Area Forecast Discussion

SYNOPSIS

Weak, transient high pressure aloft builds into Sunday, bringing a slight warm up to the region. Onshore flow at the lower levels maintains morning clouds and dry conditions through the first half of the week. Progressively cooler conditions arrive next week as an upper trough builds offshore. Rain chances returning around the middle of the week with a passing system, followed by another disturbance on Friday.

SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/

Extensive marine stratus is dissipating this afternoon, having held on for the morning and early afternoon. A weak surface gradient will continue, with light surface winds expected overnight. Closer to the Cascade crest and through portions of the adjacent valleys and foothills, a light NE wind will be present throughout the night and into Sunday morning.

Little change in the pattern tonight and Sunday. Another round of late night and morning low clouds with patchy fog over the interior while the coast stays mostly cloudy. Expect that the stratus will linger through the morning, breaking out after 12PM local time. Sunshine in the afternoon over the interior will put highs in the mid 60s to mid 70s. Highs along the coast in the mid 60s. Given the light offshore flow in the Cascades tonight into Sunday, expect that the afternoon relative humidity will be on the drier side in the Cascades and foothills locations, down to around 30%.

A few changes Monday but the forecast will remain the same. Upper level ridge moves off to the east while a weak system moves inland well north of the area. Weak low level onshore flow in the lower levels maintaining the morning clouds and afternoon sunshine pattern.

LONG TERM /TUESDAY THROUGH SATURDAY/

Ensemble guidance remains in fairly good agreement for the midweek time period as the next upper level trough remains positioned offshore. A passing disturbance on Wednesday will bring some rain chances, but with it weakening as it clips the region, it should be rather light. Another disturbance later in the brings a reinforcing round of rain chances around Friday. Otherwise, temperatures remain rather cool with highs only reaching the lower to mid 60s late in the week. While ensembles begin to depict a larger range of possibilities by the end of the week, there is pretty good agreement that the potential for significant weather impacts remain quite low through the week ahead.

AVIATION

Westerly zonal flow will prevail aloft today. Marine stratus is beginning to scatter and lift, though portions of Puget Sound will hang onto it until 21Z-22Z. Clear conditions are returning to the coastal terminals. Given the weak gradient, winds are light and variable, with some central Puget Sound terminals beginning to go NNW and will remain that way for most of the valid TAF period. Another round of low stratus clouds looks likely tomorrow morning, setting in after 12Z and lasting through Sunday morning.

KSEA...LIFR cigs have improved in recent obs to IFR. Visbys have also risen to 10SM. Cigs below 1000 feet will linger until 22Z, with scattering and clearing thereafter. Winds have been light and generally out of the NW at 3 kts or less. Expect light N winds to continue throughout the overnight period. There is still a 30% chance of IFR cigs (20% chance LIFR) tomorrow morning, beginning between 12Z-14Z. The TAF indicates a return to IFR and LIFR cigs and visbys tomorrow morning, and will likely improve after 19Z-20Z with continued N to NNE winds 4-6 kts.

21

MARINE

High pressure continues over the waters this weekend into the start of the week. Stronger onshore flow through the Strait of Juan de Fuca late Monday bring around a 50-60% chance of advisory strength winds, representing the highest chances of the next several days for meeting Small Craft Advisory thresholds. Otherwise, expect seas to build over the coastal waters late Monday as a longer-period northwest swell arrives into the region. This will bring seas from around 4 to 5 ft late Sunday up to around 7 to 9 ft over the coastal waters early next week.

FIRE WEATHER

While general low-level onshore flow will maintain cloud cover and higher humidities across the region, expect to see some light north or northeast winds near the Cascades tonight through Sunday, with Sunday afternoon RH values approaching the 30-35% range. Given the overall pattern, concerns remain minimal through much of the week ahead, with cooler temperatures and rain chances returning by the middle of next week.

SEW WATCHES / WARNINGS / ADVISORIES

WA...None. PZ...None.