508 FXUS66 KOTX 212136 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 236 PM PDT Tue May 21 2019 .SYNOPSIS... Temperatures will warm up this week with highs into the 70s by Thursday. Conditions will be generally dry through Wednesday. An approaching disturbance may trigger some showers and even a thunderstorm over north-central Washington on Thursday. By Friday this disturbance will take up residence over western Washington resulting in a good chance of showers and afternoon thunderstorms for most of the Inland Northwest. The disturbance will drop south of the area for the holiday weekend with a chance of showers mainly for Saturday. && .DISCUSSION... Tonight through Thursday: A weak shortwave disturbance is rotating counter-clockwise across the southern ID Panhandle into southeast WA this afternoon. Scattered showers will form ahead of this wave with isolated thunderstorms also possible into the late afternoon hours. Showers are expected to move south of the region by midnight. A northerly pressure gradient will strengthen overnight into Wednesday morning. The gradient will be strongest over the northern ID Panhandle with 25-35 mph wind gusts expected through the Purcell Trench by Wednesday morning. Winds will spread out over the basin with gust to around 25 mph possible. We have had a few dry days in the basin, but I still think soils are a bit too moist still for a concern for blowing dust. Models do show some instability up along the spine of the Cascades Wednesday afternoon. I left a small potential for thunderstorms, but a better potential will come for Thursday afternoon with all models indicating some potential for developing showers and thunderstorms. Best chances for thunderstorms on Thursday will again be up near the Cascade crest. Steering flow will be light from the west, which will push storms eastward into places like Mazama, Winthrop, Stehekin, Plain and Leavenworth. Heavy rain with slow moving thunderstorms will be a potential concern for flooding impacts. The main concern will be if any thunderstorms track across a recent burn scar, in which case flash flooding will be a possibility. /SVH Thursday night through Tuesday...The overall field of motion through much of the extended forecast period...including the upcoming holiday weekend...will feature a split flow regime over the western US with an upper low circulation generally to the south of the region over coastal California and the Great Basin with the northwestern US under somewhat moist south or southeasterly return flow around the northern flank of this closed low. This will create a showery regime at times with a chance of thunderstorms especially on Friday and to a lesser extent but still noticeable on Saturday afternoon and evening. Sunday and Monday a threat of showers and afternoon/early evening thunderstorms will be more confined to the far southeastern zones and by Tuesday it looks like a drier northerly flow develops over the area as the Great Basin low ejects into the central US. Friday looks like the day of most impactful weather across the region. Models are in good agreement in transiting a deepening upper low down the west side of the state on it's way to take station in the Great Basin for the rest of the holiday weekend. This track will place the forecast area under diffluent moist southerly flow on Friday with models depicting significant instability during the afternoon and evening hours as well as precipitable water values on the order of 3/4 of an inch or higher. All of these ingredients suggest a very real threat of scattered rain showers developing in the morning culminating in afternoon and evening scattered thunderstorms salted among numerous rain showers. Just about any location in the forecast area will be subject to a shower or thunderstorm on Friday but the axis of higher threat will probably be over the mountains ringing the basin. Given the weak steering flow and heavy moisture availability some of these storms could contain heavy localized downpours. It's too early to get a good handle on this threat at this time...but confidence is increasing that Friday will be the most active and potentially wet day of the extended forecast. Saturday and Sunday the area will continue to be subject to moist deformation bands but the parent closed low will be farther away and shower and thunderstorm activity will probably be sparser...but still present especially over the mountains ringing the basin. By Monday residual activity will probably be confined to the far southeastern zones and Idaho Panhandle with some isolated showers and possible thunderstorms in the Cascades. For outdoor activities this upcoming holiday weekend Saturday looks like the most active day for much of the region...while Sunday and Monday will feature a smaller threat of showers and thunderstorms. Temperatures during the entire extended period will probably run close to normal for this time of year. /Fugazzi && .AVIATION... 18Z TAFS: A low pressure system sits on the OR west coast with a deformation band extended out across the WA/OR border. Stratiform rain will fall beneath this band and westerly winds is advecting moisture into KPUW with IFR cigs expected this morning. Cigs will lift with building cumulus in the afternoon. The building cumulus will result in a shower and low chance thunderstorm threat for KLWS and KPUW after 21Z. All other TAF sites will be free of precip with VFR conditions. North to northeast winds will increase overnight and into Wednesday morning with gusts of 20-30 kts possible, especially at KCOE and KMWH. /SVH && .PRELIMINARY POINT TEMPS/POPS... Spokane 48 70 47 74 50 68 / 10 0 0 0 10 50 Coeur d'Alene 47 69 46 74 49 67 / 10 0 0 0 10 60 Pullman 43 67 44 72 48 66 / 20 0 0 0 0 50 Lewiston 49 73 50 77 54 73 / 20 0 0 0 10 60 Colville 45 77 41 82 43 75 / 10 0 0 0 10 60 Sandpoint 53 68 47 73 48 67 / 0 0 0 0 10 60 Kellogg 47 65 46 70 50 65 / 10 0 0 0 10 60 Moses Lake 51 77 51 82 53 75 / 10 0 0 0 0 40 Wenatchee 53 74 55 79 56 72 / 10 0 0 10 10 50 Omak 55 75 51 79 53 72 / 0 10 10 10 10 70 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$