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FXUS66 KOTX 022257 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 357 PM 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

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

Tonight through Tuesday night: It will be active 48 hours of weather as a strong Pacific jet slams into the PacNW delivering strong winds, moderate to heavy precipitation, and chance for thunderstorms. A trough of low pressure on the nose of the jet is beginning to swing inland today and with that, ushering a late season atmospheric river. For the next 12 hours, the main weather impacts will be steady light to moderate rainfall. This will be accompanied by breezy southerly winds with gusts near 25 mph yet these winds will pose little to no impacts.

The concern for weather impacts arrives Monday morning and continues into the early evening. As the trough swings inland, rapid cooling aloft will steepen lapse rates and transition steady stratiform precipitation toward convective showers and thunderstorms. Convective showers/t-storms will initially develop along the surface cold front as it crosses west to east between 9AM - 2PM. Once this front clears east of Idaho, convection will become more widely scattered in nature and continue on and off through the early evening across Northeastern WA, North Idaho, and along the Cascade Crest. Meanwhile, a deepening low across southern Alberta-Saskatchewan will ramp up pressure gradients across the region promoting windy conditions. The general magnitude of the pressure gradient will be on the order of 14-18 millibars, adequate to support wind advisory strength wind speeds (sustained 20-30 mph w/ gusts around 40-50 mph). The concern for stronger winds comes in the proximity of the convection (showers/t-storms) where downdrafts could tap into stronger winds aloft and briefly drive them to the surface. These would be closer to the 50-60 mph range. The threat for these winds will be more isolated and localized. Given the best chance for convection, the focus for these stronger downdraft winds will be in the central and eastern Columbia Basin, Palouse, Spokane-Cd'A Area, and into North Idaho and Northeastern WA. Wind advisories were expanded into NE WA and N ID to address the potential for these winds.

Two other areas that could experience similar winds but for reasons outside convection include the foothills of the Blue Mountains and into the Palouse where several hi-res models are showing strong enough synoptic wind speeds to come close to gusts near or above 50 mph. This includes locations like Pomeroy, Peola, Uniontown, Colton, and Moscow-Pullman. The second area is in the lee of the Cascades in the foothills around Wenatchee and on the Waterville Plateau. Similar hi-res modeling indicates a second burst of stronger speeds near sundown with gusts on the higher benches nearing 50-55 mph. This comes as the northwest jet aloft is lifting northward over the Cascades and aligning with the terrain and will need to monitored in the afternoon.

So overall impacts across the Inland Northwest will be most notable from the winds. Tree damage and power outages will be a concern.
Those traveling in high profile vehicles should be wearing of strong cross winds and keep cognizant near any showers or thunderstorms for bursts of stronger winds. Lakes will be challenging to navigate, especially for smaller vessels and kayaks.

As for precipitation amounts and impacts. Current thinking continues to support the idea that this will be mainly be a beneficial rain for the Inland Northwest which is dealing with precipitation deficits this spring. Rainfall amounts will be less than 0.25" in the lee of the Cascades increasing between 0.25-0.75" for the lowlands over the eastern third of WA and North Idaho. HRRR probabilities for 0.50" vary from 70-100%. The strong winds accompanying this system will drive up orographic rainfall amounts in the mountains with 70-100% chance for receiving over an inch of rainfall and expectations are amounts will be closer to 2" or more along the immediate Cascade Crest and a few of the higher peaks in North Idaho. Flood watches are in effect for Shoshone and Lewis Counties where there is some potential for rapid rises on streams and creeks and rock slides. Models are showing less rainfall for Lewis County compared to the last few days which is likely due to higher res modeling showing some downsloping off the Wallowas of NE Oregon. If this trend continues, may be able to adjust or remove the watch with future forecast updates.

Those venturing into the back-country of the Cascades should be prepared for crashing snow levels. Snow levels are starting near 8000 feet this afternoon and will be down closer to 4500 feet Monday evening. A few inches of slushy accumulations will be possible over Washington Pass but little to no accumulation is expected at Stevens. Main impact will be to those in the back-country who do not have proper gear, especially on the highest peaks where amounts could be upwards of 6-12".

Swift westerly flow will be over the region on Tuesday as the upper-level jet begins to buckle northward. One more wave will slip through with a continued threat for light rain. Little to no rainfall is expected in the lee of the Cascades with adequate lee side shadowing. The Cascade Crest and Idaho Panhandle will intercept a bulk of the moisture with potential for another quarter of an inch or more. Tuesday will be another windy day with gusts down 5-10 mph compared to Monday and closer to the 30-35 mph range.
Sustained speeds will remain in the 15-25 mph range for our wind prone areas across the Basin and adjacent areas.

