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Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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115 FXUS66 KOTX 191803 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 1103 AM PDT Sun May 19 2024 .SYNOPSIS... Cool and unsettled weather will continue through the weekend. Patchy frost, especially in the northern mountain valleys, will again be possible by early this morning. A brief break between weather systems Monday into Tuesday. Then a wet low pressure system is expected Late Tuesday night and Wednesday, lingering Thursday. The series of cold upper level low pressure systems through the week will keep temperatures below normal. && .DISCUSSION... Today: A cold, unstable upper low pressure system continues over the Inland northwest. Instability continued to and center trough to drive showers overnight from Deer Park-Sandpoint into the central Idaho Panhandle. This area of showers will continue to move east into the early morning hours. Showers will once again develop late morning into afternoon. Latest convective models suggest a better chance of thunderstorms with 300-500 Joules of Cape as the airmass destabilizes rapidly under a mid-May sun and cold temperatures aloft. Thunderstorms will have a general northwest to southeast drift into the late afternoon hours on the east Washington side. A few strikes of lightning, and brief, heavy showers can be expected so be on the lookout for darkening skies this afternoon for passing thunderstorm showers. Sunday night: Once the evening shower activity dies down weak high pressure builds into the region. We would expect clearing skies from the west with lingering clouds in the Idaho Panhandle. There`s some indication at least the northern mountain valleys of northeast WA could see some patchy frost early Sunday morning for 2 to 3 hours until the sun comes up. These cool temperatures around 36 have a roughly 25% chance of reaching typically colder pockets as far south as Davenport, Cheney and even Pullman, WA if the clouds clear out sufficiently. The other wildcard is some valleys will see some patchy fog to help moderate temperatures. It doesn`t hurt to be covering plants until we can get out of this cold airmass pattern or get some cloud cover in here like we will Monday night. Monday into Monday night: Overall these will be drier days except for a 20 percent chance of mountain showers in the Idaho Panhandle. Expect temperatures solidly in the 60 to middle 70s in the Central Basin, Wenatchee and Omak areas. With high clouds building in low temperatures late Monday night are expected to be 40 degrees and above in all if not most lower elevation locations. /TC Tuesday to Saturday: Much of the Inland NW will turn wetter as we head into the middle of the week as a strong low pressure system move into the region. This is followed by a decrease to more scattered showers toward the end of the work week, before another system moves in toward Friday night and Saturday. This latter system currently looks similar to the midweek system in terms of trajectory, but there are some disagreement and it does not look quiet as wet. First in the 24-hour period between 12Z Tuesday and 12Z Wednesday the center of a deep low pressure drops from northern British Columbia to northwest Washington (somewhere near Puget Sound to the Olympic National Park). The leading warm front gradually pivots into the region through Tuesday. Shower chances come to the Cascades in the morning, starting to increase over central and northeast WA and the ID Panhandle through the afternoon. It is Tuesday night into Wednesday when the low drops in and moisture deepens, tapping a a PWAT fetch of around 0.60 to 0.80 inches (or 120-150% of normal), that the steadier precipitation expands. The potential starts to increase in the evening, but really ramps up overnight into Wednesday morning over much of northern and eastern WA and the ID Panhandle, persisting near the Cascade crest too. The lower lee of the Cascades and deeper basin will see more limited rain chances. Heading into Wednesday afternoon the center of the low moves from northwest WA to northeast Oregon; then between Wednesday night into late Thursday morning it tracks toward the Yellowstone area near the MT/ID/WY border. Moisture wrapping into the low will continue to provide modest precipitation chances, though the focus starts shift toward the ID Panhandle and southeast WA as we progress into Wednesday night into Thursday morning. * Rain amounts: This system will bring a good chance of moderate to heavy precipitation to some of the area. Ensembles show the 24-hour probabilities for wetting rain between 70%-100% over all but the lower lee of the Cascades and deeper Columbia Basin. This latter region includes place like Wenatchee and Moses Lake, where the wetting rain potential is around 25-50%. If we bump amounts up to 0.25 inches, the probabilities are around 70-90% over the Cascade crest, the northern mountains, the east third of WA and the ID Panhandle. Amounts of to 0.50 inches have probabilities around 60% over the Cascade crest, northeast WA and ID Panhandle mountains, but