Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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246 FXUS66 KOTX 262236 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 336 PM PDT Sun May 26 2024 .SYNOPSIS... Dry and mild conditions on Monday with temperatures warming back into 70s and 80s. Tuesday will see a return of showers and thunderstorms to Eastern Washington and Idaho Panhandle. A few storms could be strong with gusty outflow winds. Cool and showery conditions continue through the end of the week with drier conditions at the start of the upcoming weekend. && .DISCUSSION... Tonight through Monday night: The Inland NW will continue to see relatively benign weather, but some changes will start to make themselves known toward the end of this period. Tonight a weak shortwave will exit through northeast WA/north ID by late evening while a weak ridge starts amplify over the region into Monday. Isolated shower chances near the Cascade crest and Canadian border will wane through this evening. Skies will be partly to mostly cloudy this afternoon and early evening before starting to thin at least a bit into Monday. Winds will wane through sunset into early Monday, then increase a little bit again Monday afternoon near the Cascades/Okanogan Valley. Monday night the offshore trough starts to pivot toward the region and moisture starts to increase ahead of it, along with some elevated instability. This will lead to thickening clouds, especially heading into late Monday evening into the overnight. Shower chances will also start to develop from the south-southwest overnight through the Upper Columbia Basin, Blue Mountains and Palouse up toward western Spokane county. Some guidance is more robust with this potential, with locally heavier downpours possible with that elevated instability and possible embedded t-storms. However confidence is not high enough at this point to put t-storms in the official forecast, but it will be monitored. High temperatures Monday will be slightly warmer than normal in the 70s, with a few low 80s in the deep basin and L-C valley. ...STRONG THUNDERSTORMS POSSIBLE TUESDAY AFTERNOON AND EVENING... Tuesday and Tuesday night: A robust system will move into the region this period, with showers and the potential for strong to isolated severe t-storms. As of this afternoon the Storm Prediction Center (SPC) is highlighting a Marginal Threat of severe thunderstorms for Tuesday afternoon and evening over the ID Panhandle. They have a general thunderstorm potential painted over north-central and the east third of WA; however, that does not entirely preclude these areas from seeing some stronger t-storms too. The highest risk for stronger t-storms will be late Tuesday afternoon and evening (about 1 pm to 8 pm). The largest potential impact with the strong to severe thunderstorms that might develop will be wind gusting near 35 to 60 mph. This could cause localized damage. Other impacts would be dangerous lightning, small hail and heavy downpours. With the heavier rain we will monitor for any ponding of water and debris blows/mudslides including near any recent burn scars or steeper terrain. While the storms are expected generally be moving quickly, we will have to monitor for any that may repeatedly track over the same areas and increase hydrology concerns. As it looks now here is the overall timing of precipitation. Tuesday morning lingering scattered showers and isolated embedded weak t-storms will be possible over the Columbia, Palouse the Spokane/CdA area into the northern mountains, tracking north- northeast. Chances will continue near the Cascade crest too. Tuesday afternoon the next round of showers and t-storms are forecast to develop. There may be two focus areas starting (1) a line near the Okanogan Highlands to the northern basin after midday with the cold front and (2) developing southwest of the Blue Mountains. By late afternoon to evening these two respective areas increases in coverage and strengthens as they tracks east- northeast over more of the CWA. The primary area that has likely PoPs or higher is over the Cascade crest, the NE WA and ID Panhandle mountains south toward the Blue Mountains in the late afternoon and evening. Elsewhere PoPs are in the slight to chance category, meaning a more isolated to scattered shower/t-storm threat. The lower lee of the Cascades has the lowest risk. Heading into the later evening and overnight the precipitation potential largely shifts into the mountain zones, with the highest risk lingering at the Cascade crest and over the central ID Panhandle to the Camas Prairie. Outside of the t-storms winds, the general synoptic winds will increase Tuesday afternoon into the evening and gradually decline later overnight into Wednesday morning. Speeds of 10-20 mph with gusts of 20-35 mph will be possible. With the details outlined here is a look at the ingredients for Tuesdays system and some caveats that make precise coverage still tricky. At the start of the day the areas will be in southwest flow with the parent trough off the coast and the cold front pushing into the Cascades. Moisture will deepened ahead of it to provide fuel for the convection, with PWATs around 0.75 to 1.0 inches (or 140-160% of normal). Surface dew