textproduct: Medford

KEY MESSAGES

* An unusually strong system for early September will bring wetting rain to the region with gusty winds along the coast.

* Rock slides are possible along highways 42, 138 and 38.

* Pockets of frost are possible Tuesday night east of the Cascades.

* Temperatures will be 15 to 25 degrees cooler than normal Wednesday and Thursday.

DISCUSSION

It was a breezy day Monday afternoon with a closed low spinning off the Oregon coast. That closed low will approach the Oregon coast this afternoon and bring a 20 to 50 percent chance of showers to the region. The thunderstorm probabilities are <10%, so pretty unlikely we'll observe and lightning within our portion of Oregon and California.

This closed low will become a short wave and pass through southern Oregon Tuesday evening. Another wave and surface low will strengthen off the coast Wednesday. The ECMWF ensembles have a small spread of how strong the low gets, although they all stay above 990mb. This low and cold front will bring wind and precipitation to the region during Wednesday afternoon and evening.

The one thing that did stand out was the extreme forecast index(EFI) producing values of 1 over the marine waters with a shift of tails(SOT) of 2. This would suggest this is a very strong wind event for September and there is a smaller set of ensemble members predicting significantly stronger wind gusts than what is in the model climatology for this time of year. These wind EFI and SOT values don't push inland, so these really stronger winds will be along the immediate coast with gusts to 45 mph. Also for clarity, these winds are pretty typical for a winter storm in December, although it is fairly unusually in early September.

Aside from the winds, this first front will bring wetting rain to locations west of the Cascades. As the front moves eastwards, it looks like areas east of the Cascades will likely see wetting rain Wednesday night into Thursday. Again, the EFI and SOT products are highlighting locations east of the Cascades from 0Z Thursday to 0Z Friday. Some precipitation records will likely fall on the 2nd and 3rd.

As for the impacts, rock slides along highways 38,42 and 138 are possible with this first rain of the season. Also, roads could be slick on Tuesday as a lighter rain brings up some oil to the surface. Those slick roads could continue early into the front passage Wednesday afternoon. It's also not out of the question for a few power outages along the coast as this wind hits some stressed vegetation.

Eventually, temperatures will cool behind the front with lows falling into the 40's east of the Cascades and upper 40's and lower 50's west of the Cascades. There is some concern for frost according to the NBM as it's forecasting lows in the mid 30's in Klamath Falls. However, most deterministic guidance has lows in the lower 40's with the scattered to widespread cloud cover. However, frost could occur in some locations Tuesday night before the big cold front. Temperatures will be 15 to 25 degrees cooler than normal for early September. Mount Shasta City may only hit 57 degrees on Wednesday and 59 degrees on Thursday for highs, which would be record minimum maximums for the respective days, cooler than normal indeed.

Finally, ensembles keep this upper level low over us for a while. It looks like it could just linger into Monday with spotty showers persisting across Oregon. Kind of hard to believe we could see 5 to 6 days of rain around the region in early September. Ensemble members vary greatly on location and intensity of the QPF from Friday onward, although there should be something within southern Oregon and northern California.

-Smith

AVIATION (06Z TAFs)

A mix of LIFR and IFR stratus at the coast and into the coastal valleys will thicken into Tuesday morning, and there will be a slight chance of showers or drizzle. Overnight into Tuesday morning, a mix of MVFR and VFR ceilings will fill in the Umpqua Valley, including Roseburg. After 18Z, late Tuesday morning, ceilings in these areas will lift to VFR and breaks will form in the overcast. Areas of MVFR will linger at the coast through the day and into Tuesday evening.

Elsewhere, conditions will remain VFR, with gusty southwesterly to westerly breezes (except southerly in the Shasta Valley). Also expect scattered afternoon/evening showers (mainly light) from the coast to the Cascades. There a low chance (10-15%) for thunderstorms to develop in Siskiyou County and north along/near the Cascades.

AVIATION (12Z TAFs)

A mix of LIFR and IFR stratus at the coast and into the coastal valleys will thickenm and push inland this morning, and there will be a slight chance of showers or drizzle. After 18Z, late Tuesday morning, ceilings in these areas will lift to VFR and breaks will form in the overcast. Areas of MVFR will linger at the coast through the day and into Tuesday evening.

Elsewhere, conditions will remain VFR, with gusty southwesterly to westerly breezes (except southerly in the Shasta Valley). Also expect scattered afternoon/evening showers (mainly light) from the coast to the Cascades. There a low chance <10% chace of thunderstorms in Siskiyou County and along/near the Cascades.

MFR WATCHES/WARNINGS/ADVISORIES

OR...Frost Advisory from 4 AM to 8 AM PDT Wednesday for ORZ029>031.

CA...Frost Advisory from 4 AM to 8 AM PDT Wednesday for CAZ085.

PACIFIC COASTAL WATERS...Gale Warning from 5 AM to 7 PM PDT Wednesday for PZZ350-356-370-376.