winds a little bit stronger for Monday afternoon over the southeast portion of the forecast area. Winds across the foothills of the Blue Mountains will be flirting with high winds, sustained at 30-40 mph and gusts up to 55 mph possible. Confidence isn't high enough to go with a High Wind Warning. Strong winds look to downslope off of the Northeast Blue Mountains onto the Palouse. Locations such as Pomeroy, Uniontown, and Pullman/Moscow will see the potential for 50+ mph wind gusts. Winds in the lee of the Cascades and across the basin into the Spokane/Coeur d'Alene area will see the potential for 25-30 mph winds with gusts of 45- 50 mph. Strongest winds are expected with the cold front passage in the afternoon. A Wind Advisory is in effect starting late Monday morning through Monday evening.

* Thunderstorms: As previously mentioned, the potential for thunderstorms will be greatest with the surface front from late morning into the afternoon hours on Monday. This will be the period of the best combination of moisture, instability, and forcing -- the three ingredients need for convection. Instability will be a limiting factor, but the combination of dynamics along the cold front and aloft should be enough to overcome this deficiency. Convective Allowing Models (CAMs) show the bes potential for thunderstorms developing from the Moses Lake Area to the Central Panhandle Mountains and points northward. This is where the best instability will be as the front pushes through.
The probability for thunderstorms is between 15-30%. The Palouse to the Camas Prairie will see a 10-20% chance for thunderstorms as well. Unidirectional bulk shear of around 40 kts may result in convection becoming organized in a linear fashion that would increase the potential enhanced wind gusts.

* Impacts: Moderate impacts from winds are expected regionwide.
Synoptically driven winds will result in strong cross winds, especially for high profile vehicles. Major highways such as I-90, US-2, US-95, US-195, and US-12 will be routes that could see strong cross winds on Monday. Tree damage and scattered power outages will be likely. Winds may be strong enough to topple trees, but these impacts are expected to be isolated in nature, The caveat is if thunderstorms do organize because it won't take much for strong convection to mix down damaging wind gusts on a broader scale. Rain over the Cascade crest and in the Idaho Panhandle may result in minor flooding impacts well. /SVH

Tuesday...Ensemble model guidance is in very good agreement that the Inland NW will be under zonal flow with a 125kt westerly jet lifting into southern BC during the day. The jet is also expected to push another weak atmospheric river at least across the southern portions of WA into the southern half of the ID Panhandle. The amount of precipitable water will pale compared to the system today-Monday, however it will assuredly bring another round of precipitation but it shouldn’t be heavy due to a stronger rain-shadow effect in the lee of the Cascades and significantly weaker atmospheric dynamics. Rainfall amounts will generally be a tenth of an inch or less with close to nothing across the western Columbia Basin. Deep instability will also be lacking with the upper level cold pool fixed well north of the Canadian border so we aren’t expecting thunderstorms at this point. It will also remain breezy on Tuesday, however nothing like the speeds forecast for Monday. We should see wind gusts of 30-35 mph in the afternoon across portions of central Washington.

Wednesday-Friday...There is very good ensemble agreement that a strong ridge will develop by midweek over the entire Western US in response to a deep trough forming between 140-150w. The result will be a significant warming and drying trend. 850 mb temperatures will climb from the 7-12C range on Tuesday to the 19-22C range on Friday. This translates to highs warming from the 60s to lower 70s on Tuesday to the 80s and potentially the lower 90s on Friday. However by Friday there are some significant model differences in the placement of the ridge axis. The Euro ensembles place it over the Cascades as do the Canadian runs, however the GEFS runs push the axis into western Montana with the aforementioned trough nearing the coast. The offshoot of these differences will be subtle in terms of temperatures, however it will influence whether or not we see precipitation develop in response to some ejecting energy riding NE off the offshore trough. The GEFS solutions continue to suggest this will spark some precipitation both Thursday and Friday afternoon, most likely in the form of thunderstorms. If we look at this from a cluster perspective (rather than ensemble specific perspective), the odds look a little greater with nearly 30% of the runs showing some precipitation in this area Thursday afternoon and evening and a little lesser chance Friday afternoon and evening. Suffice it to say confidence is too low to place in the forecast grids for now, but this will need to be monitored.

