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/

Current radar imagery continues to show scattered showers over portions of the CWA. Rainfall rates associated with any of these cells is generally light, although some cells over portions of Whatcom and Skagit counties are producing brief periods of moderate rainfall. At this time, thunder remains a non-issue.

That will not remain the case, however, as the potential begins to increase as the upper level low begins to move over W WA beginning late this morning. 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

Southwesterly flow aloft will become more westerly and then northwesterly throughout the day as an upper level trough moves inland across the Pacific Northwest. Scattered shower activity continues across western Washington early this morning and will continue throughout the day with isolated chances for thunderstorms possible again this afternoon and evening. The greatest probabilities of thunderstorm development remain along Puget Sound and the Chehalis Valley, however confidence in extent of activity remains too low for inclusion in any single TAF at this time. Current conditions at area terminals remain largely VFR early this morning, but could see conditions lower to more widespread MVFR conditions across the area terminals between 12-15Z. Conditions look to largely rebound to VFR by this afternoon, though brief reductions to MVFR conditions will still be possible within any heavier shower activity. Latest hi-res guidance hints at a weak convergence zone developing across the central Sound between 03-06Z tonight as onshore flow increases. SW winds persist between 4-8 kt this morning and will increase to 8-12 kt by late this morning. Winds may briefly become variable and shift to the north across the central Sound within the vicinity of the PSCZ tonight.

KSEA...VFR conditions will lower to MVFR for a couple hours between 12-15Z. Showers will be possible at the terminal though the day, with isolated thunderstorms possible this afternoon and evening, generally between 19-00Z. A PSCZ may sink southward into the terminal tonight between 03-06Z and could bring a brief reduction in ceilings and visibilities to the terminal, as well as a quick shift of winds to the north. Otherwise, expect southwest winds to persist throughout much of the day and increase to 8-12 kt later this morning, before easing again late tonight.

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MARINE

A trough of low pressure will move onshore today, with increasing onshore flow expected along the central and eastern Strait of Juan de Fuca this afternoon and evening. A few isolated thunderstorms will be possible across the area waters this afternoon and evening which could briefly bring down visibilities. A weak surface ridge will then traverse the waters on Sunday, before another frontal system moves into the waters on Monday. Southerly winds will increase across the coastal waters Sunday night into Monday, and will likely bring the next round of headlines to the area waters. Onshore flow will increase in the system's wake with Tuesday's push of westerly winds looking like the strongest at this time along the Strait. Onshore flow will continue into Wednesday as high pressure builds back into the coastal waters. A weak system may then move across the area waters on Thursday.

Seas will generally range between 6-8 ft today, before briefly subsiding towards 4-6 ft on Sunday. Seas then look to build back to 6-8 ft Tuesday and persist at this range through Thursday.

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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...None.