Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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232 FXUS66 KOTX 302217 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 317 PM PDT Tue Apr 30 2024 .SYNOPSIS... Unsettled and cool weather will continue through the end of the week, with a subtle warming trend Thursday into Friday. The weekend forecast isn`t certain at this point but its trending toward another surge of very cool and wet weather, especially on Sunday. Cool weather with scattered showers are expected to continue into early next week. && .DISCUSSION... Today and tomorrow: Widespread shower activity is ongoing under the influence of a mid level low and cold pool. There have been quite a few reports of graupel under these showers. They have been slow moving under the influence of weak vertical shear. Low level instability and surface relative vorticity look too weak for any embedded rotation in these showers. Thunder chances have ramped up a bit on the HREF compared to the older run in eastern WA, ID Panhandle, and the Blue Mountains (10-30% chance of an isolated strike). Temperatures are chilly today with highs peaking in the upper 40s to upper 50s. The upper low migrates into northwestern MT overnight and brings in drier northerly flow aloft, especially central WA. Extreme eastern WA and the ID Panhandle will still be under the influence of the trough tomorrow so theres additional shower chances. Temperatures aloft are warmer so I wouldn`t expect much in terms of thunder chances. This is reflected by the HREF probability of thunder only showing up with a 10% contour in Clearwater County tomorrow afternoon. Temperatures tomorrow will be a few degrees warmer in the 50s to 60s. /Butler Thursday and Friday...The deep and highly unstable upper level lows that region has been subject to the last couple of days will likely move east of the area. On Thursday the core of the low is expected to head into eastern MT with another moving well of the coast. A shortwave ridge is expected to build between these lows. By Thursday evening the ridge axis will likely move onto the WA coast and over eastern WA on Friday. Mid-level lapse rates on Thursday will still be sufficient for the production of diurnal convection, especially over extreme eastern WA and north Idaho. However given the lack of synoptic scale forcing, much of the lifting will need to come from orographic ascent which suggests the coverage will be considerably more limited than what we are seeing today. Suspect over central WA the chances of precipitation will be near 0. With the ridge moving atop the Inland NW on Friday the precipitation chances will be minimal over most locations, however we cant rule out a small chance of showers over the ID Panhandle mountains. As for temperatures, 850 mb readings will range from 4-7C on Thursday climbing to 7-9C on Friday. This translates to highs generally in the 60s with a chance of 70 degree or warmer weather over the LC Valley and lower Columbia Basin. Saturday and Sunday...this is where the model confidence begins to tumble considerably. The aforementioned upper level low off the coast is expected to head inland during this period. Where the low goes is quite variable. The bulk of the GEFS ensemble members take the low well to our south, whereas the bulk of the Canadian and EC solutions bring it much farther north. The GEFS would likely deliver warm and dry weather if it comes to fruition while the other solutions suggest significantly cooler and unsettled weather. On Saturday the range in temperatures over Spokane shows highs generally in the 60s from the GEFS solutions with highs struggling to get out of the 50s from the majority of Canadian and Euro solutions. As cool as unsettled Saturday could be, Sunday is looking worse. The ENS 500 mb forecast has a 546 low over NE Oregon with the Canadian showing a similar solution. Meanwhile the GEFS mean has the low over northern NV, a huge difference. In these situations it can pay to look at the cluster breakdown rather than the ensemble means from each model. If anything this lends even more credence to the cooler/wetter solutions as only about 25% of the ENS and Canadian member show anything near this with most of the GEFS solutions supporting warm/dry. The latest NBM temperatures are catching onto this trend and have already dropped Spokane`s high temperatures around 8F from the previous run which is a very significant change this far out in the forecast. So how cool could it be on Sunday? The latest Canadian operational model has an 850 mb temperature of -2C and a high temperature of 41F. The Euro model has a high of 46. We have trended the forecast cooler than the NBM but not likely cool enough. We also raised the pops a bit for Sunday based on this cooler/wetter solution but the NBM QPF isnt likely high enough. So although there are no guarantees for those with outdoor plans on Sunday, you might want to prepare for a cool and wet day. Monday and Tuesday...Model consensus improves significantly as the deep upper low is forecast to drift off to our east however a broad weak trough is forecast to persist over the region resulting in cool and unsettled weather. Highs will likely be in the 50s to lower 60s. fx && .AVIATION... 18Z TAFS: Another round of showers is developing as the atmosphere destabilize. Isolated thunderstorms may also develop, but the main threat will be over the higher terrain of SE Washington (Blue Mountains), and up over NE Washington around Republic and Colville. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is a 20% chance of thunderstorms on Tuesday mainly across southern Washington into the Lewiston area, and up over the mountains of NE Washington. Confidence is any of the TAF sites being impacted is too low to include in the TAF forecast. ----------------------- 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 34 59 34 62 37 66 / 20 10 0 10 10 0 Coeur d`Alene 33 55 35 60 37 64 / 10 20 10 10 10 0 Pullman 31 54 32 58 35 63 / 20 20 10 20 10 0 Lewiston 37 61 36 66 39 71 / 30 20 10 20 10 0 Colville 29 61 31 64 34 66 / 20 10 0 20 10 10 Sandpoint 34 53 37 59 37 62 / 30 50 20 40 20 20 Kellogg 34 49 36 56 38 62 / 30 60 20 40 20 10 Moses Lake 33 65 34 67 37 69 / 10 0 0 10 0 0 Wenatchee 38 62 39 65 42 66 / 10 0 0 10 0 10 Omak 34 65 38 68 39 70 / 10 0 0 10 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$