Scientific Forecaster Discussion
000 FXUS66 KOTX 231230 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 430 AM PST Thu Nov 23 2017 .SYNOPSIS... Wet and unsettled weather will linger through the week as a series of storm systems brings several rounds of precipitation to the region. Snow levels will be quite high through today allowing motorists to travel over mountain passes on wet pavement. Cooler and unsettled conditions this weekend will result in lowering snow levels just as travelers are heading home this weekend. Windy conditions are expected today across the exposed areas of the Basin, Palouse, and north into the Spokane-Coeur D'Alene area. && .DISCUSSION... Today through Saturday morning...Satellite observations are telling the story this morning. The ridge axis is pushing to the east and flattening as the upper level low begins to move into the region. The fire hose (atmospheric river) that has been over the region will sag south and give one more shot of deep Pacific moisture today. In addition there is a pretty decent wave running up along the cold front that will enhance the precipitation across the region today. The cold front will move across the region this afternoon and be southeast of the forecast by this evening. Southwest-west winds will result in rain shadowing off the Cascades and will keep rain amounts to a minimum for the lower east slopes and the western basin. For the remainder of the forecast area we can expect another period of light to moderate stratiform rain. Rain amounts from one to two tenths will be possible for the lower elevations outside of the basin and for the Okanogan Highlands. The Cascades and Panhandle mountains will see from a half inch to just over an inch through Friday morning. Precipitation will turn to showers behind the front with the focus across the Cascades and Panhandle mountains. Snow levels will lower to 3000-4000 feet across the north and just above 4000 feet across the south. The mountains will get a chance for some snow tonight and Friday with the higher Cascades picking up 3-7 inches of fresh snow and the Panhandle mountains picking up 1-3 inches. Temperatures today will be 10-20 degrees above normal, before dropping 10-15 degrees Friday behind the cold front. These temps will still be on the warm side of normal Next we look at winds with the cold front this afternoon. Pre-frontal winds will be on the increase this morning out of the south- southwest with sustained winds 10-20 mph. As the front moves through the region winds will increase out of southwest with sustained winds 20-35 mph with gusts 35-45 mph and locally to 50 mph. These higher winds will be mainly across the Columbia basin, the Palouse, The West Plains, and the Spokane-Coeur D'Alene corridor. Ridge top winds will also be very strong with gusts 45-55 mph and locally gusts to 60 mph and more. * Impacts: A Wind Advisory has been issued for a portion of the forecast area from 11 AM this morning to 5 PM this afternoon. Driving on almost any roads will be difficult and especially for high profile vehicles. Tree damage is likely and downed power lines will be possible. * Moderate rain on a deep snow pack will result in significant run off into area stream and small rivers. Flood watch are currently in affect of the Stehekin river in the Cascades and for Paradise Creek near Moscow ID. All small rivers and streams will continue to have high flows until the temperatures cool down on Friday. Lastly, be prepared for snow at pass levels in the Cascades. Tobin Saturday through Sunday: A shortwave disturbance will ride up a longwave trough of lower pressure in the Gulf of Alaska with a surface low developing on the Pac NW coastline. Moist isentropic ascent will increase over the region Saturday evening. The timing of the precip is slower than previous model runs, but all models agree on delaying the passage of the warm front. With the surface low quite a bit offshore, best chances for precip will be across the northwest portion of the forecast area. Easterly winds will increase creating downsloping off of the higher terrain in the Central Panhandle Mtns with the southeast portion of forecast area likely receiving little to no precip Saturday night. Strong warm air advection will keep snow levels high with snow mainly limited to the mountains of eastern WA. Even these mountain locations will see snow transitioning over to rain as snow levels increase through the night. Temperatures will likely be quite mild over the southeast portion of the region with 850 mb temps climbing to between 11 and 12 Celsius and winds remaining breezy. There is some question what the precip type will be in the Methow Valley. The NAM is more aggressive with cold air remaining damned up into this valley. However, looking at a NAM model sounding at this location, it shows a layer of low stratus hanging around through the afternoon. I am not sold that we will see enough cloud cover to prevent temperatures from warming up in the afternoon, so I am going with rain and above freezing temperatures up the Methow Valley. The timing of the cold front is in question. The 00Z GFS is much quicker with a Sunday morning frontal passage, whereas the 00Z ECMWF continues with the consistent message of slowing down this weather system. I am leaning more toward the ECMWF solution with a Sunday afternoon cold front passage. The cold front will switch winds more westerly with precip likely across extreme eastern WA and into the ID Panhandle. It also will result in increased winds. Breezy to windy conditions are expected, but not as strong as what we have forecasted for Thanksgiving. Sunday night through Wednesday: The upper level trough of lower pressure over the Gulf of Alaska will eject across the region Sunday night into Monday. Models are consistent with the bulk of the upper level dynamics now diving further south. So, this will be a period where temperatures will drop, but it does not look as wet as previously forecast. Lapse rates will steepen with colder air moving in aloft. This will mean that the region overall will be under a more unstable atmosphere and any little ripple that comes across will have the potential to set up some showers. The Panhandle will be favored for these shower and should see snow levels lowering with snow the favored precip type at the passes. The longer range medium range model guidance is looking a bit more ridgy by the middle of next week with trend toward drier weather. /SVH && .AVIATION... 12Z TAFS: The biggest concern for today and this evening will be increasing south-southwest associated with an incoming cold front. LLWS is in the forecast this morning for many of the TAF sites as the cold front approaches the region. Widespread wing gusts 30-40 kts are expected at all the TAF sites between 20-03z. At KGEG wind gusts 40-45 kts will be possible. Winds are expected to decrease significantly after 04-06z. Expect short periods of moderate to heavy rainfall as the front moves from west to east today. For early this morning stratus/fog will drop conditions to IFR/LIFR at KMWH/KEAT. So far conditions at theses sites have fluctuated wildly and expect that to continue through 18z. Conditions will settle down around 06z...but expect fog/stratus to re-develop vcnty KMWH/KEAT. Tobin && .PRELIMINARY POINT TEMPS/POPS... Spokane 58 36 46 32 45 42 / 90 30 10 10 20 50 Coeur d'Alene 57 36 44 32 45 40 / 100 40 10 10 10 60 Pullman 59 37 47 34 49 46 / 100 60 10 0 20 40 Lewiston 62 39 51 36 52 49 / 70 60 10 0 20 20 Colville 54 34 44 32 43 37 / 70 10 20 10 20 80 Sandpoint 52 35 42 30 42 38 / 100 30 20 20 10 80 Kellogg 51 34 41 31 44 41 / 100 70 30 10 10 50 Moses Lake 62 35 50 31 44 41 / 20 0 0 10 20 50 Wenatchee 57 36 48 33 41 35 / 20 10 10 10 40 60 Omak 55 34 45 33 41 35 / 40 10 10 10 20 90 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Wind Advisory from 11 AM this morning to 5 PM PST this afternoon for Coeur d'Alene Area-Idaho Palouse. WA...Wind Advisory from 11 AM this morning to 5 PM PST this afternoon for Spokane Area-Upper Columbia Basin-Washington Palouse.