Area Forecast Discussion

SYNOPSIS

Very hot temperatures today will persist overnight into Wednesday. Highs in the mid to upper 90s expected inland with chances of highs up to 100 degrees F on Wednesday. Monsoonal moisture will move in on Wednesday which will complicate the forecast. Thunderstorm and shower chances increase on Wednesday, especially in the afternoon with additional daytime heating. Widespread lightning not out of the question. Showers persist into Thursday and cooler temperatures expected through the remainder of the week.

SHORT TERM...Now through Thursday

Today has been on the hot side as high pressure over the region has built. Overnight lows were higher than forecast with temperatures reported around 65 degrees F in urban areas. This created a launching pad for today's highs where temperatures are n the widespread 90s inland and in the mid to upper 50s along the Coast. Even along the Cascades, some sites are reporting highs of 90-91 degrees as of 2 PM. Will see a continued warm up my a few more degrees today. One caveat is in the southern Willamette Valley where some bleed over monsoonal moisture is filtering in. Satellite is showing cloud cover moving in which may inhibit some daytime warming around Eugene and the foothills of the Willamette Natl Forest. With that said, still increasingly hot and there appear to be enough breaks where there is ample time for warming. Radar shows virga also in the region. Overnight temperatures will fall but are not expected to see significant relief outside of the coast. These overnight temperatures combined with today's high sparked the need for additional Heat Advisories along the south Washington Cascade and central Oregon Cascade foothills through at least tonight.

Moving into Wednesday we are going to see a bit of a transition. While some models would suggest the pattern is fairly cut and dry, the overall pattern is actually quite messy which is leading to some levels of uncertainty. While models want to maintain the overall high pressure over the region, a combination of monsoonal moisture influx and a low pressure system off of the Oregon/California border will drive a much more active weather pattern. Will see southerly flow maintaining which, when combined with warm overnight temperatures, will once again bring an influx of warm air to the region. Each model run has shown the high temperature forecast increase steadily now with multiple areas seeing forecast highs near or greater than 100 degrees F. However, that is not the whole picture as there is that moisture to deal with. Will see higher than normal humidity on Wednesday which will make the air appear hotter than it normally would if the atmosphere was drier. Major HeatRisk exists through much of the Willamette Valley.

This incoming moisture will not only impact how hot conditions feel, but will also change the sensible weather. High resolution models have begun to show more widespread and potentially intense precipitation starting as early as Wednesday morning with higher confidence on it starting Wednesday afternoon. One thing to note though is that it likely will not be in the form of rain, but rather as thunderstorms. Convective indices like Most Unstable CAPE (MUCAPE) are showing elevated convection around 1500 J/kg over the Cascades and even some inland sites. This is only amplified when taking into account daytime heating which would help us surpass any caps that form at the surface. Therefore there is around a 15-20% chance of a widespread thunderstorm situation over all of northwest Oregon and southwest Washington. Over the Cascades though that is higher with closer to a 30-35% chance. The highest probabilities will be over the Willamette Natl Forest on Wednesday afternoon and evening. Some models are showing a few cells popping up on Wednesday morning along the coast, but there is much higher consensus on the afternoon time frame. With this in mind, these storms are forecast to be wet with limited wind. However, widespread lightning (similar to last week) cannot be ruled out. It is critical that if you hear thunder or see lightning that you take cover immediately inside of a sturdy structure, get off of the water, and limit outdoor activities like golfing or playing baseball.

Storms and showers are expected to persist overnight, though will become lighter as we lose daytime heating. Isolated showers into Thursday morning. -27

LONG TERM...Thursday night through Tuesday

By Thursday, the ridge begins to weaken as a broader trough moves toward the Pacific Northwest. This should promote a return to more noticeable onshore flow and begin a gradual cooling trend that carries into late week. Temperatures are still likely to remain somewhat above seasonal normals Thursday and Friday, but not to the degree seen earlier in the week. Looking beyond Friday, forecast guidance currently does not show a strong signal for meaningful precipitation, while temperatures trend toward the lower 80s through the weekend and into early next week. -12

AVIATION

VFR flight conditions expected inland today with BKN/OVC high clouds in central OR. Any lingering marine stratus along the coast should dissipate and improve to VFR by 18-19z Tue. A resurgence of marine stratus is likely again tonight into Wednesday morning with IFR/LIFR cigs. At KONP, stratus returns after 00-03z Wed with LIFR conditions favored with a 40-60% chance. Stratus could return at KAST anytime after 06z Wed with a 20-40% chance of IFR and LIFR conditions, higher chances after 12z Wed. North to northwest winds generally less than 10 kts.

KPDX AND APPROACHES...VFR conditions prevail with northwest winds less than 8 kts. -03

MARINE

A typical summertime pattern will continue through the middle of this week with persistent north to northwest winds over the coastal waters. Winds then shift southwest to westerly Thursday into the weekend as weak low pressure develops over the waters. Winds will remain light less than 10-15 kts. Seas will be 3-6 ft at 8-9 seconds through the weekend.

On Wednesday, there is a chance for showers and thunderstorms across all of the waters through the majority of the day and overnight hours. There could be frequent lightning and gusty and erratic winds with any thunderstorm that forms. -03

PQR WATCHES / WARNINGS / ADVISORIES

OR...Heat Advisory until 11 PM PDT Wednesday for ORZ108>115-120>122. Heat Advisory until 11 PM PDT this evening for ORZ117-124. WA...Heat Advisory until 11 PM PDT Wednesday for WAZ204>210. PZ...None.