Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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949 FXUS66 KOTX 180944 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 244 AM PDT Wed Sep 18 2024 .SYNOPSIS... Isolated light rain showers will persist over north Idaho through early this morning. Conditions will trend drier toward the end of the work week, then light rain chances mainly for the mountains Sunday into Monday. A ridge of high pressure will deliver above normal temperatures early next week. && .DISCUSSION... Today through Friday night: Lingering showers will continue in the Idaho Panhandle through the morning, before the system shifts eastward, drying the region out by the afternoon. As the coastal system continues to drop south through Thursday, the PacNW will remain in a more zonal flow. With this zonal flow, expect quieter weather with temperatures near seasonal normals (highs in the 70s and lows in the upper 40s to low 50s), dry, and lighter winds. Friday brings more sunshine and a gradual cooling trend, with temperatures 2 to 5 degrees cooler than previous days. /KM Saturday through Wednesday: There is good agreement amongst the model ensembles for the general weather pattern in the extended part of the forecast. The general consensus is for a warm front to push across the Northwest late Sunday into Monday. Within the warm sector will be a healthy plume of moisture directed into the region with P- wats of over an inch. What appears to be lacking is dynamics aloft and instability to ring out that moisture. The parent low looks to remain well away from the Inland Northwest in the Gulf of Alaska. Main forcing mechanism will be some weak moist isentropic ascent and orographics. Precipition will be favored over the mountains and specifically across the Cascade crest and into the Idaho Panhandle with flow aloft out of the northwest. The National Blend of Models (NBM) is probably under doing the precipitation potential over the mountains as P-wats will be between 150-180% of normal. It won`t take too much to get moisture to precipitate out, and sometimes model guidance will underplay this potential in scenarios like this where dynamics is weak. In any event, clouds look to increas and thicken Sunday into Sunday night with mostly cloudy skies continuing into at least early Monday. Ridging of high pressure then builds in over the region for Tuesday with what looks to be a fairly sharp bump up in temperatures going from low to mid 70s Sunday into Monday into the upper 70s to low 80s by Tuesday. Wednesday may also be a mild day, but there is more uncertainty toward the end of this period. The model ensemble cluster analysis suggests a 40-45% chance for a cold front to sweep across on Wednesday. This would bring breezy to windy conditions. The cold front may not bring much in the way of precipitation either with a mild concern for elevated fire weather conditions around this time. There is another 30% of the ensembles that shows a weaker cold front passage and is delayed that would suggest either a Wednesday night or Thursday timing for frontal passage. Then around 25% of the ensemble clusters that suggests a scenario where the ridge holds firm and the region continues a warming and drying trend further. This quarter of the ensemble members suggests a warmer solution, particularly so west of the Cascades but also across eastern Washington and into the Idaho Panhandle. There is high confidence that we aren`t looking at a wetting rain scenario with below normal temperatures. The extended looks at least near average or above average for temperatures. /SVH && .AVIATION... 06Z TAFS: Light showers will continue through 07-09z from near KSFF/KCOE south to near KPUW. Not much impact from these for the TAF sites, although will be monitoring the potential for IFR stratus Wednesday morning due to a moist boundary layer and upslope boundary layer winds into KGEG/KSFF/KCOE. Current probabilities are under 50% at these TAF sites so prevailing conditions were kept VFR. Any stratus that develops will lift and erode by 17-18z, with flat stratocumulus in the afternoon. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: HREF model shows a 60-70% chance of IFR stratus between 12-16z south of Spokane (Cheney, Spangle areas), decreasing to a 40% chance over GEG, with a 20% chance at KSFF/KCOE. With favorable low level winds and some light showers further moistening the boundary layer these probabilities seem reasonable. JW && .PRELIMINARY POINT TEMPS/POPS... Spokane 76 49 76 46 72 44 / 0 0 0 0 0 0 Coeur d`Alene 73 49 74 47 70 43 / 0 0 0 0 0 0 Pullman 69 44 73 45 66 40 / 10 0 0 0 0 0 Lewiston 77 54 81 53 75 49 / 10 0 0 0 0 0 Colville 78 39 76 37 71 35 / 0 0 0 0 0 0 Sandpoint 71 45 71 44 68 39 / 0 0 0 0 10 10 Kellogg 68 49 72 49 65 45 / 10 0 0 0 10 10 Moses Lake 79 46 79 46 73 43 / 0 0 0 0 0 0 Wenatchee 80 54 79 54 73 49 / 0 0 0 0 0 0 Omak 83 51 81 48 75 46 / 0 0 0 0 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$