textproduct: Seattle/Tacoma

SYNOPSIS

An upper level low off the coast will keep temperatures seasonable again today, with slight chances for showers and possibly an isolated thunderstorm or two over the North Cascades this evening. Upper level ridging returns Monday, bringing another round of hot and dry conditions with well above normal temperatures are expected by mid-week.

LONG TERM /WEDNESDAY THROUGH SATURDAY/

The ridge will remain the dominant weather feature for much of the long term, however, models are starting to advertise some mitigating features that could start to rein in temperatures by the end of the week. This is going to depend largely on the track of the upper low that retreated from the area as stated above. Models show a fairly uniform northerly trek for the system and as it moves, it could bring in a couple of shortwave disturbances with the first being early Thursday morning. While this feature is expected to be dry, some strengthening northerlies/onshore flow will help to actually trigger a cooling trend in daytime highs. Temps look to peak on Wed, with mid to upper 80s for interior lowlands...maybe as high as 90 for the Cascade valleys...before falling back into the lower to mid 80s for Thu. A secondary shortwave nearly 24 hours later will have a similar effect, bringing temps down further to the upper 70s to around 80. Saturday raises some interesting questions, as while there is general agreement on a transition over to a more troughy pattern, the orientation of the trough will play a big role as to whether or not W WA could see the return for some chances for precip. No consensus has emerged at this time, so confidence remains low. However, this looks to at least keep the cooling trend intact, with daytime highs a degree or two cooler than Friday.

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AVIATION

Southerly flow aloft will increase today as an upper low offshore drifts towards the coast. Areas of MVFR/IFR stratus will redevelop early this morning and fill in eastward, though some uncertainty remains over how far inland the stratus will expand. Ceilings will then improve this afternoon to VFR. A few showers may develop this afternoon over the Olympic Peninsula and interior areas mainly north of KPAE.

KSEA...VFR conditions early this morning with mid and high clouds at times. The presence of the upper trough offshore complicates the forecast in terms of low ceiling development this morning. MVFR/IFR ceilings are possible between 14z-18z, but confidence is low. Surface winds light northerly shifting W/NW Sunday afternoon.

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MARINE

An onshore flow pattern will continue into next week. Onshore flow will vary in intensity, with westerly pushes through the Strait of Juan de Fuca each evening. An SCA remains in effect this evening for the central and eastern Strait of Juan de Fuca. Onshore flow will then continue into midweek.

Although primarily dry conditions are expected, there is less than a 10% chance of thunderstorms into Sunday as well.

Seas of 2 to 4 feet for the Coastal Waters through Tuesday will build to 4 to 6 feet by midweek.

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FIRE WEATHER

A slight chance of dry thunderstorms is forecast for the north Cascades this evening thanks to a low pressure system parked offshore sending a weak impulse into the area. While this will steepen lapse rates and offering some ascent, it will likely be weak and perhaps more terrain driven. With no significant source of moisture, there is some concern over dry lightning strikes, however, given the poor dynamics and waffling of model runs as to this prospect, confidence remains low. Any storms that happen to develop will be capable of gusty and erratic winds as a by-product of the steep lapse rates and downdraft CAPE, as well as lightning on dry fuels. The probabilities are capped at 10-15%.

Later this week, temperatures will be climbing and humidity dropping off, with poor overnight recoveries. Temperatures will be well into the upper 80s to mid 90s, with near 100 degrees in the Cascade valleys as periods of light offshore flow develop during the day. Elevated concerns for hot, dry and unstable conditions will persist through much of the mid to late week period.

21/18

SEW WATCHES/WARNINGS/ADVISORIES

WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.