152 FXUS66 KOTX 080601 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 1001 PM PST Mon Jan 7 2019 .SYNOPSIS...A wet storm system will arrive Tuesday into Wednesday. Expect snow in the Cascades and near the Canadian border, rain over southeast Washington and Lewiston area, with a wintry mix featuring freezing rain over parts of the Columbia Basin, Northeast Washington, and the north Idaho Panhandle. Drier weather is expected for the end of the week. && .DISCUSSION... Tonight through Wednesday...The last weather system which brought snow to our CWA last night and this morning has since moved out and is quickly being followed up by an upper level transient ridge. This is bringing mostly clear skies this afternoon but expecting this to be short lived as a fast approaching warm front will spread high clouds over the CWA late tonight and into Tuesday morning. Depending on how quickly these clouds move over our area will dictate how much radiational cooling we will see tonight, however, there is good general consensus between models that low temperatures will bottom out in the low to mid 20s tonight over the I-90 corridor and down through the Palouse. Mid and low-level clouds will eventually form over our area come Tuesday morning trapping cold air near the surface and setting the stage for a very interesting winter weather event for Tuesday afternoon/evening. Strong upper level southwesterly flow ahead of this warm front will bring isentropic lift to our area throughout the day eventually saturating the atmosphere and creating precipitation by the afternoon hours. There is good agreement between models that precipitation will initially start out as snow over our CWA despite the warm air advection aloft. However, by the evening hours warmer air aloft will begin switching precipitation to freezing rain over an area extending from Grant, Lincoln, Spokane, and into Kootenai Counties. At this point, it is not entirely clear how long the freezing rain will last into the night since some models transition over to rain quite quickly while others keep surface temperatures just below freezing all night long. Just a degree or two could make the difference between 0.01 inches of freezing rain to 0.2 inches. For now, we have gone with a model blend of 0.05 inches for Spokane with closer to 0.1 for areas further north over southern Stevens, Pend Oreille, and Bonner counties before the transition to rain occurs. Given the potential for freezing rain over the area, have issued a Winter Weather Advisory for these areas. By early Wednesday morning, these counties should rise above freezing with the threat of freezing rain continuing for northern valleys until late morning. Beyond this, light rain showers will continue for the remainder of the day, mainly for our northern counties, with snow levels around 3000 ft near the Canadian border to roughly 5000 ft over the southern half of our CWA. Wednesday night and Thursday...Latest models are in good agreement in maintaining a moist southwest flow into the region Wednesday night...with the forecast area in the warm sector of the storm system during the evening...but subject to a cold front passage later in the night. This cold front will not herald significantly cooler temperatures...it is not a strong cold front...but it will help maintain valley rain and mountain snow through Wednesday night as it transits the region...with a transition of a more orographic regime featuring a drying basin and continued showers over the orographically favorable high terrain through Thursday in the post frontal regime. Temperatures during this time frame will remain quite mild for January with overnight lows staying above freezing in most valley and basin locations and Thursday's high popping back up into the lower to mid 40s. /TW Thursday night through Monday...Models remain in overall agreement in building a strong ridge centered over Idaho or Montana through much of the extended period. The forecast area will come under deep southerly flow under the western flank of the ridge as this ridge evolves...but it will be a largely dry southerly flow with the next upstream Pacific moisture feed running south to north in this southerly flow over the coastal regions...so at least aloft there will probably be only transient patches of high clouds. However...given that Thursday's cold front will not be from the trajectory which will favor an air mass exchange with dry Canadian air...there will still be a considerable amount of low level moisture about the region as the ridge builds. This will probably lead to areas of fog and low clouds around the region through the weekend which may be slow to break during the days but will also retard strong radiational cooling at night. So the extended forecast looks pretty much dry around the region but with low clouds and some fog and temperatures continuing to run above average fro mid January. Way out on or about next Monday the GS model tries to break this ridge down with an initial short wave...while the ECMWF maintains the dry pattern. This is too far out to have any confidence in either solution at this time...but experience suggests that once these strong January ridges get established they are rather difficult to get rid of without a series of initial outrider waves. So Monday is worth a mention of slight chance of snow or valley/rain snow...but with little confidence. /Fugazzi && .AVIATION... 06Z TAFs: High level clouds are moving in from the southwest and will lower aft 15Z. Some patchy or shallow fog is possible around KGEG, KSFF, KCOE aft 09Z, but confidence isn't high. Precipitation will start to move into central WA aft 18Z, and will be in the form of snow for KEAT and KMWH. As precip moves east into southeastern WA it should quickly change to rain for portions of the Columbia Basin and Palouse/LC region, impacting KPUW and KLWS. Aft 00Z the threat of -fzra enters the forecast...mostly along the I-90 corridor. Confidence is low for any particular TAF location. Have slightly better confidence of sleet for KEAT developing in the evening. && .PRELIMINARY POINT TEMPS/POPS... Spokane 23 34 31 40 36 44 / 0 40 70 60 50 20 Coeur d'Alene 23 34 31 41 35 43 / 0 30 70 60 60 30 Pullman 24 36 34 42 37 44 / 0 40 80 60 40 20 Lewiston 29 41 37 50 41 51 / 0 30 60 30 20 0 Colville 16 34 30 41 32 44 / 0 20 80 80 80 30 Sandpoint 21 32 29 40 35 40 / 10 20 90 90 90 70 Kellogg 22 35 31 41 37 42 / 20 20 80 80 70 50 Moses Lake 27 36 33 41 33 45 / 0 50 80 60 40 0 Wenatchee 25 31 28 37 33 42 / 0 70 80 70 60 20 Omak 20 31 28 37 33 41 / 0 20 80 80 80 20 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday for Northern Panhandle. Winter Weather Advisory from 1 PM Tuesday to 7 AM PST Wednesday for Coeur d'Alene Area. WA...Winter Weather Advisory from 9 AM Tuesday to noon PST Wednesday for East Slopes Northern Cascades. Winter Weather Advisory from 4 PM Tuesday to 10 AM PST Wednesday for Northeast Mountains-Okanogan Highlands. Winter Weather Advisory from 9 AM Tuesday to noon PST Wednesday for Okanogan Valley-Waterville Plateau-Wenatchee Area. Winter Weather Advisory from 1 PM Tuesday to 7 AM PST Wednesday for Spokane Area. && $$