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Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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084 FXUS66 KOTX 101757 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 1057 AM PDT Sat Aug 10 2024 .SYNOPSIS... Showers and thunderstorms will expand across the region this afternoon. Lightning from thunderstorms will be capable of starting new fires. Thunderstorms will also come with brief heavy rain and gusty outflow winds. There is a growing threat for nocturnal thunderstorms over southeast Washington into the Idaho Panhandle Sunday night into Monday morning. Thunderstorms will also be possible in the afternoon and again on Tuesday. Most of them will occur over the northern third of eastern Washington into the northern half of the Idaho Panhandle. Quieter weather looks likely for the middle of the week but thunder chances will rise once again by the end of the work week. Daytime temperatures will begin the week slightly warmer than normal but should drop to average or slightly below by midweek. && .DISCUSSION... Morning update: adjust sky cover to decrease it some in the immediate term, then gradually increase it across the region for the rest of the day as convection starts to expand and the threat of showers/t-storm chances start to increase with the upper trough. /Solveig Previous Discussion: Today and Tonight: A broad upper level trough of lower pressure will be over the Inland Northwest today. A shortwave disturbance will also swing through on the backside of this trough that is expected to force convection over the Cascades, northern mountains, and more so into the basin compared to yesterday. Instability parameters support the potential for scattered thunderstorms primarily over the higher terrain, and more isolated in nature over the Columbia Basin. Showers and thunderstorms will be relatively slow moving at 10-15 mph to the east. Brief heavy rain over burn scars and in steep terrain will be a threat for flash flooding and/or debris flows. Pwats are modest at best between 0.8 to 0.9 inches. There will be rain under the cores, but not expected to be torrential. It will take a low probability set of circumstances for a slow moving thunderstorm and training storms over a susceptible are for a flash flood or debris flow to occur. Probability is so low that a flood watch isn`t warranted. Another threat will be for new fire starts. Lightning is a certainty, although coverage is of some question. I`m not convinced that lightning density rises to the level of being abundant enough for a red flag warning. Thunderstorms will also contain rainfall as previously stated and this moisture will help to extinguish new fires. Confidence isn`t high enough for a red flag warning for lightning today. Like with convection yesterday, these storms have a high potential for outflow winds with gusts up to around 40 mph possible. Convection will decrease into the evening with the loss of surface heating, but I expect isolated showers to continue over extreme eastern Washington and into the Idaho Panhandle overnight. Sunday and Sunday night: Another shortwave disturbance will round the backside of the trough over BC late Sunday. There will be isolated thunderstorms mainly over the Cascades and northern mountains of eastern Washington and the Norther Panhandle Sunday afternoon. What looks more concerning is the potential for nocturnal thunderstorms Sunday night into Monday morning. The NAM tends to depict elevated convection well, and has had a good track record recently. This mesoscale model shows an area of 100-300 J/kg of MUCAPE being drawn up across southeast Washington into the Central Panhandle Mountains out ahead of that shortwave that settles in over western Washington and forms a closed low circulation. The position of where this low sets up will determine where the axis of best elevated convection occurs. Either way, the synoptic scale set up looks favorable for high based thunderstorms that will be moving quicker and contain less rainfall. Lightning with this batch of convection will likely be more efficient at causing new fires. Instability is on the lower end for scattered thunderstorms though, so there is question of how much lightning will occur. Upper level dynamics looks good though with a diffluent 500 mb wind flow pattern. This will lead to diverging motions aloft with favorable positive vorticity advection as well. Temperatures will remain steady with highs ranging from the mid 80s to mid 90s. Temperatures look to remain more mild Sunday night with increased cloud cover due to the elevated convection and lows will struggle to cool below 60 degrees across the forecast area. /SVH Monday...The unsettled weather will likely continue into the beginning of the new work week as there is good model consistency is showing a shortwave trough ejecting northeast through the Inland NW ahead of western WA trough. High level Total Totals suggest there will be two main areas of convection. The first