Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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070 FXUS66 KOTX 151740 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 1040 AM PDT Thu Aug 15 2024 .SYNOPSIS... The next weather system will move into the Inland Northwest from the southwest this afternoon. Thunderstorms Thursday afternoon into Thursday night will bring the potential for gusty outflow winds. This will be followed by wetter showers and thunderstorms on Friday particularly across the Idaho Panhandle and northern mountains. More showers and thunderstorms are possible late Saturday afternoon and night for central WA. && .DISCUSSION... Today through Friday night: A low pressure circulation will bring an increasing chance for showers and thunderstorms to the Inland Northwest today into tonight. The center of the low is currently moving into southern Oregon with a diffluent wind flow pattern aloft over eastern Oregon. The prognoses is for the center of the low to move into northern Oregon this afternoon with favorable dynamics aloft rotating into eastern Washington and the Idaho Panhandle at this time. The upper level wave takes on a negative tilt as it does so, which should add in strengthening upward motions particularly over the Idaho Panhandle and into extreme eastern Washington late this afternoon into this evening. The low continues its northward march into eastern Washington tonight precipitation morphing into more of a stratiform deformation band across the northern panhandle and into northeast Washington. There will be embedded convective elements that will be present with isolated thunderstorms remaining possible, but overall, precipitation looks to take on more of a steady moderate precipitation event overnight. The low then looks to begin its exit into southeastern BC late Friday. Moisture wrapping around the low will develop the Cascades and northern mountains. The atmosphere will be fairly moist throughout the atmospheric column across these areas by Friday afternoon. Sun breaks will heat the surface with covective temperatures quickly being met and convection expected to contain periods of heavy rainfall. This is multifaceted low pressure system with multiple hazards that will be possible beginning this afternoon and continuing into Friday. * Today`s Convection: Mid levels of the atmosphere will destabilize through the morning into the afternoon out ahead of the advancing low. Altocumulus Castellanus (ACCAS) clouds are expected to develop across the region and any one of these will be capable of developing into a quick shower. Best instability with the NAM indicating up to 200-400 J/kg of MUCAPE will be over southeast Washington into the southern to central Idaho Panhandle. This is where the best chances for elevated showers and thunderstorms are expected with a 30-50% chance expected. This includes places like Pullman/Moscow, Lewiston-Clarkston Valley, Camas Prairie, and Silver Valley; however, all areas will see at least a 10-20% chance for at least showers as the afternoon progresses. * Evening and Overnight Convection: Surface based convection looks to fire off over northeast Oregon and in the southern Idaho Panhandle as this mid level band clears the area by the late afternoon hours. This convection looks to become strong with 500- 1000 J/kg of surface based CAPE to work with. Model guidance has also trended up with shear profiles on the order of 20-30 kts in the 0-6 km layer. This is marginal at best for seeing severe thunderstorms. DCAPEs though look to be between 800-900 J/kg for the Camas Prairie, into southeast Washington, and across much of the Central Panhandle Mountains. The concern will be for strengthening outflow winds from convection with the advancing cold pool. The question remains though if the gust front will outrun the convection or sustain it, and it becomes stronger as it advances north and westward. Movement of storms will be a little unusual in the sense that they will be moving off of the higher terrain the Panhandle (east to west) toward communities located in the Palouse region and Spokane/Coeur d`Alene areas. Wind gusts in the neighborhood of 40-50 mph will be possible with stronger convective cells. Convection then looks to transition into more of a stratiform band of precipitation that will deliver primarily beneficial moderate rainfall across the Northern Panhandle into the Northeast Mountains and Okanogan Highlands. * Friday`s Convection: Models indicate 500-1000 J/kg of surface based CAPE from the east slopes of the northern Cascades and across the northern mountains for Friday