there is even a 20-30% chance even over the lower east third of WA. Also the EFI table show this to be anomalously wet system and the signal for that has only increased in the past couple days. So, yes this system appears will bring a good soaking to much of the area (except south-central WA, which does not have as high a risk, sorry.) * Hydrology: With all the rain projected, some main stem rivers across far eastern WA and ID show rises heading into the mid to later week time frame, but none of them are projected to rise to even action stage much less flood stage at this point. However some of the smaller creek/streams will need to be monitored (Paradise Creek is on the more prominent ones that comes to mind, but right now it isnt forecast to reach action stage, but it will be monitored. There could be some ponding of water on roads and some debris or mudslides, especially in the steeper terrain. The main potential for any of this is over north- central through eastern WA and the ID Panhandle. * Snow: I would be amiss if I did not mention the snow risk with this system. Yes overall it is tiny. But models are showing some snow around the higher mountains. The risk for anything measurable is low, but I would not be surprised to see some around Stevens or Sherman Pass. The exception is Washington Pass/North Cascades Highway Mountain Pass along SR 20; many ensemble show the 24-hour potential for 1 inch between 50-60% with the 48-hour potential higher. Some guidance, such as the ECMWF right now shows 2 inches of more, but 24/48 hour probabilities are at around 35% and 45%, respectively. With all the said, road temperatures are generally not conducive to much accumulation this time of year. * Thunderstorms, winds, temperatures: There is a limited chance for some embedded t-storms, mostly Wednesday afternoon over northeast WA and the ID Panhandle, but those chances are only around 10-15%. So this will not be a big threat by any means. Winds will be a little bit breezy between Tuesday and Wednesday, with most models pointing to the lee of the Cascades and central WA as having the highest potential with gusts near 20-25 mph. Elsewhere gusts near 15-20 mph will be possible. Lastly temperatures: Tuesday will be relatively mild with highs in the mid-60s to near 70, but Wednesday those milder readings will be mostly held to south-central WA as the clouds and rain elsewhere which have highs struggling to get out of the 50s, if not even the lower 50s. For Thursday, secondary weak upper impulses drop in from the north, keeping the potential for showers alive. The highest chances will be found over the northeast WA and the ID Panhandle in the afternoon. The overall potential wanes heading into Thursday night and Friday morning. Then the next low starts to approach later Friday into Saturday, leading to another increase in the precipitation potential. Marginally afternoon breeziness remains each afternoon, particularly over central WA. Temperatures will be held back on Thursday over the eastern third of WA and ID, with highs still in the 50s to mid-60s, while upper 60s to low 70s are seen closer to lee of the Cascades. Temperature recover a little bit Friday into Saturday, but remain below normal. However at this point I do not see any significant threat of morning lows being cold enough for frost in this period. That is at least true outside of the mountains; so if you are heading into the mountains be aware the could be some low temperatures near freezing, especially the higher elevations Cascades. /Solveig && .AVIATION... 18Z TAFS: An upper level cold trough of lower pressure over the Inland Northwest will promote a convection through the day today. Covective temperatures are already being achieved over the higher terrain of northeast Washington into the northern Panhandle with isolated showers forming. Expect coverage of convection to expand as the surface continues to warm up into the afternoon. Thunderstorms will be single cell pulse type featuring infrequent lightning, brief heavy rain, and small hail. Thunderstorms are most likely to impact KCQV-KDEW-KSZT-KGEG-KSFF-KCOE today. Convection is expected to dissipate clouds see a clearing trend between 02-04Z this evening. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is low with a 20-25% chance that thunderstorms will develop at KGEG-KSFF- KCOE terminals today. Lower chances that thunderstorms will develop at KEAT-KEPH-KMWH (5% chance) and KPUW-KLWS (10-15% chance). There is low to moderate confidence for ceilings producing MVFR conditions at KCOE Monday morning. /SVH ----------------------- 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 62 38 69 43 66 45 / 50 20 10 0 10 80 Coeur d`Alene 57 38 65 41 64 46 / 60 40 20 0 10 90 Pullman 56 36 63 41 63 44 / 30 10 10 0 10 80 Lewiston 64 44 70 46 72 50 / 20 10 20 0 10 70 Colville 63 33 69 38 66 41 / 60 40 20 0 30 80 Sandpoint 55 36 62 40 63 44 / 80 60 50 10 20 80 Kellogg 52 40 60 43 62 47 / 70 50 50 10 20 80 Moses Lake 68 41 75 44 68 45 / 10 10 0 0 10 30 Wenatchee 66 44 73 49 65 48 / 10 10 0 0 20 30 Omak 69 40 75 44 68 45 / 30 20 0 0 30 60 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$