area will occur near the north Cascade/Okanogan Highlands. This region will largely depend on the ascent associated with the ejecting shortwave combined with good mid-level instability. The second areas will likely occur over SE WA into NC ID...under a burgeoning low-level Theta-e ridge axis combined with good mid-level instability and weaker lifting associated with the ejecting trough. This setup is similar to what the region experienced on the overnight and morning hours on August 5th and could result in considerable lightning. Not surprisingly, the NBM underplays this event, much like it has for most of the past week during non- surface based convection. We have adjusted the forecast accordingly. By afternoon, the models are decent agreement that the aforementioned shortwave trough will migrate into NW MT with the deeper trough moving onshore Cascades. This setup will still allow convection to develop with the best chances reserved for the northern third of eastern WA and the northern half of the ID Panhandle. These thunderstorms will primarily form in an atmosphere with 120-140 percent of the normal precipitable water and that suggests they wont be dry. The movement will be fast enough (15-20 mph) that the flash flooding risk wont be great, however the motion will be about half that value near the north Cascades in western Okanogan County so that bears watching. With the deeper trough heading inland, the temperatures west of the Cascades should drop resulting in a strong cross-Cascade temperature gradient. This suggests we should see some gap winds develop by late afternoon or evening. Tuesday...All of the ensemble runs take to the upper level trough and push it eastward through the day while digging it slightly southward. This should place most of the upper level energy south of our forecast area and thus wed expect to see a quieter day for convection. The CAPE values also are quite a bit less than what we should see on Monday, also supporting a quieter day. Nonetheless it wont necessarily be dry. The best chances again will lie over the northern third of WA into the ID Panhandle. The cross-cascade thermal gradient will remain tight and thus we could be looking at more gusty winds in the lee of the Cascades. For Wednesday and Thursday there is decent agreement between the ensembles with longwave troughing rebuilding off the coast and weak southwest flow over eastern WA and N ID. This pattern will show little support for synoptic scale forcing and deep instability and thus the thunderstorm chances will be small. However watch for Thursday if the offshore trough gets close enough there could be some weak offshore flow into the Cascade Crest combined with uncapped instability. By Friday the odds will increase for renewed thunderstorm chances as the offshore trough heads eastward. How far east is the question. Nearly a third of the ensemble runs push the trough east of the Cascades and this could lead to an active thunderstorm day, but the majority of the models keep the trough west of the Cascades. Aside from the thunderstorm chances, we are likely looking at near normal to slightly above normal temperatures to begin the week and then bottoming out on Wednesday or Thursday. That equates to highs in the mid 80s to mid 90s on Monday dipping into the 80s for most valley locations on Wednesday and then slowly warming thereafter. fx && .AVIATION... 18Z TAFS: Showers and thunderstorms will increase in coverage this afternoon. Development of convection will favor the Cascades and northern mountains of eastern Washington early this afternoon, but will move off of the higher terrain and across the basin after 22Z. Wenatchee, Omak, Ephrata, and Moses Lake airports are expected to see impacts from convection with lightning, brief heavy rain, and gusty outflow winds up to 40 kts possible. There is potential for convection to move into the Spokane-Coeur d`Alene corridor and Palouse region in the evening after 00Z. Convection will wane with the loss of surface heating with lingering showers expected overnight. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is high for VFR conditions except if a thunderstorm moves over an airport. In this scenario, IFR to MVFR conditions with reduced visibility is possible. There is a 20-30% chance for thunderstorms at KEAT and KOMK and KCOE. Chances for thunderstorms decrease to around 15-20% at KEPH-KMWH-KGEG-KSFF and around 10% at KPUW and KLWS. ----------------------- 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 90 60 90 64 87 56 / 20 20 0 20 20 10 Coeur d`Alene 87 57 87 61 85 55 / 20 30 10 20 10 10 Pullman 87 56 86 60 83 51 / 20 20 0 20 20 10 Lewiston 97 67 96 67 93 62 / 20 20 0 20 20 20 Colville 89 49 88 55 87 47 / 30 30 10 10 20 10 Sandpoint 85 55 84 59 84 52 / 30 30 10 10 20 10 Kellogg 85 60 84 63 83 57 / 30 30 10 10 20 20 Moses Lake 93 60 91 63 89 55 / 20 20 0 0 10 0 Wenatchee 92 68 91 66 89 62 / 30 10 0 0 10 0 Omak 91 62 94 64 92 61 / 30 20 10 10 20 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$