Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
195 FXUS66 KSEW 281101 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 401 AM PDT Sun Apr 28 2024 .SYNOPSIS...Unsettled weather with a series of frontal systems crossing the region through the beginning of next week. Multiple rounds of showers are expected, with the chance of thunderstorms Sunday and Monday and moderate snow in the Cascades. The pattern will dry out some mid week, with more showers possible Thursday and Saturday. Warmer temperatures will also return second half of this week. && .SHORT TERM /TODAY THROUGH TUESDAY/...An occluded front passed through the region yesterday, that brought showers across the region. Post front, there are a few bands of showers still on radar in the Cascades and along the coast this morning (via a couple of surface troughs in these regions). Between systems (with an approaching shortwave trough from Canada), the showers will remain isolated through the Sunday morning hours, and begin to increase in coverage this afternoon as an upper-level low tracks southward from Canada by Monday. This system is unusually cold aloft for a late April system, with 500 mb temperatures down at -35 C, which has some implications for the impacts expected in the next 48 hours. The precipitation Sunday/Monday breaks down into higher elevations,and lower elevations. For highland areas (Cascades and Olympics), the cool air aloft is expected to work its way towards the surface tonight/Monday morning, with morning lows expected to be in the low 30s and upper 20s. This will drop snow levels to just under 2,000 feet. Southwest/west flow aloft is expected to tap moisture (in addition to a jet max driving moisture from the Pacific via westerlies). All things considered, snow will be the primary precipitation falling in the mountains from Sunday through Monday. A Winter Weather Advisory continues for the Cascades at elevations greater than 3,500 feet for moderate snowfall amounts, though impacts may be enhanced given the late season timing of this system. With cold air coming from the north, the precipitation should transition from rain to snow during the late Sunday evening/Monday morning time frame. When all set and done, most areas will likely see 4 to 8 inches of snow (despite the warmer surfaces that may compact some of the snow down on the surface). There`s a 20% chance that snowfall at Snoqualmie and Stevens Pass exceeds 6 inches. There still will be some snow flakes for areas between 2,000 feet and 3,500 feet (i.e. Snoqualmie Pass), but the snow is not expected to stick on roadways. For areas in the advisory (including Stevens Pass), minor travel impacts are expected due to slick roads. For the lowlands, post frontal isolated showers are expected to increase this afternoon as the low approaches from Canada. There`s a slight chance for thunder this afternoon, which may be limited due to slightly warmer temperatures aloft today. With the cooler air aloft in place on Monday (along with 200 J/kg CAPE, 0-3 km SRH around 100 m2s2, and 40 kt of 0-6 km shear), the thunder chances will be significantly greater. Some of the stronger storms may be able to produce small hail and gusty winds. Again, coverage will be scattered, with breaks in between bands/clusters of showers. Tuesday will start to gradually dry out as the low departs the region to the southeast. Expect high temperatures in this period to top out in the mid 50s in the low lands, with 40s in the highlands. Southwest winds will remain light around 5 to 10 mph, with higher gusts possible near waterways today, however. .LONG TERM /WEDNESDAY THROUGH SATURDAY/...The drying trend from Tuesday continues into Wednesday with a break in troughs. There still some uncertainty as to if a trough will pass through on Thursday with showers, but it appears that ensembles/deterministic models are pointing towards Friday being dry (via a ridge), and potentially a trough passing through Saturday with showers. Temperatures will start to increase on Wednesday through the remainder of the week, with highs in the low 60s for lowlands (50s for mountains), and lows in the 40s. Winds will remain light. HPR && .AVIATION...West-southwest flow aloft as upper-level troughing remains anchored over the northeastern Pacific. Southwest surface flow once again today with a 70% chance for wind gusts from 20 to 25 kt at the Puget Sound terminals and HQM from 18 to 02Z. Conditions have improved to mostly VFR this morning, with a few locations still holding on to MVFR. Mostly VFR with CIGs from 3000- 4000 feet generally this afternoon with a gradual lowering to MVFR levels possible (35% chance) after 06Z this evening for the Puget Sound terminals. Scattered showers with a lightning strike or two possible this afternoon and evening thanks to the weak instability associated with the aforementioned troughing over the Pacific Northwest. KSEA...MVFR this morning, with breezy southwest winds from 10 to 15 kt, with wind gusts increasing to 20 to 25 kt from 20-02Z (25% chance for wind gusts reaching 25 kt). Expect CIGs to gradually rise to 3000 to 4000 feet this afternoon with southerly wind gusts developing once again. There`s currently a 70% chance for wind gusts of at least 20 kt from 18-02Z. CIGs look to lower after 06Z tonight with a 40% chance for MVFR conditions to return. Davis && .MARINE...Unsettled conditions continue over the area waters with breezy to windy conditions today as another round of Small Craft Advisories go into affect later this morning for Puget Sound with southerly winds increase and through the central and East Strait of Juan de Fuca as westerly flow strengthens. Steep seas from 6 to 8 feet will build to 8 to 10 feet this afternoon. Scattered showers and thunderstorms will be possible over the area waters this afternoon and Monday afternoon, with brief and localized wind gusts to SCA thresholds possible, though not widespread enough to warrant advisories at this time over the coastal waters. Seas remain elevated through Tuesday from 8 to 10 feet followed by a gradual decrease to 4 to 6 feet into the latter portion of the week. Davis && .HYDROLOGY...No river flooding is anticipated over the next seven days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory from 11 PM this evening to 11 AM PDT Monday for West Slopes North Cascades and Passes-West Slopes North Central Cascades and Passes-West Slopes South Central Cascades and Passes. PZ...Small Craft Advisory until 5 AM PDT early this morning for Central U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm- Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 11 AM this morning to 5 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 1 PM this afternoon to 5 AM PDT Monday for East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 8 AM this morning to 8 PM PDT this evening for Puget Sound and Hood Canal. && $$