Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
011 FXUS66 KOTX 011200 CCA AFDOTX Area Forecast Discussion National Weather Service Spokane WA 500 AM PDT Sat Jun 1 2024 .SYNOPSIS... A chance for light showers will return for the northern mountains today, but most locations will mainly only see thickening clouds and possible some sprinkles. A dynamic system pushes across the region Sunday into Monday, bringing widespread rain, followed by gusty winds, and the potential for thunderstorms on Monday. Next week will see a warming and drying trend. Confidence is increasing for temperatures warming into the 80s and 90s. && .DISCUSSION... ...DYNAMIC SPRING STORM TO BRING WIDESPREAD RAIN SUNDAY INTO MONDAY FOLLOWED BY A CHANCE FOR THUNDERSTORMS AND WINDY CONDITIONS... Today and Tonight: A weak shortwave trough of lower pressure will push across the Inland Northwest today. Clouds will increase and thicken over much of the region. Moisture will be lacking with this precursor shortwave though ahead of the "big show" spring storm system over the following 48 hours. Best chances for showers (at 30- 50%) will be across the northern mountains. Otherwise, most valley locations and the basin will largely only see the potential for sprinkles. There is a 20-25% for thunderstorms near the Canadian border today with this weak system. A westerly pressure gradient will tighten across the Cascades late this afternoon into this evening. Winds will increase into the Wenatchee Area out of the Wenatchee River Valley, on the higher benches of Wenatchee and Chelan, on the Waterville Plateau, and into the western portion of the basin; these areas will see the potential for wind gusts of 25-35 mph into the evening. Sunday through Monday night: A dynamic spring storm system remains on track to bring the potential for moderate impacts to portions of the region. There are three variables of note that will be monitored as this storm system approaches: (1) rain, (2) wind, (3) thunderstorms. A strong 150+ kt jet streak will be directed into the Northwest. The upper level trough will take on a slight negative tilt and the timing of this feature during the day on Monday could play a big role in organizing convection across the Inland Northwest. It will also bring with it a moderate strong Atmospheric River (AR 2 to 3 level event) that will be focused more so across the state of Oregon, but a good portion of the moisture will penetrate across eastern Washington and into the Idaho Panhandle. Ensembles have sped up the arrival of rainfall with the AR moving in during the day on Sunday. The earlier timing makes sense as the strong westerly jet should act to bring in the moisture plume fairly quickly. Widespread stratiform rain will be upon the region by Sunday afternoon. Strong westerly flow across the Cascades will keep rainfall amounts down in the lee of the Cascades with the focus of heavier rainfall along the Cascade crest and in the favorable upslope areas of the Idaho Panhandle (see rainfall section for amounts and probabilistic guidance). The next potential for widespread impacts will be from the winds that pick up as the rain shuts off, or becomes convective late in the morning into the afternoon and evening. The convective element with the cold front passage increases the potential for moderate impacts with winds. About 1/4 of the ECMWF ensemble members indicate the potential for 50 mph wind gusts on the Spokane West Plains and on the Palouse. This is largely due to a deep mixing layer right as the mid level cold front clears the area and the surface front moves through. Deterministic model guidance shows up to 300 J/kg of surface based CAPE as early as 10AM-12PM late Monday morning. Potential is increasing for convection to form along the cold front with 25-45 kts of unidirectional shear in the 0-6 km layer. A CAPE of near 300 J/kg with this shear profile would be sufficient for convection to organize along the cold front. There may not be a lot of lightning due to relative low Equilibrium Levels (EL) up to around 20 kft, but strong dynamics with the advancing vorticity maximum at the base of the upper level trough should be sufficient to make up for the lack of instability. The NAM is also showing around 500 J/kg of downdraft CAPE. Putting it all together and there is good potential for enhanced winds with convection that forms by late morning into the afternoon. * Rain Amounts: This is the part of the forecast where confidence is highest. Overall, precipitation accumulations have increased a bit since 24 hours prior. In general, the Cascade crest is looking at storm total rainfall of between 1.75-3.0 inches south of Lake Chelan and between 1.0-1.75 inches north of Lake Chelan. Places such as Omak, Wenatchee, and Moses Lake will see downsloping off the Cascades with much less in the way of total accumulations in the range of 0.05-0.20 inches. Rainfall amounts will then precipitously increase over extreme eastern Washington and in the Idaho Panhandle with around 0.5-1.0 inches expected for Colville, Spokane, Pullman, and Lewiston. Then between 1.0-2.0 inches expected in the Idaho Panhandle with the westerly slopes of the higher terrain likely wringing out the most moisture and may eclipse the 2 inches mark. A near certainty that the Cascade crest will see at least an inch and around an 80% chance for at least 1.5 inches. The Idaho Panhandle has a 30-60% chance for at least an inch and around a 10% chance for more than 1.5 inches. * Winds: Increased wind speeds a bit for the afternoon. Synoptically driven winds in the absent of convection are expected to be sustained 25-30 mph with gusts of up to 40-50 mph across the exposed areas of the basin. This will includes in the lee of the Cascades across the Wenatchee Area, into the Moses Lake Area, Spokane Area, Palouse, and lower Asotin and Garfield County. Winds look strong enough that a wind advisory may be needed if models continue their trend upwards over the weekend. * Thunderstorms: As previously mentioned, the potential for thunderstorms will be greatest with the surface front from late morning into the mid afternoon hours. This will be the period of the best combination of moisture, instability, and forcing -- the three ingredients need for convection. Instability will be a limiting