Temperatures over the next few days will be on the cool side of normal with highs only in the 50s to 60s. We will not have to worry about freezing temperatures unless you are on the higher peaks.
Lows will be mild tonight remaining in the 40s and 50s then become chilly dipping into the mid to upper 30s to mid 40s Tuesday and Wednesday mornings. /sb

Wednesday to Sunday: The Inland NW will be drier and turning warmer most of this period. A mid-level ridge gradually amplifies over the region, somewhat held back by impulses riding through southern Canada before Friday into Saturday. Then models start to diverge on whether the ridge will persist or more troughy weather comes in. From Wednesday to Friday look for transient higher clouds, with a bit more in the way of high clouds Thursday as one of those waves slide by southern Canada. A bit of flat cumulus is possible around the mountains in the afternoon. Temperatures are forecast to reach the upper 60s and 70s Wednesday, with some areas approaching 80 in the deeper basin and L-C Valley. Thursday will see more upper 70s and 80s, then mid-80s to mid-90s Friday into Saturday. Saturday looks like the warmest day with some areas even may areas pushing near 90 and perhaps near 100 in the deeper basin. Ensembles show about a 15-25% chance of the 100 degree readings occurring there; less than likely but still not out of the realm of possibility. Any of this heat may cause some impacts and some precautions may be needed to stay cool.

By late Saturday into Sunday some deeper moisture stats to returns, with shortwave disturbance pushing near or through the region. This will lead to some increased shower chances and may some embedded t-storms. These first come to the Blues, Camas Prairie and Cascade crest Saturday night and expand to more of the region Sunday, but PoPs are still higher around the mountains.
Some guidance, particularly the Canadian model, shows a strong system and broader precipitation chances Sunday (into next Monday). However this model solution has been very inconsistent.
Precise temperatures next Sunday carry some lower confidence as models show a model range in possible values. However the forecast indicates some cooling over Saturday, with values remaining easily above normal based on current solutions. /Solveig

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
00Z TAFS: The leading warm front and plume of subtropical moisture has reached the Inland Northwest with light rain reported across most terminals as of 00z. As rain continues to fall, there is a 40-70% chance for MVFR conditions as the boundary layer moistens.
Probabilities have come down compared to previous runs. Areas with moderate rainfall carry the greatest risk for visibility down to 2-3SM. If this persists for several hours, conditions could deteriorate to IFR for several hours. This comes with a 10-30% chance. Rain intensities look to fluctuate at times between 9-13Z for some areas. 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 across the eastern third of WA and into North Idaho. Widely scattered shower and isolated t-storms continue across NE WA and N ID through Monday evening but strong winds region-wide will be the most impactful weather element to aviation.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence has decreased some for timing and magnitude of the MVFR conditions.
This is a rare pattern for June so forecast continues to lean in this direction but backed off on timing with 00z package.
Confidence is low to moderate for IFR conditions. There is also uncertainty with the duration and/or breaks in precipitation intensities within that 9-13z time-frame. Conditions could be quite varying after 15z with developing convection. Would not rule out lightning strikes and isolated stronger wind gusts near convective downdrafts.

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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 50 62 43 66 43 70 / 100 90 10 30 0 0 Coeur d'Alene 49 59 41 61 44 68 / 100 100 20 40 10 0 Pullman 50 58 42 62 44 67 / 100 100 10 30 0 0 Lewiston 56 68 48 71 53 79 / 100 100 10 20 0 0 Colville 47 62 38 63 37 71 / 100 90 20 60 10 0 Sandpoint 49 57 40 57 42 66 / 100 100 50 60 40 0 Kellogg 49 55 42 57 46 66 / 100 100 50 60 40 0 Moses Lake 52 68 48 74 44 77 / 80 50 10 10 0 0 Wenatchee 52 63 47 69 46 75 / 80 50 10 30 0 0 Omak 49 69 44 71 42 75 / 90 60 10 30 0 0

OTX WATCHES/WARNINGS/ADVISORIES
ID...Wind Advisory from 9 AM to 7 PM PDT Monday for Northern Panhandle.

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 through Monday evening for Central Panhandle Mountains-Lewis and Southern Nez Perce Counties.

WA...Wind Advisory from 9 AM to 7 PM PDT Monday for Northeast Mountains.

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

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