points increase to the upper 50s to mid-50s, highest over the eastern third of WA and ID. Between 18Z Tuesday-03Z Wednesday, SBCAPE values between 100-1500 J/kg expand over portions of eastern WA and ID. The higher end of that range is found over the WA/ID border eastward the MT border. MUCAPE values between 1500-2500 J/kg are found around the central Panhandle southward. Additionally 0-6km bulk shear values average around 50kts, which helps develop more organized t-storms. With the cold front/upper trough moving toward the eastern third of WA and ID toward the late afternoon and evening, all these things converge to bring this stronger t-storm potential. A potential inhibiting factor is cloud cover and temperatures. Highs are forecast to be in the 70s to low 80s (similar to Monday). However the cloud cover increasing of the system may inhibit temperatures from reaching their full potential and also may limit the overall convective potential. With that said, the upper trough/cold front look like decent kickers as are other ingredients. Shower/t-storms are likely in the region, but we will have to continue to fine-tune precise coverage and timing. /Solveig Wednesday through Friday: A trough will be established over the region on Wednesday delivering cooler than normal temperatures and continued threat for showers and thunderstorms. Initially, these will be more of the hit or miss variety forming with the assistance of afternoon heating however by the late afternoon and evening, several deterministic models are hinting of an embedded shortwave rounding the base of the trough. This could create more organized bands of showers and embedded thunderstorms if it comes to fruition. These would be more of the weak variety storms with lightning, brief downpours of small hail/rain, and wind gusts up to 30 mph as the main threats. It will be cold enough aloft to support snow down to 4000 feet so those venturing in the backcountry should dress accordingly. Our heaviest cells will be capable of a quick 0.25" of rainfall but this should be close to the higher limits given the lack of shear and PWATS decreasing near 0.50" The trough begins to drift eastward on Thursday with shortwave ridging moving toward the WA Coast near 130W yet there will be enough cool air aloft for one more afternoon of convective showers and weak thunderstorms. By Friday, consensus of the ensembles is for the ridge to continue to shift inland with the ridge axis nearing the Cascades Friday evening. Can`t rule out one more day of afternoon showers in the Idaho Panhandle but this comes with moderate uncertainty and National Blend of Models are trending toward drier conditions. Blustery west to southwest winds will continue on Wednesday. Sustained winds across the Columbia Basin and through the Cascade Gaps will remain in the 15-20 mph range with gusts up to 30 mph. Outside these windier locations, winds will be more in the 5-15 mph range with gusts 20-25 mph. Winds decrease further on Thursday but still come with some breezes up to 25 mph in the lee of the Cascades and lower Columbia Basin. Winds continue to wane Friday. Temperatures start off in the 60s Wednesday then 60-70s for Thursday and Friday with readings increasing a few degrees each consecutive day. The combination the cool trough and drier air will bring another concern for frost to our sheltered northern valleys. Thursday morning looks to be the coldest morning. Here are probabilities for Thursday morning lows of 34F or colder at communities most vulnerable: Winthrop - 15% Republic - 40% Colville - 40% Priest Lake area - 50% Next weekend: Forecast leans toward a warming and drying pattern under the influence of a ridge of high pressure. Closer examination of the 100 member ensemble suggest there are differences with the amplification of the ridge and duration over the Inland Northwest. There is high confidence for mild temperatures but uncertainty if readings will remain in the 70s-80s or just 80s. The placement of the ridge will have ramifications for the extent of cloud cover and potential for increasing rain chances by Sunday into Monday. The ensembles are nearly split 50-50 with the differing solutions lending low confidence for which scenario will play out. /sb && .AVIATION... 00Z TAFS: One disturbance exits and high pressure builds in overnight into Monday. Isolated showers near the Cascade crest and northeast mountains wane after 02-06Z. Otherwise look for BKN-OVC middle to high clouds this evening, starting to thin overnight then remaining variable into Monday. VFR conditions expected. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high confidence for VFR conditions to continue across the TAF Sites. ----------------------- 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 45 74 51 76 45 63 / 0 0 10 20 20 20 Coeur d`Alene 45 73 51 75 47 61 / 0 0 0 20 30 30 Pullman 43 73 50 73 45 60 / 0 0 10 20 50 10 Lewiston 49 81 57 81 53 69 / 0 0 10 20 50 10 Colville 38 72 44 73 38 62 / 0 0 10 40 40 50 Sandpoint 43 71 48 73 45 58 / 0 0 0 40 60 50 Kellogg 46 71 53 75 48 57 / 0 0 0 30 70 40 Moses Lake 45 79 52 78 45 69 / 0 0 10 20 0 10 Wenatchee 48 77 55 73 47 64 / 0 0 0 10 0 10 Omak 43 76 50 76 43 68 / 0 0 10 20 20 20 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$