333 FXUS66 KOTX 180013 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 455 PM PDT Wed Apr 17 2013 .SYNOPSIS... A warming trend will occur through Thursday, but another weather system is expected to bring valley rain and high mountain snow on Friday...with showers lingering both Saturday and Sunday. A cold front is expected to pass through Northeast Washington and North Idaho Sunday for cooler temperatures. A warming and drying trend is likely beginning next Monday as high pressure begins to build over the region. && .DISCUSSION... Tonight and Thurs: Our biggest challenge will be timing how quickly the clouds increase overnight along and ahead of the warm front currently approaching the Pacific NW coast. Clouds have been slow to thicken, but are expected to move across Ern Wa and N Id overnight and into Thurs. These inbound mid and high clouds certainly will play a major role in overnight low temps, as well as high temps tomorrow. With a rather dry air mass in place now and through tonight, based on the latest sfc obs and fcst soundings, it's going to take more much synoptic-scale forcing for ascent (especially isentropic ascent over the warm front) to give us any chance of pcpn. And given the slow expected saturation, combined with stable short wave ridging, it's really not until midday Thurs that our pcpn chances increase significantly and pose a light rain threat for the lower elevations. A few hundredths of pcpn in the Cascades and N Id will be likely however. bz Thursday night through Saturday: The Inland Northwest becomes wet. A frontal wave (now pushing toward 140W) arrives late Thursday night. Deep lift within the leading isentropic ascent and LFQ of the jet develops in earnest late Thursday night into Friday morning. The lift persists into the afternoon with the incoming cold front and mid-level shortwave and a second low (now near southeast Alaska) dropping through British Columbia. While the Pacific low weakens as it moves inland that B.C. low strengthens. During the system's progression inland it taps subtropical moisture. PWATs rise to 130-180% of normal (or the 0.50 to 0.75 inch range) ahead of the cold front. Precipitation chances increase to likely for a good part the Inland Northwest by early Friday and PoPs remain high through the day. The highest precipitation amounts focus at the Cascade crest and the Idaho Panhandle southwest into the Blue Mountains. Yet some modest amounts (one-quarter to one-third inch) are also projected over the eastern third of Washington. A west-northwest flow will, however, lead to a modest rain shadow in the lee of the Cascades. This area will have a better chance of remaining dry. I kept a slight chance of thunderstorms across east Washington and north Idaho through early evening, with some low-grade CAPE and negative LIs on the northeast side of the jet. The afternoon passage of the front would also work to bring some thunder risk. By Friday evening the cold front moves east, but through the night and continuing into Saturday an inverted surface trough lingers near the Idaho-Montana border. In addition a secondary shortwave approaches from British Columbia and northeast Washington and northern Idaho remains in favorable lift in the diffluent LFQ of the jet. The core of the deep moisture plume does shift south, but PWATs remain respectable between 95-120% of normal (or the 0.25 to 0.45 inch range). So precipitation chances will remain high across Idaho as well as the eastern third of Washington, particularly closer to Idaho border, and at the Cascade crest. However precipitation amounts are not expected to be as high for much of the region compared with those generated by the primary system. The northwesterly flow focuses some of the higher amounts over the central Panhandle. Snow levels, based on 850mb temperatures rising to between 3 and 6 C and wet-bulb zero heights, are projected to be around 5000-6500 feet ahead of the cold front. They drop to around 4000-5000 feet behind the cold front and locally near 3500 closer to Cascades through Okanogan Highlands Saturday morning. Lastly, the dynamic front and mixing along it in proximity to the jet will make for breezy conditions. At this time, speeds are not expected to warrant any highlights. Yet gusts between 20 and 30 mph are possible, with locally higher amounts in the exposed terrain. Temperatures are projected to be milder. However persistent cloud cover and precipitation on Friday, especially over the Panhandle, will keep temperatures from reaching their full potential. This means values near to a degree or so under seasonal norms. Milder temperatures are expected toward the lee of the Cascades and deeper Columbia Basin thanks to down-sloping and more sun. /J. Cote' Saturday night through Sunday night: Another shortwave upper level trough of lower pressure will push across the region out of British Columbia. This will bring a chance for more unsettled weather through the weekend. Best chances will be mainly be across the Idaho Panhandle. This disturbance will usher in some colder Polar Continental air, which will result in dropping snow levels across the Idaho Panhandle. Mid slopes across the ID Panhandle mountains stand a good chance of picking up some light snowfall. This could result in some accumulating snowfall over Lookout Pass Saturday night into Sunday. I cannot rule out some accumulating snowfall in the valleys (especially above 2500 feet), but the timing is not good for valley accumulations as it looks like the coldest air will not move in until during the day on Sunday when insolation will help to warm up temps. We could see a brisk northerly breeze as this system passes through for Sunday with below normal temperatures. The northerly breeze will result in a wind chill factor making it feel even colder than the ambient temperature. Sunday night is expected to be our coldest night of the period. Our official start of the growing season for low elevation areas including the Wenatchee Area, Moses Lake Area and L-C Valley is April 23 based off of the average last freeze for the season. At the moment, These areas are not expected to see widespread freezing temperatures, but I wouldn't be surprised if our low temps are not cold enough as we should see clearing skies and weakening winds. Some pockets of freezing temps will be possible across these areas. Monday through Wednesday: High pressure offshore looks to push into the region for next week. This will result in a decreasing chance for precip and a warming trend. However, the 00Z GFS and ECMWF model guidance brings in the ridge a bit quicker than their ensemble means, so there is some uncertainty with how warm we will actually get and when we will see the warmest temps. However, we should see above normal temps by around mid week. /SVH && .AVIATION... 00Z TAFS: A deep fetch of Pacific moisture will begin to spill into the Inland NW during then next 24 hours. Look for a gradual lowering of cigs aft 12z Thursday...lowering btwn 6-8k agl aft 18z Thursday and possibly down to 5k agl arnd 00z. The low level air mass is very dry and pcpn will mainly fall in the form of sprinkles through 00z and incr in intensity thur nt and fri. /sb && .PRELIMINARY POINT TEMPS/POPS... Spokane 30 55 41 56 38 57 / 10 20 50 80 30 30 Coeur d'Alene 28 54 38 53 36 56 / 10 20 60 80 40 40 Pullman 30 53 39 55 37 55 / 0 20 60 90 50 30 Lewiston 34 59 43 60 42 61 / 0 10 30 70 30 20 Colville 31 60 40 61 35 62 / 10 10 50 90 40 30 Sandpoint 27 52 36 51 35 55 / 10 20 50 90 50 50 Kellogg 28 48 36 49 36 50 / 10 20 70 100 60 60 Moses Lake 35 64 43 68 40 67 / 10 10 20 20 10 10 Wenatchee 40 59 44 66 41 62 / 10 0 20 20 10 10 Omak 33 62 40 66 37 65 / 10 10 20 20 10 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$