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Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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450 FXUS66 KOTX 111220 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 520 AM PDT Sun Aug 11 2024 .SYNOPSIS... Several more days of showers and thunderstorms over portions of the region. Focus for thunderstorms will be over the Cascades, and northern mountains, as well as over extreme southeastern Washington into the southern Panhandle Sunday night through Monday night. Lightning from thunderstorms will be capable of starting new fires. Thunderstorms will also come with brief heavy rain and gusty outflow winds. Quieter weather looks likely for the middle of the week, but thunder chances will rise once again by the end of the work week and into the weekend. Daytime temperatures will begin the week slightly warmer than normal but should drop to average or slightly below by midweek. && .DISCUSSION... Today through Monday: A shortwave trough of lower pressure will drop south from the Vancouver Island today and swing across the northern Cascades to the Okanogan Highlands into tonight. A second shortwave disturbance will take a similar track for Monday afternoon into Monday evening. The disturbance for today looks more potent and will feature a robust vorticity maximum, and is apparent by darkening on water vapor satellite over this area. The NAM has been performing well in modeling the instability parameters more accurately depicting dew point temperatures in the mid 40s to around 50, and near the Cascade crest, up into the mid to upper 50s. This equates to around 500-1000 J/kg of surface based CAPE over the mountains of the northern Washington Cascades to the Okanogan Highlands for both today and again on Monday. There`s enough instability at mid levels up to around 200 J/kg of MUCAPE for the threat of lightning to continue through tonight as well. Lightning flashes are expected to be more frequent in nature in the afternoon and evening, and more likely to be isolated overnight into the morning hours on Monday. Downdraft CAPEs of 1000-1500 J/kg will result in the potential for strong outflow winds. Conditions look similar to the past couple of days and we`ve received reports of 40+ mph winds with wind damage from convective cells. Damaging winds are isolated in nature and have come with very localized microbursts. There will be the threat of brief heavy rain as well, but the threat for flooding type impacts looks minimal as storms will have some motion to them off to the northeast on the order of 10-15 mph. The potential for new fire starts and fanning of flames from outflow winds will be the higher risk over the next couple of days. See the fire weather section below for additional details including a red flag warning that will be in effect today through Monday. Monday night through Tuesday night: A trough of lower pressure will swing through on Tuesday. There is an axis of mid level instability extending from northeast Oregon into the southern Idaho Panhandle for Monday night into Tuesday morning. An axis of isolated to scattered high based thunderstorms is expected over extreme eastern Washington into the Idaho Panhandle. These thunderstorms will be moving more quickly than storms expected over the northwest portion of the forecast area. The faster storm motion will result in relatively drier storms with new fires possible from lightning strikes. Thunderstorms are once again expected over the northern Cascades (primarily north of Lake Chelan) into the Okanogan Highlands on Tuesday. Thunderstorm coverage looks to be less than what is expected today and Monday. Similar threat can be expected though with convection with lightning strikes and gusty outflow winds. The ejecting trough will increase the westerly winds. Expect winds to increase through the Cascade gaps late Monday afternoon, and looks to remain breezy across the Columbia Basin into Tuesday. Exposed area across the basin are expected to see sustained winds of up to 15 mph and gusts up to around 25 mph. The trough pushing through on Tuesday will drop temperatures from the upper 80s to low 90s Sunday and Monday into the 80s regionwide for Tuesday. /SVH Wednesday and Thursday... The ensemble models are in very good agreement for this period with a weak shortwave ridge expected over the Inland NW on Wednesday followed by a rebuilding trough off the coast or just inland and the return of mid-level southerly flow. This regime should deliver dry weather on Wednesday with increasing moisture on Thursday. Whether or not this increased moisture will deliver precipitation is questionable. The base of the trough and the bulk of the synoptic forcing will generally lie south of the Inland NW in the morning, but then there are hints it will eject northeast in the afternoon. If this comes to fruition, we should see an increasing chance of showers and thunderstorms over SE WA and NC ID by afternoon as well as a smaller chance near the Cascades. Temperatures will be around to slightly cooler than normal on Wednesday with highs primarily in the 80s while on Thursday there could be some lower 90s over the LC Valley and lower Columbia Basin. From Friday into the weekend, there is a strong signal that an unusually deep 500 mb