Thunderstorms developed over the east slopes of the Cascades and the Columbia Basin in a rich feed of wrap around moisture from an upper low over Oregon during the night of May 17th. 72 inches.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS OTX wrote it up.
Thunderstorms developed over the east slopes of the Cascades and the Columbia Basin in a rich feed of wrap around moisture from an upper low over Oregon during the night of May 17th. These storms dumped a general quarter to half inch of rain over most of the western basin, but individual cells locally produced higher amounts with Ephrata measuring 0.72 inches. Local flash flooding and debris flows were noted from these storms.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Spokane, WA wrote its last area forecast discussion before the first event at May 17, 1:23 PM PST, 6 hours 7 minutes earlier.
Area Forecast Discussion National Weather Service Spokane WA 223 PM PDT Thu May 17 2018 .SYNOPSIS... Chances for showers and thunderstorms will remain this evening. The arrival of a cold front on tonight and Friday will produce breezy north and northeast winds and cooler temperatures closer to average. More showers but fewer thunderstorms can be expected for Saturday...but Sunday looks like a dry break period. A warming trend will develop next week but with more showers and thunderstorms possible. && .DISCUSSION... Tonight through Friday...A broad upper level trough with multiple weak centers of circulation exists over much of the western United States...with plenty of moisture wrapping around the main center of circulation faintly visible on satellite over southwest Montana or the northeast shoulder of Idaho. Small sub-synoptic impulses are generating slow moving showers and thunderstorms this afternoon over much of the region...especially the eastern half of the forecast area...but the sunny portion of the deep basin and east slopes of the Cascades will probably generate enough surface based instability to trigger some early evening showers and thunderstorms over those locations as well. Radar indicates the most coherent bands of showers and storms along the Idaho/Washington border and over Ferry County. In addition to the plethora of other flood products still in effect...the presence of these slow moving and heavy rain producing cells has necessitates a general Flash Flood Watch for Okanogan County through this evening...with special attention to recent burn scars in the region. While thunderstorms will decrease with the sunset tonight...multiple high resolution models suggest the remnants of these areas of storms will drift into the Cascades overnight tonight and early Friday as a mass of showers, dynamically assisted by yet another spur upper low...best resolvable on Water Vapor satellite currently over British Columbia. This trough will bring a surface cold front out of Canada as it drops in tomorrow and allow a temporary drying trend...or at least a decrease in prevalence of showers and a more stable air mass largely precludingany thunderstorms on Friday...but things will still be unsettled with the only other effect of this weak front being increased north winds down the Purcell Trench and slightly increased winds down the Okanogan. Cooler conditions in the wake of the front will be most noticeable in the western basin on Friday with the current upper 70s getting knocked back into the mid 60s. Friday night through Sunday...Latest models are actually in fairly good agreement with the overall flow regime through most of the extended period. On Saturday the upper low from Canada will stop further southward progression...sit over the forecast area and be the focal point for more showers over the entire area ...with a chance of thunderstorms in the mix mainly over the southeast. The low will eject out of the area to the northeast Saturday night and Sunday as the next Pacific trough approaches the coast. This wave will allow warm advection in southerly flow ahead of it leading to the next warming trend...and also kicking off more showers and thunderstorms but limited to the Cascades on Sunday. Sunday appears to present the best day of the weekend for outdoor activities elsewhere with dry and warming conditions. Monday through Thursday...A rather fascinating weather pattern becomes established over the area for the next week. The next incoming Pacific trough will dive into the Great Basin and then pinch off into a stationary closed low as part of a Rex Block pattern with the High component over western Canada. The forecast area will reside under an easterly flow regime beneath the far northern flank of the Great basin upper Low. A persistent area of high precipitable water will be entrenched over the forecast area and and continuously recharged as wrap-around feed over the top of Great Basin closed low...quite possibly moisture which loops up and crosses the Rockies from the Gulf of Mexico...sort of an Atlantic marine push. Meanwhile the temperature will increase every day through Wednesday. With noticeably increased and anomalously high humidity and high temperatures in the 80s by midweek the region may feel a bit like Hawaii only without the palm trees. This pattern will also be conducive to a daily thunderstorm threat especially on the mountains ringing the basin...and with weak steering flow in a very moist air mass these storms will bring a threat of heavy rain in their cores. Only way out, on or about Thursday is the possibility of a break in this pattern with a weak cooling trend. /Fugazzi && .Hydrology...With the cooling trend expected this weekend mountain snow melt will slow down...however