Saturday and Sunday...For the weekend weather the model confidence continues to decrease. Each ensemble package is different.
Although they all show a strong ridge over the region, the placement of the axis is vastly different. The EC pushes it into eastern Washington with the GEFS and Canadian solutions move it into central or western MT with the trough moving onto the coast.
The Canadian runs have changed significantly over the past 24 hours while the Euro runs have remained persistent. The closer the trough is to the Inland NW, the wetter and cooler the forecast will be. It still looks like Saturday will be the hottest day of the week with high temperatures ranging from the mid 80s to mid 90s. And it could be even hotter than that over portions of central Washington. There is a 20-30% chance of seeing triple digit heat for Wenatchee, Moses Lake, Omak, and Othello. There will also be a small chance in the LC valley. By Sunday all the solutions either flatten the ridge or move it farther east of our region. This should bring cooling temperatures, but to what extent is questionable. If the Canadian solutions verify we’d cool off by at least 10F, but this scenario only represents only 15% of the total ensemble solutions, so we aren’t sold. A tougher question is will we see precipitation? Certainly we would if the more aggressive Canadian solutions wins out, however the other 85% of the runs show little if any precipitation right now. fx

HYDROLOGY
An Areal Flood Watch remains in effect for Lewis and Shoshone Counties in Idaho. There will be an enhanced potential for increases on small streams and creeks for these areas. For the rest of the forecast area, rain will be largely beneficial due to antecedent dry soils and low base river flows. Flooding is not anticipated at this time on larger rivers but will need to be monitored. Another potential impact will be for rock slides on steeper slopes in the Cascades and across the Idaho Panhandle. /SVH

AVIATION
18Z TAFS: A wet Pacific storm system will move in over the Inland Northwest today bring widepsread rain and deteriorating conditions to all airports. Timing has been roughly 1 hour slower for Idaho and terminals in far NE WA. Rain will move into KOMK- KEAT- KEPH- KMWH between 18-20Z and then into KCQV- KSZT- KGEG- KSFF- KCOE- KPUW- KLWS between 20-23Z this afternoon. Expect MVFR conditions as the boundary layer moistens up with the rain. Winds will increase a bit with gusts up to 15-24 kt ahead of the incoming rain. Moderate rainfall with visibility down to 2SM is expected for extreme eastern Washington and the Idaho Panhandle later in the afternoon after 00Z resulting in periods of IFR conditions through the night. Rain ends from west to east 12-16z however this will come with increasing winds and quick development of convective showers. These showers will bring potential for strong wind gusts from 16-22Z and could mature into thunderstorms with isolated lightning strikes and small hail but by in large, enhanced wind gusts under rain cores will be the largest concern which could approach 50kts between 18-21z while tracking across the eastern third of WA and into North Idaho.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in moderate rainfall 00Z Monday-06Z Tuesday resulting MVFR conditions. Confidence is low to moderate for IFR conditions with a 20-30% chance for visibility down to 2SM during this same period. Rain may let up and become light between 06-12Z Saturday night with the potential for the potential of ceilings lifting above 3 kft and VFR conditions.

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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 66 50 62 43 66 44 / 70 100 90 10 30 0 Coeur d'Alene 64 50 59 42 60 46 / 70 100 100 20 40 10 Pullman 64 50 58 42 62 46 / 70 100 100 20 30 10 Lewiston 72 57 69 50 71 54 / 60 100 100 10 20 0 Colville 66 44 62 35 62 36 / 80 100 100 20 40 10 Sandpoint 62 49 57 40 56 43 / 70 100 100 50 60 40 Kellogg 62 51 55 44 57 48 / 70 100 100 50 50 30 Moses Lake 68 51 67 47 74 44 / 80 80 50 10 10 0 Wenatchee 66 54 63 49 68 47 / 80 80 50 10 30 0 Omak 68 49 68 43 70 42 / 80 90 60 10 20 0

OTX WATCHES/WARNINGS/ADVISORIES
ID...Wind Advisory from 8 AM to 11 PM PDT Monday for Central Panhandle Mountains-Coeur d'Alene Area-Idaho Palouse-Lewis and Southern Nez Perce Counties-Lewiston Area.

Flood Watch from this afternoon through Monday evening for Central Panhandle Mountains-Lewis and Southern Nez Perce Counties.

WA...Wind Advisory from 8 AM to 11 PM PDT Monday for Lower Garfield and Asotin Counties-Moses Lake Area-Spokane Area-Upper Columbia Basin-Washington Palouse-Waterville Plateau- Wenatchee Area.

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