factor, but the combination of dynamics along the cold front and aloft should be enough to overcome this deficiency. Model trends have increased the potential for organization of convection with cold front passage. * Impacts: Best potential for moderate impacts will be with any thunderstorms that develop because they will be capable of becoming organized and would enhance the potential for wind gusts at the surface. It`s possible that in this scenario that we would see 50+ mph wind gusts with confidence increase a bit for this scenario. It will be interesting to see how Convective Allowing M models (CAMs) depict this potential as we get that data for Monday on Saturday. There is the potential for moderate impacts with just the synoptically driven winds with the passage of the cold front as well. Difficult travel is expected for high profile vehicles. Winds will also have the potential for light weight objects to become airborne. See the "Hydrology Section" below for hydro impacts. /SVH Tuesday...Ensemble model guidance is in fair agreement that the Inland NW will be under zonal flow with a 120kt westerly jet poised over southern BC. The jet is also expected to push a weak system over the Cascades in the morning and into north Idaho by afternoon. This system will be much weaker than the Sunday/Monday one. It will also feature a sizable rain shadow given persistent moderate westerly flow in the mid-atmosphere. This will me especially apparent in the afternoon as moderate cold air advection develops at 700 mbs in the lee of the Cascades. Rainfall amounts will generally be a tenth of an inch or less. The instability wont be sufficiently deep for thunderstorms, or at least it looks like that at this point. It will also remain breezy on Tuesday, however nothing like the speeds forecast for Monday. We should see wind gusts of 30-35 mph in the afternoon across portions of central Washington. Wednesday-Friday...There is very good ensemble agreement that a strong ridge will develop by midweek over the entire Western US in response to a deep trough forming between 140-150w. The result will be a significant warming and drying trend. 850 mb temperatures will climb from the 8-12C range on Tuesday to the 20-23C range on Friday. This translates to highs in the warming from the mid 60s to mid 70s on Tuesday to the 80s and potentially the lower 90s on Friday. However by Friday there are some significant model differences. Most of the GFS ensemble solutions move the aforementioned offshore trough nearly to 130w on Friday. This still results in similar temperatures, however it also introduces a small chance of showers and thunderstorms, mainly for the SE corner of WA into NC ID. Cluster analysis suggests this scenario is supported by 75% of the GFS members and around 30% of the Canadian members. Meanwhile a dry solution is supported by all of the ENS solutions. Saturday and Sunday...Model differences continue between the unsettled GFS solutions (nearly 60% of its runs), and the much drier and warmer ENS solutions. If the latter models verify it would remain dry through and very warm through the weekend. By Sunday there the models are not in good agreement. The ENS solutions have a 578 dm ridge centered over the Cascades, Canadian solutions push the ridge into western MT and the GFS runs have a much weaker ridge over central Montana. So needless to say, confidence is not high, however the majority of the ensembles support it being dry next weekend. As for temperatures there is a wide variation in solutions. The Canadian runs have 850 mb temperatures in the lower to middle 20s which would support highs in the 90s across most valley locations on Saturday with a slight cooling for Sunday. Meanwhile the GFS runs are nearly 10F cooler with an increasing chance of precipitation across the eastern third of WA and the ID Panhandle on Saturday expanding across the northern mountains and Cascades on Sunday. So stay tuned and hopefully we will see some better model agreement as we get closer in time, but right now odds favor a very warm and dry scenario. fx && .HYDROLOGY... There is a Hydrologic Statement in effect focusing on rises to stream, creeks, and some mainstem rivers. Base flows remain low across the region, and a good portion of the Inland Northwest is within a moderate drought (including the Cascades) and even a severe drought in the central portion of the Panhandle. Due to the ongoing drought conditions, the rain that is expected will largely be beneficial. With that said, the amount of water with a little bit of snow left to be melted in the highest elevations of the Cascades will result in a steep rise on the Stehekin River. Flooding is not anticipated at this time but will need to be monitored. Another potential impact will be for rock slides on steeper slopes in the Cascades and across the Idaho Panhandle. /SVH && .AVIATION... 12Z TAFS: A shortwave trough of lower pressure will pass across the Inland Northwest today. Clouds will lower and thicken this morning; however, ceilings will largely remain above 6 kft with VFR conditions. Best chances of showers today will be across the Cascades and over the northern mountains. A chance of sprinkles is possible at KPUW-KGEG-KSFF-KCOE. There will also be a 20% chance for thunderstorms near the Canadian border this afternoon. Thunderstorms are expected to remain north of airports from the Methow Valley, Omak, Colville, and Sandpoint. Winds will pick up out of the west into KEAT Saturday afternoon and evening with sustained winds up to 15 knots and gusts up to 25 knots. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in VFR conditions and thunderstorms remaining north of TAF sites. /SVH ----------------------- 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 72 49 64 52 63 43 / 20 10 90 100 90 20 Coeur d`Alene 69 48 63 51 59 43 / 20 10 80 100 100 30 Pullman 69 48 64 51 59 41 / 10 10 80 100 100 20 Lewiston 77 55 72 57 69 49 / 10 10 70 100 100 20 Colville 69 41 64 45 63 36 / 30 20 90 100 90 30 Sandpoint 66 47 61 50 57 41 / 20 20 90 100 100 60 Kellogg 68 51 61 52 56 43 / 10 20 80 100 100 60 Moses Lake 78 49 67 53 70 46 / 10 0 80 80 50 10 Wenatchee 75 54 65 54 65 48 / 10 0 90 70 50 20 Omak 75 48 67 51 69 44 / 20 0 90 80 60 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$