low will drop down from the Gulf of Alaska and take residence somewhere off the West Coast. The situational awareness tables suggest the low will be in the tenth percentile (or stronger) for lows this time of year which is a far cry from the weather pattern weve seen for much of this summer. Unfortunately although the notion of the low is consistent, where it takes residence and how quickly it heads inland is anything but. Thus this will be a low confidence forecast in terms of when and where we could see thunderstorms however there is no arguing this pattern is conducive for an unsettled weather pattern. Taking a look at the operational models is interesting to say the least although this practice isnt a great idea 5-7 days out due to high variability of specifics. In any event the 8/11 00z GFS run shows a strong 563 dm negatively tilted trough sweeping NE through the region Friday night which could potentially trigger widespread showers and thunderstorms especially given the forecast of 50 or more knots of 0-6 km shear. However the corresponding EC runs keep the low off the coast through the entire weekend which would result in a drier and significantly warmer forecast. Breaking up the ensemble runs into clusters suggest about 40-50 percent of the runs bring the low inland which would result wet weather for much of the area with highs only in the 70s for the weekend. Meanwhile another 30 percent of the runs leave the low well off the coast which would keep highs in the 80s to lower 90s. The other runs are somewhere in between. In any event, stay tuned as we get closer to this portion of the forecast and hopefully we will see a little better agreement between the ensembles. fx && .FIRE WEATHER... A red flag warning will be issued for the east slopes of the central to northern Cascades of Washington, Methow Valley, and Okanogan Valley/Highlands. This will be for abundant lightning and gusty outflow winds from thunderstorms, and will be in effect from 1 PM today through 1 PM Monday. Uncertainty remains on lightning coverage over this portion of the forecast area.The combination of favorable upper level dynamics, instability, and available moisture is expected to result in widely scattered thunderstorm development with frequent cloud to ground lightning strikes. Density of lightning strikes will be highest during the afternoon and evening hours with a lull expected late tonight into Monday morning; however, a redevelopment of thunderstorms with abundant lightning is expected Monday afternoon. There will be a mix of wet and dry thunderstorms across these areas. The frequency of lighting is expected to be enough for new fire starts despite wetting rains that occurs under the cores of thunderstorms. Gusty outflow winds of 40+ mph is also expect from thunderstorms that may fan flames and result in the rapid spread of any new or ongoing fires. A secondary threat for lightning will be over southeast fire districts and into north Idaho both tonight, but more so Monday night with high based nocturnal thunderstorms. These thunderstorms will be fast moving and more likely to contain dry lightning. There is low confidence on coverage of lightning with these thunderstorms. /SVH && .AVIATION... 12Z TAFS: An area of low pressure will drop south across the Vancouver Island today with diffluent upper level flow over the northwest portion of the forecast area. Surface heating will destabilize lower level of the forecast area with widely scattered thunderstorms over the Cascades that will include across the Methow Valley State Airport (KS52) and Omak Airport (KOMK). A frontal boundary at mid levels will slide in over southeast Washington to the northern Idaho Panhandle with a band of mid level clouds forming this afternoon. Mid level instability will increase across far southeast Washington into the southern Idaho Panhandle tonight. This is expected to produce ACCAS clouds across KLWS and KPUW. Convection with isolated thunderstorm activity is expected to continue in the vicinity of KS52 and KOMK overnight. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is high for VFR conditions. General haziness from regional fires may result in brief MVFR conditions with reduced visibility, particularly in the Methow Valley and possibly at KPUW. Confidence is high for thunderstorms near the Cascades this afternoon into this evening. ----------------------- 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 89 62 88 57 80 54 / 10 10 10 0 10 0 Coeur d`Alene 86 61 86 55 79 52 / 10 10 10 10 10 0 Pullman 86 57 84 52 75 48 / 10 10 20 20 20 0 Lewiston 96 67 93 63 86 60 / 10 20 20 20 20 0 Colville 88 51 87 48 81 44 / 20 30 20 10 10 10 Sandpoint 84 57 83 53 78 50 / 10 10 10 10 10 0 Kellogg 84 64 84 58 75 54 / 20 10 20 20 10 0 Moses Lake 91 61 90 56 84 53 / 0 0 10 0 0 0 Wenatchee 91 67 88 62 84 60 / 10 10 10 10 0 0 Omak 93 64 91 61 89 59 / 20 20 30 20 20 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...Red Flag Warning from 1 PM this afternoon to 11 PM PDT Monday for East Portion of North Cascades National Park/Lake Chelan National Recreation Area (Zone 697)-East Washington Central Cascades (Zone 696)-East Washington North Cascades (Zone 698)-Methow Valley (Zone 704)-Okanogan Highlands and Kettle Mountains (Zone 699)-Okanogan Valley (Zone 703). && $$