afternoon. Dew points increase into the lower to mid 50s across these areas: Methow Valley, Okanogan Valley, Republic, Northport, etc. Model soundings show a moist, skinny CAPE profile, and weak steering flow. Thunderstorms that develop are expected to be tied to the terrain and will slow moving or nearly stationary, particularly in the Cascades around and north of Lake Chelan. The concern will be for flash flooding and debris flows in and around burn scars, as well as in steep terrain. Convection will dissipate Friday evening leaving a fairly moist boundary layer in the mountain valleys. The added moisture from rainfall looks to result in fog forming. Friday will be cooler with high temperatures cooling from the 80s to low 90s on Thursday into the mid 70s to low 80s for Friday. /SVH Saturday: We will see mostly sunny skies to start the day. Clouds will begin to increase in the afternoon across central WA. The trough will push a wave up into central WA and an associated surface low. Models are indicating enough instability for showers and thunderstorms to move into portions of central WA during the late afternoon and evening hours, and then keep pushing north through the night to the Canadian border. Areas of main concern are the east slopes of the Cascades, Wenatchee area, the Waterville Plateau and the Okanogan and Methow Valleys. NBM probability of precip was pretty good, but have `massaged` a few locations to have higher chances Saturday evening/night. There is a 20-30% chance of thunderstorms for the aforementioned areas. The chance of showers extends further east into eastern WA and portions of north ID, and that is possible, but the best moisture and instability for showers/thunder threat remains further west. Sunday through Thursday: The Inland Northwest will remain sandwiched between a trough along the west coast and a ridge over the rockies/plains region. There is a 20-30% chance of residual showers in the Cascades on Sunday, but then after that the forecast is mostly dry. The next chance of showers is Wednesday/Thursday with a 20% chance mainly across the mountains. Wednesday 60% of ensembles are showing the trough having a more direct impact on the region, but by Thursday that goes down to a 50% chance. Temperatures will fluctuate in the 80s to around 90, with a trend towards upper 70s and low 80s perhaps by Thursday. Sunday afternoon and evening will see southerly wind gusts up to 20 mph...locally 25 mph across central WA. The remaining period will see afternoon/evening gusts to about 15 mph for those typically breezier locations in central WA. /Nisbet && .AVIATION... 18Z TAFS: An upper level shortwave will move into eastern Washington and north Idaho from the south this afternoon. This is bringing scattered high based showers across southeast Washington and the southern and central Panhandle this morning. There is a chance for sporadic embedded thunderstorms with these showers in the vicinity of KLWS and KPUW. Surface based thunderstorms will develop over the higher terrain of northeast Oregon and the Idaho Panhandle late in the afternoon into the evening. Some of these thunderstorms may become strong with gusty outflow winds up to 40 kts impacting airports in extreme eastern Washington namely KLWS- KPUW-KCOE-KSFF-KGEG. Thunderstorms will also be capable of heavy rain that brings a brief period of MVFR conditions after 01Z. Moderate rain with lowering ceilings is expected across the northern Idaho Panhandle into northeast Washington overnight into Friday morning. Isolated embedded thunderstorms will be possible overnight as well. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence for VFR conditions today becoming moderate overnight in the northern Idaho Panhandle and Northeast Washington. One caveat when MVFR conditions may occur today is with heavy rain under thunderstorms in the late afternoon and evening hours. Moderate confidence for thunderstorms developing at KLWS and KPUW today and low confidence for thunderstorms across the remainder of TAF sites (KEAT-KMWH-KGEG-KSFF-KCOE)./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 87 58 80 55 90 58 / 20 60 30 10 10 20 Coeur d`Alene 85 57 78 53 87 57 / 20 80 50 10 10 20 Pullman 80 52 78 51 90 53 / 20 70 20 0 10 10 Lewiston 88 63 86 62 98 63 / 20 70 20 10 10 10 Colville 88 49 77 44 87 47 / 0 50 60 30 10 30 Sandpoint 84 54 74 51 84 54 / 20 70 80 20 0 20 Kellogg 83 57 74 57 86 59 / 40 80 70 10 10 10 Moses Lake 87 57 83 55 92 56 / 20 20 10 0 20 40 Wenatchee 87 62 82 62 89 63 / 20 20 10 10 30 60 Omak 93 62 86 58 92 61 / 0 20 40 20 10 50 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$