667 FXUS66 KOTX 211143 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 443 AM PDT Tue May 21 2013 .SYNOPSIS... The mild spring weather will come to an end. A strong cold front will sweep through the region Tuesday through early Wednesday with showers likely, a chance of thunderstorms, breezy winds and cooler temperatures. Cool and showery conditions will prevail through much of the week, but will improvement a bit for the holiday weekend. && .DISCUSSION... Today and tonight: a strong cold front moves into the region, with increasing rain and the potential for thunderstorms, with the added threat of snow coming to some of the low lands late tonight into Wednesday morning. This morning satellite imagery showed the cold front advancing onto the Pacific Northwest coast and radar showed a swath of rain extending about 100-150 miles east of it. Deep layer lift with the cold front and supporting jet stream coming around the upper low is expected to sustain the precipitation. So precipitation that is just coming into the Cascades is expected to expand across central Washington through the morning and early afternoon. More widely scattered showers are expected to develop across eastern Washington and north Idaho this afternoon, before the steadier band of precipitation comes here late this afternoon and evening with the cold front. The center of the upper low and linked strong vorticity max comes onshore near northwest OR this evening, before swinging toward the central WA/OR border overnight. This will sustain the deep layer lift and keep precipitation chances high tonight across the eastern third of Washington, northern Idaho and west to southwest across the Okanogan through Cascades. As for the thunderstorm potential, convective parameters indicate increasing CAPE and unstable LI value through this morning and afternoon. The highest CAPE and most unstable LI values and so highest thunderstorm threat is projected to be from the Okanogan Highlands into the northeast mountains and Idaho Panhandle. However some low-grade CAPE and the High Level Total Totals instability parameter indicating values between 30-36 C back into the Okanogan Valley through Waterville Plateau and Cascades, some thunder cannot be ruled out here either. Lastly models indicate some modest 0-6km shear across the region. Where instability is best values are between 35-50kts. This support the potential for a few organized and stronger thunderstorms, with a threat of hail and strong winds. However is the cold front passes quicker this potential will be limited. Temperatures will vary greatly from the Idaho Panhandle to the Cascades today. The incoming cold front and precipitation is expected to keep temperatures across the western Basin through Cascades relatively cool and below normal. The eastern third of Washington and northern Idaho, however, will see more sun and less precipitation ahead of the cold front until later in the day. This will hold temperatures closer to or slightly above normal. Overnight into Wednesday morning colder air begins to punch in with the center of the upper low. Across the Cascades through the Palouse and Camas Prairie, models show 850 mb temperatures dropping to between 0 and -2 C. This suggests snow levels dropping to near 3000 feet across this region. So some snow remains in the forecast, even around Pullman. Accumulations should be limited by surface temperatures generally remaining above freezing. The exception will be in the higher Cascades and potentially around the Blue Mountains and higher Camas Prairie, including near Winchester. /J. Cote' Wednesday and Wednesday Night: As we progress into the overnight hours we will continue to see widespread rain for the Inland NW and potentially moderate precipitation for parts of NE Washington and the Northern Idaho Panhandle especially in the higher elevations. A major swing in temperatures will also be accompanied by the strong cold front. Mentioning the higher amounts of QPF we can base this off of not only the front slowing its progression in the mentioned areas...but also a strong jet will be supportive of higher precip. intensity. The key to all of this relating to potential impacts of flooding will be the snow levels. For this forecast the snow levels were only slightly adjusted but with current forecast levels much of the precip that falls in the higher elevations will fall as wet snow rather than rain and this will help minimize the flooding impacts. If all precip were to fall as rain the flooding potential for rivers and streams would be elevated due to higher amounts of direct runoff...but given the current scenario the snowfall will act as a storage mechanism for much of the liquid and let it slowly melt and contribute to streams as temperatures warm back up. Other changes that were made to the forecast would be even further increasing the POPs for the NE part of Washington and the ID Panhandle for Wednesday. Although POPs were increased the over QPF amounts were trimmed back a little especially for the east slopes of the Northern Cascades as new model runs show slightly lower amounts. QPF was also trimmed for NE part of the forecast area...but remain high for the time frame which will still lead to rises in the streams and rivers. All rivers minus the Okanogan River near Tonasket is currently forecast to remain below flood stage and the Okanogan will only reach minor flood stage. In general this period will see temps drop significantly from the previous day (nearly 30F in some areas) and some breezy conditions as the front pushes through the area. /Fliehman Thursday through Monday...Unsettled and cool conditions will continue through the extended period. Models are in very good agreement in holding a deep upper level low right over or very close to the forecast area through Friday...which is a high confidence recipe for showery conditions just about everywhere in the forecast area. The latest GFS and EC models both sense a secondary deformation area transiting the region from south to north on Friday which may enhance the density of showers on this day especially across the northern tier zones. For the upcoming holiday weekend model consensus begins to break down but both models depict a weakening of the coherence of this upper low...an opening up and broadening trend into a general trough over the region (GFS) or a migration of the upper low center into southern Canada (ECMWF). Both solutions suggest a continued unsettled and showery regime...but showers may not be as prevalent as when the upper low was directly over the region. At this time the weekend does not look like a wash-out...but none the less iffy for outdoor activities...with guarded optimism that the densest shower activity will occur over the mountains north and east of the basin. Temperatures through the upcoming week will start out well below normal but slowly add a degree or two each day and top out around normal by next Sunday and Monday. /Fugazzi && .AVIATION... 12Z TAFS: A cold front moves from the Cascades to Idaho trough the next 12-18 hours, while the center of an upper low swings in from the west/southwest tonight into Wednesday morning. The front will bring an increasing area of rain near the western TAF sites this morning and early afternoon. Scattered showers will develop further east this afternoon, while a steadier band of precipitation is expected to come by late afternoon and evening with the cold front. A chance of thunderstorms also exists throughout much of the region, with the best chance for organized activity north and east of KGEG. Storms may be locally strong with gusty winds/small hail. Tonight the incoming upper low is expected to keep precipitation chances high across eastern TAF sites. Local MVFR cigs are possible in the more persistent precipitation. Colder air accompanying the upper low will allow some snow to mix in toward Wednesday morning but surface temperatures are expected to limit the accumulation threat below 3000 feet. /J. Cote' && .PRELIMINARY POINT TEMPS/POPS... Spokane 73 41 51 36 61 42 / 30 90 90 40 30 30 Coeur d'Alene 75 41 48 36 59 41 / 30 90 90 50 40 40 Pullman 72 38 51 35 60 41 / 10 80 80 30 40 40 Lewiston 80 44 59 41 66 46 / 30 60 70 30 30 40 Colville 76 45 55 37 66 41 / 50 90 90 60 50 40 Sandpoint 76 43 50 36 59 39 / 30 90 90 80 40 50 Kellogg 75 40 44 34 55 40 / 30 90 90 70 50 50 Moses Lake 71 43 62 40 68 44 / 70 50 30 30 30 30 Wenatchee 63 43 58 42 65 45 / 60 60 40 40 50 30 Omak 69 43 59 38 66 41 / 80 80 70 50 50 30 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$ |