the warming trend beginning early next week will allow another acceleration. So for the foreseeable future little relief is expected for the river flooding issues plaguing the region. In the immediate term...the thunderstorm activity around the area today and this evening will maintain a threat of localized flash flooding. The following rivers are currently under riverine Flood Warnings... Moyie River at Eastport AFFECTING Boundary County Lake Pend Oreille AFFECTING Bonner County St Joe River at St Maries AFFECTING Benewah County Kettle River near Ferry above Curlew AFFECTING Ferry County Methow River near Pateros AFFECTING Okanogan County Okanogan River near Tonasket AFFECTING Okanogan County Pend Oreille River below Albeni Falls AFFECTING Bonner and Pend Oreille Counties Similkameen River near Nighthawk AFFECTING Okanogan County The Flood Warning has been ended for the following river... Stehekin River at Stehekin AFFECTING Chelan County A Flash Flood warning continues through this evening for Okanogan County in general for wet thunderstorms. The Flash Flood Warning will continue indefinitely for the Lost River community in the Methow Valley for possible levee failure. An Areal Flood Warning continues over Okanogan County for the lake shore of Lake Osoyoos...and the possibility of renewed rainfall and snow melt related flooding on the Salmon Creek and Chewuck River. An Areal Flood warning remains in effect for a sliver of Ferry and Stevens county along the Kettle River from the Canadian border to Lake Roosevelt, in addition to the riverine Flood Warning for the Kettle River near Ferry. && .AVIATION... 18Z TAFS: Showers and thunderstorms are likely at all TAF sites over the next 24 hours. These storms will be slow moving. Cigs should be mainly VFR or MVFR, but storms could produce IFR conditions for a few hours. I do not expect fog to form on Friday morning. RJ && .PRELIMINARY POINT TEMPS/POPS... Spokane 52 62 49 67 49 75 / 70 50 30 40 20 0 Coeur d'Alene 50 61 47 67 47 75 / 80 50 20 30 20 10 Pullman 49 59 48 67 48 73 / 50 60 40 50 10 10 Lewiston 53 64 53 73 52 79 / 40 60 40 50 10 10 Colville 52 65 48 70 47 78 / 70 70 20 20 20 10 Sandpoint 49 64 45 65 44 74 / 90 40 20 30 20 10 Kellogg 46 59 43 64 45 73 / 70 50 40 40 20 10 Moses Lake 56 66 52 73 49 79 / 60 50 40 20 10 0 Wenatchee 55 64 53 71 52 77 / 70 80 40 30 20 10 Omak 55 65 52 71 51 77 / 70 70 40 20 20 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. &&
Hazardous Weather Outlook National Weather Service Spokane WA 820 AM PDT Thu May 17 2018 IDZ001>004-026-027-WAZ036>038-042-043-181530- Northern Panhandle-Coeur d'Alene Area-Idaho Palouse- Central Panhandle Mountains-Lewiston Area- Lewis and Southern Nez Perce Counties-Spokane Area- Northeast Mountains-Okanogan Highlands- East Slopes Northern Cascades-Okanogan Valley- 820 AM PDT Thu May 17 2018 This Hazardous Weather Outlook is for portions of North Idaho... North Central Idaho...North Central Washington and Northeast Washington. .DAY ONE...TODAY AND TONIGHT Area rivers remain high, with some flooding in the Idaho Panhandle and extreme northern Washington. Although the probability for widespread hazardous weather is low, scattered thunderstorms are possible today. Some of these storms may slow moving and contain brief heavy rain. .DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY Many area rivers will see rises and become very high through the week with additional snow melt, with some flooding in the Idaho Panhandle and extreme Northern Washington. WAZ031>035-041-044-181530- Northeast Blue Mountains-Lower Garfield and Asotin Counties- Washington Palouse-Moses Lake Area-Upper Columbia Basin- Wenatchee Area-Waterville Plateau- 820 AM PDT Thu May 17 2018 This Hazardous Weather Outlook is for portions of Central Washington...East Central Washington...North Central Washington and Southeast Washington. .DAY ONE...TODAY AND TONIGHT Area rivers remain high, with major flooding on some rivers in the area. Although the probability for widespread hazardous weather is low, scattered thunderstorms are possible today. Some of these storms may slow moving and contain brief heavy rain heightening the possibility of flash flooding on recent burn scars. .DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY Many area rivers will remain in flood through the next week as mountain snow melt continues.
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SPOKANE WA
426 PM PDT THU MAY 17 2018
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0420 PM HEAVY RAIN 3 SE TWISP 48.33N 120.08W
05/17/2018 M0.47 INCH OKANOGAN WA TRAINED SPOTTER
THUNDERSTORM WITH SMALL HAIL AND HEAVY RAIN.
0.47 INCHES OF RAIN IN PAST 35 MINUTES.
&&
JWThe Spokane radar (KOTX), 191 km away, was scanning. The volume nearest the first event began at 03:30 UTC.
1 debris flow, 1 flash flood. Local standard time, as filed.
Heavy rain from thunderstorms during the evening of May 17th resulted in a debris flow of rocks and mud onto the Malaga-Alcoa Highway betrween mileposts 10 and 12.
Flooding from heavy rain producing thunderstorms occurred in Ephrata during the late evening of May 17th. Basin Street, the main road through the downtown business district was flooded with 4 to 8 inches of water with first floor damage to multiple businesses in the downtown area. A portion of the ceiling in the Department of Corrections building collapsed due to standing water on the roof.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Washington in 2018Every state in 2018This day in weather historyWarnings in Washington, May 2018
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 758689, 758680.