Area Forecast Discussion

SYNOPSIS

Low pressure will move directly over western Washington today, bringing widespread showers with the chance for isolated thunderstorms this afternoon and evening. Any storms that develop will be capable of lightning, gusty winds, and brief periods of heavy rain. While the threat for thunder ends this evening, a generally troughy pattern will keep weather cool and unsettled for the first half of the upcoming week. A warming and drying trend takes hold late next week.

SHORT TERM /TODAY THROUGH MONDAY/

General instability associated with the leading edge of such a feature coupled with cold air aloft will certainly lead to another day of thunderstorm potential...and models and SPC outlook has rightly locked on to this. However, there are some offsetting factors that look to limit the amount of activity that may spring up this afternoon and early evening. First, while there is cold air aloft, the air at the surface is not terribly toasty either as daytime highs are progged to remain largely in the mid to upper 50s. In turn, these cooler surface conditions will keep any LI and available CAPE pretty low. These offsetting factors actually serve to help highlight where the better chances for thunder will emerge...mainly around Puget Sound, the south Sound area and the SW interior/Chehalis valley. NBM ProbThunder values echo this, but might be a little too generous...at times indicating a 30-40 pct chance for thunder in these areas. To take into account offsetting factors, trimmed to allow 30 pct to be the roof and even that being limited to the 2-6 PM PDT window. This upper low ejects out of the area overnight tonight, taking the bulk of activity with it, although and potential for thunder looks to wrap up by sunset or maybe an hour after.

A shortwave ridge following in the wake of the aforementioned low will bring a few degrees of warming as daytime highs creep into the upper 50s to lower 60s but is not strong enough to remove much of the moisture in the area. Clouds look to remain in place, albeit with some breaks...and even an isolated shower or two will be possible, mainly in the Cascades.

A frontal system reaches the coastline Sunday night but hesitates to push inland until Monday afternoon...when the best chances for rainfall emerge. Temps take a minor hit to reflect this, only dropping a degree or two from Sunday's highs, but still remaining within the upper 50s to lower 60s. No thunderstorm threat appears to accompany this frontal passage at this time.

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LONG TERM /MONDAY NIGHT THROUGH FRIDAY/

A series of upper level lows will keep troughy conditions over W WA into the first half of the upcoming week...and as such keep cooler temperatures and the chances for precipitation in the forecast. As these seem to follow one right after the other, it will be difficult to differentiate from rains associated with the upper low exiting Monday night and the fresh system entering into the area Tuesday into Wednesday. Latest forecast and model data suggest a brief dip in PoPs Tuesday night into early Wednesday morning, but will need to monitor future runs to see if consistency will build confidence in this window or if it shifts about both in areal terms and temporally. Cooler daytime highs will remain the rule for both of these days with Tuesday still ranging in the upper 50s to lower 60s while Wednesday sees a slight warming with lower 60s area-wide.

As this all happens over land, upper level ridging will continue to build over the eastern Pacific. Once this feature starts moving eastward Wednesday night, models do show it beginning to scour out any lingering moisture...although how quickly varies model to model. Wide agreement that Thursday and Friday will be dry thanks to this feature as its impacts spread eastward and the warming trend for these two days starting to show signs of being able to leap over tall buildings in a single bound. Daytime highs Thursday see plenty of upper 60s to lower 70s for the interior and Friday jumps further with mid 70s to near 80 expected...making up the distance from below normal to above normal very quickly.

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AVIATION

An upper trough axis over the coastal waters this morning will shift inland across the interior this afternoon then east of the Cascades tonight. Westerly flow aloft will become north to northwesterly tonight. A combination of daytime heating and cool temperatures aloft will destabilize the air mass this afternoon for scattered showers and isolated thunderstorms. Low level convergence will enhance this across central Puget Sound. Shower coverage will diminish this evening. MVFR ceilings can be expected around heavier showers and/or isolated thunderstorms. Remnant low level moisture and light surface winds will lead to greater coverage of MVFR ceilings across the region on Sunday morning. This will gradually lift to VFR Sunday afternoon.

KSEA...Tempo MVFR ceilings expected around some of the heavier showers today. Confidence in thunderstorm activity in the vicinity of the terminal has increased and is thus reflected in a prevailing line for 23Z in the TAF. Gusty and erratic winds can be expected around any storms that do develop. More predominant MVFR ceilings are expected Sunday AM as remnant low level moisture remains in the area. These will lift to VFR Sunday afternoon. Surface winds southwesterly 9 to 13 knots this morning into the afternoon. The convergence zone is expected to shift into the vicinity of the terminal for few hours this evening for a brief to switch to NW-NE winds. Winds will switch back to prevailing southerlies after 06Z.

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MARINE

A weak trough over the coastal waters this morning will shift inland tonight as weak high pressure builds over the coastal waters. Another trough and associated frontal system will approach the outer coastal waters Sunday evening then slow and dissipate. Onshore flow will increase Tuesday and Wednesday as broad high pressure gradually expands into the coastal and offshore waters. Onshore weakens late next week as thermally induced low pressure begins to expand northward across Western Oregon.

Coastal seas are expected to remain below 10 feet through the period.

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FIRE WEATHER

Although lightning will be possible today and rainfall amounts may not quite make it up to the 0.25 in threshold separating wet and dry storms, Min RH values exceeding 60 pct should limit any fire weather concerns today. As the generally troughy pattern remains in place over W WA in the first half of the upcoming week, again, some limiting moisture is expected to remain in place.

As upper level ridging builds into the area starting Thursday, conditions, including fuels, will need to be monitored as temperatures are expected to climb to above normal values entering into the week 2 time period.

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SEW WATCHES / WARNINGS / ADVISORIES

WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.